EEL 3211C - BASIC ELECTRIC ENERGY CONVERSION

LABORATORY MANUAL

Revised

Revision 4.1.0

Patrick Shea

Edited by Jerome Thompson

I. COURSE OBJECTIVES

A. What are we going to do?

One of the main purposes of this lab is to familiarize the student with the main areas of study of conventional electric energy conversion:

1) Power Measurement & Instrumentation 2) Single and Three Phase Transformers 3) DC Motors & Generators 4) Induction Machines

B. How are we going to do it?

In order to study these areas, the course will consist of the following experiments:

1) Alternating Current. 2) Three Phase Circuits 3) Magnetic Circuits and Single Phase Transformers 4) Three Phase Transformers 5) AC Induction Motors 6) DC Machines

These experiments will enable the student:

1) To gain an understanding of the characteristics of the principles governing electric energy conversion devices.

2) To understand the validity of the most elementary models of said devices

3) To gain a working knowledge of power instrumentation and the limitations of each instrument. C. What else?

Another objective of this lab is to help the future professional, in our case the engineer, to develop well rounded

laboratory skills. These skills will enable this professional to do the following:

1) To follow instructions. 2) To translate a schematic diagram into a working piece of hardware. 3) To use appropriately instrumentation on machines. 4) To take data systematically in table form from an experiment. 5) To debug a faulty circuit set up.

6) To report IN A PROFESSIONAL MANNER the results of the experiment, with all the supporting evidence such as the purpose, theory, procedure & data, analysis of data, and conclusions.

The first five skills will be learned in the laboratory environment, and the sixth one will be mastered outside the classroom following the guidelines given in this manual and during class.

II. SAFETY RULES AND REGULATIONS

The electrical power laboratory constitutes a hazardous environment; it has the potential to seriously injure and even kill! Theodore Bernstein has made some research on the area of electrocution. Some facts taken from his 1983 paper are the following:

• Threshold of perception for a finger-tapping contact at 60 Hz is about 0.2 mA.

• Let-go Current (60 Hz) for women = 6 mA, for men = 9 mA.

• Asphyxia can be caused by 60 Hz currents of 40-60 mA across the chest.

• Ventricular Fibrillation can be caused by currents of 150 mA for 1 sec.

These current levels are in the mA range; however, in this lab, you will handle excitation currents of up to 860 mA (!) and armature currents of up to 6 or 7 A!

So, strict adherence to the following rules will greatly decrease the probability that accidents will occur.

All personnel using the laboratory are encouraged to constantly THINK SAFETY. The following rules should be followed on top of basic common sense.

1) Always assume all circuits are energized unless you know with certainty they are not.

2) Before starting the experiment, follow Power Lab Safety (PLS) rules. Check that:

a) all switches and circuit breakers are in the open position, SW & CB = 0 b) all power supplies (AC & DC) are set to zero volts, PS = 0

3) Never energize a test circuit without the instructor's permission.

4) If you need to make changes to an energized circuit, follow the PLS rules procedure:

a) Set power supplies to zero volts, PS = 0

b) Turn-off your circuit, Ckt = OFF

c) Make necessary changes,

d) Turn-on your circuit, Ckt = ON

e) Proceed with your experiment.

5) Never touch moving parts of machinery, nor even get near bare metal in energized circuits.

6) Never work with electrical circuits with wet or moist hands.

7) If you know or suspect that an accident has occurred, de-energize all affected circuits. Do not jeopardize your own safety in doing so.

8) Do not work on a cluttered lab table. Keep your work area organized and as free as possible from clutter. It is important for safety reasons and it makes it easier to trace out your test circuit.

Do not wear clothing or jewelry which would cause you to get involved in an accident (e.g. long necklaces (particularly metal ones), neckties, rings, bracelets, and loose-fitting garments. Shoes, preferably rubber soled ones, must be worn in the lab. Long hair also presents a hazard near moving parts of machinery.

9) Think out ahead of time the consequences of closing or opening a switch, and if you do not know the function of a switch, terminal, etc., DO NOT GUESS! Ask the instructor.

10) Do not "fiddle with" equipment not directly involved in your experiment, and never engage in horseplay or any other activity which would distract others.

11) At the end of the experiment, neatly put the equipment (meters, cables, etc.) you used in their appropriate place.

Learn the following definitions of these three signs:

WARNING: a hazard that could result in injury or death to personnel. CAUTION: a hazard that could result in damage to or destruction of part or the entire product. NOTE: very important information.

III. LABORATORY POLICIES

1) Preparation: The student is expected to read and understand the laboratory procedure before starting the experiment, and to have answers for any questions contained in any assigned PRELAB and/or Quizzes.

2) Laboratory decorum:

a) No eating, drinking or smoking is allowed in the laboratory

b) Students are never allowed to be in the laboratory without supervision.

c) Be on time for your assigned lab. Starting Fall 2017 there is a 5 min grace period after the start of your lab in which you are still allowed to enter and participate in lab. After that the lab door will be closed and you will not be allowed to participate in the current lab session. You may participate in another session – if another one is available (please be on time for that make up lab), but you will receive a 10 point deduction on your lab report. No excuses, be responsible!

3) Teams: Students will work in teams of two ideally (3 if an odd number of students are present in lab).

4) The TA or Faculty Instructor may send announcements through the class Canvas webpage. It’s the responsibility of the student to check Canvas in time. Lack of timely response due to tardiness or not seeing announcements will not be tolerated.

5) Experiments:

a) It is the student's responsibility to go through his or her class notes, textbook and the fundamentals provided in the lab manual before coming to the lab. Lack of preparation can affect the Lab participation grades.

b) If you have any doubts discuss with the TA, rather than doing mistakes because that leads to greater loss of grades than not being thorough with the theory. The TA may ask questions about any step or calculations done during the lab. If a student is found not sure about what he/she has done, the grades for lab participation will get adversely affected.

c) Students MUST follow the instructions very thoroughly in the order indicated.

d) Setting up the circuit for the experiment takes the longest, so BE PATIENT, AND DON'T RUSH YOUR TEAMMATES.

e) At the end of the experiment, leave cables & meters arranged as neatly as possible. Failure to clean up your work station and shut equipment off will result in a lowered participation grade.

6) Reports:

a) Reports are due before you come into your next scheduled lab (this means there may be times when your lab is not due for 1-2 weeks). Therefore even if you are physically not present for your assigned lab (whatever the case may be), your lab report will still be due at the start time of that lab.

b) Reports will be graded based on content AND presentation.

c) Each student will be responsible for a report for every lab.

d) You have one week after the lab report has been graded to dispute your score (Ex. Do not approach or message the TA asking for points back on a lab from the beginning of the semester).

IV. LAB REPORT INFO

GENERAL INFORMATION ABOUT THE LAB REPORTS

In the EEL3211C lab, you will be required to write formal lab reports using modified IEEE format. The following information is provided to help you to do so. Read and follow the instructions carefully. Points will be taken off at the TA’s discretion if this format is not followed. For an example of this modified IEEE format refer to the EEL3211 Lab docs link in the Files section of Canvas.

**Note: Please do not use your partner’s lab reports or a friend’s lab report from a previous semester for submission via

Canvas. When you turn in your lab reports it is checked for plagiarism via Turnitin. Show integrity and do your own work. ***

Lab Report Outline:

On the header place your name, partner’s name, lab #

1) Purpose

Explain what you are trying to demonstrate in the whole lab.

2) Theory (supporting information)

Explain in brief the concepts related to this lab i.e. electromagnetics, AC/DC voltage, motor/generator functionality, etc. You may use an internet database or books supporting your information (if written in your own words and cited after the conclusion)

3) Procedure & Data Analysis

The lab has been broken down into sections; hence, the report should also be broken into exact same sections.

All steps, diagrams, calculations, answers to questions and results for each section should be together. Use step numbers (e.g. 1, 2, 3...) to give a short description of what YOU did (include circuit diagrams).

Present data neatly in the appropriate format: tables & graphs.

Use the screen captures for graphical input into your reports. It is suggested that you use Windows Snipping Tool

• Copy/Paste all photos or data tables to Excel or Word

• Store the data on your USB drive and share data with teammates.

• Number and label tables, figures, and graphs.

• Show calculations done clearly without missing steps. Highlight the result.

• Compare your measured results with other calculated results. When possible, show the accuracy of your results to the expected values (% Error)

• Do NOT include your original (draft) data sheet here.

4) Conclusions

• Itemize (in bullet form, or use numbers) your conclusions on all sections.

• Refer back to the purpose of the experiment and say how much was achieved.

5) References For any outside support data utilized, please note using IEEE text notation. For the reference section, it is suggested to utilize the following website to cite information. http://www.makecitation.com/ieee_books_style.php Link active as of 5/31/2015

B. How to Write a Good Report.

1) Remember, you are writing for the convenience of the reader, not the author.

2) Whenever possible, the report should "stand alone" (i.e. not require reference to information external to the report itself). If this is not practical, reference to the appropriate source should be made in a clear and complete manner.

3) The mechanical aspects of report writing require clarity, neatness, and conciseness for the obvious reason that these details aid the reader in understanding what you are trying to communicate. Follow these guidelines:

a) Paper : White 8.5 inch x 11 inch

b) Reports must be typed

c) If plots are not computer generated, use graph paper to plot data.

d) Number and title all graphics (brief description)

e) Titles for tables on top

f) Titles for figures and graphs belong on the bottom

g) Always clearly label axes (variable name and its units) on all your graphs

h) Correct spelling and grammar are essential

i) Avoid double-spacing

j) Plot Convention: Y vs. X. Which means Y on the vertical axis, and X on the horizontal axis!

Points will be taken off if these guidelines are not followed.

Grading Criteria:

Laboratory reports will be graded on the following two criteria:

A) Content—the actual information. • Purpose • Theory • Procedure and Data Analysis • Conclusion • References

B) Presentation—how the information is presented.

• Following directions • Organization • Neatness • Legibility

• Conciseness

•

“Do’s Don'ts”:

1) Do read the material that pertains to the experiment BEFORE coming to lab. Focus mainly on the purpose of the lab, and on the procedure section.

2) Do the pre-labs if they are assigned.

3) Follow the instructions on the procedure section, STEP-BY-STEP!

4) Respect your teammate’s right to understand what you all are doing.

5) Don't get upset at the lab instructor because of the malfunctioning of the machines and/or meters. At

times, there is no way for the instructor to order a machine and/or fix a malfunctioning machine in a short period of time.

6) Don't ask the lab instructor to review material (such as the dot convention on transformers; that is Circuits-I material!) or to review the theoretical principles of any other experiment. The theory for labs is covered during the in-class section of this course. The reason for this is that there is no time to review the material for the experiments because most of them are quite lengthy; that is the purpose of the pre-labs: to make you go back to your notes and text book to review the material for that experiment.

V. Overview of the LVDAC-EMS

The Lab-Volt Data Acquisition and Management for Electromechanical Systems (LVDAC-EMS) is a computer-based system for measuring, observing, and analyzing electrical and mechanical parameters in electromechanical systems and power electronics circuits. The LVDAC-EMS system consists of a data acquisition interface (DAI) module and the LVDAC-EMS software.

The DAI module consists of an isolation unit and a data acquisition unit. These units convert the high-level voltages and currents found in electric power systems and power electronics circuits into digital numbers that are used by the personal computer which runs the LVDAC-EMS software. The LVDAC-EMS software is a complete set of instruments that run on Microsoft Windows. Available instruments are voltmeters, ammeters, power meters, an eight-channel oscilloscope, a phasor analyzer, a harmonic analyzer, and a spectrum analyzer. Furthermore, build-in capabilities for data storage and graphical representation, as well as programmable meter functions, allow unimaginable possibilities for studying and analyzing electromechanical systems and power electronic circuits.

Once running, the LVDAC-EMS system uses the data transmitted by the DAI module to calculate the values indicated by the meters and update the waveforms, phasors, harmonic components, and frequency spectra displayed by the Oscilloscope, Phasor Analyzer, Harmonic Analyzer, and Spectrum Analyzer, respectively.

Introduction

This section is divided into three subsections. The subsection, Description of a Data Acquisition System, explains the advantages of using a data acquisition system for the study and analysis of electromechanical systems and power electronics circuits. It also explains the operation of data acquisition systems. The subsection, Data Acquisition System of the LVDAC-EMS System, describes the function of the Data Acquisition Interface module in the LVDAC-EMS system. It also introduces the two versions of the Data Acquisition Interface (DAI) module that can be used in the LVDAC-EMS system.

Description of a Data Acquisition System

Studying electrical power circuits, especially three-phase power circuits, involve measuring different parameters as well as observing voltage and current waveforms. Voltage, current, power (active, reactive, apparent), impedance, motor speed, and torque are some of the many parameters usually measured, and phasor analysis can be essential for detailed three-phase study.

Because most conventional instruments only present one type of information, measuring all the different parameters requires a variety of instruments. The instruments are often limited with fixed scales and input ranges, most oscilloscopes only have two channels, and torque/speed measuring devices are not commonplace items. The amount of information that must be collected becomes enormous, and most data analysis, calculation, and plotting of graphs must be done by hand. Therefore, in-depth study of many circuits is not always easy to do with conventional equipment.

A data acquisition system is a computer-based system that can gather and analyze information from several external sources, and perform different calculations on the acquired data. A single computer can thus replace a variety of meters and instruments, display several waveforms simultaneously, analyze waveforms and data to extract important information, record data, and plot graphs. Generally, data acquisition systems gather information represented by electrical signals. Some information, such as the input or output voltage of an electrical device is already in electrical form. Other information can be converted to electrical form by a transducer. For example, the speed of a motor can be converted into an electrical signal by a speed sensor.

The electrical signal from a speed sensor is called an analog signal because it is analogous to the speed; if the speed increases, the voltage increases, and vice versa. The voltage of an analog signal can vary continuously

and take on any value within a certain range.

Computers are digital devices that use discrete numbers to store and process data. A data acquisition system therefore requires a circuit that converts continuous analog signals to discrete digital values. The type of circuit used for this purpose is called an analog-to-digital converter, or ND converter. The sampling and conversion process is illustrated in Figure 1.

Figure 1: Sampling and analog-to-digital (A/D) conversion of an analog signal.

The analog signal is first sampled at regular intervals by a sample-and-hold circuit, which holds each sampled level until the analog-to-digital (ND) converter has converted it to a digital number. The rate at which the signal is sampled is called the sampling rate. The higher the sampling rate, the more faithfully the digital numbers produced will follow the original signal. High sampling rates, however, generate lots of numbers and these may fill the computer memory very quickly, so the sampling rate should not be too high. In theory, the lowest sampling rate that can be used is equal to twice the frequency of the highest frequency component in the analog signal. In practice, most systems use a higher sampling rate than that.

When a data acquisition system must acquire data from several different sources, a single ND converter can be used along with a multiplexer, as shown in Figure 2. The multiplexer is a switch that selects each analog input, or channel, in turn. Each time the multiplexer selects a new analog input, the signal present at the input is sampled and converted to a digital number.

The number of channels sampled by the multiplexer affects the sampling rate per channel. If the ND converter can convert 100 000 samples per second, a single channel could be sampled at that rate. However, if two channels were used, each channel would be sampled at 50 000 samples per second, and four channels would be sampled at 25 000 samples per second.

Figure 2: Input configuration of a typical multi-channel A/D converter.

Depending on the application, a data acquisition system may sample signals continuously, or it may take a certain number of samples and then stop sampling until commanded to take another batch of samples. In either case, the digital numbers representing the samples can be processed and analyzed by the computer to extract useful information. Usually, this information can be presented on the computer screen in different ways, which are selected by the user of the system.

Data Acquisition System of the LVDAC-EMS System

In the LVDAC-EMS system, data is acquired through an isolation unit and a data acquisition unit. Both units are enclosed in the Data Acquisition Interface (DAI) module.

The isolation unit isolates and converts the high-level voltages and currents applied to the voltage and current inputs of the DAI module into low-voltage signals. Each low-voltage signal is proportional to, and electrically isolated from, the high-level electrical signal present at the corresponding input. The low-voltage signals, and other signals coming from low-voltage inputs of the DAI module, are internally routed to the data acquisition unit.

The data acquisition unit contains the circuitry needed for analog signal sampling and A/D conversion. It converts the low-voltage signals into corresponding digital data. The digital data is then read and analyzed by the LVDAC-EMS software running in the personal computer. The results are displayed on the computer screen according to the representation selected by the user. The display can be a panel of meters showing the values of the measured parameters, an oscilloscope showing the waveforms of the measured parameters, etc.

Figure 3: Overview of the data acquisition and management process in the LVDAC-EMS system.

A standard USB port cable is used to connect the DAI module to the personal computer. Figure 3 gives an overview of the data acquisition and management process.

Data Acquisition Interface module

The front panel of the Data Acquisition Interface (DAI) module, Model 9062-1, is shown in Figure 4. It consists of three high-voltage inputs (El to E3), three high-current inputs (I1 to I3), a torque input (ANALOG INPUT T), a speed input (ANALOG INPUT N), two ANALOG OUTPUTS, eight AUXILIARY ANALOG INPUTS, and a SYNC. INPUT. Access to inputs El to E3 and I1 to I3 is made through 4-mm safety banana jacks mounted on the front panel. Access to the ANALOG INPUTS T and N, ANALOG OUTPUTS, AUXILIARY ANALOG INPUTS, and SYNC. INPUT is made through miniature banana jacks also mounted on the front panel.

Inputs El to E4 and I1 to I4 are fully protected against over-voltage and short-circuit conditions. Furthermore, these inputs are electrically isolated from the circuitry in the DAI module through voltage and current isolators. This allows direct connection to electrical power circuits. Each of these isolators has two measuring ranges (high and low), the range selection being controlled by the LVDAC-EMS software. The following table shows the rating of the low and high ranges of the voltage and current isolators for the various versions of the DAI module.

Figure 4: Front panel of the DAI module, Model 9062-1.

ANALOG INPUTS T and N are used for measuring the torque and speed of a motor. ANALOG INPUT T is designed to receive the output signal of a torque sensor having a sensitivity of 0.3 N•m/V (2.66 lbf•in/V).

Similarly, ANALOG INPUT N is designed to receive the output signal of a speed sensor having a sensitivity of 500 r/min / V. For example, ANALOG INPUTS T and N of the Data Acquisition Interface module can be connected to the corresponding outputs of the Lab-Volt Prime Mover / Dynamometer, Model 8960.

The ANALOG OUTPUTS provide a voltage whose amplitude can be set between +10 V and -10 V using the LVDAC-EMS software. Each voltage can be used to control a device in an electrical power circuit through the LVDAC-EMS software.

AUXILIARY ANALOG INPUTS 1 to 8 are low voltage inputs (±10 V max.). These inputs can be connected to the outputs of voltage isolators, current isolators, torque sensors, speed sensors, etc. to measure various type of parameters. Each of these inputs is programmable through the LVDAC- EMS software. The SYNC. INPUT is a digital input (TTL levels). It is used for external synchronization of the oscilloscope in the LVDAC-EMS system.

All data exchanges between the DAI module and the computer running software LVDAC-EMS are made through a USB port connection. The USB port connector is located in the upper right corner of the module's front panel. Note that the data exchanged between the DAI module and the computer not only includes the digitized input and output signals but also control signals for range selection on the isolators and status verification.

The DAI module, Model 9062-1 which is what we use, requires low-voltage AC power (24 V). This voltage must be applied to one of the LOW POWER INPUT jacks on the front panel of the DAI module. The POWER ON LED confirms the presence of input power. Since the interface is isolated we will be powering it on before we begin the experiment and leaving it on for the duration of the lab.

Overview of the Power Supply.

The Power Supply module, Model 8821-2X, provides the necessary ac/dc power, both fixed and variable, single-phase and three-phase, to perform all of the laboratory experiments in the Lab-Volt training program in electrical power technology.

The Power Supply module is a rugged piece of equipment designed to take a lot of abuse. It incorporates features that protect it against damage from overloads and accidental short-circuits. Most important, these features include safety measures to adequately protect the student against accidental shock. Besides the main 3-phase, on-off circuit breaker on the front panel, all of the outputs have their own circuit breakers.

They can be reset by a common reset button located on the front panel. The Power

Supply module has five outputs:

A fixed-voltage, 3-phase, 4-wire ac output (terminals 1, 2, 3, and N on the front panel)

A fixed-voltage dc output (terminals 8 and N on the front panel)

A variable-voltage, 3-phase, 4-wire ac output (terminals 4, 5, 6, and N on the front panel)

A variable-voltage dc output (terminals 7 and N on the front panel)

A fixed-voltage, low power ac output

Figure 5: Front Panel of the Model 8821-20 Power Supply (120/208 V -60 HZ).

The variable-voltage ac and dc outputs are controlled by the single control knob on the module front panel. A built-in ac/dc voltmeter indicates all the variable ac and the variable and fixed dc output voltages according to the position of the voltmeter selector switch. The rated current of each output may be exceeded considerably for short periods of time without harming the power supply or tripping the circuit breakers. This feature is particularly useful in the study of dc motors under overload or starting conditions where high currents may be drawn. All of the power sources may be used simultaneously providing that the total current drawn does not make the main 3-phase, on-off circuit breaker trips. Your power supply, if handled properly, will provide years of reliable operation and will present no danger to you.

PRELAB QUESTIONNAIRE # 1

Answers to this questionnaire are due within the first five minutes of the first lab. Do not hand write.

1) What is the prerequisite class for this lab?

2) What is the approach for each one of the experiments? Your answer should be at least a paragraph.

3) This lab requires additional research outside of what is taught in lab to fully understand lab concepts. (Yes, No)

4) What are the maximum and minimum number of students per group that are allowed?

5) Threshold of perception for a finger tapping contact at 60 Hz is _ mA.

6) Let-go current (@ 60 Hz) for women = mA, for men = mA.

7) Asphyxia can be caused by 60-Hz currents of mA across the chest.

8) Ventricular fibrillation can be caused by currents of mA for 1 sec.

9) What are the maximum current levels that we will handle in this lab for: i) excitation currents, and ii) armature currents?

10) What are The Lab Safety Rules, and when do you use them?

11) What are the five steps of the Lab Safety procedure when working with an energized circuit, and when do you use it?

12) How much time do you have after the official start of lab before you are considered late?

13) Can you enter your assigned lab late? What should you do if you are late for a lab?

14) How many points are immediately deducted from your lab if you are late?

15) What does the signs WARNING, CAUTION, AND NOTE mean?

16) Theory will be reviewed by the TA. (True, False)

17) When are lab reports due?

18) Can lab reports be accepted AFTER the last day of classes?

19) Which are the two criteria for grading lab reports?

20) By what means does the TA communicate announcements about labs to students and whose responsibility is it to regularly check for said announcements?

21) What is the plot convention for lab figures?

22) What format must you use for your lab reports?

23) Are you allowed to approach the TA or Professor for extra points on your lab report(s) at the end of

the semester (True, False)

Lab 0 – Introduction to LabVolt Work Stations Introduction: This familiarization exercise consists of a step by step procedure that shows how to use the Lab-Volt

computer based Metering window and Data Table. There exercise can be performed using either the actual or virtual Lab-

Volt Electromechanical System (EMS).

CAUTION!

High voltages are present in this laboratory exercise! Do not make or modify any banana jack connections with the power

on unless otherwise specified!

Section 1 Introduction to Measuring Current and Voltage

1.1 Make sure all power supplies from the Power Supply Module (PS) are off (O) and the variable control is set to its lowest, counterclockwise position.

1.2 Verify the USB cable is connected between your workstation PC and the DAI.

1.3 Turn on the 24V AC power supply. Launch the LVDAC-EMS application on your workstation. Connect the 24V cable, gray cable with unique ends, from the power supply to the DAI.

**Note: The 24V power supply must be turned on prior to loading the LVDAC software or you may experience errors when using the LVDAC metering windows**

1.4 Set up the circuit in Figure 1.1 (use the variable DC supply 7-N). Vary the resistance of the circuit using the values shown in Table 1.1 and measure the values for I1 and E1. We will be measuring E1 across R1 and I1 as the supplied current from the power supplies using the Data Acquisition Interface (DAI).

Fig 1.1

Table 1.1

The Resistive Load Module Figure A shows the Resistive Load module. This module consists of three identical sections. Each

section has three resistors of different values which can be connected to electrical circuits through a pair of terminals. To

insert a resistor in an electrical circuit, the terminals of the section in which this resistor is located are connected to the

circuit, and the toggle switch associated with this resistor is set to the I (on) position. In Figure A, for example, the toggle

switch associated with the resistor at the extreme left of the module front panel is set to the I (on) position, while the toggle

switches associated with all other resistors are set to the O (off) position. This allows a resistor to be inserted in a circuit,

Line Voltage

(V)

Resistance (R1)

(Ω)

120

1200

600

400

300

240

using the corresponding section terminals (red terminals). Several combinations of switch positions are possible, allowing

you to place different resistance values in a circuit.

Figure A. Resistive Load Module

1.5 The DAI is powered by the 24V 3A AC source below the main power supply switch. It uses a cable with a special connector. The DAI should remain powered on throughout the entire lab. The measurement equipment cannot energize the circuit being tested. You must establish the habit of making sure all power supplies are completely reduced to 0 (turn counterclockwise, CCW) before turning on any switches.

1.6 Once the circuit wiring has been verified by your TA, turn on the main power switch and adjust the

voltage to the required level.

1.7 Launch the LVDAC-EMS software on the PC. Display the Metering application by opening the file ES12-1.dai located under the C:\Program Files\Lab-Volt\Samples directory. This is a convenient starting point for this lab, but the metering application is very customizable.

1.8 Launch the Data Table window to record your results for E1 and I1.

Note: The Data Table window must be opened to allow data to be recorded.

1.9 Repeat step 1.9 for all resistance values in Table 1.1.

Section 2 Introduction to AC and Oscilloscopes/Phasor Analyzer 2.1 Set up the circuit in Figure 2.1 using Table 2.1 for the assigned values and measure the values for I1 and

E1. We will be measuring E1 across R1 and I1 as the supplied current from the power supplies using the Data Acquisition Interface (DAI). The variable AC sine source that you will be using with these machines is 4-N. You will be only using this for the AC source (not 1-N) throughout the rest of this semester.

Figure 2.1

Table 1.2

Line Voltage (V)

Resistance (R1) (Ω)

Inductance (L1) (Ω)

120 300 300

2.2 Note that this is an AC circuit and you must be careful to connect the DAI with the proper polarities.

Have your TA check the circuit to assure it is wired correctly before proceeding!!

2.3 You must establish the habit of making sure all power supplies are completely reduced to 0 (turn counterclockwise, CCW) before turning on any switches.

2.4 Once the circuit wiring has been verified by your TA, turn on the main power switch and adjust the

voltage to the required level.

2.5 Launch the Data Table window to record your results for E1 and I1.

Note: The Data Table window must be opened to allow data to be recorded.

2.6 Launch the Oscilloscope and Phasor Analyzer. Display E1 and I1 on CH1 and CH2,

respectively. Display at least two complete cycles of the sine waves. Set a convenient vertical scale to display peak amplitudes easily. Take a screen shot of Oscilloscope and Phasor Analyzer views.

2.7 Measure the peak voltage, the peak current, and the period for the displayed

waveforms.

2.8 Turn off the power supply.

Lab 1: Alternating Current

Objective

After completing this unit, you will be able to explain and demonstrate the amplitude, frequency and phase of alternating voltage and current waveforms. You will also demonstrate concepts related to instantaneous power.

Fundamentals

Alternating current (AC) is universally used throughout the world for driving motors and for powering electrical equipment. As its name suggests, an alternating voltage is one which is continually reversing (alternating) its polarity. When speaking of AC voltages, it is quite correct to consider them as being DC voltages which are continually in the process of changing their value and polarity. The number of times that the polarity passes from positive to negative and then from negative to positive in one second is called frequency. The normal AC line frequency in North America is 60 Hz, while most countries in Europe, and several others, have an AC line frequency of 50 Hz.

Besides reversing polarity periodically, AC voltages also change in value from instant to instant, in a way that depends on the type of power supply. It is possible to obtain a square wave, a triangular wave, or other types of waveforms for the voltage. Theory and practical evidence has shown though, that the type of waveform best suited for running electrical machinery is the sine wave. This periodic waveform permits us to obtain the highest efficiency from transformers, motors, and generators. It also results in the quietest operation. Although sine waves seem more complicated than triangular or square waves, they make calculations of voltages and currents in electrical circuitry simpler. The value of a sine wave can be calculated for any instant of its cycle using the sine function, and this value always repeats after one complete cycle.

Sine waves having maximum values other than unity can be calculated using simple proportion. Negative values indicate that the polarity of the voltage or current has reversed. At any given instant in time, a sine wave will be at a given position, measured in degrees from a reference point. Consider two identical generators adjusted to exactly the same frequency. Suppose now that the second generator is turned on a short instant after the first. Using the sine wave from the first generator as a reference, we can say that the second waveform is lagging the reference by several degrees. The separation in time between the two AC waveforms is the phase shift. Phase shifts are often measured using phase angles. The term lagging or leading phase shift is used to indicate whether the waveform reaches maximum after or before the reference.

The fundamental property of capacitors is to oppose voltage changes across their terminals. The opposition to voltage changes is proportional to the capacitor's capacitance (C). When capacitance is added to an AC circuit, an effect similar to that produced by circuit resistance is observed. That is, there is opposition to the flow of current. This effect is due to capacitive reactance (XC), which is defined as the opposition created by capacitance to the flow of alternating current.

Capacitance is a measure of the amount of electrical charge that a capacitor can store in the dielectric between its two conducting plates when a given voltage is applied across them. The measurement unit for capacitance is the farad (F), which is an extremely large quantity. Most typical capacitors have values in the range of microfarads and picofarads depending on whether they are used in electric power circuits or electronics.

When a DC voltage is applied suddenly to a capacitor a large current will flow. It will continue

to flow at a decreasing rate until the capacitor has charged up to the value of the source

voltage, ES. The current will then have dropped to zero because the voltage across the

capacitor is no longer changing, and the capacitor is neither charging nor discharging. The

current can be quite large if the voltage across the capacitor changes quickly. If the source

voltage increases at a rapid rate, a large charging current will flow into the capacitor, and the

capacitor will act as a load and store energy. Conversely, if the source voltage decreases at a

rapid rate, a large discharging current will flow out of the capacitor. The capacitor acts as a

momentary source of power just like a generator and releases energy. The ability to store

electric energy comes from the electric field set up between the plates of the capacitor. The

quantity of energy stored depends on the capacitor's capacitance and the applied voltage.

When a capacitor is charging, it receives and stores energy, but does not dissipate it. When it

discharges, the stored energy will be released until the voltage across the capacitor drops to

zero. These facts help in understanding the behavior of a capacitor when it is connected to an

ac power source.

When the AC voltage increases, the capacitor stores energy, and when the voltage decreases, the

capacitor releases the stored energy. During the storing period, the capacitor is a load on the source,

but during the releasing period, the capacitor actually returns power back to the source. This

produces the interesting situation in which the capacitor acts periodically as a source of power

returning energy to the source that gave it power in the first place. In AC circuits, power flows back

and forth between the capacitor and the power source and nothing useful is accomplished.

If a wattmeter was connected to measure the power in the circuit, it would indicate zero since no power is dissipated in the capacitor. The wattmeter actually tries to indicate positive when the capacitor charges and negative when it discharges, but the power flow reversal takes place so quickly that the meter pointer cannot follow. The real power, or active power as it is more properly known, that is associated with an ideal capacitor, is therefore zero. However, there will be a voltage drop across the capacitor and current will flow in the circuit. The EI product is known as apparent power in volt-amperes (VA). For the special cases of purely capacitive and inductive AC circuits, the apparent power is known as reactive power in VAR (volt-amperes reactive). The instantaneous power waveform will show that there are instances of negative power peaks in reactive circuits, which corresponds to the fact that power swings back and forth between the load and the source. As you will learn in this unit, the capacitive phase shift between the voltage and current is directly linked to the alternating power flow direction.

Inductors are frequently called chokes or coils. The entire electrical industry revolves around coils, which are found in motors, generators, relays, and numerous other electrical devices. The fundamental property of inductors is to oppose changes in the current flowing through its coil. The opposition to current changes is proportional to the inductor's inductance (L). The inductance is a measure of the amount of energy that an inductor stores in the magnetic field set up when a current flows through its coil, and the measurement unit for inductance is the henry (H).

When inductance is added to an AC circuit, an effect similar to that of capacitance is observed: there is opposition to the flow of current. This effect is referred to as the inductive reactance (XL), which is defined as the opposition created by inductance to the flow of alternating current. When current flows through a coil of wire, a magnetic field is set up and this field contains energy. As the current increases, the energy contained in the field also increases. W hen the current decreases, the energy contained in the field is released. The magnetic field eventually returns to zero when the current is zero. The situation is analogous to the capacitor, except that in a capacitor, it is the voltage that determines the amount of stored energy, while in the inductor, it is the current. In AC circuits, power flows back and forth between the inductor and the power source and nothing useful is accomplished; just like in the case for capacitors. As you will later see, the alternating power flow in inductive AC circuits is related to the inductive phase shift between current and voltage in a manner similar to capacitive phase shift.

If a wattmeter was connected to measure the power consumed by an ideal inductor, it would indicate zero. In practice, however, all coils dissipate some active power and the wattmeter will indicate a small amount of power because the coil wire always has resistance, and therefore, dissipates power as a resistor does.

There will be a voltage drop across the inductor and current will flow in the inductive AC circuit in a way very similar to the purely capacitive circuit. The apparent power (EI product) will equal reactive power in the case of the ideal inductor, and the instantaneous power waveform will show negative power peaks like capacitive AC circuits do. In order to distinguish between

capacitive reactive power and inductive reactive power, a negative sign is usually associated with capacitive VAR and a positive sign with inductive VAR.

Both capacitors and inductors store energy and cause a phase shift of 90 degrees between voltage and current. Capacitors store energy in an electric field set up by the application of a voltage, while inductors store energy in a magnetic field set up by a current that flows in a coil of wire.

Phasors Review

The relationships between current and voltage of an impedance load depend on its real or reactive elements. Resistors, capacitors and inductors can have the same impedance but will not have the same voltage/current phasor representation. For resistors, voltage and current across the resistor are in the same phase. For capacitors, current leads the voltage by 90 degrees. Lastly for inductors, the current lags the voltage by 90 degrees. These relationships are shown in Figure A, Figure B, and Figure C respectively.

Depending on the set up of the circuit, i.e. parallel or in series, the voltage or current respectively will be the same in all impedance elements. Using the voltage-current vector relationships above, it is possible to find the phasors for all elements and for total impedance as shown in Figure D and Figure E. Figure D Figure E

EQUIPMENT REQUIRED Workstation PC (PC) Lab-Volt Data Acquisition and Management Electro-Mechanical System (LVDAM-EMS)

Power Supply Module (PS)

Data Acquisition Interface Module (DAI) Resistive Loads Module Capacitive Module

Inductance Module

PROCEDURE

CAUTION! High voltages are present in this laboratory exercise! Do not make or modify any banana jack connections with the power on unless otherwise specified! Section 1 Sine Waves 1.1 Make sure all power supplies from the Power Supply Module (PS) are off (O) and the variable control is set to its lowest, counterclockwise position. 1.2 Verify the USB cable is connected between your workstation PC and the DAI. 1.3 Launch the LVDAM-EMS application on your workstation. Connect the 24V cable, gray cable with unique ends, from the power supply to the DAI. Turn on the 24V AC power supply. 1.4 Set up the circuit shown in Figure 12 with the values shown in Table 12. 1.5 Once all team members have verified the circuit, turn on the main power. 1.6 Open metering, phasor analyzer and oscilloscope windows on the LVDAC software and collect data for E1, I1, I2 and I3 respectively.

1.7 Compare the arithmetic sum of IR and IC i.e. ∑(I) with the geometric sum (IC 2+IR 2). Compare those values to the measured value of IS. Explain theoretically why the geometric sum gives a more precise value of IS for this circuit (Hint: think about the reactance of the resistors and capacitors and how it impacts the voltage and current). Show in analysis section of your lab report.

Figure 12

Table 12 Line

R C

Voltage

(Ω) (Ω)

(V)

120 60 60

1.8 Determine the phase angle. θ= arctan (IC / IR) (degrees).

1.9 What is the Impedance angle, φ? Does this make sense with a capacitive load? 1.10 Turn off all the power and turn the voltage control knob fully CCW. Section 2 Power factor correction/Vectors and Phasors in AC Circuits

Figure 13

Table 13

Line

R L Voltage

(Ω) (Ω) (V)

120 100 100

2.1 Set up the circuit in Figure 13 using Table 13 for the assigned values 2.2 Open the metering configuration as in Section 1.

2.3 Turn on the power supply and adjust the voltage to the value in Table 13. Use the data table to measure the current and voltage. 2.4 Use the measured values of voltage and current to calculate the apparent power. S = E * I 2.5 What is the Impedance angle, φ? Does this make sense with an inductive load?

2.6 Use the following equations to determine the power factor cos(φ) and the reactive power.

Hint: cos φ = P/S, Q = √(S2-P2)

2.7 Using a Power triangle diagram correctly label all relevant components (P, Q , S and φ )

with their respective magnitudes. Show this in the analysis section of your lab report.

2.6 Launch the phasor analyzer to see the relationship between voltage and current. Take screen shot of Oscilloscope and phasor Analyzer views. 2.7 Turn off the power. 2.8 Set up the circuit shown in Figure 14 with the values shown in Table 14. Energize the circuit without C1 being activated in the circuit (switches down). Record power factor at the source. 2.9 Now insert C1 into the circuit (switches up). Record the power factor, reactive power delivered (QS) and apparent power (SS) at the source. Draw the power triangle for the source. How much Reactive

power does the capacitor deliver? Hint: Qc = Qsinitial-Qsfinal Show work.

2.10 Compare the pf value with the capacitor and without the capacitor. Which one is better and why?

Figure 14

Table 14

Line R C L

Voltage (Ω) (Ω) (Ω)

(V)

120 100 80 100

2.11 Using the phasor analyzer, verify the values obtained by calculations. Take screen shot of Oscilloscope and phasor Analyzer views.

2.12 Turn off all the power and turn the voltage control knob fully CCW. 2.13 Set up the circuit shown in Figure 15 with the values shown in Table 15. Pay special attention to the value of IS. Hint: When constructing the circuit in figure 15 complete the outer connections first i.e. Source, I1, E2, E3, etc. then you can simply connect the RLS in parallel with the voltmeters respectively.

Figure 15

Table 15

Is R C L (A) (Ω) (Ω) (Ω)

1 80 60 60

2.14 Compare the arithmetic sum of ER, EC, and EL to the measured value of ES.

2.15 Compare ES with √[ER2+(EL-EC)2] (V).

2.16 Determine the phase angle Φ, Active Power and Reactive Power. 2.17 Turn off all the power and turn the voltage control knob fully CCW. Remove all leads and cables and return them to the rack on the side of the Lab-Volt equipment.

Lab 2: Three-Phase Circuits

Objective After completing this unit, you will be able to solve balanced three-phase AC circuits connected in wye and delta configurations, and demonstrate the difference between line and phase voltage. You will also be able to determine active, reactive, and apparent power, and establish the phase sequence of a three-phase ac supply. You will use voltage and current measurements to verify the theory and calculations presented in the exercises. Fundamentals

Most electrical power generation and power transmission in the world today utilizes some

form of three phase AC circuits. Three phase AC systems have two distinct advantages over there single phase counterparts: (1) it is possible to get more power per kilogram of metal from a three phase machine and (2) the power delivered to a three phase load is constant at all times instead of pulsing as it does in single phase systems. Three-phase systems also make use of induction motors easier by allowing them to start without special auxiliary starting windings.

In most cases, three-phase circuits are symmetrical and have identical impedances in each of the circuit’s three branches (phases). Each branch can be treated exactly as a single-phase circuit, because a balanced three-phase circuit is simply a combination of three single-phase circuits. Therefore, voltage, current, and power relationships for three-phase circuits can be determined using the same basic equations and methods developed for single-phase circuits. Non-symmetrical, or unbalanced, three-phase circuits represent a special condition and their analysis is more complex. A three-phase ac circuit is powered by three voltage sine waves having the same frequency and magnitude and which are displaced from each other by 120°. The phase shift between each voltage waveform of a three-phase ac power source is therefore 120° (360°/3 phases). Figure 1 shows an example of a simplified three-phase generator (alternator) producing three-phase ac power. A rotating magnetic field produced by a rotating magnet turns inside three identical coils of wire (windings) physically placed at a 120° angle from each other, thus producing three separate ac voltages (one per winding). Since the generator’s rotating magnet turns at a fixed speed, the frequency of the ac power that is produced is constant, and the three separate voltages attain the maximal voltage value one after the other at phase intervals of 120°.

Figure 1. Simplified three- phase generator

The phase sequence of the voltages of a three-phase power supply is the order in which they follow each other and reach maximum. These voltages are shown with the phase sequence EA, EB, and EC, which in shorthand form is the sequence A-B-C. Phase sequence is important because it determines the direction in which three-phase motors turn. If the phases are connected out of sequence, the motor will turn in the opposite direction, and the consequences could be quite serious. For example, if clockwise rotation of a motor is the normal direction to make an elevator go up, connecting the phase wires incorrectly would result in the elevator going down instead of

up, and vice-versa and a serious accident could occur.

Wye and delta configurations The windings of a three-phase ac power source (e.g., the generator in Figure 1) can be connected in either a wye configuration, or a delta configuration. The configuration names are derived from the appearance of the circuit drawings representing the configurations, i.e., the letter Y for the wye configuration and the Greek letter delta (Δ) for the delta configuration. The connections for each configuration are shown in Figure 2.

Figure 2. Types of three phase configurations

Each type of configuration has definite electrical characteristics. As Figure 2a shows, in a wye-connected circuit, one end of each of the three windings (or phases) of the three-phase ac power source is connected to a common point called the neutral. No current flows in the neutral because the currents flowing in the three windings (i.e., the phase currents) cancel each other out when the system is balanced. Wye connected systems typically consist of three or four wires (these wires connect to points A, B, C, and N in a), depending on whether or not the neutral line is present. Figure 2b shows that, in a delta-connected circuit, the three windings of the three-phase ac power source are connected one to another, forming a triangle. The three line wires are connected to the three junction points of the circuit (points A, B, and C in b). There is no point to which a neutral wire can be connected in a three-phase delta-connected circuit. Thus, delta-connected systems are typically three-wire systems. Distinction between line and phase voltages, and line and phase currents The voltage produced by a single winding of a three-phase circuit is called the line-to-neutral

voltage, or simply the phase voltage Ephase. In a wye-connected three-phase ac power source, the

phase voltage is measured between the neutral line and any one of points A, B, and C, as shown in

a. This results in the following three distinct phase voltages: EA-N, EB-N, and EC-N. The voltage

between any two windings of a three-phase circuit is called the line-to-line voltage, or simply the

line voltage Eline. In a wye-connected three-phase ac power source, the line voltage is √3

(approximately 1.73) times greater than the phase voltage (i.e., Eline = √3 Ephase). In a delta-

connected three-phase ac power source, the voltage between any two windings is the same as

the voltage across the third winding of the source (i.e Eline = Ephase), as shows Figure 2b. In both

cases, this results in the following three distinct line. EA-B, EB-C and EC-A.

The three line wires (wires connected to points A, B, and C) and the neutral wire of a three-phase power system are usually available for connection to the load, which can be connected in either a wye configuration or a delta configuration. The two types of circuit connections are illustrated in Figure 3. Circuit analysis demonstrates that the voltage (line voltage) between any two line wires, or lines, in a wye-connected load is √3 times greater than the voltage (phase voltage) across each

load resistor. Furthermore, the line current Iline flowing in each line of the power source is equal

to the phase current Iphase flowing in each load resistor. On the other hand, in a delta-connected

load, the voltage (phase voltage) across each load resistor is equal to the line voltage of the source. Also, the line current is √3 times greater than the current (phase current) in each load resistor. The phase current in a delta-connected load is therefore ξ͵ times smaller than the line current.

Figure 3. Types of load connections

The relationships between the line and phase voltages and the line and phase currents simplify the analysis of balanced three-phase circuits. A shorthand way of writing these relationships is given below:

• In wye-connected circuits: Eline = √3 Ephase and Iline = Iphase

• delta-connected circuits: Eline = Ephase and Iline = √3 Iphase

Power in balanced three-phase circuits The formulas for calculating active, reactive, and apparent power in balanced three-phase circuits are the same as those used for single-phase circuits. Based on the formula for power in a single-phase circuit, the active power dissipated in each phase of either a wye- or delta-connected load is equal to: Pphase = Ephase * Iphase * cos (Θ)

• Pphase is the active power dissipated in each phase of a three-phase circuit, expressed in watts (W).

• Ephase is the phase voltage across each phase of a three-phase circuit, expressed in volts (V).

• Iphase is the phase current flowing in each phase of a three-phase circuit, expressed in

amperes (A).

• Θ is the angle between the phase voltage and current in each phase of a three-phase circuit, expressed in degrees (°) .

Therefore, the total active power PT dissipated in a three-phase circuit is equal to:

PT = 3* Pphase =3* Ephase * Iphase * cos (Θ) where PT is the total active power dissipated in a three-phase circuit, expressed in watts (W) In purely resistive three-phase circuits, the voltage and current are in phase, which means that cos(Θ) = 1. Therefore, the total active power PT

dissipated in purely resistive three-phase circuits is equal to: 3* Ephase * Iphase.

EQUIPMENT REQUIRED

Workstation PC (PC) Lab-Volt Data Acquisition and Management Electro-Mechanical System (LVDAC-

EMS) Power Supply Module (PS) Data Acquisition Interface Module (DAI) Resistance Module Capacitance Module

PROCEDURE

CAUTION! High voltages are present in this laboratory exercise! Do not make or modify any banana jack connections with the power on unless otherwise specified!

Section 1 Three Phase Circuits

1.1 Make sure all power supplies from the Power Supply Module (PS) are off (O) and the

variable control is set to its lowest, counterclockwise position.

1.2 Verify the USB cable is connected between your workstation PC and the DAI. Turn on

the 24V AC supply for the DAI.

1.3 Set up the circuit in Figure 20. Once you’ve verified that the circuit has been correctly constructed with groupmates turn the power supply on and adjust the voltage control knob to the 100% mark (completely clockwise). Measure the line-to-neutral voltages of the power supply (E4-N, E5-N, E6-N) then calculate the average voltage.

1.4 Open the Oscilloscope and Phasor Analyzer measurement instruments. Record data. Is

the phase shift between each voltage sine wave of the three-phase ac power source equal to 120°?

1.5 Turn the power off then construct the delta circuit in Figure 21. Turn the power supply

on and adjust the voltage control knob to the 100% mark (completely clockwise). Measure the line-line voltages (E4-5, E5-6, E4-6) and calculate the average line-line voltages.

1.6 Open the Oscilloscope and Phasor Analyzer measurement instruments. Record data. Is

the phase shift between each voltage sine wave of the three-phase ac power source equal to 120°?

1.7 Calculate the ratio of the average line-to-line voltage to the average line-to-neutral

voltage.

1.8 Turn the AC power source off.

Figure 20

Figure 21

Section 2 Voltage, current, and power measurements in a wye-connected circuit

2.1 Construct the circuit shown in Figure 22 (V= 120V; R1, R2 and R3 = 300 Ω). Once

you’ve verified that the circuit was correctly constructed with groupmates turn the power supply on. Note: L1, L2 and L3 correspond to the 3 variable phase voltages (4,5 and 6) used in the 2 previous circuits.

Figure 22. Wye-connected, three-phase circuit supplying power to a three-phase resistive load.

2.2 In the metering window make necessary change so that you can record the rms voltage of ER1, ER2, ER3 and Eline (i.e. E1, E2, E3 and E4) and currents IR1, IR2, IR3 and IN (I1, I2, I3

and I4). Are the load voltages approximately equal? Compare the individual load currents. Are they balanced? What does this tell you about the characteristic of the Wye-connected circuit? 2.3 Turn off the AC Power supply.

2.4 Calculate the ratio of Eline: Ephase

2.5 Disconnect the neutral line, then turn the three-phase ac power source in the Power Supply on. What effect did disconnecting the neutral line have on the measured voltages and currents in the metering window? 2.6 Calculate the active power dissipated in each phase of the circuit and the total active power PT dissipated in the circuit using the voltages and currents recorded in the previous steps.

2.7 Calculate PT = 3 (Ephase* Iphase) dissipated in the circuit using the average phase voltage and current. Compare this data with the calculated total active power calculated in step 2.6

2.8 Turn off AC Power supply. Section 3 Delta connected circuits/ Two- Watt meter method

Figure 23. Three-wire, delta-connected, three-phase circuit set up for power measurements using the two-wattmeter method.

3.1 Construct the circuit shown in figure 23 (V = 120, R = 200Ω, C=240Ω). Verify that the

circuit has been correctly constructed with group members and turn the power supply on.

3.2 Calculate the power factor at the source using recorded data cos(Θ) = P/S. 3.3 Using the two-wattmeter method calculate PF delivered by the source.

Θ = tan-1√3 [(W1-W2)/(W1+W2)]

3.4 Compare values obtained in steps 3.3 and 3.4 3.5 Modify the switch settings on the Resistive Load and Capacitive Load modules with the

following values: R1= 300Ω C1 = 600Ω R2, R3= 600Ω C2, C3=1200Ω The circuit at the load end is now unbalanced.

3.6 Repeat steps 3.2-3.5 3.7 Turn off power supply and disassemble your circuit.

Lab 3 Single Phase Power Transformers

Fundamentals

Magneto motive Forces and Flux

Magnets possess a magneto motive force which generates flux, forming a magnetic field surrounding the magnet between its north and south poles. Magnetic reluctance is analogous to electrical resistance (although it does not give rise to heat dissipation in magnetic circuits).

Although the relationship between flux and mmf suggests a linear relationship, non-linearity arises for higher values of MMF because the magnetic circuit begins to saturate i.e. the microscopic magnetic dipoles begin to orient themselves along the direction of the magnetic field and no more change can be done to the magnetic circuit so that reluctance starts to increase non-linearly.

Saturation curve of an iron core

Voltage Regulation The load on a large power transformer in a sub-station will vary from a very small

value in the early hours of the morning to a very high value during the heavy peaks of maximum industrial and commercial activity. The transformer secondary voltage will vary somewhat with the load, and because motors, incandescent lamps, and heating devices are all quite sensitive to voltage changes, transformer regulation is of considerable importance. The secondary voltage also depends upon whether the power factor of the load is leading, lagging, or unity. Therefore, it should be known how the transformer will behave (its voltage regulation) when connected to a capacitive, an inductive, or a resistive load. Transformer voltage regulation in percent is determined with the following formula

𝑽𝒐𝒍𝒕𝒂𝒈𝒆 𝑹𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 = 𝟏𝟎𝟎 ∗ (𝑬𝑵𝑳 − 𝑬𝑭𝑳)

𝑬𝑭𝑳

ENL = No load voltage

EFL= Full load voltage The result (a percentage value) obtained gives an indication of transformer behavior under load. The smaller the voltage regulation percentage, the smaller the secondary voltage variation with load, and the better the voltage regulation. Note that ENL is measured with the secondary winding open while EFL measured when nominal current flows in the secondary winding. Several factors affect a transformer's operation. The resistance and inductive reactance of its windings cause internal voltage drops that vary with the amount of current flowing in the windings. If the secondary is lightly loaded, current through the

winding resistance and reactance is small and the internal voltage drops are not significant. As the load increases, current and internal voltage drops also increase. If a transformer were perfectly ideal, its windings would have neither resistance nor inductive reactance to cause voltage drops. Such a transformer would have perfect regulation under all load conditions and the secondary voltage would remain absolutely constant. But practical transformer coils are made of real wire, and thereby, have resistance and inductive reactance.

Simplified equivalent circuit of a practical transformer

Equipment Needed Workstation PC (PC) Lab-Volt Data Acquisition and Management Electro-Mechanical System (LVDAC-

EMS) Power Supply Module (PS) Data Acquisition Interface Module (DAI) Single-Phase Transformer Module Three-Phase Transformer Module Resistance Module

Following Parts from LabVolt MagTran kit.

Mounting Base - 1 number 133mm Laminated Bar – 3 numbers 133mm Laminated Bar with Hook – 1 number Coil (275 turns/1100 turns) – 2 numbers Search coil for Flux meter – 1 number 0.75 mm Fiber Spacer – 1 number

Section 1 Saturation curve and voltage ratio of a Transformer

1.1 Set up the circuit shown in figure 1. Arrange the laminated bars and tighten the screws to reduce air gaps in the magnetic circuit as much as possible.

CAUTION: High voltages are present in this lab exercise! Do not make any connections with the power on! Also be sure to connect each ground terminal (green) on the components to the power supply ground.

Figure 1 Saturation curve circuit

1.2 On the power supply, turn the main control to 0. Then turn on the power and adjust

voltage E to 30V ac. If the core vibrates strongly, tighten the clamping screws. Check the value of the exciting current I. It should be between ~0.30A ac – 0.50A ac. If it is much greater than 0.50A ac, try tightening the screws some more or slightly rearrange the laminated bars to minimize the air gaps.

1.3 If the frequency in step 1.2 were doubled, would the exciting current increase or decrease? Explain in lab report. (Hint: Consider the effective impedance of an inductor and ohm’s law).

1.4 Without making any other changes to your set up, reduce the primary voltage to 0 V then gradually increase the voltage using the values given in Table 1 (note: continue in 5 voltage increments between 30 and 55 volts). Record the values of exciting current and determine the voltage required to get an exciting current of 2A. Table 1

1.5 Using the data table, draw the saturation curve. What is the exciting current I when the nominal voltage (30 V ac) is applied to the primary winding?

1.6 Set up the circuit shown in figure 2 (exclude coil B from the circuit). The secondary winding of coil A is connected only to the voltmeter (an open circuit). Tighten the screws to minimize the air gaps

in the magnetic circuit.

Figure 2 Voltage ratio circuit

1.7 Turn on the power and adjust E1 to 30 V ac. Measure the value of voltage E2. 1.8 Turn off the power. Calculate the voltage ratio of E2/E1. Verify data.

Section 2: AC Induction and Magnetic Coupling

2.1 Set up the circuit shown in Figure 3. Tighten the screws to reduce the air gaps to a minimum. Insert

the search coil on either laminated bar adjacent to coil B. Adjust the scale on your flux meter

accordingly.

Primary Voltage Exciting current

V ac A ac

10

15

20

25

30

…

55

CAUTION: High voltages are present in this lab exercise! Do not make any connections with the power on! Also be sure to connect each ground terminal (green) on the components to the power supply ground.

Figure 3 Circuit to observe the effect of air gaps on ac induction and magnetic coupling

2.2 On the power supply, turn the main control to 0. Then turn the power supply gradually until E1 = 30 V ac. Measure current I1, voltage E2. Record the results in Table 2. Turn off power.

Table 2

Gap (g) E1 I1 E2 Flux

mm V ac A ac V ac Φ

0 30

0.75 30

1.5 30

2.25 30

2.3 Turn the screws on the mounting base counterclockwise (CCW) to slacken the bar inside coil B. Insert a 0.75 mm Fiber Spacer to create two gaps having a length 0.75mm. The fiber spacer passes inside the coil. Tighten the screws. Turn on the Power and adjust the voltage E1 to 30 V ac. Measure I1 and E2 and record data in table 2.

2.4 Turn off power. Repeat the same process (adjusting E1 to 30 V) and add another spacer (creating a larger gap) until you have completed Table 2.

2.5 Remove the spacers and tighten the screws so that the magnetic circuit has minimal gaps again. Bring the 178mm laminated bars to your set up so that it lies like a shunt across the left and right legs of the iron core without coils.

2.6 Turn on the power supply and adjust the voltage to 30 V ac. Measure current I1 and voltage E2. 2.7 Why does E2 decrease when the shunt is in place? Why does I1 decrease when the shunt is in place?

Answer in lab report. 2.8 Observation: Vary the horizontal position of the shunt and then lift the shunt vertically. What is the

effect on the voltage E2? Turn off power. 2.9 Set up the circuit shown in figure 4 with no gaps and a lamp connected to terminals 3 and 4 of coil B.

Make sure the screws are tightened. Turn on the power supply, and gradually raise voltage until E1 = 30 V ac. Observe that the lamp lights up. Measure I1, E1 and I2 and record results in Table 3. Calculate the Power (PL) delivered to the lamp and record in table 3. Turn off power.

Figure 4 Circuit to observe energy transferred through air gaps

Table 3

Gap (g) E1 I1 E2 I2 PL

mm V ac A ac V ac A ac W

0 30

0.75 30

1.5 30

2.25 30

2.1.1 Turn the screws on the mounting base counterclockwise (CCW) to slacken the bar inside coil B. Insert

a 0.75 mm Fiber Spacer to create two gaps. Tighten the screws. Turn on the Power and adjust the voltage

E1 to 30 V ac. Measure I1, E2 and I2. Record data in table 3. Repeat until the table is complete. Turn off

power.

2.1.2 Why does E2 decrease while I1 increases? Explain in lab report.

2.1.3 Remove the spacers and tighten the screws so that the magnetic circuit has minimal gaps. Place

the magnetic shunt so that it lies across the left and right legs of the iron core. Turn on the power supply

and adjust voltage E1 to 30 V ac. Note the effect on the brightness of the lamp as the shunt is moved closer

to and further away from the legs.

2.1.4 With the shunt in physical contact with the legs, measure I1, E2 and I2. Calculate the power delivered

to the lamp.

2.1.5 Compare the power delivered to the lamp with the shunt in place to the power delivered with no

shunt (PL in first row of Table 3) explain the difference.

2.1.6 If the lamp in the circuit burned out, would there still be considerable power delivered to coil B?

Explain.

2.1.7 Disassemble the circuit

Section 3 Transformer Regulation in a Single Phase Transformer

3.1 Open the setup configuration file ES17-8.dai (File > Open > scroll down to ES17-8.dai). Set up the

transformer loading circuit shown in figure 6. Ensure that all switches on the resistive load module are

open.

Figure 6 Transformer regulation circuit

3.2 Turn on the power supply and adjust the voltage control to 120V. Measure and record the no-load

value of E1, I1, E2 and I2 of the transformer. Adjust the switches on the resistive load modules to obtain

the values given in Table 5. Repeat this same procedure for the inductive and capacitive loads.

Table 5

Line Voltage (V) R(Ω)

120 open

120 1200

120 600

120 400

120 300

120 240

3.3 Display the Graph window, select E2 as the Y-axis parameter, and I2 as the X-axis parameter.

Make sure the line graph format and the linear scale are selected. Observe the curve of secondary

voltage versus current. What happens to the secondary voltage as the resistive load

increases/decreases?

3.4 Calculate the voltage regulation of the resistive circuit using the no load (R = infinity) and

full load R = minimum value) output voltages using the equation shown below:

𝑽𝒐𝒍𝒕𝒂𝒈𝒆 𝑹𝒆𝒈𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 = 𝟏𝟎𝟎 ∗ (𝑬𝑵𝑳 − 𝑬𝑭𝑳)

𝑬𝑭𝑳

3.5 Repeat steps 3.3 and 3.4 for the capacitive and inductive loads. How does the secondary

voltage vary as the capacitive and inductive loads are increased respectively? What

differences did you observe between the three load curves?

3.6 Ensure that the Power Supply is turned off, and that the voltage control knob is turned fully

counterclockwise. Remove all leads and cables.

Lab 4: Three-Phase Transformers

• Objectives: - Being familiar with operating characteristics of three phase transformers. - Being able connect transformer windings in wye and delta configurations, and verify that

windings are connected with the proper phase relationships. - Voltage and current measurements will be used to study transformer operation and working

characteristics.

• Introduction: The main features of three-phase circuits that are important to recall are that there are two types of

connections, wye and delta. Furthermore, in wye-connected three-phase circuits, the line voltages are

greater than the phase voltages by the factor √3 and the line and phase currents are equal. On the other

hand, in delta-connected three-phase circuits, the line currents are greater than the phase currents by the

factor √3 and the line and phase voltages are equal.

A three-phase transformer can be a single unit or three single-phase units, and the primary and secondary

windings can be connected in either wye or delta to give four types of connections, delta-delta, wye-wye,

delta-wye, and wye-delta. Usually, three-phase power systems has a line voltage of 208 V (380 V or 415 V

in some countries), and standard 120-V voltage (220 V or 240 V in some countries) can be obtained

between a line wire and the neutral wire as shown in Figure 9-1. The wye connected secondary provides

three-phase 120/208-V power using 4 wires as shown, and the primary side of the transformer may be

connected in delta like in the figure, or in wye. One big advantage of using a delta configuration for the

primary is that only three wires are needed to distribute the three phases.

Figure 9-1. Commercial Three-Phase 120/208-V Power System

An advantage of a delta-delta connection is that two single-phase transformers (instead of three) can be

operated in what is known as the open-delta or "V" configuration if one of the three transformers becomes

damaged or is removed from service. The open-delta transformer bank still delivers phase voltages and

currents in the correct relationship, but the capacity of the bank is reduced to 57.7% (1/√3) of the total

nominal capacity available with three transformers in service.

In the delta-delta and wye-wye configurations, the line voltage at the secondary is equal to the line voltage

at the primary times the inverse of the turns ratio. In the delta-wye configuration, the line voltage at the

secondary is equal to the line voltage at the primary times the inverse of the turn ratio times √3. In the

wye-delta configuration, the line voltage at the secondary is equal to the line voltage at the primary times

the inverse of the turn ratio times 1/√3.

Regardless of how the windings in a three-phase transformer are connected, precautions must be taken

to ensure that the secondary windings are connected with the proper phase relationships. For a wye

configuration, this means that the voltage measured across any two secondary windings (line voltage)

must be √3 times greater than the voltage across either winding (phase voltage). If not, the connections

must be reversed before continuing.

With a delta configuration, the voltage measured between the ends of two series connected secondary

windings must equal the voltage across either winding. If not, the connections must be reversed. When

one end of the third winding is connected, the voltage measured across all three series-connected

windings must equal zero before connecting them together to close the delta. It is extremely important to

verify that the voltage within the delta equals zero before the delta is closed. If not, the resulting current

will be very high and damage the windings.

Part 1: Three-Phase Transformer Connections

• Objectives: When you have completed this exercise, you will be able to connect three-phase transformers in delta-

delta and wye-wye configurations. You will measure winding voltages to verify that the secondary windings

are connected with the proper phase relationships, and you will verify that the voltage within a delta

equals zero before the delta is closed.

• Discussion As mentioned earlier, four common ways of connecting transformer windings to form a three-phase

transformer are: delta-delta, wye-wye, delta-wye, and wye-delta, as shown in Figures 9-2 and 9-3. In order

to set up a wye connection, first connect the three components (windings) together at a common point

for interconnection with the neutral wire, then connect the other end of each component in turn to the

three line wires. To set up a delta connection, connect the first component in series with the second, the

second in series with the third, and the third in series with the first to close the delta loop. The three line

wires are then separately connected to each of the junction nodes in the delta loop.

Figure 9-2. Delta-Delta and Wye-Wye Connections

Figure 9-3. Delta-Wye and Wye-Delta Connections

Before a three-phase transformer is put into service, the phase relationships must be verified. For a wye

configuration, the line voltages at the secondary windings must all be √3 times greater than the

corresponding phase voltages. If not, winding connections must be reversed. To verify that phase

relationships are correct for a wye configuration, the voltage between two windings (EAB) is measured as

shown in Figure 9-4 (a) to confirm that it is √3 times greater than the line-to-neutral voltage across either

winding (for example EAN). The voltages between the third winding and the others (EBC and ECA) are then

measured to confirm that they are also √3 times greater than the phase voltage (EAN), as shown in Figure

9-4 (b).

Figure 9-4. Confirming Phase Relationships in a Wye-Connected Secondary.

For a delta configuration, the line voltages at the secondary windings must all be equal. If not, winding

connections must be reversed. To verify that phase relationships are correct for a delta configuration, the

voltage across two series connected windings (ECA) is measured as shown in Figure 9-5 (a) to confirm that

it equals the voltage across either winding (EAB and EBC). The third winding is then connected in series, and

the voltage across the series combination of the three windings is measured to confirm that it is zero

before the delta is closed, as shown in Figure 9-5 (b). This is extremely important for a delta configuration;

because a very high short-circuit current will flow if the voltage within the delta is not equal to zero when

it is closed.

Figure 9-5. Confirming that the Delta Voltage Equals Zero

• Procedure 1. Connect the Three-Phase Transformer module in the delta-delta configuration shown in Figure 9-

6.

Figure 9-6. Three-Phase Transformer Connected in Delta-Delta

1. Turn on the power and adjust the voltage control to obtain the line-to-line voltage ES equals 200V on the analog/digital meter on the power supply. Use E1 to measure the winding voltages and record the results. Make sure to record the angle associated with each value as well.

E1-2

E1-7

E1-12

E3-5

E3-10

E3-15

Do the measurements confirm that the secondary windings have the proper phase relationship?

----- Yes ----- No

2. Are the voltages within the secondary delta equal to zero, thus confirming that it is safe to close the delta?

----- Yes ----- No

Note: The value of E3-15 will not be exactly zero volts because of small imbalances in the three-phase line

voltages. If it is more than 5 V, the winding connections must be checked carefully.

3. When the winding connections are confirmed to be correct, close the delta on the secondary side of the transformer. Connect E1, E2, and E3 to measure the line-line voltages at the secondary. Turn on the power and adjust the voltage control to obtain ES equals 200V on the analog/digital meter on the power supply. Note that the transformer is connected using the 1:1 ratio, so the primary and secondary voltages should be equal.

4. Observe the voltage phasors on the Phasor Analyzer. Does the display confirm they are equal with a 120-degree phase shift between each of them?

----- Yes ----- No

5. Turn off the power. Connect E2 to measure the line voltage E1-2 on the primary side. Turn on the power and adjust the voltage control dial to obtain ES equals 200V. Compare the voltage phasor of E1-2 on the primary side with that of E3-5 on the secondary side. Does the Phasor Analyzer display show that the voltages are equal and in phase, except for possibly a small difference due to transformer reactance?

----- Yes ----- No

6. Turn off the power and connect the Three-Phase Transformer module in the wye-wye configuration shown in Figure 9-7.

Figure 9-7. Three-Phase Transformer Connected in Wye-Wye

7. Turn on the power and adjust the voltage control to obtain ES equals 200V. Use E1 to measure the winding voltages and record the results. Make sure to record the associated angle.

E1-6

E1-11

E6-11

E1-2

E6-7

E11-12

E3-8

E3-13

E8-13

E3-5

E8-10

E13-15

8. Do the measurements confirm that the secondary windings have the proper phase relationship? ----- Yes ----- No

9. Are the line-to-line voltages on the primary and secondary sides of the transformer √3 times greater than the line-to-neutral i.e. phase values?

----- Yes ----- No

10. Observe the voltage phasors on the Phasor Analyzer. Does the display confirm they are equal with a 120o phase shift between each of them?

----- Yes ----- No

11. Turn off the power without modifying the setting of the voltage control. 12. Connect E2 to measure phase voltage E1-2 on the primary side. Turn on the power and compare

the voltage phasor of E1-2 on the primary side with that of E3-5 on the secondary side. Does the Phasor Analyzer display show that the voltages are equal and in phase, except for possibly a small difference due to transformer reactance?

----- Yes ----- No

Part 2: Voltage and Current Relationships

• Objectives When you have completed this exercise, you will be familiar with the voltage and current ratios of three-

phase transformers connected in delta-wye and wye-delta configurations. Measurements of primary and

secondary voltages will demonstrate that these configurations create a phase shift between the incoming

and outgoing voltages.

• Discussion As seen in the previous exercise, primary and secondary voltages in delta-delta and wye-wye connections

are in phase and the voltage at the secondary is equal to the voltage at the primary times the inverse of

the turns ratio. In delta-wye and wye-delta connections however, there will be a 30o phase difference

between the primary and secondary voltages. Also, in the delta-wye configuration, the line voltage at the

secondary is equal to the line voltage at the primary times the inverse of the turn ratio times √3. On the

other hand, in the wye-delta configuration, the line voltage at the secondary is equal to the line voltage at

the primary times the inverse of the turn ratio times 1/√3.

The 30o phase shift between the primary and secondary does not create any problems for isolated groups

of loads connected to the outgoing lines from the secondary. However, if the outgoing lines from the

secondary of a three-phase transformer have to be connected in parallel with another source, the phase

shift might make such a parallel connection impossible, even if the line voltages are the same. Recall that

in order for three-phase circuits and sources to be connected in parallel, line voltages must be equal, have

the same phase sequence, and be in phase when the parallel connection is made.

Figure 9-8 shows a three-phase transformer, with a turns ratio equal to 1:1, connected in the delta-wye

configuration and feeding a three-phase load. The voltage across each primary winding EPRI equals the

incoming line voltage, but the outgoing line voltage ESEC is √3 times that voltage because the voltage across

any two secondary windings is √3 times greater than the voltage across a single secondary winding. Note

that if the three-phase transformer had a turns ratio of 1:10, the line voltage at the secondary would be

10 x √3 times greater the line voltage at the primary, because the inverse of the turns ratio is multiplied

by the √3 factor. The line current in the secondary is the same as the phase current, but the line current in

the primary is √3 times greater than the corresponding phase current.

Figure 9-8. Three-Phase Delta-Wye Configuration

• Procedure:

1. Connect the Three-Phase Transformer module in the wye-delta configuration shown in Figure 9-9. Make sure that the voltage within the delta is zero before closing the delta.

Figure 9-9. Three-Phase Transformer Connected in Wye-Delta

2. Turn on the power and adjust the voltage control dial until ES equals 200V. Connect E1, E2, and E3 to measure the line voltages at the primary and record the results. Find also the average value of the line voltage given. Make sure to record the associated angle.

E1-6

E11-1

E6-11

EAVG

3. Observe the voltage phasors on the Phasor Analyzer. Are they approximately equal with a 120o phase shift between each of them?

----- Yes ----- No

4. Turn off the power without modifying the setting of the voltage control. Connect E1, E2, and E3 to now measure the line voltages at the secondary. Turn on the power and record the line voltages as well as the average value of the line voltages. Make sure to record the associated angle.

E3-5

E8-10

E13-15

EAVG

5. Observe the voltage phasors on the Phasor Analyzer. Does the display confirm they are equal with a 120o phase shift between each of them?

----- Yes ----- No

6. Turn off the power without modifying the setting of the voltage control. Connect E2 to measure line voltage E1-6 on the primary side. Turn on the power and compare the voltage phasor of E1-6

on the primary side with that of E3-5 on the secondary side. Does the Phasor Analyzer display confirm a phase shift of around 30o between the two?

----- Yes ----- No

7. Calculate the ratio AVG ESEC / AVG EPRI using the values recorded in steps 2 and 4. Is it

approximately equal to 1/√3? ----- Yes ----- No

8. Turn off the power and connect the Three-Phase Transformer module in the delta-wye configuration shown in Figure 9-10. During this step ensure that node 12 is connected to node 1. Also ensure that node 11 is connected to node 7. Set the Resistive Load module so that R=1200Ω, and connect I1, I2, and I3 to measure the three-line currents to the load.

9. Connect E1, E2, and E3 to measure the line voltages at the primary, turn on the power, and adjust

the voltage control to obtain the line-to-line voltage of ES equals 200V. Record the value of the line voltages, as well as the average value.

E1-2

E6-7

E11-12

EAVG

Figure 9-10. Three-Phase Transformer Connected in Delta-Wye

10. Observe the voltage and current phasors on the Phasor Analyzer. Does the display confirm that the voltage and current phasors are in phase?

----- Yes ----- No

11. Turn off the power without modifying the setting of the voltage control. Connect E1, E2, and E3 to now measure the line voltages E3-8, E8-13, and E13-3 on the secondary side. Turn on the power. Does the Phasor Analyzer display show that the voltage phasors lead the current phasors by 30o?

----- Yes ----- No

Note: Since the currents in the secondary are in phase with the voltages in the primary, the Phasor

Analyzer display is equivalent to observing all voltage phasors at the same time, except for the

difference in scale between the parameters.

12. Return to the Metering application and record the measured values for the line voltages at the secondary, and the average value. Make sure to record the associated angle.

E3-8

E8-13

E13-3

EAVG

13. Calculate the ratio AVG ESEC/AVG EPRI using the values recorded in steps 9 and 12. Is it approximately equal to √3?

----- Yes ----- No

14. Turn off the power and connect I1 and I2 to measure the line and phase currents on the primary side of the delta-wye configuration by opening the circuit at points X and Y shown in Figure 9-10. Remember to reconnect the load resistors at the secondary when I1 and I2 are disconnected.

15. Turn on the power and calculate the ratio ILINE / IPHASE for the primary circuit using the measured currents. Is the ratio approximately equal to √3?

----- Yes ----- No

16. Is the line current on the primary side approximately equal to the line current on the secondary side?

----- Yes ----- No

• CONCLUSION You connected a 1:1 three-phase transformer in wye-delta and delta-wye configurations and saw that

the line voltage between primary and secondary either increased or decreased by a √3 factor. You

also confirmed that the outgoing line voltages at the secondary were shifted 30o with respect to the

incoming line voltages at the primary.

Part 3: Open Delta Connection

• Objective When you have completed this exercise, you will be able to connect two transformers in an open-

delta configuration to supply a balanced three-phase load. You will also be able to demonstrate that

the maximum power in the open-delta configuration is 57.7% (1/√3) the capacity of a normal delta-

delta configuration.

• Discussion The open-delta connection allows three-phase balanced loads to be supplied using only two

transformers. This configuration is useful if the amount of load power is not excessive, or when one

of the three transformers must be taken out of service because of damage or some other reason. The

most important thing to note is that the power capacity in the open-delta configuration is 57.7% of

the total capacity of the normal delta-delta configuration, or 86.6% of the capacity of the two

remaining transformers. The reason for this is quite simple, and Figure 9-11 will be used to illustrate

the explanation.

Figure 9-11. Open-Delta Configuration

In a standard delta configuration, the line current is √3 times greater than the current flowing in the

phase winding. When one of the transformers is absent, full line current flows through the phase

windings, since line and phase currents are the same in an open-delta configuration. The large

increase in current will cause the phase windings to overheat and will damage the transformer unless

load power is reduced. The line current must therefore be reduced by √3, meaning that the power

capacity in the open-delta configuration is limited to 57.7% of the power capacity normal delta-delta

configuration. The following example illustrates the calculation of maximum power.

When three 50-kVA transformers are connected in delta-delta configuration, the total capacity of the

bank is their sum, or 150 kVA. For two transformers in an open-delta configuration, the capacity is

150 kVA//3, or 86.6 kVA, which is the same as 86.6% of the total capacity of two transformers (0.866

x 100 kVA ' 86.6 kVA).

• PROCEDURE 1. Connect the Three-Phase Transformer module in the delta-delta configuration shown in Figure 9-

12. Close the deltas at primary terminals 7, 11 (point X) and secondary terminals 10, 13 (point Y) last, and use separate wires for the connections. Connect E1 and I1 at the primary and E2, I2, and I3 at the secondary as shown. Make sure the current within the secondary delta equals zero before applying full power to the circuit.

Figure 9-12. Demonstrating an Open-Delta Configuration

2. Connect the Resistive Load module as shown, and set R to 600Ω. Connect E3 in parallel with one of the resistor as shown in Figure 9-12. Turn on the power and adjust the voltage control to obtain the line-to-line voltage ES equals 200V. Record the line and phase (winding) values indicated by the meters as well as the apparent powers indicated by meters PQS1 and PQS2.

E1 =___V

E2 =___V

E3 =___V

I1 =___A

I2 =___A

I3 =___A

S1 =___VA

S2 =___VA

S3 =___VA

3. Do the meters show that the primary and secondary line voltages are equal (E1, E2), and the current in the primary windings equals that flowing in the secondary windings (I1, I2)?

----- Yes ----- No

4. Is the line current to the load shown by I3 around √3 times greater than phase current I2 in the secondary winding?

----- Yes ----- No

Note: Observe also that the load voltage shown by E3 is √3 times less than line voltage E2 at the secondary.

5. Carefully open the primary delta at point X by disconnecting the wire at primary terminal 11, and observe the change in line and phase currents. Exercise caution when you open the delta since high voltage is present on the wire.

6. Do the phase currents at the primary and secondary (I1,I2) increase by a large amount, as well as the values of S1 and S2?

----- Yes ----- No

7. Is the increase approximately equal to √3? ----- Yes ----- No

8. With the open-delta configuration, does the phase current in the secondary now equal the line current to the load?

----- Yes ----- No

9. Close the delta at point X, and open the secondary at point Y by disconnecting the wire at terminal 13. Once again exercise caution with the open connection. Do you observe the same results as in the previous steps?

----- Yes ----- No

Lab 5 AC Induction Motors

UNIT OBJECTIVE

After completing this unit, you will be able to demonstrate and explain the operation of ac induction

motors using the Squirrel-Cage Induction Motor module and the Capacitor-Start Motor module.

DISCUSSION OF FUNDAMENTALS

As you saw in Unit 1, a voltage is induced between the ends of a wire loop when the magnetic flux linking the loop varies as a function of time. If the ends of the wire loop are short-circuited together, a current flows in the loop. Figure 4-1 shows a magnet that is displaced rapidly towards the right above a group of conductors. The conductors are short-circuited at their extremities by bars A and B and form a type of ladder.

Figure 4-1. Magnet Moving Above a Conducting Ladder.

Current flows in the loop formed by conductors 1 and 2, as well as in the loop formed by conductors 2 and 3. These currents create magnetic fields with north and south poles as shown in Figure 4-2.

Figure 4-2. Current in the Conductors Creates

Magnetic Fields.

The interaction between the magnetic field of the magnet and the magnetic fields produced by the currents induced in the ladder creates a force between the moving magnet and the electromagnet (the conducting ladder). This force causes the ladder to be pulled along in the direction of the moving magnet. However, if the ladder moves at the same speed as the magnet, there is no longer a variation in the magnetic flux. Consequently, there is no induced voltage to cause current flow in the wire

loops, meaning that there is no longer a magnetic force acting on the ladder. Therefore, the ladder must move at a speed which is lower than that of the moving magnet for a magnetic force to pull the ladder in the direction of the moving magnet. The greater the speed difference between the two, the greater the variation in magnetic flux, and therefore, the greater the magnetic force acting on the conducting ladder. The rotor of an asynchronous induction motor is made by closing a ladder similar to that shown in Figure 4-1 upon itself to form a type of squirrel cage as shown in Figure 4-3. This is where the name squirrel-cage induction motor comes from.

Figure 4-3. Closing a Ladder Upon Itself Forms a Squirrel Cage.

To make it easier for the magnetic flux to circulate, the rotor of a squirrel-cage induction motor is placed inside a laminated iron cylinder. The stator of the induction motor acts as a rotating electromagnet. The rotating electromagnet causes torque which pulls the rotor along in much the same manner as the moving magnet in Figure 4-1 pulls the ladder.

EXERCISE OBJECTIVE

When you have completed this exercise you will be able to demonstrate the operating characteristics of a three-phase induction motor using the Four-Pole Squirrel-Cage Induction Motor module.

DISCUSSION

One of the ways of creating a rotating electromagnet is to connect a three-phase power source to a stator made of three electromagnets A, B, and C, that are placed at 120° to one another as shown in Figure 4-4.

Figure 4-4. Three-Phase Stator Windings.

When sine-wave currents phase shifted of 120° to each other, like those shown in Figure 4-5, flow in stator electromagnets A, B, and C, a magnetic field that rotates very regularly is obtained.

Figure 4-5. Three-Phase Sine-Wave Currents Flowing in the Stator Windings.

Figure 4-6 illustrates the magnetic field created by stator electromagnets A, B, and C at instants numbered 1 to 6 in Figure 4-5. Notice that the magnetic lines of force exit at the north pole of each electromagnet and enter at the south pole. As can be seen, the magnetic field rotates clockwise.

The use of sine-wave currents produces a magnetic field that rotates regularly and

whose strength does not vary over time. The speed of the rotating magnetic field is known as the synchronous speed (nS) and is proportional to the

frequency of the ac power source. A rotating magnetic field can also be obtained using other combinations of sine-wave currents that are phase-shifted with respect to each other, but three-phase sine-wave currents are used more frequently.

When a squirrel-cage rotor is placed inside a rotating magnetic field, it is pulled

around in the same direction as the rotating field. Interchanging the power connections to two of the stator windings (interchanging A with B for example) interchanges two of the three currents and reverses the phase sequence. This causes the rotating field to reverse direction. As a result, the direction of rotation of the motor is also reversed.

Figure 4-6. Position of the Rotating Magnetic Field at Various Instants. (From Electrical

Machines, Drives, and Power Systems by Theodore Wildi. Copyright © 1991, 1981 Sperika

Enterprises Ltd.)

Referring to what has been said in the Discussion of Fundamentals of this unit, one can easily deduce that the torque produced by a squirrel-cage induction motor

increases as the difference in speed between the rotating magnetic field and the rotor increases. The difference in speed between the two is called slip. A plot of the speed versus torque characteristic for a squirrel-cage induction motor gives a curve similar to that shown in Figure 4-7. As can be seen, the motor speed (rotor speed) is always lower than the synchronous speed nS because slip is necessary for the motor to develop

torque. The synchronous speed for the Lab-Volt motors is 1800 r/min for 60-Hz power, and 1500 r/min for 50-Hz power.

Figure 4-7. Speed Versus Torque Characteristic of a Squirrel-Cage Induction Motor.

The speed versus torque characteristic of the squirrel-cage induction motor is

very similar to that obtained previously for a separately-excited dc motor.

However, the currents induced in the squirrel-cage rotor must change

direction more and more rapidly as the slip increases. In other words, the

frequency of the currents induced in the rotor increases as the slip increases.

Since the rotor is made up of iron and coils of wire, it has an inductance that

opposes rapid changes in current. As a result, the currents induced in the rotor

are no longer directly proportional to the slip of the motor. This affects the

speed versus torque characteristic as shown in Figure 4-8.

Figure 4-8. The Motor Inductance Affects the Speed Versus Torque Characteristic.

As the curve shows, the no-load speed is slightly less than the synchronous speed nS, but as the load torque increases, motor speed decreases. For the

nominal value of motor torque (full-load torque) corresponds a nominal operating speed (full- load speed). Further increases in load torque lead to a point of instability, called breakdown torque, after which both motor speed and output torque decrease. The torque value at zero speed, called locked-rotor torque, is often less than the breakdown torque. At start-up, and at low speed, motor current is very high and the amount of power that is consumed is higher than during normal operation.

Another characteristic of three-phase squirrel-cage induction motors is the fact

that they always draw reactive power from the ac power source. The reactive power even exceeds the active power when the squirrel-cage induction motor rotates without load. The reactive power is necessary to create the magnetic field in the machine in the same way that an inductor needs reactive power to create the magnetic field surrounding the inductor.

Procedure Summary

In the first part of the exercise, you will set up the equipment in the Workstation, connect the equipment as shown in Figure 4-9, and make the appropriate settings on the Prime Mover / Dynamometer.

In the second part of the exercise, you will apply the nominal line voltage to the squirrel-

cage induction motor, note the motor direction of rotation, and measure the motor no-load speed. You will then increase the mechanical load applied to the squirrel-cage induction motor by steps. For each step, you will record in the data table various electrical and mechanical parameters related to the motor. You will then use this data to plot various graphs and determine many of the characteristics of the squirrel-cage induction motor.

In the third part of the exercise, you will interchange two of the leads that supply power to

the squirrel-cage induction motor and observe if this affects the direction of rotation.

PROCEDURE

CAUTION!

High voltages are present in this

laboratory exercise! Do not make

or modify any banana jack

connections with the power on

unless otherwise specified!

Setting up the Equipment

1. Install the Power Supply, Prime Mover / Dynamometer, Four-Pole Squirrel- Cage Induction Motor, and Data Acquisition Interface (DAI) modules in the EMS workstation. Mechanically couple the Prime Mover / Dynamometer to the Four-Pole Squirrel-Cage Induction Motor.

2. On the Power Supply, make sure the main power switch is set to the O (off) position, and the voltage control knob is turned fully counterclockwise. Ensure the Power Supply is connected to a three-phase power source.

3. Ensure that the USB port cable from the computer is connected to the DAI module. Connect the LOW POWER INPUTs of the DAI and Prime Mover / Dynamometer modules to the 24 V - AC output of the Power Supply. On the Power Supply, set the 24 V - AC power switch to the I (on) position.

4. Start the Metering application.

• In the LVDAC-EMS, open setup configuration file ACMOTOR1.DAI then select meter layout 2. (File > Open > acmotor1). Once the Metering window is open select View > Layout > Layout 2.

• In Meter 3 (M3) click on “AI-8/n” then select Encoder AB under input/function.

• In Meter 4 (M4) click the torque correction button “C” then select OK.

• In Meter 10 (M10) click on “MD(PQS1…”. In the Meter Settings window scroll down in the Input/Function window and select “Pm (AI-7/T, AI-8/n)”.

5. Connect the equipment as shown in Figure 4-9.

Note: The dotted line between the squirrel cage motor and the Dynamometer symbolizes the timing belt which should already be assembled.

The “T” and “N” on the Dynamometer are the torque and speed outputs which have already been connected to the DAI via the small red.

The wires connected to the squirrel cage induction motor from I1, I2, E1 and E2 correspond to nodes

1,2 and 3 on the face plate of the motor itself. Ensure that nodes 4,5 and 6 on the induction motor are connected.

Figure 4-9. Squirrel-Cage Induction Motor Coupled to a Dynamometer.

6. Set the Prime Mover / Dynamometer controls as follows:

• MODE switch . . . . . DYN.

• LOAD CONTROL MODE switch…MAN.

• LOAD CONTROL knob . . . . . . . . . MIN. (fully CCW)

• DISPLAY switch . . . . . . . . . . . . . . . . . TORQUE (T)

Note: If you are performing the exercise using LVSIM®-EMS, you can zoom in the Prime Mover / Dynamometer module before setting the controls in order to see additional front panel markings related to these controls.

Characteristics of a Squirrel-Cage Induction Motor

7. Turn on the Power Supply and set the voltage control knob so that the line voltage indicated by meter E1 is equal to the nominal line voltage of the squirrel-cage induction motor.

Note: The rating of any of the Lab-Volt machines is indicated in the lower left corner of

the module front panel. If you are performing the exercise using LVSIM®-EMS, you can obtain the rating of any machine by leaving the mouse pointer on the rotor of the machine of interest. Pop-up help indicating the machine rating will appear after a few

seconds.

What is the direction of rotation of the squirrel-cage induction motor?

Record the motor speed indicated in meter N of the metering window

Is the no-load speed almost equal to the speed of the rotating magnetic field (synchronous speed) given in the Discussion?

8. On the Prime Mover / Dynamometer, adjust the LOAD CONTROL knob so that the mechanical power developed by the squirrel-cage induction motor (indicated by meter Pm in the Metering window) is equal to 175 W (nominal motor output power).

Record the nominal speed, torque, and line current of the squirrel-cage induction motor in the following blank spaces. The line current is indicated by meter I1.

nNOM = ______r/min (compare with calculated nominal speed).

TNOM =___Nm

Nominal current= ___A

On the Prime Mover / Dynamometer, turn the LOAD CONTROL knob fully counterclockwise. The torque indicated on the Prime Mover / Dynamometer display should be 0 N-m

(0 lbf-in).

9. Record the motor line voltage ELINE, line current ILINE, active power P, reactive power Q, speed n, and output torque T in the Data Table.

On the Prime Mover / Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.3 N-m (3.0 lbf-in) increments up to 1.8 N-m (15.0 lbf-in). For each torque setting, record the data in the Data Table. Note: The metering window in the LVDAC software displays torque in units of N-m. The meter on the Dynamometer displays torque in units of lbf-in. Therefore, your increments may be different depending on which display you base your recordings on.

On the Prime Mover / Dynamometer, carefully adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.1 N-m (1.0 lbf-in) increments until the motor speed starts to decrease fairly rapidly (breakdown torque region) ~19-21 lbf-in. Do not increase the load to the point that the motor locks up. For each additional torque setting, record the data in the Data Table.

Once the motor speed has stabilized, record the data in the Data Table. Note: The nominal line current of the Four-Pole Squirrel-Cage Induction Motor may be exceeded while performing this manipulation. It is, therefore, suggested to complete the manipulation within a time interval of 5 minutes or less.

10. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover / Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise, and turn off the Power Supply.

Does the motor line current increase as the mechanical load applied to the squirrel-cage induction motor increases?

11. In the Graph window, make the appropriate settings to obtain a graph of the motor speed (obtained from meter N) as a function of the motor torque (obtained from meter T). Name the x-axis as Squirrel-Cage Induction-Motor Torque, name the y-axis as Squirrel-Cage Induction-Motor Speed. Put this graph in your lab report.

Exercise 2 Effect of Voltage on the Characteristics of Induction Motors

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to use the Four-Pole Squirrel-Cage Induction Motor module to demonstrate how the voltage applied to an induction motor affects its characteristics.

DISCUSSION

It is desirable to have a strong rotating magnetic field in induction motors to obtain the strongest magnetic force possible between the stator and the rotor. This results in a powerful motor because this allows a high torque to be developed. To increase the strength of the rotating magnetic field, it is necessary to increase the ac voltage applied to the stator windings of the induction motor (motor voltage). However, when the motor voltage is increased too much, the motor current (current in the

stator windings) is large even at no load because the iron in the stator of the motor begins to saturate. When the motor is saturated, the strength of the rotating magnetic field almost ceases to increase as the no-load motor current is increased. To determine the nominal voltage of an induction motor, a voltage versus current graph as shown in Figure 4-17 is usually plotted when the motor operates without load. This graph is similar to the saturation curve of a transformer or dc motor. The nominal voltage is selected so that the motor operating point is located near or in the knee of the saturation curve.

Figure 4-17. No-Load Voltage Versus Current Characteristic of an Induction Motor.

It is also possible to plot the speed versus torque characteristic for different motor voltages. Figure 4-18 shows an example of speed versus torque characteristics for both nominal and reduced motor voltages.

Figure 4-18. Speed Versus Torque Characteristics for Nominal and Reduced Motor Voltages.

As shown in Figure 4-18, both the locked-rotor torque and the breakdown torque decrease greatly when the motor voltage is reduced. In practice, the torque decreases by a factor equal to the square of the reduction factor of the motor voltage. For example, the torque is reduced by a factor of four when the motor voltage is reduced by a factor of two (i.e. decreased to one half its original value). In some circumstances, the motor voltage is reduced intentionally to obtain small variations in the speed of an induction motor. Furthermore, reducing the motor voltage allows the starting current of the

motor to be lowered.

Procedure Summary

In the first part of the exercise, you will set up the equipment in the Workstation and connect the equipment as shown in Figure 4-19.

In the second part of the exercise, you will vary the voltage applied to the windings of

the squirrel-cage induction motor at no load while measuring and recording the winding current. You will plot a graph of the winding voltage versus the winding current and observe the effect of saturation.

In the third part of the exercise, you will set up the circuit shown in Figure 4-20 and

make the appropriate settings on the Prime Mover / Dynamometer. You will then set the voltage applied to the squirrel-cage induction motor below the nominal value to see the effect this has on the no-load speed.

In the fourth part of the exercise, you will vary the load applied to the

squirrel-cage induction motor operating with reduced voltage. For each load setting, you will record in the data table various electrical and mechanical parameters related to the motor. You will then use this data to plot various graphs and determine many of the characteristics of the squirrel-cage induction motor when it operates with reduced voltage.

PROCEDURE

CAUTION!

High voltages are present in this

laboratory exercise! Do not make

or modify any banana jack

connections with the power on

unless otherwise specified!

Setting up the Equipment

1. Install the Power Supply, Prime Mover / Dynamometer, Four-Pole Squirrel- Cage Induction Motor, and Data Acquisition Interface (DAI) modules in the EMS workstation.

2. On the Power Supply, make sure the main power switch is set to the O (off) position, and the voltage control knob is turned fully counterclockwise. Ensure the Power Supply is connected to a three-phase power source.

3. Ensure that the USB port cable from the computer is connected to the DAI module.

Connect the LOW POWER INPUTs of the DAI and Prime Mover / Dynamometer modules to

the 24 V - AC output of the Power Supply. On the Power Supply, set the 24 V - AC power switch to the I (on) position.

4. Start the Metering application. In the Metering window, open setup configuration file ACMOTOR1.DAI then select meter layout 2.

5. Connect the equipment as shown in Figure 4-19.

Note: The windings of the Four-Pole Squirrel-Cage Induction Motor are connected in delta to allow a greater voltage to be applied to the windings.

Figure 4-19. Delta Connection of the Stator

Windings of the Four-Pole Squirrel-Cage

Induction Motor.

6. Turn on the Power Supply and set the voltage control knob so that the voltage applied to each of the squirrel-cage induction motor windings (indicated by meter E1) is equal to 50% of the nominal voltage of these windings.

Note: The nominal voltage and current of the windings of the Four-Pole Squirrel-Cage Induction Motor are indicated on the module front panel.

Record the winding voltage and the winding current (indicated by meter I1) in the Data Table.

7. On the Power Supply, turn the voltage control knob in 5% increments up to the 100% position to increase the winding voltage by steps. For each voltage setting, record the winding voltage and current in the Data Table.

Note: The nominal line current of the Four-Pole Squirrel-Cage Induction Motor is exceeded while performing this manipulation. It is, therefore, suggested to complete the manipulation within a time interval of 5 minutes or less.

When all data has been recorded, turn the voltage control knob fully counterclockwise and turn off the Power Supply.

8. In the Graph window, make the appropriate settings to obtain a graph of the motor winding voltage (obtained from meter E1) as a function of the motor winding current (obtained from meter I1). Name the x-axis as Squirrel-Cage Induction-Motor Winding Current, name the y-axis as Squirrel-Cage Induction-Motor Winding Voltage, and insert graph in your lab report.

Exercise 3 Two-Phase and Single-Phase Operation of a Three-Phase

Squirrel-Cage Induction Motor

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to demonstrate the main operating characteristics of single-phase induction motors using the Capacitor-Start Motor module.

DISCUSSION

It is possible to obtain a single-phase squirrel-cage induction motor using a simple electromagnet connected

to a single-phase ac power source as shown in Figure 4-21.

Figure 4-21. Simple Single-Phase Squirrel-Cage Induction Motor.

The operating principle of this type of motor is more complex than that of the three- phase squirrel-cage induction motor. The simple induction motor of Figure 4-21 can even be considered as an eddy-current brake that brakes in an intermittent manner since the sinusoidal current in the stator electromagnet continually passes from peaks to zeros. One could even wonder how this motor can turn since it seems to operate similarly as an eddy-current brake.

However, when the rotor of the simple induction motor of Figure 4-21 is turned manually, a torque which acts in the direction of rotation is produced, and the motor continues to turn as long as ac power is supplied to the stator electromagnet. This torque is due to a rotating magnetic field that results from the interaction of the magnetic field produced by the stator electromagnet and the magnetic field produced by the currents induced in the rotor. A graph of speed versus torque for this type of motor is shown in Figure 4-22. The curve shows that the torque is very small at low speeds. It increases to a maximum value as the speed increases, and finally decreases towards zero again when the speed approaches the synchronous speed nS.

Figure 4-22. Speed Versus Torque Characteristic of a Single-Phase Induction Motor.

The low torque values at low speeds are due to the fact that the currents induced in the rotor produce magnetic fields that create forces which act on the rotor in various directions. Most of these forces cancel each other and the resulting force acting on the rotor is weak. This explains why the single-phase induction motor shown in Figure 4-21 must be started manually. To obtain torque at low speeds (starting torque), a rotating magnetic field must be produced in the stator when the motor is starting. In Unit 1 of this manual, you saw that it is possible to create a rotating magnetic field using two alternating currents, I1 and I2, that are phase shifted 90°

from one another, and two electromagnets placed at right angles to each other.

Figure 4-23. Adding a Second Electromagnet to the Simple Induction Motor of Figure 4-21.

Figure 4-23 shows the simple induction motor of Figure 4-21 with the addition of a second electromagnet placed at right angle to the first electromagnet. The second electromagnet is identical to the first one and is connected to the same ac power source. The currents I1 and I2 in the electromagnets (winding currents) are in phase because the coils have the same impedance. However, because of the inductance

of the coils of the electromagnets, there is a phase shift between the currents and the ac source

voltage as illustrated in the phasor diagram of Figure 4-23.

Since currents I1 and I2 are in phase, there is no rotating magnetic field produced in the stator. However, it is possible to phase shift current I2 by connecting a capacitor in series with the winding of electromagnet 2. The capacitance of the capacitor can be selected so that current I2 leads current I1 by 90° when the motor is starting as shown in Figure 4-24. As a result, an actual rotating magnetic field like that previously illustrated in Unit 1 is created when the motor is starting. The capacitor creates the equivalent of a two-phase ac power source and allows the motor to develop starting torque.

Figure 4-24. Adding a Capacitor Allows the Induction Motor to Develop Starting Torque.

Another way to create a phase shift between currents I1 and I2 is to make a winding

with fewer turns of smaller-sized wire. The resulting winding, which is called auxiliary winding, has more resistance and less inductance, and the winding current is almost in phase with the source voltage. Although the phase shift between the two currents is less than 90° when the motor is starting, as shown in Figure 4-25, a rotating magnetic field is created. The torque produced is sufficient for the motor to start rotating in applications not requiring high values of starting torque.

Figure 4-25. Phase Shift Between the Winding Currents when an Auxiliary Winding Is Used.

However, the auxiliary winding cannot support high currents for more than a few seconds without being damaged because it is made of fine wire. It is therefore connected through a centrifugal switch which opens and disconnects the winding from the motor circuit when the motor reaches about 75% of the normal speed. After the centrifugal switch opened, the rotating magnetic field is maintained by the interaction of the magnetic fields produced by the stator and the rotor.

Procedure Summary

In the first part of the exercise, you will set up the equipment in the Workstation and connect the equipment as shown in Figure 4-26.

In the second part of the exercise, you will observe both two-phase and single-phase

operation of the three-phase squirrel-cage induction motor using the Phasor Analyzer.

In the third part of the exercise, you will observe the operation of a single-

phase induction motor using a capacitor-start motor and the Phasor Analyzer.

ROCEDURE

CAUTION!

High voltages are present in this

laboratory exercise! Do not make or

modify any banana jack connections

with the power on unless otherwise

specified!

Setting up the Equipment

1. Install the Power Supply, Four-Pole Squirrel-Cage Induction Motor, Capacitor-Start Motor, Capacitive Load, and Data Acquisition Interface (DAI) modules in the EMS workstation.

2. On the Power Supply, make sure the main power switch is set to the O (off) position, and the voltage control knob is turned fully counterclockwise. Ensure the Power Supply is connected to a three-phase power source.

3. Ensure that the USB port cable from the computer is connected to the DAI module.

Connect the LOW POWER INPUT of the DAI module to the 24 V - AC output of the Power Supply.

On the Power Supply, set the 24 V - AC power switch to the I (on) position.

4. Start the Metering application. Exit out of the acmotor1 metering window file (do not save settings). On the LVDAC-EMS desktop select

File > New > Click the metering window icon.

5. Connect the equipment as shown in Figure 4-26.

Figure 4-26. Three-Phase Squirrel-Cage Induction Motor.

Two-Phase and Single-Phase Operation of a Three-Phase Squirrel-Cage Induction Motor

6. Turn on the Power Supply and set the voltage control knob so that the voltage applied to each of the motor windings (indicated by meter E1) is equal to the nominal voltage of these windings.

Note: The nominal voltage and current of the windings of the Four-Pole Squirrel-Cage

Induction Motor are indicated on the module front panel.

Does the squirrel-cage induction motor start readily and rotate normally?

7. In the Phasor Analyzer window, select voltage phasor E1 as the reference phasor then select proper sensitivities to observe voltage phasor E1 and current phasors I1, I2, and I3. These phasors represent the ac source line- to-neutral voltage and the line currents in the three-phase squirrel-cage induction motor. Are phasors I1, I2, and I3 all equal in magnitude and separated by a phase angle of 120°, thus showing they create a normal rotating magnetic field?

8. Turn off the Power Supply. Open the circuit at point A shown in Figure 4-26. Make sure that VOLTAGE INPUT E1 of the DAI module remains connected to the ac power source.

9. Turn on the Power Supply. Does the squirrel-cage induction motor start readily and rotate normally? In the Phasor Analyzer window, observe current phasors I2 and I3. Is there a phase shift between current phasors I2 and I3 to create a rotating magnetic field?

Open the circuit at point B shown in Figure 4-26.

11. Turn on the Power Supply, set the voltage control knob to about 50%, wait approximately 5 seconds, then turn off the Power Supply and turn the voltage control knob fully counterclockwise. Does the squirrel-cage induction motor start readily and rotate normally?

12. Use the Capacitive Load module to connect a capacitor to the motor circuit as shown in the first line of the table in Figure 4-27. Set the capacitance of the capacitor to the value indicated in the figure (all switches up on a Capacitive load).

13. Turn on the Power Supply and slowly set the voltage control knob to 100%. While doing this, observe phasors I2 and I3 in the Phasor Analyzer window as the voltage increases.

Does the squirrel-cage induction motor start to rotate? Briefly explain why (refer to theory given in lab manual). The motor may rotate too slowly at 15.4 µF so you may need to add another capacitive load in parallel to increase motor speed.

Figure 4-27. Adding a Capacitor to the Motor Circuit.

14. On the Capacitive Load module, open the switches to disconnect the capacitor from the motor circuit and cut off the current in one of the two windings of the squirrel-cage induction motor.

Does the squirrel-cage induction motor continue to rotate, thus showing that it can

operate on single-phase ac power once it has started?

Turn off the Power Supply and turn the voltage control knob fully counterclockwise.

Lab 6

SEPARATELY- EXCITED, SERIES, SHUNT AND COMPOUND DC MOTORS

OBJECTIVES

1) To demonstrate how the field current affects the characteristics of a separately-excited dc motor using the DC Motor/Generator module.

2) To demonstrate the main operating characteristics of series, shunt and compound motors.

EQUIPMENTS

EMS Workstation Model 8110, DC Motor/Generator Model 8211, Resistive Load Model 8311, Power Supply Model 8821, Prime Mover/Dynamometer Model 8960 and Data Acquisition Interface Model 9062.

INTRODUCTION

Separately-Excited DC Motor

It is possible to change the characteristics of a separately-excited dc motor by changing the strength of the

fixed magnetic field produced by the stator electromagnet. This can be carried out by changing the

current that flows in the stator electromagnet. This current is usually referred to as the field

current (IF) because it is used to produce the fixed magnetic field in the dc motor. A rheostat connected in series with the electromagnet winding can be used to vary the field current.

Figure 4.1 illustrates how the speed versus armature voltage of a separately- excited dc motor is affected

when the field current is decreased below its nominal value. Constant K1 becomes greater. This

means that the motor can rotate at higher speeds without exceeding the nominal armature voltage.

It is also possible to set the field current of a separately-excited dc motor above its nominal value for short

time intervals. The effect on the speed versus armature voltage relationship is reversed, i.e. constant K1 becomes smaller. As a result, the motor can develop a higher torque during these time intervals but the speed at which the motor can rotate, without exceeding the nominal armature voltage is reduced.

Figure 4.1 Decreasing Current IF below its Nominal value Affects Constants K1

Series Motor

The series motor is a motor in which the field electromagnet is a series winding connected in series with

the armature as shown in Figure 4.2. The strength of the field electromagnet varies as the

armature current varies. As a result, K1 vary when the armature current varies. Figure 4.2 shows the speed versus torque characteristics of a series motor when the armature voltage is fixed. This characteristics shows that the speed decreases non linearly as the torque increases, i.e. as the armature current increases.

Figure 4.2 Series Motor and its Speed versus Torque characteristics

The series motor provides a strong starting torque and a wide range of operating speeds when it is supplied by a fixed-voltage dc source. However, the speed, torque and armature current depend on the mechanical load applied to the motor. The series motor has non-linear operating characteristics as suggested by the speed versus torque relationship in Figure 4.2. As a result, it is difficult to operate a series motor at a constant speed when the mechanical load fluctuates. Finally, a series motor must never run with no mechanical load because the speed increases to a very-high value which can damage the motor (motor runaway).

Shunt Motor

The shunt motor is a motor in which the field electromagnet is a shunt winding connected in parallel with

the armature, both being connected to the same dc voltage source as shown in Figure 4.3. For a

fixed armature voltage, constant K1 is fixed, and the speed versus torque characteristics is very similar to that obtained with a separately-excited dc motor powered by a fixed-voltage dc source,

as shown in Figure 4.3. As in a separately-excited dc motor, the characteristics K1 of a shunt motor

can be changed by varying the field current with a rheostat. It is difficult to change the speed of a shunt motor by changing the armature voltage because this changes the field current and thereby the motor characteristics in a way that opposes speed change.

Figure 4.3 Shunt Motor and its Characteristics

The main advantage of a shunt motor is the fact that only a single fixed-voltage dc source is required to

supply power to both the armature and the shunt winding. Finally when the shunt winding opens

accidentally, the field current IF becomes zero, the motor speed increases rapidly and motor runaway occurs as suggested by the speed versus field current characteristics shown in Figure 4.3.

Compound Motor

It is possible to combine shunt and series winding to obtain a particular speed versus torque characteristics. For example, to obtain the characteristics of decreasing speed when the motor torque increases, a series winding can be connected in series with the armature so that the magnetic flux it produces adds with the magnetic flux produced by shunt winding. As a result, the magnetic flux increases automatically with increasing armature current. This type of dc motor is

referred to as a cumulative compound motor because the magnetic fluxes produced by the series and shunt windings add together. Shunt and series winding can also be connected so that the magnetic fluxes subtract from each other. This connection produces a differential-compound motor which is rarely used because the motor becomes unstable when the armature current increases. Figure 4.4 shows a compound motor and its speed versus torque characteristics (cumulative compound).

Figure 4.4 Compound Motor and its Speed versus Torque Characteristics

Figure 4.5 is a graph that shows the speed versus torque characteristics of the various types of dc motors. As can be seen, the separately-excited dc motor and shunt motor have very similar characteristics. The main feature of these characteristics is that the motor speed varies little and linearly as the torque varies. Finally, the characteristics of a cumulative compound motor are a compromise of the series and shunt motor characteristics. It provides the compound motor with a fairly wide range of operating speed but the speed does not vary linearly as the torque varies.

Figure 4.5 Speed versus Torque Characteristics of Various DC Motors

PROCEDURE

CAUTION

High voltages are present in this laboratory exercise! Do not make or modify any banana

jack connections with the power on unless otherwise specified!

1. Install the Power Supply, Prime Mover/Dynamometer, DC Motor/Generator and Data Acquisition Interface (DAI) modules in the EMS Workstation.

Note: If you are performing the experiment using the EMS system,

ensure that the brushes of the DC Motor/Generator are adjusted to the

neutral point. To do so, connect an ac power source (terminals 4 and N

of the Power Supply) to the armature of the DC Motor/Generator

(terminals 1 and 2) through CURRENT INPUT I1 of the Data Acquisition

Interface module. Connect the shunt winding of the DC Motor/Generator

(terminals 5 and 6) to VOLTAGE INPUT E1 of the Data Acquisition

Interface module. Start the Metering application. Turn on the Power

Supply and set the voltage control knob so that an ac current (indicated

by meter I1) equal to half the nominal value of the armature current

flows in the armature of the DC Motor/Generator. Adjust the brushes

adjustment lever on the DC Motor/Generator so that the voltage across

the shunt winding (indicated by meter E1) is minimum. Turn off the

Power Supply, exit the Metering application and disconnect all leads and

cables.

Mechanically couple the Prime Mover/Dynamometer to the DC Motor/Generator using a timing belt if it has not been done already.

2. On the Power Supply, make sure that the main switch of the Power Supply is set to the O (OFF) position, and the voltage control knob is turned fully counter clockwise.

3. Connect the DAI LOW POWER INPUT of the DAI and Prime Mover/Dynamometer modules to the 24V-AC output of the Power Supply. On the Power Supply, set the 24V-AC power switch to the I (on) position.

4. Open the dcmotor1.dai file and make the following changes to the metering window:

• In M2, click on “AI-8/n”. Then in the meter settings window under

input/function select Encoder AB then select Apply > OK. • In M9, click on “MD(PQS1 + PQS2)”. Then in the meter settings window under

input/function select Pm(AI-7/T,AI-8/n).

5. Set up the separately-excited dc motor circuit shown in Figure 4.6.

Attention! - Note: If you are performing the exercise with a line voltage 240 V, use the

resistive load module to connect a 960-Ω resistor in series with the rheostat.

Figure 4.6 Separately-Excited DC Motor Coupled to a Dynamometer

6. Set the Prime Mover/Dynamometer controls as follows:

MODE switch ………………………………..…………….…. DYN. LOAD CONTROL MODE switch …………………………… MAN. LOAD CONTROL knob ………………..……… MIN. (fully CCW) DISPLAY switch

……………………………………….SPEED (N)

Note: If you are performing the experiment using LVSIM-EMS, you can

zoom in the Prime Mover/Dynamometer module before setting the

controls in order to see additional front panel markings related to these

controls.

Speed versus Armature Voltage Characteristics of a Separately-Excited DC Motor

7. Turn on the Power Supply.

On the DC Motor/Generator, set the FIELD RHEOSTAT so that the field current IF indicated by meter I2 in the

Metering window is equal to the value given in table 4.1. Note: if the resistance on the rheostat

is not sufficient, connect a resistive load is series with node 8 on the DC motor and then connect the negative terminal of the resistor to neutral.

Line Voltage Field Current If V dc mA

(voltage control knob 100 %)

210

Table 4.1 Field Current of the Separately-Excited DC Motor

8. In the Metering window, select the torque correction function for meter T. Meter T now indicates the dc motor output torque.

Record the dc motor speed n, armature voltage EA, armature current IA, field current IF, and output torque

T (indicated by meters N, E1, I1, I2 and T respectively) in the Data Table.

On the Power Supply, set the voltage control knob to 10%, 20%, 30% etc. up to 100% in order to increase

the armature voltage EA by steps. For each voltage setting, wait until the motor speed stabilizes and then record the data in the Data Table.

9. When all data has been recorded in Table 4.1 (Results Section), turn the voltage control knob fully counterclockwise and turn off the Power Supply.

10. In the Graph window, make the appropriate settings to obtain a graph of the dc motor

speed n (obtained from meter N) as a function of the armature voltage EA (obtained from meter E1). Plot the Graph 4.1 (Results Section).

11. Use the two end points to calculate the slope K1 of the relationship obtained in Graph 4.1 (Results Section). The values of these points are indicated in data table (Table 4.1-Results Section).

Describe how does decreasing the field current IF affect the speed versus voltage characteristics and

constant K1 of a separately-excited dc motor.

In the Data table window, clear the recorded data.

Speed versus Torque Characteristics of a Series Motor

12. Modify the connections so as to obtain the series motor circuit shown in Figure 4.7.

13. Turn on the Power Supply and set the voltage control knob so that the armature voltage EA indicated by meter E1 is equal to the any value when the dc motor speed is 1500 r/min. The series motor should start to rotate.

Figure 4.7 Series Motor Coupled to a Dynamometer

14. In the Metering window, make sure the torque correction function of meter T is selected.

Record the motor speed n, output torque T, armature voltage EA, and armature current IA

(indicated by meters n, T, E1 and I1 respectively) in the Data Table.

On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 Nm increments up to 2.0 N-m.

If you are using the new 8960-20 Dynamometer (with the big digital screen) follow these steps if not skip the steps in red:

• Make sure the load control knob is fully knob is fully CCW.

• Press the function button in the Dynamometer until the digital screen reads out “2Q CT Brake”.

• Before adjusting the voltage control knob press the “start/stop” button on the dynamometer/power supply.

• Adjust the load control knob slowly because there is a slight delay between the mechanical control knob input to the digital display of the torque.

For each torque setting, readjust the voltage control knob of the Power Supply so that the armature

voltage EA remains equal to the value set in the previous step, wait until the motor speed stabilizes and then record the data in the Data Table.

Note: It may not be possible to maintain the armature voltage to its original value

as the torque is increased. The armature current may exceed the rated value while

performing this manipulation. It is therefore suggested to complete the

manipulation within a time interval of 5 minutes or less.

15. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply.

In the Data Table window, confirm that the data has been stored. The data must be filled

in Table 4.2 (Results Section).

16. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.2 (Results Section).

Briefly describe how the speed varies as the mechanical load applied to the series motor increases, i.e. as

the motor torque increases.

Compare the speed versus torque characteristics of the series motor to that of the separately-excited dc

motor.

17. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables.

Speed versus Torque Characteristics of a Shunt Motor

18. Set up the shunt motor circuit shown in Figure 4.8. Make sure that the LOAD CONTROL knob on the Prime Mover/Dynamometer is set to the MIN. position (fully CCW).

Figure 4.8 Shunt Motor Circuit

19. Turn on the Power Supply, set the armature voltage EA to the value recorded in step 13.

20. Set the FIELD RHEOSTAT on the DC Motor/Generator so that the field current IF is equal to the value indicated in Table 4.2.

Line Voltage Field Current If V dc mA

(voltage control knob 100 %)

210

Table 4.2 DC Motor Field Current

21. In the Metering window, make sure the torque correction function of meter T is selected.

Record the motor speed n, output torque T, armature voltage EA, armature current IA and field current IF (indicated by meters n, T, E1, I1 and I2 respectively) in the Data Table.

On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 Nm increments up to 2.0 N.m. For each torque setting, readjust the voltage control knob of the Power Supply so that the armature voltage EA remains equal to the value set in the previous step, wait until the motor speed stabilizes and then record the data in the Data Table.

Note: It may not be possible to maintain the armature voltage to its original value

as the torque is increased. The armature current may exceed the rated value while

performing this manipulation. It is therefore suggested to complete the

manipulation within a time interval of 5 minutes or less.

22. When all data has been recorded, set the LOAD CONTROL knob on the Prime

Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply.

In the Data Table window, confirm that the data has been stored. The data must be

filled in Table 4.3 (Results Section).

23. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.3 (Results Section).

24. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables.

Speed versus Torque Characteristics of a Cumulative Compound Motor

25. Set up the cumulative compound motor circuit shown in Figure 4.9. Make sure that the LOAD CONTROL knob on the Prime Mover/Dynamometer is set to the MIN. position (fully CCW).

Figure 4.9 Cumulative-Compound Motor Circuit

26. Turn on the Power Supply, set the armature voltage EA to the value recorded in step 13.

27. Set the FIELD RHEOSTAT on the DC Motor/Generator so that the current in the shunt winding is equal to the value indicated in Table 4.2.

28. In the Metering window, make sure the torque correction function of meter T is selected.

Record the motor speed n, output torque T, armature voltage EA, armature current IA and field current IF (indicated by meters n, T, E1, I1 and I2 respectively) in the Data Table.

On the Prime Mover/Dynamometer, adjust the LOAD CONTROL knob so that the torque indicated on the module display increases by 0.2 Nm increments up to

2.0 N.m. For each torque setting, readjust the voltage control knob of the Power Supply so that

the armature voltage EA remains equal to the value set in the previous step, wait until the motor speed stabilizes and then record the data in the Data Table.

Note: It may not be possible to maintain the armature voltage to its original value

as the torque is increased. The armature current may exceed the rated value while

performing this manipulation. It is therefore suggested to complete the

manipulation within a time interval of 5 minutes or less.

29. When all data has been recorded, set the LOAD CONTROL knob on the Prime Mover/Dynamometer to the MIN. position (fully CCW), turn the voltage control knob fully counterclockwise and turn off the Power Supply.

In the Data Table window, confirm that the data has been stored. The data must be filled

in Table 4.4 (Results Section).

30. In the Graph window, make the appropriate settings to obtain a graph of the series motor speed (obtained from meter N) as a function of the series motor torque (obtained from meter T). Plot the Graph 4.4 (Results Section).

31. Set the 24 V – AC power switch to the O (off) position and remove all leads and cables.