lab instructions ees 612 electrical machines and actuators ...jkoch/courses/ees612... · 3...

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Faculty of Engineering, Architecture and Science

Department of Electrical and Computer Engineering

LAB INSTRUCTIONS

EES 612 – Electrical Machines and Actuators

Experiment # 4: DC Motor Control

1. Introduction

Pulse-Width Modulation (PWM) is the most widely used method for controlling electric

machines of DC or AC type. A few typical applications include: 1) speed control of vehicles,

elevators and escalators, hoist and conveyors, and pumps and fans; 2) motion control of disc

drive heads, robot arms, surgical tools, and machine tools; 3) current control of electromagnets

and other static loads, and 4) illumination control of displays and monitors.

The PWM method relies on periodic, intermittent (on-off), connection of the load to the power

source. If the mentioned switching process is exercised fast enough, that is, if the switching

period is sufficiently small, then an inertial load responds only to the average of the pulsating

voltage and not to its fluctuations. Moreover, the average voltage can be controlled by the ratio of

the “on time” to the switching period . The ratio, , is known as the “duty ratio” or

“duty cycle”. Electronic semiconductor switches are employed due to their ease of control, high

on/off speed, and long lifespan. This experiment is concerned with the PWM technique and its

enabling solid-state hardware.

2. Pre-lab Assignment

2.1) For the H-bridge converter of Fig. 2.1, draw the missing waveforms in Fig. 2.2 and explain

whether the switching process illustrated by Fig. 2.2 corresponds to a half- or to a full-bridge

operation. Then derive an expression for the average of the load voltage , as a function

of .

Fig. 2.1. H-Bridge Converter.

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Fig. 2.2. A possible switching scenario for the H-bridge converter.

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2.2) Now draw the missing waveforms of Fig. 2.3 and explain whether they correspond to a half-

bridge operation or to a full-bridge operation. Then, derive an expression for the average of the

load voltage as a function of .

Fig 2.3. Another possible switching scenario for the H-bridge converter.

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3. Lab Work

General Safety Note

To prevent injury to persons or damage to equipment, the power source must be turned

OFF prior to the completion (or change) of any circuit connections.

Equipment

L298 H-Bridge Driver Module (hereinafter, referred in this document to as the “H-

Bridge Box”)

Bench-top power supply

Function generator

4-channel oscilloscope

55- inductor

15- Resistor

Digital multimeter

Tachometer

Experiments

3.1. Half-bridge energization of a resistive load

3.1.1) Connect the power supply to the terminals +12V and COM- on the rear panel of the H-

bridge box. Observe the polarities.

3.1.2) Connect the electrical terminals VA and VB of the H-bridge box to the terminals of the

15- resistor. The resistor will act as the load in this experiment.

3.1.3) Set the toggle switches of the H-bridge box as per Table 3.1.1.

Table 3.1.1. Toggle switch positions for Experiment 3.1.

Toggle Switch Position

1 HB CW/HB CCW HB CW

2 Full/Half Half

3 Brake/Run Brake

3.1.4) Connect the TTL pulse output of the function generator to the BNC of the H-bridge

box.

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3.1.5) Connect the oscilloscope channels to the other BNCs of the H-bridge box, according to

Table 3.1.2 below. This is to monitor the signals , , , and , as labeled on the

schematic diagram below:

Fig. 3.1.1. H-Bridge Converter.

Table 3.1.2. Signals to be monitored in Experiment 3.1.

Signal BNC Oscilloscope Channel

1 g1 Ch1

2 VA Ch2

3 VB Ch3

4 IL Ch4

3.1.6) Turn on the power supply and set its output voltage to 15 V. This results in

for the H-bridge converter of Fig. 3.1.1.

3.1.7) Set the oscilloscope channels Ch1-Ch4 to the DC coupling mode and the oscilloscope

trigger to Source 1. Turn on the function generator and set its frequency range to 100

kHz and its waveform to Square. Monitor the waveform (i.e., Ch1 of the

oscilloscope). Set the switching frequency and duty ratio to and ,

respectively. Note that these two parameters correspond to a 5-kHz even-symmetrical

pulse train for .

3.1.8) Change the switch Brake/Run from “Brake” position to “Run” position and, using the

oscilloscope, monitor and plot on Fig. 3.1.2 the waveforms , , for two

periods. To monitor , utilize the mathematical functionalities of the

oscilloscope.

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Fig. 3.1.2. Load voltage waveform for a duty ratio of 50%.

3.1.9) Maintain the frequency, but change the duty ratio in steps of 0.1, according to Table

3.1.3. Using the multimeter, measure the corresponding DC (average) and AC voltages

that drop across the resistor, and complete Table 3.1.3. To change the duty cycle, press

down the SYMMETRY button on the function generator and then adjust the

CENTER CAL SYMMETRY knob to obtain the desired duty ratio.

Table 3.1.3. Resistor voltage versus duty ratio, for .

0.2 0.3 0.4 0.5 0.6 0.7 0.8

DC voltage across

the resistor

AC voltage across

the resistor

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3.1.10) Turn off the function generator and the power supply. Change the switch Brake/Run

from “Run” position to “Brake” position.

3.2. Half-bridge energization of a resistive-inductive (RL) load with a high switching

frequency

3.2.1) Repeat Experiment 3.2, but for . Reflect the results on Fig. 3.2.1 and Table

3.2.1.

Fig 3.2.1. Load voltage waveform for a duty ratio of 50%, for .

Table 3.2.1. Resistor voltage versus duty ratio for the RL load and .

0.2 0.3 0.4 0.5 0.6 0.7 0.8

DC voltage across

the resistor

AC voltage across

the resistor

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3.2.2) Change the switch Brake/Run from “Run” position to “Brake” position.

3.3. Full-bridge energization of a resistive-inductive (RL) load

For this experiment, the test setup is the same as that in Experiment 3.3. Also, monitoring Ch1 of

the oscilloscope, set the switching frequency to . However, in contrast to Experiment

3.3, leave the Full/Half toggle switch at “Full” position.

3.3.1) Change the switch Brake/Run from “Brake” position to “Run” position, and monitor

the waveform , , for at least two periods, for , , and

. For the three aforementioned values of duty cycle, plot the corresponding set

of waveforms on Fig. 3.3.1, Fig. 3.3.2, and Fig. 3.3.3, respectively.


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3.3.2) Maintain the frequency, but change the duty ratio in steps of 0.1, according to Table

3.3.1. Use the multimeter and measure the corresponding DC (average) and AC

voltages that drop across the resistor; complete Table 3.3.1.

Table 3.3.1. Resistor voltage versus duty ratio, for .

0.2 0.3 0.4 0.5 0.6 0.7 0.8

DC voltage across

the resistor

AC voltage across

the resistor

3.3.3) Turn off the function generator and the power supply. Change the switch Brake/Run from

“Run” position to “Brake” position.

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3.4. Full-bridge energization of a DC motor

For this experiment, the test setup is the same as that in Experiment 3.3, with the exception that

the RL load is replaced with a DC motor.

The DC motor is a part of the H-bridge box, and its shaft has protruded out of the front panel of

the H-bridge box. To energize the motor, connect by short wires the electrical terminals VA and

VB to the electric terminals MTR+ and MTR-, respectively.

3.4.1) Turn on the power supply and the function generator. Change the switch Brake/Run

from “Brake” position to “Run” position, and monitor the waveforms ,

, for at least two periods, for , , and . For the three

aforementioned values of duty cycle, plot the corresponding set of waveforms on Fig.

3.4.1, Fig. 3.4.2, and Fig. 3.4.3, respectively.

Fig. 3.4.1. DC motor armature voltage waveform, for a duty ratio of 20%.

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3.4.2) Slowly change the duty ratio from its minimum (of approximately zero) to its

maximum (of approximately unity) and observe that:

a) The motor stops at .

b) The direction of rotation is different for the values of larger than compared

to those smaller than .

c) The motor runs faster (irrespective of direction) as differs more from .

3.4.3) Change the duty ratio in steps of and, using the tachometer, measure the

corresponding shaft speeds. Complete Table 3.4.1.

Table 3.4.1. DC motor speed versus duty ratio.

0.2 0.3 0.4 0.5 0.6 0.7 0.8

motor speed in

rpm

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3.4.4) With the duty ratio set at (or, alternatively, at ), corresponding to a fast

rotation, change the Brake/Run switch to the “Brake” position, observe the response

of the dc motor. Repeat the experiment disconnecting the positive terminal from the dc

motor and compare the result with the first experiment. Explain the difference.

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4. Conclusions and Remarks

4.1) Comment on the AC and DC voltage measurements reported in the Tables 3.1.3, 3.2.1 and

3.3.1.

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4.2) Compare the DC voltage measurements you reported in Tables 3.2.1 and 3.3.1 with the

values that their formulas (that you derived in the Pre-Lab part) predict. Provide reasons for the

discrepancies.

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4.3) Comment on the measurements of Table 3.4.1.

Design of experiment and development of manual: A. Yazdani

Design and development of equipment: A. Yazdani and J. Koch

Last Updated May 1, 2015—RO

lab instructions ees 612 electrical machines and actuators ...jkoch/courses/ees612... · 3...

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