scalar speed control of induction motors

8/9/2019 Scalar Speed Control of Induction Motors

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Lebanese University

Faculty of Engineering III

Scalar Speed Control of Induction Motors

Using Pulse Width Modulation

Rami Al Halabi

Communication and Mini Project

Semester VII 2014/2015

Dr. Youssef Harkous


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Scalar Speed Control of Induction Motors

Using Pulse Width Modulation

Keywords

Open loop control, induction motor drive, pulse width modulation, scalar control

Abstract

The objective of research was to present the open loop speed control of variable-voltage variable-

frequency (VVVF) induction motor drive. The main issues regarding a two-level PWM controller were

designed for controlling an induction motor. The speed response results were compared. The study

used Simulink program and adjustment of controller parameters were set consistently with the

mathematic equation of the motor. In this study, the 1.6 kW model of three-phase induction motorwas used in the simulation. The results showed that the variable frequency PWM controller can drive

the induction motor system more at varying speed, with a small variation of output torque.


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Table of Contents

List of Figures 3

Introduction 4

Chapter 1: Asynchronous Motors

Introduction 5

Structure and Operation 5

Torque and Speed Characteristics 7

Scalar Control of Speed and Torque 9

Chapter 2: PWM and Inverters

Introduction 10

Pulse Width Modulation 10

Generation of PWM Signals 11

Using Insulated-Gate Bipolar Transistor (IGBT) Bridges 12

Chapter 3: Simulation

Introduction 13

Control Part 13

High Power part 14

Chapter 4: Results and Discussion

Introduction 15

Speed 15

Torque 16

Current 16

Fourier Analysis 17

IGBT Currents 18

PWM Output at Different Frequencies (Vab) 18

Conclusion 19

References 20


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List of Figures

Fig. 1.1 Basic Structure of 3-phase induction motor 5

Fig. 1.2 Creation of rotating magnetic field 6

Fig. 1.3 Wound rotor with slip rings 6

Fig. 1.4 Squirrel cage rotor with bars 6

Fig. 1.5 Equivalent circuit of AC induction motor 7

Fig. 1.6 Torque as function of slip characteristics 8

Fig. 1.7 Torque curves at varying frequencies 9

Fig. 2.1 Induction as function of PWM voltage 10

Fig. 2.2 Intersection method for generating a single-phase PWM signal 11

Fig. 2.3 Generating a 3-phase 6-pulse PWM signal 11

Fig. 2.4 3-Arm IGBT Bridge 12

Fig. 3.1 Simulation (part 1) 13

Fig. 3.2 Simulation (part 2) 14

Fig. 4.1 Rotor Speed Characteristics at Different Frequencies 15

Fig. 4.2 Torque Characteristics at Different Frequencies 16

Fig. 4.3 Current Variation at Different Frequencies 16

Fig. 4.4 Magnitude Fourier Analysis at Different Frequencies 17

Fig. 4.5 Fourier Phase Analysis at Different Frequencies 17

Fig. 4.6 Complementary IGBT Currents 18

Fig. 4.7 Vab at Different Frequencies 18


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Introduction

As our industrial technologies evolve and develop, we find ourselves in need for more control over the

machines we use. Quality control is such a big aspect of industry nowadays, and so the tolerance of errors

allowed to occur has much decreased. The human race has evolved over the last 160 years from using steam

powered or horse powered mechanical contraptions, to using servomotors and induction motors.

DC motors were introduced in the 1870s and played a critical role in the second industrial revolution.

They were widely used in locomotives and trams but their high maintenance and high costs made them less

favorable in industry. The invention of AC motors by Nikola Tesla in 1887, and the standardization of the AC

power as the power of choice for generation and distribution turned the industry’s focus towards AC motors,

and in particular, the low cost and low maintenance induction motor.

The industrial and domestic application of the induction motor continued to grow, but with it, the need

for more customizability of these motors also grew. Induction motors have speeds that depend on the feeding

frequency and the load being powered. And so, motor drives were developed to help control the speed and

torque of these motors.

The first method of control was the open-loop variable-frequency control. This method consisted of

varying the frequency of the feeding voltage. However, a problem arose because the torque of such motors

grew very high at low frequencies. In an effort to keep the torque constant throughout operation, the open-

loop variable-voltage variable-frequency (VVVF) control was developed. This method kept a constant voltage

to frequency ratio, in an attempt to keep the torque constant. It was widely used for its simplicity and ease of

implementation. It was known as scalar control of speed, because it focus only on the magnitude and not the

phase of the supplied parameters.

Still, some industrial applications required even more precise control. And for that purpose more

advanced systems were developed. The space-vector control model focused on the magnitude and phase ofthe feeding signal, thus improving the performance of the machine. Closed-loop control system were also

developed to provide a better control mechanism.

I will focus in this paper on introducing the AC induction motor and its open-loop VVVF drive. A

simulation is also provided for the functioning of this motor.

The paper will first recall what we need of the induction motor characteristics and equations

(Chapter1), then, I will cover Pulse Width Modulation (PWM) in Chapter 2. The remaining two chapters are

dedicated of the simulation of the system and discussing the results.


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Fig. 1.1 Basic Structure of 3-phase induction motor

Chapter 1:

Asynchronous Motors

1. Introduction

Asynchronous motors, also known as induction motors are the most widely used type of motors in industry.

These motors operate on AC electricity, thus the current needed in the rotor to produce torque is supplied

from the stator by electromagnetic induction, and does not require mechanical commutation or separate

excitation like in DC and Universal motors. There are two types of rotor structures, wire-wound and squirrel

cage rotors.

Three-phase squirrel-cage induction motors are widely used in industry for their low cost, low maintenance

and reliability. Single-phase versions of these motors are widely used in household appliances and small-scale

compressors and pumps. Although the basic functioning mode of these motors was fixed-speed, these motors

are increasingly being used with VFD (Variable Frequency Drive) for variable speed applications.

2. Structure and Operation:

The induction motor consists of two main parts: the stator – a magnetic circuit with a three-phase winding

that creates a rotating magnetic field – and the rotor, which is connected to the mechanical part of the system

through the shaft.

2.1. Stator:

It consists of a steel frame containing slots, enclosing a hollow cylindrical core (made up of laminations of

silicon steel). The laminations are to reduce hysteresis and eddy current losses. Three similar coils having

mutual geometrical angles of 120 degrees create the rotating magnetic field. The ability of the three-phasesystem to create the rotating field utilized in electric motors is one of the main reasons why three phase

systems dominate in the world electric power supply systems. The stator is used a source to induce current in

the rotor by supplying a magnetic field through the air gap.

The three-phase current in the stator winding creates a rotating magnetic field in the air gap. This magnetic

field rotates a speed called synchronous speed Ns. The synchronous speed of an induction motor is based on

the supply frequency and the number of poles in the motor winding and can be expressed as:

= 2 × 60 ×

Where,

= ℎ ()

= ()

=

2=


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Fig. 1.2 Creation of rotating magnetic field

Fig. 1.4 Squirrel cage rotor with barsFig. 1.3 Wound Rotor with Slip Rings

The three-phase current passing through the stator windings creates this rotating magnetic field. These

current, while varying in magnitude and sign, produce an illusion of a rotating magnet.

We tend to create multiple distributed coils for every poles instead of one coil. This action reduces the

harmonics in the total magnetic induction present in the air gap.

2.2. Rotor:

The rotor of an induction machine can be either a wire wound rotor (3-phase shorted coils) or a squirrel cage

rotor with short-circuited aluminum bars. The speed of a wire-wound rotor can be controlled to a limited

extent by the introduction of a starting rheostat. The squirrel cage motor on the other hand cannot be

controlled by changing the rotor circuit.


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Fig. 1.5 Equivalent circuit of AC induction motor

3. Torque and Speed Characteristics:

In what continues, I will only refer to the squirrel cage induction motor.

3.1 Slip:

Synchronous motors contain a magnet or an electromagnet in their rotors, so the speed of rotation of the

rotor is equal to the mechanical speed of rotation of the magnetic field. Whereas in induction motors, if the

speed of rotation of the rotor is equal to that of the RMF (Rotating Magnetic Field), then there would be no

induced current in the rotor and thus, no flux. This is why the speed of the rotor is always slightly less than the

synchronous speed. The term “slip” defines the amount of difference between these two speeds:

= −

× 100

Where,

= ()

= ℎ

=

The slip varies between almost 0% at no load, and 3 – 6 % at nominal load conditions.

3.2 Equivalent Circuit of AC Induction Motor:

The equivalent circuit is a single-phase representation of a multiphase induction motor that is valid in steady-

state balanced-load conditions. It is expressed simply in terms of the following components:

Stator resistance and leakage reactance ( , )

Rotor resistance, leakage reactance, and slip ( , ′ , ′ )

Magnetizing reactance ( )

Air gap power is equal to electromechanical power output plus rotor copper losses.


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Fig. 1.6 Torque as function of slip characteristics

3.3 Torque Equation:

The equation defining the torque, by using the equivalent circuit parameters and power equations can be

expressed as:

Where,

=

= = ()

= ℎ (Ω)

= = ()

= (Ω)

′

= (Ω)

We can observe from the above formula that the torque is a function of slip. Drawing these characteristics we

obtain:

The maximum value of the breakdown torque (N.m) is attained at


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Fig. 1.7 Torque curves at varying frequencies

4. Scalar Control of Speed and Torque:

Scalar control is based on the varying of the magnitude of the control variables (V and f ), without regard to

the phase values and effects. The voltage of the machine can be varied to control the flux, the slip or

frequency can be varied to control the torque. However, the voltage or frequency still have effects on the

other output variable (V has an effect on torque, and f has an effect on flux).

Scalar-controlled drives gives inferior performance compared to vector controlled drives. However, scalar

controls are easier to implement.

4.1. Open-Loop V/Hz Control:

This method is by far the most used method of control of induction motors because of its simplicity, and the

wide use of induction motors in industry.

We know that the flux in the air gap of the motor is directly related to the ratio V/f , so, to keep the flux

constant –thus avoiding saturation –we have to keep the ratio V/f constant.

In order to be able to control the frequency of the driver, we need an inverter to generate a pulse width

modulated signal of varying frequency.


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Chapter 2:

PWM and Inverters

1. Introduction

In order to control the speed of an induction motor, we require a variable frequency drive, that is able to

provide enough power, and behave as a sinusoidal current in the stator windings. Pulse Width Modulation

(PWM) is a technique used to supply power to an inductive device, so that the current behaves in a sinusoidal

manner. PWM Inverters can be used to create controlled PWM signals from DC voltage sources. This type of

drives is widely used to control induction motors, either with scalar or vector control techniques.

2. Pulse Width Modulation:

Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a technique used to encode a

message into a pulsing signal. Although this modulation technique can be used to encode information for

transmission, its main use is to allow the control of the power supplied to electrical devices, especially toinertial loads such as motors.

The average value of voltage (and current) fed to the load is controlled by turning the switch between supply

and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total

power supplied to the load.

The PWM switching frequency has to be much higher than what would affect the load (the device that uses

the power), which is to say that the resultant waveform perceived by the load must be as smooth as possible.

Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from

few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio

amplifiers and computer power supplies.

The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty

cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in

percent, 100% being fully on.

The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there

is practically no current, and when it is on and power is being transferred to the load, there is almost no

voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases

close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set

the needed duty cycle.

Fig. 2.1 Induction as function of PWM voltage


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3. Generation of PWM Signals:

A simple method to generate the PWM

pulse train corresponding to a given

signal is the intersective PWM: the

signal (here the red sinewave) is

compared with a sawtooth waveform

(blue). When the latter is less than the

former, the PWM signal (magenta) is in

high state (1). Otherwise it is in the low

state (0).

To drive a 3-arm IGBT bridge – which will be explained next – we need a 6 pulse PWM signal, in which pulses 1

and 2 are complementary, so are 3 and 4, and 5 and 6. These pulses will open the gates of the bridge to allow

DC voltage to be transferred to the output, thus producing a 3-phase PWM waveform of high power.

Fig. 2.2 Intersection method for generating a

single-phase PWM signal

Fig. 2.3 Generating a 3-phase 6-pulse PWM

signal


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4. Using Insulated-Gate Bipolar Transistor (IGBT) Bridges:

An IGBT Bridge is a device that converts

DC voltage to a PWM waveform following

the impulses it is given. This kind of

bridges can be used to drive high power

inductive loads by generating a high

power 3-phase PWM signal from a DC

voltage source (usually a rectified AC

source).

The greatest use of the IGBT bridge is that

it can provide varying frequency PWM

output by varying the frequency of the 6

complementary PWM pulses that are supplied

each to one of the transistors. Every two

transistors found in the same column are fed by complementary pulses to produce the forward and reverse

half of the output signal. The presence of 3 pairs of transistors fed by ±120° pulses are sufficient to produce a

3-phase output.

Fig. 2.4 3-Arm IGBT Bridge


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Chapter 3:

Simulation

1. Introduction

To study the time response of the motor current and torque, we setup a Simulink simulation of the drive and

the motor.

2. Control Part:

The first part of the block diagram is the control circuit that generates the PWM driving signal for the bridge to

operate. This part shows a linear relation between f and V. We also generate three sine signals to be

compared inside the Double Signal Generator to generate the pulses necessary to drive the IGBT Bridge.

Staring from the right, we have the frequency control signal, which in turn provides the ω value for the sin

functions and the voltage control through the gain block, which has a gain of 7.66. This is the value at which

the V/Hz ratio stays.

Next, the three sin waves with ±120° phase shift are MUXed and multiplied by a modulation index of 0.9.

These three signals are fed to the Double Pulse Generator, which compares them to a sawtooth signal of

carrier frequency of 1080 Hz, thus generating six complementary pulses to operate the six IGBTs of the bridge.

The IGBT Bridge is the power electronics device that allows us to generate the high power – high voltage

signal to drive the asynchronous motor.

Fig. 3.1 Simulation (part 1)


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2. High Power Part:

The second part of the simulation is the asynchronous motor; this part allows us to simulate the response of

the induction motor to the PWM voltage feeding it.

The induction motor used is a squirrel-cage induction motor with a nominal line-line voltage of 380 volts. This

motor is a 4-pole motor.

The scopes connected allow us to view the stator current, electromagnetic torque, rotor speed and Fourier

analysis of the supply voltage. The multimeters below allows us to the output of the bridge, along with the

internal functioning of two complementary diodes inside the bridge.

The sampling time for the simulation is = 1 − 5 .

The simulation parameters are:

Type: Variable-step

Solver: ode23tb(stiff/TR-BDF2)

Fig. 3.2 Simulation (part 2)


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Chapter 4:

Results and Discussion

1. Introduction

The simulation is run for 0.8 seconds to determine the rotor speed, torque and current steady state values.

We then run in for two periods to view the PWM output and the functioning of the bridge in detail.

2. Speed:

The speed characteristics show the relation between the frequency and the rotor speed. The ratio of speed

over frequency takes the values of 28.5, 28.75, and 28.7 for the values of frequency 70, 60 and 50 Hz,

respectively.

Therefore, we can see that the output speed of the motor is directly proportional to the frequency of the

feeding voltage.

The transient phase lasts 0.18, 0.25 and 0.275 for the values of frequency 50, 60 and 70 Hz.

The synchronous speeds of the three runs are 1500, 1800 and 2100 rpm for increasing values of frequency.

The values of the slip are 3.33, 4.16 and 4.76 % for increasing values of frequency. Thus, we deduce that slip

increases as we increase the frequency.

Fig. 4.1


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3. Torque:

We notice that the values of torque vary between 10 and 15 N.m, but they stay around the same values. The

transient time for the frequencies varies in accordance with the transient time of the speed graph.

5. Current:

The current that flows in the motor is pseudo-sinusoidal due to the effect of the PWM voltage inside inductive

loads. We notice that the staring current is almost 10 times the value of the steady-state current.

The amplitude of the current also increases with the increase of frequency.

Fig. 4.3

Fig. 4.2


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6. Fourier Analysis:

The Fourier analysis of the voltage between lines A and B, shows that the phase shift varies between 7 and 11

degrees of phase shift after the transient phase ends.

The magnitude of the Fourier analysis takes the values of 275, 355 and 400 as the frequency increases.

We notice that the harmonics at 50 Hz and 70 Hz are much higher than the harmonics at 60 Hz. This happens

because the nominal frequency of the motor is 60 Hz. This is why the green curves (60 Hz) have much less

oscillation.

Fig. 4.4

Fig. 4.5


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Conclusion

The squirrel-cage induction motor is, and will be in the foreseeable future, the motor of choice of most

industrial and domestic applications. The high reliability and low maintenance of this motor, along with its

ability to run in chemically and industrially “dirty” environments. These applications however have differentoperation conditions for the motor being used. One of the most important parameters to be controlled is

speed.

Rotor speed can be easily controlled by using the open-loop constant V/Hz technique, which varies the

DC feeding voltage of the inverter (and ultimately the motor) as a function of the chosen frequency. This

techniques requires the generation of a reference signal to control the bridge which provides power to the

motor.

This technique, while being extremely simple to implement, still has disadvantages when compared to

closed loop control. We observe a slight difference of the output torque, which, in theory, should stay

constant. We also notice harmonics in the current and voltage fed to the motor.

Therefore, depending on the application at hand, open-loop scalar speed control can be the right

choice for a simple to implement control system, or it might not be enough, in which case we convert to the

more advances space vector control method.


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References

[1] R. Echavarria, S. Hota, M. Oliver, “A Three Phase Motor Drive IGBTs and Constant V/F Speed Control with

Slip Regulation”. IV IEEE International Power Electronics Congress Technical Proceedings, pp 87-91, 1995.

[2] O. Ruksaboon, C. Thongchaisuratkrul, “Speed Control of Variable-voltage variable-frequency Induction

Motor Drive Using Two-layered PI Controller”, King’s Mongkut University of Technology North Bangkok

[3] B.K. Bose, “Modern Power Electronics and AC Drives”, Prentice-Hall, NJ, USA, pp 30-47, 2001

[4] W. Emar, H. Sarhan, R. Al-Issa, I. TTrad, M, Awad, “V/F Control of Squirrel Cage Induction Motor Drives

Without Flux or Torque Measurement Dependency”,2011

[5] K. Aditya, A.Newwel, “Implementation of Close Loop Speed Control with VVVF Control and Slip Regulation

on LIM”, Engineering, Technology & Applied Science Research Vol. 4, No. 2, pp 596-599, 2014

[6] P.K. Behera, M.K. Behera, A.K. Sahoo, “Speed Control of Induction Motor using Scalar Control Technique”,

International Journal of Computer Applications, 2014

Figures

http://www.mpoweruk.com/motorsac.htm

http://electriciantraining.tpub.com/14177/css/14177_91.htm

http://openticle.com/2012/09/26/ac-motor-types/

http://avstop.com/ac/apgeneral/typesofacmotors.html

http://en.wikipedia.org/wiki/Induction_motor#mediaviewer/File:IMEQCCT.jpg

http://en.wikipedia.org/wiki/Induction_motor

http://en.wikipedia.org/wiki/Pulse-width_modulation

http://en.wikipedia.org/wiki/Pulse-width_modulation#mediaviewer/File:Pwm.svg

http://200.126.14.82/web/help/toolbox/powersys/pwmgenerator.html

http://www.diyelectriccar.com/forums/showthread.php/mes-dea-tim600-explosion-help-52606p3.html

scalar speed control of induction motors

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