Lab Volt PWM AC Motor Drives [PDF]

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Power Electronics

PWM AC Motor Drives

Student Manual 29979-00

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Power Electronics

PWM AC Motor Drives Student Manual 29979-00

A

POWER ELECTRONICS

PWM AC MOTOR DRIVES

by the Staff of Lab-Volt Ltd.

Copyright © 2009 Lab-Volt Ltd. All rights reserved. No part of this publication may be reproduced, in any form or by any means, without the prior written permission of Lab-Volt Ltd.

Legal Deposit – Third Trimester 2009 ISBN 978-2-89640-358-5 2-89289-274-0 (1st Edition, 1992) SECOND EDITION, AUGUST 2009

Printed in Canada August 2009

Foreword Semiconductor technology has long influenced developments in low-power electronics fields such as instrumentation and telecommunications. It was some time, however, before applications related to the vast domain of electric power control became available. Today, many reliable, flexible, and efficient power electronics systems are used in all spheres of industry. Applications can be found in the field of both dc and ac motor control, as well as in high-voltage electric power generation and transmission. The Power Electronics hands-on training system from Lab-Volt offers a comprehensive program in the field of Power Electronics. It comprises a variety of training modules and manuals that cover most important aspects and techniques relevant to the field, through the use of line commutated switches (SCRs) and self commutated switches (IGBT, MOSFET, GTO, etc.). The subject matter is approached from a practical point of view. Following a discussion of theoretical concepts in each laboratory exercise, the student is guided through a step-by-step, hands-on exercise procedure. A conclusion and a set of review questions terminate each exercise. When a circuit element or circuit set-up is introduced, the related phenomena are explained, and the student verifies the theory using the procedures given in the exercise. Each exercise builds on that which was previously carried out. This "builtin" progression in difficulty promotes efficient learning.

III

Acknowledgements We sincerely thank Mr Theodore Wildi, Professor Emeritus of Electrical Engineering at Laval University, for his contribution to many of the manuals in the Lab-Volt Power Electronics series.

IV

Table of Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII Exercise 1

Saturation and Effect of Frequency in Magnetic Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 The phenomenon of saturation in magnetic circuits. Saturation curve of magnetic circuits. Effect of frequency in magnetic circuits.

Exercise 2

Three-Phase Voltage-Source Inverter Induction-Motor Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 Squirrel-cage induction motor speed control using a three-phase voltage-source inverter (VSI). Torque versus magnetic flux in an induction motor. Voltage versus frequency in a VSI induction-motor drive. The concept of constant voltage-to-frequency (V/f) ratio. Obtaining a constant V/f ratio by manually varying the dc voltage at the input of the voltage-source inverter.

Exercise 3

Constant V/f Ratio PWM-Inverter Induction-Motor Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 Squirrel-cage induction motor speed control using a three-phase voltage-source inverter (VSI). 180° modulation versus pulse-width modulation (PWM) in VSI induction-motor drives. Operation of the PWM voltage-source inverter. Obtaining a constant voltage-tofrequency (V/f) ratio in a VSI induction-motor drive by using synchronous pulse-width modulation (PWM).

Exercise 4

Operation of a Synchronous Motor as a Stepper Motor . 4-1 Introduction to the stepper motor. Operation of a three-phase synchronous motor as a stepper motor in a 180°-modulation voltage-source inverter (VSI) motor drive. Synchronous motor speed control.

Appendices

A Circuit Diagram Symbols . . . . . . . . . . . . . . . . . . . . . . . . A-1 B Impedance Table for the Load Modules . . . . . . . . . . . . B-1 C Equipment Utilization Chart . . . . . . . . . . . . . . . . . . . . . . C-1

Bibliography We Value Your Opinion!

V

VI

Introduction Various symbols are used in many of the circuit diagrams given in the exercises of this manual. Each symbol is a functional representation of a device used in power electronics. The use of these symbols greatly simplifies the circuit diagrams, by reducing the number of interconnections shown, and makes it easier to understand the circuit operation. Appendix A of each manual of the Lab-Volt Power Electronics series lists the symbols used, the name of the device which each symbol represents, and a diagram showing the equipment, and in some cases the connections, required to obtain the device. The exercises in this manual can be carried out with the following ac network line voltages: 120 V ac, 220 V ac, and 240 V ac. The values of the components in the various circuits used often depend on the line voltage. For this reason, each component in the circuit diagrams is identified with a capital letter and a subscript number. A table accompanying the circuit diagram indicates the value of each component for the various line voltages (120, 220, and 240 V ac). Appendix B of this manual provides a table which gives usual impedances which can be obtained using the 120-V ac, 220-V ac, and 240-V ac versions of the load modules of the Power Electronics Training System. Before performing the exercises in this manual, the student should be familiar with the operation of the Chopper / Inverter Control Unit (inverter modes of operation) and IGBT Chopper / Inverter module. The student should also know the operation of three-phase diode rectifiers and three-phase inverters built with power IGBT’s. Refer to the manual Familiarization with the Lab-Volt Power Electronics Equipment, part number 29971-E0, if you are performing the exercises of this manual using the Lab-Volt Data Acquisition and Management System (LVDAM). Refer to the manual Familiarization with the Lab-Volt Power Electronics Equipment, part number 29971-E0, if you are performing the exercises of this manual using the MOSFET Chopper / Inverter, Model 8837-0X, instead of the IGBT Chopper / Inverter, Model 8837-AX.

VII

Exercise

1

Saturation and Effect of Frequency in Magnetic Circuits EXERCISE OBJECTIVE •

To understand the phenomenon of saturation in magnetic circuits.

•

To learn the effect of frequency in magnetic circuits.

DISCUSSION Saturation in magnetic circuits When an ac voltage is applied to a magnetic circuit, such as a coil, a transformer or a motor, an ac current flows in the circuit. This ac current creates an alternating magnetic flux which is proportional to the ac voltage. The ac current in the magnetic circuit is also proportional to the ac voltage, and therefore to the alternating magnetic flux, as long as the maximum value of the alternating magnetic flux does not start to saturate the core of the magnetic circuit. When the core of the magnetic circuit starts to saturate, it takes more and more ac current to increase the alternating magnetic flux. As a result, the ac current in the magnetic circuit is no longer proportional to the ac voltage as shown in Figure 1-1. This phenomenon is called saturation. Eventually, when the core of the magnetic circuit becomes highly saturated, a slight increase of the ac voltage, and therefore of the alternating magnetic flux, causes a great increase of the ac current.

Figure 1-1. Saturation curve of a magnetic circuit.

1-1

Saturation and Effect of Frequency in Magnetic Circuits Figure 1-2 (a) shows an example of the waveforms of the voltage and current in a magnetic circuit when there is no saturation. The voltage is a sine wave and the current is a slightly distorted sine wave. The distortion is due to the hysteresis loop of the magnetic material which is present even in the absence of saturation. The waveform of the magnetic flux is shown as a dotted line in Figure 1-2 (a). It is a sine wave, as is the waveform of the voltage. Figure 1-2 (b) shows an example of the waveforms of the voltage and current in the same magnetic circuit when saturation occurs (the voltage has been increased). The voltage is still a sine wave, but the current no longer resembles a sine wave. The waveform of the current contains high peaks. These peaks of current are necessary to produce the magnetic flux corresponding to the peak value of the magnetic flux. The waveform of the magnetic flux, which is shown as a dotted line in Figure 1-2 (b), is again a sine wave, as is the waveform of the voltage. Note: The saturation phenomenon is less evident when there is an air gap in the magnetic circuit. Consequently, the distortion of the current waveform (peaks of current) is less severe in an ac machine (core with an air gap) than in a transformer (core without air gap).

The effect of frequency in magnetic circuits Suppose that an ac voltage having an arbitrary wave shape is applied to a coil that produces the flux in a magnetic circuit. If the resistance of the coil is negligible, the following equation gives the relationship between the voltage, frequency, and magnetic flux: E = kfNMmax. where

E is the rms value of the ac voltage across the coil, k is a constant whose value depends on the waveform of the voltage E and the distribution of the winding over the core of the coil, f is the frequency of the voltage E, N is the number of turns on the winding of the coil, Mmax. is the maximum, or peak, value of the alternating magnetic flux.

This equation clearly shows that for a given frequency, the flux Mmax. is proportional to the voltage E. In a given magnetic circuit, the waveform of the voltage E and the distribution of the winding over the core of the coil do not change and the number of turns N on the winding of the coil is fixed. In this case, the values of k and N are fixed. Consequently, for a given voltage E, the flux Mmax. now only depends on the frequency f related to the voltage E. The flux Mmax., and therefore the ac current in the magnetic circuit, decreases when the frequency f increases, and vice versa. Therefore, using a higher frequency in a given magnetic circuit allows higher voltages E to be applied before saturation occurs. Conversely, using a lower frequency causes lower voltages E to produce saturation. To illustrate this phenomenon, Figure 1-3 shows the saturation curves of a magnetic circuit for various frequencies. In this figure, the saturation curve with the highest frequency (f1) has the highest saturation voltage.

1-2

Saturation and Effect of Frequency in Magnetic Circuits

Figure 1-2. Waveforms of the voltage, current, and flux in a magnetic circuit, with and without saturation.

1-3

Saturation and Effect of Frequency in Magnetic Circuits

Figure 1-3. Saturation curves of a magnetic circuit for various frequencies.

Procedure Summary In the first part of this exercise, you will set up in the Mobile Workstation the equipment required to carry out this exercise. In the second part of this exercise, you will use the circuit shown in Figure 1-4 to observe saturation in a magnetic circuit. In this circuit, a variable-voltage ac power supply is connected to a step-up transformer. You will increase the ac voltage applied to the primary winding while observing the waveform of the current in the primary winding and the waveform of the voltage across the secondary winding using a current isolator and a voltage isolator, respectively. The waveform of the voltage across the secondary winding is a faithful copy of the waveform of the voltage across the primary winding. In the third part of this exercise, you will use the circuit shown in Figure 1-7 to observe the effect of frequency in a magnetic circuit. In this circuit, the variablevoltage ac power supply used in the circuit of Figure 1-4 is replaced with a 180°modulation single-phase inverter. You will vary the frequency while observing the waveforms of the current in the primary winding and voltage across the secondary winding of the transformer. You will measure the voltage across the secondary winding of the transformer for various currents in the primary winding of the transformer, using three different frequencies. From these measurements, you will plot the saturation curves of the transformer at these frequencies. Note: This exercise is quite long. However, it can be divided in two parts and carried out in two laboratory periods. In the first laboratory period, carry out the PROCEDURE sub-sections entitled: – –

1-4

Setting up the equipment Observing saturation in a magnetic circuit

Saturation and Effect of Frequency in Magnetic Circuits In the second laboratory period, carry out the PROCEDURE sub-sections entitled: – –

Setting up the equipment Effect of frequency in a magnetic circuit

EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix C of this manual, to obtain the list of the equipment required to carry out this exercise. 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

G

1. Install the Power Supply, the Enclosure / Power Supply, the Chopper / Inverter, the Power Diodes, the Single-Phase Transformer, the Smoothing Inductors, the AC Voltmeter, and the Capacitive Load modules (2) in the Mobile Workstation.

G

2. Install the Chopper / Inverter Control Unit and the Current/Voltage Isolator in the Enclosure / Power Supply.

G

3. Make sure that the main power switch of the Power Supply is set to the O (OFF) position. Connect the Power Supply to a three-phase wall receptacle.

G

4. Plug the Enclosure / Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the I (ON) position.

G

5. Make sure that the toggle switches on the Capacitive Load modules are all set to the O (open) position.

Observing saturation in a magnetic circuit

G

6. Connect the modules as shown in Figure 1-4.

1-5

Saturation and Effect of Frequency in Magnetic Circuits

Figure 1-4. A simple magnetic circuit (step-up transformer).

G

7. Make the following settings: On the Power Supply Voltage selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-N On the oscilloscope Channel-1 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (AC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . . . 5 V/DIV. (AC coupled) Vertical Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHOPped Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ms/DIV. Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LINE Trigger Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . positive (+) Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . HF REJection

G

8. On the Power Supply, make sure that the voltage control knob is set to the 0 position, then set the main power switch to the I (ON) position. Slowly set the voltage control knob of the Power Supply halfway between the 40 and 50 positions (45% of the ac network line voltage).

1-6

Saturation and Effect of Frequency in Magnetic Circuits On the oscilloscope, make the appropriate settings to position the traces of channels 1 and 2 in the upper and lower halves of the screen, respectively. The traces of channels 1 and 2 represent the waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer, respectively.

G

9. Sketch the waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer in Figure 1-5.

Figure 1-5. Waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer.

Determine the peak values of the current in the primary winding and the voltage across the secondary winding of the transformer using the waveforms sketched in Figure 1-5. Note these values in Figure 1-5. Read the voltage indicated by the AC Voltmeter. It is the ac rms voltage across the secondary winding of the transformer (ES). Note this voltage in the following space. ES =

V ac

Is the voltage ES much higher than the nominal voltage of the secondary winding?

G Yes

G No

According to the waveforms sketched in Figure 1-5, is the iron core of the transformer saturated?

G Yes

G No

1-7

Saturation and Effect of Frequency in Magnetic Circuits G 10. On the Power Supply, set the voltage control knob so that the peak current in the primary winding of the transformer (IPEAK) is equal to the value given in the following table: LINE VOLTAGE

IPEAK

V ac

mA

120

250

220

125

240

125

Table 1-1. Peak current in the primary winding of the transformer (IPEAK).

Sketch the waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer in Figure 1-6.

Figure 1-6. Waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer (IPEAK high).

G 11. Determine the peak value of the voltage across the secondary winding of the transformer using the voltage waveform sketched in Figure 1-6. Note this value in Figure 1-6 as well as the value of the current IPEAK. Measure the voltage ES on the AC Voltmeter and note the result in the following space. ES =

1-8

V ac

Saturation and Effect of Frequency in Magnetic Circuits Is the voltage ES much higher than the nominal voltage of the secondary winding?

G Yes

G No

Compare the waveforms of the current in the primary winding obtained for the two voltages applied to the transformer.

Explain why these waveforms are different.

On the Power Supply, set the main power switch to the O position. Effect of frequency in a magnetic circuit

G 12. Connect the modules as shown in Figure 1-7. G 13. Make the following settings: On the Chopper / Inverter Control Unit DC SOURCE 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX. MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3- 180° On the Chopper / Inverter module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O On the Power Diodes module Interconnection Switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I On the oscilloscope Channel-1 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (DC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . . . 5 V/DIV. (DC coupled) Vertical Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHOPped Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ms/DIV. Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EXTernal Trigger Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . positive (+) Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . HF REJection 1-9

Saturation and Effect of Frequency in Magnetic Circuits

Figure 1-7. A single-phase inverter supplying power to a simple magnetic circuit (step-up transformer).

G 14. On the Power Supply, make sure that the voltage control knob is set to the 0 position then set the 24-V ac power switch and the main power switch to the I (ON) position. The POWER ON LED on the Chopper / Inverter module should light up to indicate that the module is correctly powered. Slowly set the voltage control knob of the Power Supply to the 20 position (20% of the ac network line voltage). On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob so that the period of the waveforms displayed on the oscilloscope screen is equal to 20 ms. This sets the operating frequency of the single-phase inverter to 50 Hz. The waveforms displayed on the oscilloscope screen 1-10

Saturation and Effect of Frequency in Magnetic Circuits represent the current in the primary winding of the transformer (channel 1) and the voltage across the secondary winding of the transformer (channel 2). These resemble square waves since the single-phase inverter uses 180° modulation.

G 15. Sketch the waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer in Figure 1-8.

Figure 1-8. Waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer.

Determine the peak values of the current in the primary winding and the voltage across the secondary winding of the transformer using the waveforms sketched in Figure 1-8. Note these values in Figure 1-8.

G 16. On the Power Supply, set the voltage control knob so that the current IPEAK is equal to the value given in the following table: LINE VOLTAGE

IPEAK

V ac

mA

120

250

220

125

240

125

Table 1-2. Peak current in the primary winding of the transformer (IPEAK).

1-11

Saturation and Effect of Frequency in Magnetic Circuits G 17. Sketch the waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer in Figure 1-9.

Figure 1-9. Waveforms of the current in the primary winding and the voltage across the secondary winding of the transformer (IPEAK high).

Determine the peak value of the voltage across the secondary winding of the transformer using the voltage waveform sketched in Figure 1-9. Note this value in Figure 1-9 as well as the value of the current IPEAK. Does the phenomenon observed in the previous section of this exercise occurs when a single-phase inverter supplies power to the transformer?

G Yes

G No

On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob to the MAX. position while observing the waveforms of the current and voltage on the oscilloscope screen. Describe what happens. Explain.

G 18. On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob so that the period of the waveforms displayed on the oscilloscope screen is again equal to 20 ms. On the oscilloscope, set the sensitivities of channels 1 and 2 to 0.5 V/DIV. and 1 V/DIV., respectively.

1-12

Saturation and Effect of Frequency in Magnetic Circuits On the Power Supply, set the voltage control knob so that the current IPEAK is equal to the value given in the following table: LINE VOLTAGE

IPEAK

V ac

mA

120

30

220

15

240

15

Table 1-3. Peak current in the primary winding of the transformer (IPEAK).

G 19. Note the current IPEAK and the peak voltage across the secondary winding of the transformer (VPEAK) in the first row of Table 1-4. IPEAK

VPEAK

mA

V

Table 1-4. VPEAK versus IPEAK when the frequency is equal 50 Hz.

G 20. On the Power Supply, set the voltage control knob to increase the current IPEAK in six steps until it reaches the maximum value given in Table 1-5. For each step, record the current IPEAK and voltage VPEAK in Table 1-4. Note: Vary the sensitivities of channels 1 and 2 of the oscilloscope as necessary.

1-13

Saturation and Effect of Frequency in Magnetic Circuits LINE VOLTAGE

IPEAK

V ac

mA

120

400

220

200

240

200

Table 1-5. Maximum peak current in the primary winding of the transformer.

G 21. On the Power Supply, set the voltage control knob to the 20 position. On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob so that the period of the waveforms displayed on the oscilloscope screen is equal to 30 ms. This sets the operating frequency of the single-phase inverter to approximately 33 Hz. On the oscilloscope, set the sensitivities of channels 1 and 2 to 0.5 V/DIV. and 1 V/DIV., respectively. On the Power Supply, set the voltage control knob so that the current IPEAK is equal to the value given in the following table: LINE VOLTAGE

IPEAK

V ac

mA

120

30

220

15

240

15

Table 1-6. Peak current in the primary winding of the transformer (IPEAK).

G 22. Record the current IPEAK and the voltage VPEAK in the first row of Table 1-7.

1-14

Saturation and Effect of Frequency in Magnetic Circuits IPEAK

VPEAK

mA

V

Table 1-7. VPEAK versus IPEAK when the frequency is approximately equal to 33 Hz.

G 23. On the Power Supply, set the voltage control knob to increase the current IPEAK in six steps until it reaches the maximum value given in Table 1-8. For each step, record the current IPEAK and voltage VPEAK in Table 1-7. Note: Vary the sensitivities of channels 1 and 2 of the oscilloscope as necessary.

LINE VOLTAGE

IPEAK

V ac

mA

120

400

220

200

240

200

Table 1-8. Maximum peak current in the primary winding of the transformer.

G 24. On the Power Supply, set the voltage control knob to the 20 position. On the oscilloscope, set the time base to 2 ms/DIV. On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob so that the period of the waveforms displayed on the oscilloscope screen is as close as possible to 10 ms. This sets the operating frequency of the single-phase inverter to approximately 100 Hz. On the oscilloscope, set the sensitivities of channels 1 and 2 to 0.5 V/DIV. and 1 V/DIV., respectively. On the Power Supply, set the voltage control knob so that the current IPEAK is equal to the value given in the following table:

1-15

Saturation and Effect of Frequency in Magnetic Circuits Note: Do not take the overshoot in the current waveform into account when setting IPEAK. LINE VOLTAGE

IPEAK

V ac

mA

120

30

220

15

240

15

Table 1-9. Peak current in the primary winding of the transformer (IPEAK).

G 25. Record the current IPEAK and the voltage VPEAK in the first row of Table 1-10. Note: Do not take the overshoot in the voltage waveform into account when measuring VPEAK.

IPEAK

VPEAK

mA

V

Table 1-10. VPEAK versus IPEAK when the frequency is approximately equal to 100 Hz.

G 26. On the Power Supply, set the voltage control knob to increase the current IPEAK in five steps until it reaches the maximum value given in Table 1-11. For each step, record the current IPEAK and voltage VPEAK in Table 1-10. Note: Vary the sensitivities of channels 1 and 2 of the oscilloscope as necessary.

1-16

Saturation and Effect of Frequency in Magnetic Circuits LINE VOLTAGE

IPEAK

V ac

mA

120

300

220

150

240

150

Table 1-11. Maximum peak current in the primary winding of the transformer.

G 27. From the results in Tables 1-4, 1-7, and 1-10, plot in Figure 1-10 the relationships between IPEAK and VPEAK (saturation curves) for the three frequencies used.

Figure 1-10. Relationships between IPEAK and VPEAK (saturation curves) for various frequencies.

Briefly explain why VPEAK virtually stops to increase once IPEAK reaches a certain value.

1-17

Saturation and Effect of Frequency in Magnetic Circuits For a given voltage, briefly explain why the current decreases when the frequency is increased.

G 28. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch and the 24-V ac power switch to the O position. Set the rocker switch on the Enclosure / Power Supply to the O position. Remove all leads, cables, and probes. CONCLUSION In this exercise, you observed that in a magnetic circuit (transformer) the ac current is no longer proportional to the ac voltage when saturation occurs. You also observed that the peak value of the current increases greatly when saturation occurs. You saw that for a given ac voltage across a coil, the ac current in the coil decreases when the frequency increases. You measured the voltage across the secondary winding of the transformer for various currents in the primary winding of the transformer, at three different frequencies. From these measurements, you plotted the saturation curves of the transformer at these frequencies. From these curves, you found that using a higher frequency in a magnetic circuit allows higher ac voltages to be applied before saturation occurs. REVIEW QUESTIONS 1. What is the relationship between the voltage and the current in magnetic circuits when there is no saturation?

2. What happens to the relationship between the voltage and the current in magnetic circuits when there is saturation?

1-18

Saturation and Effect of Frequency in Magnetic Circuits 3. Briefly explain what causes saturation in magnetic circuits.

4. Describe what happens to the ac voltage, ac current, and alternating magnetic flux of a magnetic circuit when the frequency is increased.

5. When a magnetic circuit is saturated, what are the two solutions which can be used so that the magnetic circuit is no longer saturated? Explain.

1-19

1-20

Exercise

2

Three-Phase Voltage-Source Inverter Induction-Motor Drive EXERCISE OBJECTIVE •

To understand the operation of an induction-motor drive using a three-phase voltage-source inverter.

•

To understand the concept of constant voltage-to-frequency ratio.

DISCUSSION Introduction The induction motor is used more and more today because its cost is low, and it is rugged and easy to maintain. However, the speed of an induction motor mainly depends on the frequency of the ac power source which supplies it. Therefore, a variable-frequency ac power source is required to control the speed of an induction motor. Self-commutated inverters are devices which convert dc power into ac power. Such inverters allow single-phase, two-phase, and three-phase ac power sources with variable frequency and voltage to be obtained. Therefore, self-commutated inverters are well suited for induction-motor drives. The three-phase voltage-source inverter induction-motor drive Figure 2-1 shows a three-phase voltage-source inverter driving a three-phase squirrel-cage induction motor. Such a drive is usually referred to as a three-phase voltage-source inverter induction-motor drive. In the drive of Figure 2-1, the inverter uses 180° modulation, but other types of modulation such as the 120° modulation, programmed-waveform modulation, and pulse-width modulation, can be used. Figure 2-2 shows the same induction-motor drive using the symbol representing a voltage-source inverter (VSI). Varying the dc power supply voltage allows the ac voltage at the inverter outputs, and therefore the ac voltage applied to the induction motor, to be varied. Varying the operating frequency of the inverter varies the frequency of the ac voltage applied to the induction motor. When the frequency increases, the speed of the induction motor increases and vice versa. Reversing the connections of any two of the inverter outputs reverses the direction of rotation of the motor. In practice, however, the connections do not need to be physically reversed because the INVERTER CONTROL UNIT can accomplish the same result by electronically changing the phase sequence of the ac voltages at the outputs of the inverter.

2-1

Three-Phase Voltage-Source Inverter Induction Motor-Drive

Figure 2-1. A three-phase voltage-source inverter induction-motor drive.

Figure 2-2. Simplified diagram of the three-phase voltage-source inverter induction-motor drive shown in Figure 2-1.Torque versus magnetic flux in an induction motor

The torque developed in an induction motor is proportional to the current induced in the rotor and the magnetic flux in the motor. It is, therefore, desirable to maintain the magnetic flux in the induction motor as high as possible so that the motor can develop the highest possible torque. The relationship between the voltage E, the frequency f, and the magnetic flux M in an induction motor is based on the following equation: E = kfNM where

E is the rms value of the ac voltage applied to the induction motor, k is a constant whose value depends on the waveform of the voltage E as well as the distribution of the stator windings and the number of poles of the induction motor, f is the frequency of the voltage E, N is the number of turns on the stator windings per pole of the induction motor, M is the magnetic flux per pole of the induction motor.

This equation shows that for a fixed frequency f, the flux M in the stator of the induction motor increases as the voltage E increases and vice versa. As the 2-2

Three-Phase Voltage-Source Inverter Induction Motor Drive voltage E is increased, the flux M increases and eventually reaches a certain value which makes certain parts of the core of the induction motor start to saturate. Before saturation occurs, the flux M is proportional to the magnetizing current, that is, the current which produces the magnetic flux in the induction motor. Therefore, the power dissipated as heat in the stator windings (RI2 losses) increases as the flux M is increased. Furthermore, the iron losses in the motor increases as the flux M is increased. When saturation occurs, the magnetizing current in the stator windings begins to increase considerably while the flux M increases only slightly. This causes the power dissipated as heat in the stator windings (RI2 losses) to increase considerably. Therefore, it is not desirable to increase the flux M beyond saturation because the losses in the induction motor increase considerably while the flux M increases only a little. In brief, the maximum flux M in the induction motor is mainly limited by saturation. In practice, the flux M in the induction motor is usually maintained at a level that slightly saturates the core of the motor, that is, the induction motor usually operates around the knee of the saturation curve. This allows a high flux M to be obtained with a magnetizing current which is relatively low. In other words, this allows the induction motor to develop the highest possible torque while maintaining the losses in the motor at an acceptable level. Voltage versus frequency in a VSI induction-motor drive In VSI induction-motor drives, the speed of the motor is varied by varying the operating frequency of the inverter over a fairly wide range. Typically, the frequency can range from a few hertz to more than 100 Hz. For a fixed voltage E, the flux M decreases as the frequency f increases. On the other hand, as the frequency decreases, the flux M increases and eventually reaches a certain value which makes the core of the induction motor begin to saturate. In this case, the voltage E can be decreased to decrease the flux M and prevent saturation. Therefore, in a VSI induction-motor drive, it is highly desirable that the voltage E decreases as the frequency f decreases and vice versa. In other words, the voltageto-frequency ratio, or V/f ratio, must remain constant so that the flux M remains fairly constant. Furthermore, the value of the V/f ratio determines the value of the flux M as well as the point on the saturation curve at which the induction motor operates. The value of the V/f ratio is usually set so that the flux M slightly saturates the core of the induction motor. As a result, the induction motor can develop a maximum torque over a given range of speed. Figure 2-3 shows relationships between the voltage E and frequency f for a given induction-motor drive. The V/f ratio is different for each of these relationships. One of these V/f ratios is considered as normal while the other two are respectively above normal and below normal. Two points are drawn on each voltagefrequency relationship at frequencies of 50 and 100 Hz. Figure 2-4 shows the saturation curves of the motor used in the induction-motor drive, at frequencies of 50 and 100 Hz. The points drawn at frequencies of 50 and 100 Hz on the voltage-frequency relationships of Figure 2-3 are shown on the 50 and 100-Hz saturation curves. Thus, points A to F in Figure 2-3 correspond to points A to F in Figure 2-4. 2-3

Three-Phase Voltage-Source Inverter Induction Motor-Drive

Figure 2-3. Relationship between the voltage E and the frequency f for a given induction-motor drive.

Figures 2-3 and 2-4 clearly show that when the V/f ratio is kept constant, the flux M in the stator of the induction motor is virtually constant. For instance, when the V/f ratio is normal, the induction motor operates in the knee of the saturation curve regardless of the frequency (see points B and E in both figures). The induction motor, therefore, is slightly saturated. When the V/f ratio is above normal, the induction motor operates above the knee of the saturation curve regardless of the frequency (see points C and F in both figures). In this case, the induction motor is saturated. On the other hand, when the V/f ratio is below normal, the induction motor operates below the knee of the saturation curve regardless of the frequency (see points A and D in both figures). In this case, the induction motor is not saturated.

2-4

Three-Phase Voltage-Source Inverter Induction Motor Drive

Figure 2-4. Saturation curves of the motor used in the induction-motor drive, at frequencies of 50 and 100 Hz.

Procedure Summary In the first part of this exercise, you will set up in the Mobile Workstation the equipment required to carry out this exercise. In the second part of this exercise, you will use the circuit shown in Figure 2-5 to observe the operation of a three-phase voltage-source inverter induction-motor drive. In this circuit, a variable-voltage three-phase ac power supply, a three-phase diode rectifier, an inductor L1, and resistors R1, R2, and R3 are used to build a variablevoltage dc power supply. The resistors allow the dc power supply to sink current. The motor used in this circuit is a squirrel-cage induction motor. You will vary the operating frequency and change the connections of the outputs of the inverter while observing the speed and direction of rotation of the induction motor. You will also vary the voltage applied to the induction motor while observing the waveform of the line current in one of the inverter outputs. In the third part of this exercise, you will measure the line-to-neutral voltage (phase voltage) which must be applied to the induction motor to obtain a given line current (given magnetic flux), at various frequencies. From these measurements, you will plot the relationship between the phase voltage and the frequency in an induction motor operating with a virtually constant magnetic flux.

2-5

Three-Phase Voltage-Source Inverter Induction Motor-Drive EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix C of this manual, to obtain the list of the equipment required to carry out this exercise. 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

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1. Install the Power Supply, the Enclosure / Power Supply, the Chopper / Inverter, the Power Diodes, the Four-Pole Squirrel-Cage Induction Motor, the Smoothing Inductors, the DC Voltmeter/Ammeter, the AC Voltmeter, and the Resistive Load modules in the Mobile Workstation.

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2. Install the Chopper / Inverter Control Unit and the Current/Voltage Isolator in the Enclosure / Power Supply.

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3. Make sure that the main power switch of the Power Supply is set to the O (OFF) position. Connect the Power Supply to a three-phase wall receptacle.

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4. Plug the Enclosure / Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the I (ON) position.

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5. Make sure that the toggle switches on the Resistive Load module are all set to the O (open) position.

Operation of a three-phase voltage-source inverter induction-motor drive

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6. Install the dynamo of the Speed Sensor / Tachometer on the shaft of the Four-Pole Squirrel-Cage Induction Motor. Connect the dynamo to the TRANSDUCER jack of the Speed Sensor / Tachometer. Connect the modules as shown in Figure 2-5.

2-6

Three-Phase Voltage-Source Inverter Induction Motor Drive

Figure 2-5. A three-phase voltage-source inverter induction-motor drive.

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7. Make the following settings: On the Power Supply Voltage selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-N On the Chopper / Inverter Control Unit DC SOURCE 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAXimum MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3- 180° On the IGBT Chopper / Inverter module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Interconnection switch S2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Note: If you are using a MOSFET Chopper / Inverter, the interconnection switch S1 must be set to the I position.

2-7

Three-Phase Voltage-Source Inverter Induction Motor-Drive On the Power Diodes module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I On the Speed Sensor / Tachometer ROTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . clockwise On the oscilloscope Channel-1 Sensitivity . . . . . . . . . . . . . . . . 5 V/DIV. (DC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (DC coupled) Vertical Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHOPped Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ms/DIV. Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EXTernal Trigger Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . positive (+) Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . HF REJection

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8. On the oscilloscope, make the appropriate settings to position the traces of channels 1 and 2 in the upper and lower halves of the screen, respectively. The signal displayed on the oscilloscope screen is the switching control signal applied to electronic switch Q1 of the three-phase voltage-source inverter. On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob so that the frequency of the switching control signal applied to electronic switch Q1 is 0 (the signal will settle at +4 V or 0 V). Slowly turn the DC SOURCE 1 control knob clockwise until the period of the switching control signal is approximately equal to 80 ms. On the Chopper / Inverter module, disconnect the cable connected to SWITCHING CONTROL INPUT 1, then connect it to the OUTPUT of the voltage isolator, making sure you connect the black lead of the cable to the common terminal.

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9. On the Power Supply, make sure that the voltage control knob is set to the 0 position then set the 24-V ac power switch and the main power switch to the I (ON) position. The POWER ON LED on the Chopper / Inverter module should light up to indicate that the module is correctly powered. Slowly set the voltage control knob of the Power Supply to the 30 position (30% of the ac network line voltage). The induction motor should start to rotate and the oscilloscope should now display the waveforms of the line-toneutral voltage (phase voltage) and line current provided by the three-phase voltage-source inverter. In which direction does the induction motor rotate according to the Speed Sensor / Tachometer?

2-8

Three-Phase Voltage-Source Inverter Induction Motor Drive G 10. On the oscilloscope, set the time base to 5 ms/DIV. On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob clockwise until the period of the phase voltage decreases to approximately 20 ms (frequency increase) while observing the speed indicated by the Speed Sensor / Tachometer. Describe what happens. Explain.

G 11. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch to the O position. On the Four-Pole Squirrel-Cage Induction Motor, reverse the connections of the leads connected to terminals 2 and 3. On the Power Supply, set the main power switch to the I (ON) position then slowly set the voltage control knob to the 30 position. The induction motor should start to rotate. On the Speed Sensor / Tachometer, modify the setting of the ROTATION switch, if necessary, so that the tachometer indicates the speed of the induction motor. Does the induction motor rotate in the same direction as in the previous step? Explain.

G 12. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch to the O position. On the Four-Pole Squirrel-Cage Induction Motor, reverse the connections of the leads connected to terminals 2 and 3 so that they are connected as before. On the Power Supply, set the main power switch to the I (ON) position then slowly set the voltage control knob to the 30 position. The induction motor should start to rotate clockwise. On the Speed Sensor / Tachometer, set the ROTATION switch to the clockwise position so that the tachometer indicates the speed of the induction motor.

2-9

Three-Phase Voltage-Source Inverter Induction Motor-Drive Note: For the rest of this exercise, vary the setting of the ROTATION switch as necessary so that the tachometer indicates the speed of the induction motor.

On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob counterclockwise until the induction motor stops rotating, then continue to turn this knob counterclockwise until the period of the phase voltage is equal to approximately 20 ms. While doing this, observe the speed indicated by the Speed Sensor / Tachometer. Note: Do not allow the induction motor to remain stationary too long, because a fairly high dc current may flow in the motor. This could eventually make the circuit breakers on the Power Diodes module trip.

Describe what happens. Explain.

G 13. On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob so that the induction motor rotates clockwise and the period of the phase voltage is equal to approximately 20 ms. Sketch the waveforms of the phase voltage and line current provided by the three-phase voltage-source inverter in Figure 2-6.

Figure 2-6. Waveforms of the phase voltage and line current provided by the three-phase voltagesource inverter.

G 14. On the Power Supply, slowly set the voltage control knob halfway between the 80 and 90 positions (85% of the ac network line voltage) while observing 2-10

Three-Phase Voltage-Source Inverter Induction Motor Drive the waveforms of the phase voltage and line current on the oscilloscope screen and the speed of the induction motor on the Speed Sensor / Tachometer. Sketch the waveforms of the phase voltage and line current provided by the three-phase voltage-source inverter in Figure 2-7.

Figure 2-7. Waveforms of the phase voltage and line current provided by the three-phase voltagesource inverter.

Describe what happens when the phase voltage provided by the three-phase voltage-source inverter increases.

Voltage versus frequency in an induction-motor drive

G 15. On the Power Supply, set the voltage control knob to the 10 position. Make the following settings on the oscilloscope: Channel-1 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (DC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . 0.5 V/DIV. (DC coupled) Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ms/DIV. On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob so that the induction motor continues to rotate clockwise and the period (T) of the phase voltage waveform is equal to approximately 80 ms.

2-11

Three-Phase Voltage-Source Inverter Induction Motor-Drive Note: The oscilloscope display may be unstable. If so, vary the HOLD OFF control of the oscilloscope to stabilize the display as much as possible.

G 16. On the Power Supply, slowly set the voltage control knob so that the peak line current (ILINE peak) provided by the three-phase voltage-source inverter reaches the value given in Table 2-1. This current creates a magnetic flux which slightly saturates the stator of the induction motor. LINE VOLTAGE

ILINE peak

V ac

A

120

0.9

220

0.45

240

0.45

Table 2-1. Peak line current (ILINE peak) required to slightly saturate the stator of the induction motor.

Read the value of the phase voltage (VPHASE) provided by the three-phase voltage-source inverter on the AC Voltmeter. Record this voltage in the appropriate row of the VPHASE column of Table 2-2. Calculate the frequency (f) related to the period T then record the result in the appropriate row of the f column of Table 2-2. T

f

VPHASE

ms

Hz

V ac

80 60 40 25 20 16 12 10 Table 2-2. Phase voltage required to slightly saturate the stator of the induction motor versus frequency.

G 17. On the Chopper / Inverter Control Unit, turn the DC SOURCE 1 control knob clockwise to successively obtain each of the other periods T given in Table 2-2. For each period T, carry out step 16. 2-12

Three-Phase Voltage-Source Inverter Induction Motor Drive Note: Vary the channel-1 sensitivity and the time base of the oscilloscope as necessary. The peak value of the line current (ILINE peak) required when the period (T) is 12 or 10 ms may not be reached depending on the ac network line voltage.

G 18. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch and the 24-V ac power switch to the O position. From the results in Table 2-2, plot in Figure 2-8 the relationship between the phase voltage required to slightly saturate the stator of the induction motor (VPHASE) and the frequency.

Figure 2-8. Relationship between the phase voltage required to slightly saturate the stator of the induction motor (VPHASE) and the frequency.

2-13

Three-Phase Voltage-Source Inverter Induction Motor-Drive From the relationship plotted in Figure 2-8, which condition must be met so that the magnetic flux in the stator of the induction motor remains fairly constant when the speed of rotation is varied by varying the frequency? Explain.

G 19. Set the rocker switch on the Enclosure / Power Supply to the O position. Remove all leads, cables, and probes. CONCLUSION In this exercise, you observed that in a three-phase voltage-source inverter induction-motor drive, the speed of the motor increases as the operating frequency of the inverter increases and vice versa. You also observed that the direction of rotation of the motor depends on the phase sequence of the line-to-neutral voltages (phase voltages) applied to the motor. You found that for a fixed frequency, increasing the voltage applied to the motor increases the current in the motor, and therefore the magnetic flux, and that this can eventually saturate the stator of the motor. You measured the phase voltage which must be applied to the induction motor to obtain a given line current (given magnetic flux), at various frequencies. From these measurements, you plotted the relationship between the phase voltage and frequency in an induction motor operating with a virtually constant magnetic flux. From this relationship, you found that the V/f ratio must be kept constant so that the magnetic flux in the stator of an induction motor remains fairly constant. REVIEW QUESTIONS 1. Briefly explain why inverters are well suited to build induction-motor drives.

2. Describe the relationship between the operating frequency of the inverter and the speed of the motor, in a three-phase voltage-source inverter induction-motor drive.

2-14

Three-Phase Voltage-Source Inverter Induction Motor Drive 3. What determines the direction of rotation of the motor in a three-phase voltagesource inverter induction-motor drive?

4. In a three-phase voltage-source inverter induction-motor drive, describe what may happen when the voltage applied to the motor increases while the frequency remains fixed.

5. Briefly explain why it is desirable to maintain the V/f ratio constant when varying the frequency in an induction-motor drive.

2-15

2-16

Exercise

3

Constant V/f Ratio PWM-Inverter Induction-Motor Drive EXERCISE OBJECTIVE •

To understand the operation of a pulse-width modulation (PWM) voltage-source inverter.

•

To understand the operation of a constant V/f ratio PWM-inverter induction-motor drive.

DISCUSSION 180° modulation versus pulse-width modulation (PWM) in voltage-source inverter induction-motor drives Self-commutated inverters are well suited to build induction-motor drives since they can provide ac voltages whose amplitude and frequency can be varied. When 180° modulation is used in a voltage-source inverter induction-motor drive, the dc voltage applied at the input of the inverter must be varied when the speed of rotation is varied (by varying the frequency), so as to maintain the V/f ratio constant. This maintains a fairly constant magnetic flux and prevents the induction motor from saturating. In practice, induction-motor drives built with a 180°-modulation voltagesource inverter are provided with a circuit that automatically increases the dc voltage at the inverter input when the operating frequency of the inverter is increased and vice versa. The added circuitry allows a constant V/f ratio to be obtained, but it also increases the complexity and cost of the motor drive. Pulse-width modulation (PWM) voltage-source inverters are self-commutated inverters which can provide a variable ac voltage even when the dc voltage at the inverter input is fixed. The frequency of the ac voltage at the outputs of a PWM voltage-source inverter can still be varied as in any other self-commutated inverters. In other words, a constant V/f ratio can be obtained at the outputs of a PWM voltage-source inverter without having to vary the dc voltage at the input. Consequently, no additional circuitry is required to control the dc voltage at the input of the PWM voltage-source inverter in order to obtain a constant V/f ratio. As a result, the PWM voltage-source inverter is often preferred to the 180°-modulation voltagesource inverter to build induction-motor drives since it reduces the cost and complexity of the drive. However, the fairly high switching speed required in PWM voltage-source inverters limits the maximum power of the drive. For these reasons, the PWM voltage-source inverters are more likely to be encountered in low- and medium-power induction-motor drives, the 180°-modulation voltage-source inverters being used in high-power induction-motor drives.

3-1

Constant V/f Ratio PWM-Inverter Induction Motor Drive

Figure 3-1. Control signals generated in a PWM voltage-source inverter.

3-2

Constant V/f Ratio PWM-Inverter Induction Motor-Drive Operation of the PWM voltage-source inverter Synchronous pulse-width modulation is used to produce the signals required to control the semiconductor switches in a PWM voltage-source inverter. Figure 3-1 shows an example of the control signals generated in a PWM three-phase voltagesource inverter. Figure 3-1 also shows the triangular-wave and sine-wave signals which the inverter control unit produces internally to generate the control signals. The voltages of the triangular- and sine-wave signals are compared to produce control signal 1. A positive pulse is produced whenever the voltage of the sine-wave signal exceeds that of the triangular-wave signal. Control signal 1 is simply inverted by a logic circuit to generate control signal 4. Control signals 1 and 4 are phase shifted by 120° and 240° to generate the other four control signals. The width of the pulses in the control signals varies according to the instantaneous voltage of the sine-wave signal which the inverter control unit produces internally. The pulse width increases as the instantaneous voltage of the sine-wave signal increases and vice versa. Furthermore, varying the sine-wave signal amplitude varies the ratio of the maximum to the minimum pulse width. Similarly, the rate at which the width of the pulses in the control signals varies depends on the frequency of the sine-wave signal which the inverter control unit produces internally. This rate increases as the frequency of the sine-wave signal increases and vice versa. In brief, the ratio of the maximum to the minimum pulse width depends on the amplitude of the sine-wave signal. Furthermore, the rate at which the pulse width varies is proportional to the frequency of the sine-wave signal. Figure 3-2 shows the waveforms of the line-to-line voltages at the PWM voltagesource inverter outputs, which result from the control signals shown in Figure 3-1. A line-to-line voltage is the voltage between any two outputs of the inverter. Line-toline voltages are usually referred to as line voltages. This explains why the voltage waveforms in Figure 3-2 are identified VLINE1, VLINE2, and VLINE3. The waveforms of the line voltages at the PWM voltage-source inverter outputs are bipolar pulse trains which approximate sine waves which are shifted by 120° as shown by the dotted lines in Figure 3-2. The amplitude and frequency of the sine waves which the bipolar pulse trains approximate are respectively proportional to the amplitude and frequency of the sinewave signal which the inverter control unit produces internally. In other words, the value and frequency of the line voltages at the outputs of a PWM voltage-source inverter are respectively proportional to the amplitude and frequency of the sinewave signal which the inverter control unit produces. Notice that the pulse-width modulation is said to be synchronous because there is a fixed ratio between the frequency of the triangular-wave and sine-wave signals. In Figure 3-1, the frequency of the triangular-wave signal is 9 times that of the sinewave signal. The same ratio is used in the 3- V/f MODE of the Chopper / Inverter Control Unit.

3-3

Constant V/f Ratio PWM-Inverter Induction Motor Drive

Figure 3-2. Waveforms of the line voltages at the PWM voltage-source inverter outputs.

Using a PWM voltage-source inverter to obtain a constant V/f ratio inductionmotor drive When a PWM voltage-source inverter is used to build an induction-motor drive, the inverter control unit produces a sine-wave signal whose amplitude is proportional to its frequency. Since the value and frequency of the line voltages at the outputs of a PWM voltage-source inverter are respectively proportional to the amplitude and frequency of this sine-wave signal, the V/f ratio at the outputs of the inverter is made constant. Figure 3-3 shows a typical relationship between the voltage and frequency at the outputs of a PWM voltage-source inverter. The voltage is proportional to the frequency so that the V/f ratio is constant. The voltage stops increasing when it reaches a certain value. This value often corresponds to the nominal voltage of the induction motor that is being driven.

3-4

Constant V/f Ratio PWM-Inverter Induction Motor-Drive

Figure 3-3. Typical relationship between the voltage and frequency at the outputs of a PWM voltage-source inverter.

Procedure Summary In the first part of this exercise, you will set up in the Mobile Workstation the equipment required to carry out this exercise. In the second part of this exercise, you will use the circuit shown in Figure 3-4 to compare the use of 180° modulation and pulse-width modulation (PWM) in a threephase voltage-source inverter induction-motor drive. In this circuit, a variable-voltage three-phase ac power supply, a three-phase diode rectifier, inductor L1, and resistors R1, R2, and R3 are used to build a variable-voltage dc power supply. The resistors allow the dc power supply to sink current. The motor used in this circuit is a squirrel-cage induction motor. In the third part of this exercise, you will observe the waveforms of the line-to-line voltage (line voltage) and line current at the outputs of the PWM voltagesource inverter and measure the line voltage while varying the frequency. This will allow you to understand the operation of the constant V/f ratio PWM-inverter induction-motor drive. In the fourth part of this exercise, you will measure the line voltage at the PWM voltage-source inverter outputs at various frequencies. From these

3-5

Constant V/f Ratio PWM-Inverter Induction Motor Drive measurements, you will plot the relationship between the line voltage provided by the PWM voltage-source inverter and the frequency. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix C of this manual, to obtain the list of the equipment required to carry out this exercise. 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

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1. Install the Power Supply, the Enclosure / Power Supply, the Chopper / Inverter, the Power Diodes, the Four-Pole Squirrel-Cage Induction Motor, the Smoothing Inductors, the DC Voltmeter/Ammeter, the AC Voltmeter, and the Resistive Load modules in the Mobile Workstation.

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2. Install the Chopper / Inverter Control Unit and the Current/Voltage Isolator in the Enclosure / Power Supply.

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3. Make sure that the main power switch of the Power Supply is set to the O (OFF) position. Connect the Power Supply to a three-phase wall receptacle.

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4. Plug the Enclosure / Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the I (ON) position.

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5. Make sure that the toggle switches on the Resistive Load module are all set to the O (open) position.

180° modulation versus pulse-width modulation (PWM) in a voltage-source inverter induction-motor drive

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6. Install the dynamo of the Speed Sensor / Tachometer on the shaft of the Four-Pole Squirrel-Cage Induction Motor. Connect the modules as shown in Figure 3-4.

3-6

Constant V/f Ratio PWM-Inverter Induction Motor-Drive

Figure 3-4. A three-phase voltage-source inverter induction-motor drive.

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7. Make the following settings: On the Power Supply Voltage selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-N On the Chopper / Inverter Control Unit DC SOURCE 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAXimum MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3- 180° On the IGBT Chopper / Inverter module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Interconnection switch S2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Note: If you are using a MOSFET Chopper / Inverter, the interconnection switch S1 must be set to the I position.

3-7

Constant V/f Ratio PWM-Inverter Induction Motor Drive On the Power Diodes module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I On the Speed Sensor / Tachometer ROTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . clockwise On the oscilloscope Channel-1 Sensitivity . . . . . . . . . . . . . . . . 5 V/DIV. (DC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . 0.5 V/DIV. (DC coupled) Vertical Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHOPped Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ms/DIV. Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EXTernal Trigger Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . positive (+) Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . HF REJection

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8. On the oscilloscope, make the appropriate settings to position the traces of channels 1 and 2 in the upper and lower halves of the screen, respectively. On the Power Supply, make sure that the voltage control knob is set to the 0 position then set the 24-V ac power switch and the main power switch to the I (ON) position. The POWER ON LED on the Chopper / Inverter module should light up to indicate that the module is correctly powered. Slowly set the voltage control knob of the Power Supply to the 80 position (80% of the ac network line voltage). The induction motor should start to rotate and the oscilloscope should now display the waveforms of the line-toline voltage (channel 1) and line current (channel 2) provided by the voltagesource inverter.

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9. On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob so that the speed of the induction motor decreases to approximately 2500 r/min as indicated by the Speed Sensor / Tachometer. On the Power Supply, set the voltage control knob to the 60% position. The speed of the induction motor should remain at approximately 2500 r/min. On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob counterclockwise to decrease the speed of the induction motor to approximately 500 r/min. While doing this, turn the voltage control knob of the Power Supply counterclockwise so that the peak line current (ILINE peak) does not exceed the value given in the following table:

3-8

Constant V/f Ratio PWM-Inverter Induction Motor-Drive LINE VOLTAGE

ILINE peak

V ac

A

120

1.0

220

0.5

240

0.5

Table 3-1. Peak line current (ILINE peak) required to slightly saturate the stator of the induction motor.

When varying the speed of rotation, what is the disadvantage of using 180° modulation in a voltage-source inverter induction-motor drive?

G 10. Make the following settings: On the Chopper / Inverter Control Unit MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3- V/f On the oscilloscope Channel-2 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (DC coupled) The induction motor should stop rotating. On the Power Supply, set the voltage control knob to the 100% position. The induction motor should start to rotate again at a speed of approximately 500 r/min. On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob clockwise so that the speed of the induction motor passes from approximately 500 to 2500 r/min, then turn it counterclockwise so that the speed passes from approximately 2500 to 500 r/min. While doing this, observe the waveform of the line current provided by the voltage-source inverter and the line-to-line voltage (line voltage) indicated by the AC Voltmeter. Note: Vary the time base of the oscilloscope as necessary.

When varying the speed of rotation, what is the advantage of using pulsewidth modulation in a voltage-source inverter induction-motor drive?

3-9

Constant V/f Ratio PWM-Inverter Induction Motor Drive Operation of a constant V/f ratio PWM-inverter induction-motor drive

G 11. On the Chopper / Inverter Control Unit, turn the DC SOURCE 1 control knob clockwise until the speed of the induction motor is as close as possible to 1000 r/min. Set the time base of the oscilloscope to 5 ms/DIV. Sketch the waveforms of the line-to-line voltage (line voltage) and line current in Figure 3-5.

Figure 3-5. Waveforms of the line voltage and current provided by the PWM voltagesource inverter (1000 r/min., time base = 5 ms/DIV.).

Measure and record the line voltage (VLINE) provided by the PWM voltagesource inverter on the AC Voltmeter. VLINE (1000 r/min.) =

V ac

G 12. On the Chopper / Inverter Control Unit, turn the DC SOURCE 1 control knob clockwise until the speed of the induction motor is as close as possible to 2000 r/min. Set the time base of the oscilloscope to 2 ms/DIV. Sketch the waveforms of the line voltage and current in Figure 3-6.

3-10

Constant V/f Ratio PWM-Inverter Induction Motor-Drive

Figure 3-6. Waveforms of the line voltage and current provided by the PWM voltagesource inverter (2000 r/min., time base = 2 ms/DIV.).

Measure and record the voltage VLINE provided by the PWM voltage-source inverter on the AC Voltmeter. VLINE (2000 r/min.) =

V ac

From the observations made so far, briefly describe the operation of the constant V/f ratio PWM-inverter induction-motor drive.

Relationship between the voltage and frequency at the output of a constant V/f ratio PWM-inverter induction-motor drive

G 13. On the oscilloscope, set the time base to 10 ms/DIV. On the Chopper / Inverter Control Unit, slowly set the DC SOURCE 1 control knob so that the induction motor continues to rotate clockwise and the period (T) of the line voltage waveform is equal to approximately 70 ms. Note: The oscilloscope display may be unstable. If so, vary the HOLD OFF control of the oscilloscope to stabilize the display as much as possible.

G 14. Measure the voltage VLINE provided by the PWM voltage-source inverter on the AC Voltmeter. Record this voltage in the appropriate row of the VLINE column of Table 3-2.

3-11

Constant V/f Ratio PWM-Inverter Induction Motor Drive Calculate the frequency (f) related to the period T then record the result in the appropriate row of the f column of Table 3-2. T

f

VLINE

ms

Hz

V ac

70 50 40 30 25 20 15 10 8 Table 3-2. Line voltage (VLINE) at the output of the PWM voltage-source inverter versus frequency.

G 15. On the Chopper / Inverter Control Unit, turn the DC SOURCE 1 control knob clockwise to successively obtain each of the other periods T given in Table 3-2. For each period T, carry out step 14. Note: Vary the time base of the oscilloscope as necessary.

G 16. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch and the 24-V ac power switch to the O position. From the results in Table 3-2, plot in Figure 3-7 the relationship between the voltage VLINE provided by the PWM voltage-source inverter and the frequency. From the curve plot in Figure 3-7, briefly explain why the dc voltage at the PWM voltage-source inverter input does not need to be varied when varying the speed of the induction motor.

Briefly explain why the voltage VLINE stops increasing when the frequency reaches a certain value.

3-12

Constant V/f Ratio PWM-Inverter Induction Motor-Drive

Figure 3-7. Relationship between the voltage VLINE provided by the PWM voltage-source inverter and the frequency.

G 17. Set the rocker switch on the Enclosure / Power Supply to the O position. Remove all leads, cables, and probes. CONCLUSION In this exercise, you found that when 180° modulation is used in a voltage-source inverter induction-motor drive, the dc voltage at the inverter input must be varied when varying the speed of rotation (by varying the frequency) to maintain the V/f ratio constant and to prevent the induction motor from saturating. You also found that when pulse-width modulation is used in a voltage-source inverter inductionmotor drive, the dc voltage at the inverter input does not need to be varied to maintain the V/f ratio constant when varying the speed of the induction motor.

3-13

Constant V/f Ratio PWM-Inverter Induction Motor Drive You observed that the waveforms of the line-to-line voltage (line voltage) at the PWM voltage-source inverter outputs are bipolar pulse trains. You also observed that the width of the pulses in these pulse trains and the value of the line voltage increase when the frequency increases and vice versa. You measured the line voltage at the output of the PWM-inverter induction-motor drive at various frequencies. From these measurements, you plotted the relationship between the line voltage and the frequency in a PWM-inverter induction-motor drive. From this relationship, you found that the line voltage is proportional to the frequency until the line voltage reaches a certain value which approximately corresponds to the nominal line voltage of the induction-motor which is being driven. REVIEW QUESTIONS 1. What are the advantages and disadvantages of using pulse-width modulation instead of 180° modulation in voltage-source inverter induction-motor drives?

2. Briefly describe how the control signals are generated in a PWM voltage-source inverter.

3. Briefly explain how the voltage at the outputs of a PWM voltage-source inverter can be varied without varying the dc voltage at the inverter input.

4. Describe the waveforms of the line-to-line voltage at the outputs of a PWMinverter induction-motor drive.

3-14

Constant V/f Ratio PWM-Inverter Induction Motor-Drive 5. Describe the relationship between the voltage and frequency at the output of a PWM-inverter induction-motor drive.

3-15

3-16

Exercise

4

Operation of a Synchronous Motor as a Stepper Motor EXERCISE OBJECTIVE •

To understand that a synchronous motor operates similarly to a stepper motor in a voltage-source inverter motor drive.

•

To understand the effect of frequency in a voltage-source inverter synchronousmotor drive.

DISCUSSION Introduction to the stepper motor A stepper motor is a motor whose rotor can only move to predetermined positions, which are all equally spaced. The number of predetermined positions depends on the way the motor is built. When voltages are applied to the stator windings of a stepper motor, the position at which the rotor stops depends on the initial position of the rotor and on the polarities of the currents flowing in the stator windings of the motor. Changing the polarity of the current in one stator winding makes the rotor move to the predetermined position, which precedes or follows its original position. The rotor of a stepper motor can be made to rotate step by step by modifying the polarities of the currents in the stator windings according to a predetermined sequence. Figure 4-1 (a) shows a simple stepper motor. The stator has two windings, which are labelled A and B, and the rotor is a permanent magnet. Because the rotor is a permanent magnet, this motor is usually referred to as a permanent-magnet stepper motor. When positive voltages are applied to windings A and B, as shown in Figure 4-1 (a), the currents flowing in these windings create two magnetic fields, each of which has a north (N) pole and a south (S) pole. These magnetic fields combine to form a single magnetic field. The N and S poles of the resulting magnetic field are located at positions 3 and 1 of the stator, respectively, that is, between the N poles and S poles of the two originating magnetic fields. This, therefore, forces the N pole of the rotor to settle at position 1. In Figure 4-1 (b), the polarity of the voltage applied to winding B is reversed (it becomes negative). This also reverses the polarity of the magnetic field produced by winding B. As a result, the N and S poles of the resulting magnetic field are now located at positions 4 and 2 of the stator, respectively. This forces the N pole of the rotor to move to position 2. Figures 4-1 (c) and (d) show the positions of the rotor for the two other polarity combinations of the voltages applied to windings A and B of the stepper motor. In brief, Figures 4-1 (a) to (d) show that the N and S poles of the resulting magnetic field, as well as the rotor, can move to four predetermined positions, which are shifted 90° with respect to each other, depending upon the polarities of the voltages applied to stator windings A and B of the stepper motor.

4-1

Operation of a Synchronous Motor as a Stepper Motor

Figure 4-1. Positions of the rotor versus the polarities of the voltages applied to the stator windings of a simple stepper motor.

Table 4-1 shows the polarity combinations of the voltages applied to stator windings A and B of the stepper motor shown in Figure 4-1 (a) to (d). When the voltages applied to stator windings A and B of the stepper motor are modified to obtain the polarity combinations in the order in which they are listed in Table 4-1, the rotor of the stepper motor successively moves to positions 1, 2, 3, and 4, that is, it rotates clockwise in steps, or increments, of 90°. Conversely, when these voltages are modified to obtain the polarity combinations in the inverse order to that in which they are listed in Table 4-1, the rotor of the stepper motor successively moves to positions 4, 3, 2, and 1, that is, it rotates counterclockwise in increments of 90°.

4-2

Operation of a Synchronous Motor as a Stepper Motor VOLTAGE POLARITY WINDING A

WINDING B

+

+

+

!

!

!

!

+

Table 4-1. Polarity combinations of the voltages applied to stator windings A and B of the stepper motor shown in Figure 4-1 (a) to (d).

Operation of a synchronous motor as a stepper motor Table 4-2 shows a series of polarity combinations for the voltages applied to the stator windings of a three-phase synchronous motor. When these voltages are modified successively to obtain these polarity combinations, the three-phase synchronous motor operates similarly to a stepper motor. VOLTAGE POLARITY PHASE-1 WINDING

PHASE-2 WINDING

PHASE-3 WINDING

+

!

+

+

!

!

+

+

!

!

+

!

!

+

+

!

!

+

Table 4-2. Polarity combinations for the voltages applied to the stator windings of a three-phase synchronous motor.

The three-phase synchronous motor rotates clockwise, in increments of a certain number of degrees, when the polarity combinations are generated in the order in which they are listed in Table 4-2. In this case, the waveforms of the voltages applied to the stator windings of the three-phase synchronous motor are as shown in Figure 4-2.

4-3

Operation of a Synchronous Motor as a Stepper Motor

Figure 4-2. Waveforms of the voltages applied to the stator windings of a three-phase synchronous motor to obtain the polarity combinations of Table 4-2 in the order in which they are listed.

The three-phase synchronous motor rotates counterclockwise, in increments of a certain number of degrees, when the polarity combinations are generated in the inverse order to that in which they are listed in Table 4-2. In this case, the waveforms of the voltages applied to the stator windings of the three-phase synchronous motor are as shown in Figure 4-3. The voltage waveforms in Figures 4-2 and 4-3 are bipolar square waves which are shifted by 120° with respect to each other. The only difference between the voltage waveforms in these two figures is that the phase-2 and phase-3 voltage waveforms are interchanged. The line-to-neutral voltage (phase voltage) waveforms which can be obtained at the outputs of a 180°-modulation three-phase voltage-source inverter are very similar to the voltage waveforms shown in Figures 4-2 and 4-3. Therefore, a 180°-modulation three-phase voltage-source inverter can be used to drive a threephase synchronous motor so that it operates similarly to a stepper motor.

4-4

Operation of a Synchronous Motor as a Stepper Motor

Figure 4-3. Waveforms of the voltages applied to the stator windings of a three-phase synchronous motor to obtain the polarity combinations of Table 4-2 in the inverse order to that in which they are listed.

When the operating frequency of the three-phase voltage-source inverter is 1 Hz, for example, the rotor of the three-phase synchronous motor rotates one incremental step every one sixth of a second. The direction of rotation depends on the phase sequence of the voltages applied to the stator windings of the three-phase synchronous motor (voltage waveforms of Figure 4-2 or Figure 4-3). The angular value of an increment depends on the number of poles in the three-phase synchronous motor. The value of an increment is 60° for a two-pole three-phase synchronous motor, 30° for a four-pole three-phase synchronous motor and so on. Increasing the operating frequency of the three-phase inverter increases the number of incremental steps per second, and therefore, the speed of the three-phase synchronous motor. The speed of the three-phase synchronous motor is equal to the speed at which the magnetic field created by the currents in the stator windings revolves. This is true as long as the strength of the revolving magnetic field is sufficient to maintain the N and S poles of the rotor aligned with the N and S poles of the revolving field. When the operating frequency of the three-phase voltage-source inverter increases while the output voltage remains fixed, the currents in the stator windings of the 4-5

Operation of a Synchronous Motor as a Stepper Motor three-phase synchronous motor decrease since the reactances of the windings increase. This, in turn, decreases the strength of the revolving magnetic field. When a certain frequency is reached, the strength of the revolving magnetic field becomes insufficient to maintain the N and S poles of the rotor aligned with the N and S poles of the revolving field. As a result, the speed of the rotor decreases to a very low value or the rotor simply stops rotating since it is no longer attached to the revolving magnetic field. The rotor of the three-phase synchronous motor can also stop rotating when the operating frequency of the three-phase inverter is suddenly increased. In this case, the speed of the revolving magnetic field increases suddenly but the magnetic field strength is insufficient to accelerate the rotor (on account of its inertia) and so its N and S poles do not remain aligned with the N and S poles of the revolving field. Procedure Summary In the first part of this exercise, you will set up in the Mobile Workstation the equipment required to carry out this experiment. In the second part of this exercise, you will apply dc voltages to the stator windings of a four-pole three-phase synchronous motor and vary the polarities of these voltages according to a given sequence while observing the behaviour of the motor. You will then use the circuit shown in Figure 4-5 to observe the behaviour of the three-phase synchronous motor when it is driven by a 180°-modulation three-phase voltage-source inverter. In the third part, you will vary the operating frequency of the three-phase voltagesource inverter to observe the effects it has on the behaviour and speed of the threephase synchronous motor. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix C of this manual, to obtain the list of the equipment required to carry out this exercise. 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

G

4-6

1. Install the Power Supply, the Enclosure / Power Supply, the Chopper / Inverter, the Power Diodes, the Three-Phase Synchronous Motor/Generator, the Smoothing Inductors, the DC Voltmeter/Ammeter, the AC Voltmeter, and the Resistive Load modules in the Mobile Workstation.

Operation of a Synchronous Motor as a Stepper Motor G

2. Install the Chopper / Inverter Control Unit and the Current/Voltage Isolator in the Enclosure / Power Supply.

G

3. Make sure that the main power switch of the Power Supply is set to the O (OFF) position. Connect the Power Supply to a three-phase wall receptacle.

G

4. Plug the Enclosure / Power Supply line cord into a wall receptacle. Set the rocker switch of the Enclosure / Power Supply to the I (ON) position.

G

5. Make sure that the toggle switches on the Resistive Load module are all set to the O position.

Using a three-phase synchronous motor as stepper motor

G

6. Connect the modules as shown in Figure 4-4. In Figure 4-4, the windings of a three-phase synchronous motor are labelled phase 1 (winding connected to terminals 1 and 4), phase 2 (winding connected to terminals 2 and 5), and phase 3 (winding connected to terminals 3 and 6). The polarities of the dc voltages applied to the phase-1, phase-2, and phase-3 windings of the synchronous motor are negative, negative, and positive, respectively, considering terminals 4, 5, and 6 of the windings as the reference point to determine the dc voltage polarities.

G

7. Make the following settings: On the Power Supply Voltage selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-N On the Three-Phase Synchronous Motor/Generator Toggle switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I (closed) EXCITER control knob . . . . . . . . . . . . . . . . . . . . . . . . . fully CCW On the Power Supply, make sure that the voltage control knob is set to the 0 position then set the main power switch to the I (ON) position. Slowly set the voltage control knob of the Power Supply halfway between the 0 and 10 positions (5% of the ac network line voltage). The rotor of the three-phase synchronous motor may rotate a little and stop at a certain position. Note the position at which the rotor has stopped.

4-7

Operation of a Synchronous Motor as a Stepper Motor

Figure 4-4. Using a three-phase synchronous motor as a stepper motor.

On the Three-Phase Synchronous Motor/Generator, modify the connections so that the polarities of the dc voltages applied to the windings are the same as those indicated in the first row of Table 4-3, while observing the behaviour of the rotor. Describe what happens.

4-8

Operation of a Synchronous Motor as a Stepper Motor VOLTAGE POLARITY PHASE 1

PHASE 2

PHASE 3

+

!

+

+

!

!

+

+

!

!

+

!

!

+

+

!

!

+

Table 4-3. Polarity combinations for the dc voltages applied to the windings of the three-phase synchronous motor.

G

8. On the Three-Phase Synchronous Motor/Generator, modify the connections to obtain the five other polarity combinations given in Table 4-3 in the order in which they are listed, while observing the behaviour of the rotor. Describe what happens when the polarities of the voltages applied to the windings of the three-phase synchronous motor are modified to obtain the combinations given in Table 4-3 in the order in which they are listed.

G

9. The polarities of the dc voltages applied to the phase-1, phase-2, and phase-3 windings of the synchronous motor should be negative, negative, and positive, respectively. This corresponds to polarity combination indicated in the last row of Table 4-3. On the Three-Phase Synchronous Motor/Generator, modify the connections to obtain the five other polarity combinations given in Table 4-3 in the inverse order to that in which they are listed, while observing the behaviour of the rotor. Describe what happens when the polarities of the voltages applied to the windings of the three-phase synchronous motor are modified to obtain the combinations given in Table 4-3 in the inverse order to that in which they are listed.

G 10. On the Power Supply, set the voltage control knob to the 0 position then set the main power switch to the O position.

4-9

Operation of a Synchronous Motor as a Stepper Motor Install the dynamo of the Speed Sensor / Tachometer on the shaft of the Three-Phase Synchronous Motor/Generator. Connect the dynamo to the TRANSDUCER jack of the Speed Sensor / Tachometer. Connect the modules as shown in Figure 4-5.

Figure 4-5. A three-phase synchronous motor drive built with a 180°-modulation voltage-source inverter.

4-10

Operation of a Synchronous Motor as a Stepper Motor G 11. Make the following settings: On the Power Supply Voltage selector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-N On the Three-Phase Synchronous Motor/Generator EXCITER control knob . . . . . . . . . . . . . . . . . . . . . . . mid position On the Chopper / Inverter Control Unit DC SOURCE 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . mid position MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3- 180° On the IGBT Chopper / Inverter module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Interconnection switch S2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I Note: If you are using a MOSFET Chopper / Inverter, the interconnection switch S1 must be set to the I position.

On the Power Diodes module Interconnection switch S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I On the Speed Sensor / Tachometer ROTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . clockwise On the oscilloscope Channel-1 Sensitivity . . . . . . . . . . . . . . . . 5 V/DIV. (DC coupled) Channel-2 Sensitivity . . . . . . . . . . . . . . . . 1 V/DIV. (DC coupled) Vertical Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHOPped Time Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ms/DIV. Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EXTernal Trigger Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . positive (+) Trigger Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . HF REJection

G 12. Figure 4-6 shows an example of the waveforms of the line-to-neutral voltages (phase voltages) at the outputs of a 180°-modulation voltagesource inverter. The dotted lines in this figure divide each cycle of these waveforms into six time intervals. The polarities of the phase 1, 2, and 3 voltages are indicated in the first interval of Figure 4-6. Indicate the polarities of the phase 1, 2, and 3 voltages in each of the subsequent intervals of Figure 4-6.

4-11

Operation of a Synchronous Motor as a Stepper Motor

Figure 4-6. Waveforms of the phase voltages at the outputs of a 180°-modulation three-phase voltage-source inverter.

The polarities (3) indicated in each of the time intervals of Figure 4-6 form a polarity combination. Determine the sequence of polarity combinations from the polarities indicated in each of the time intervals of Figure 4-6 (starting with the left most interval). Record your results in the following lines.

4-12

Operation of a Synchronous Motor as a Stepper Motor Compare the sequence of polarity combinations you found from the phasevoltage waveforms of Figure 4-6 to the sequence of polarity combinations obtained when reading Table 4-3 from top to bottom.

G 13. Figure 4-7 shows another example of the waveforms of the phase voltages at the outputs of a 180°-modulation voltage-source inverter. The phase-1 voltage waveform remains the same but the phase-2 and phase-3 voltage waveforms have been interchanged. The dotted lines in Figure 4-7 divide each cycle of these waveforms into six time intervals. The polarities of the phase 1, 2, and 3 voltages are indicated in the first interval of Figure 4-7. Indicate the polarities of the phase 1, 2, and 3 voltages in each of the subsequent intervals of Figure 4-7.

Figure 4-7. Waveforms of the phase voltages at the outputs of a 180°-modulation three-phase voltage-source inverter.

4-13

Operation of a Synchronous Motor as a Stepper Motor The polarities (3) indicated in each of the time intervals of Figure 4-7 form a polarity combination. Determine the sequence of polarity combinations from the polarities indicated in each of the time intervals of Figure 4-7 (starting with the left most interval). Record your results in the following lines.

Compare the sequence of polarity combinations you found from the phasevoltage waveforms of Figure 4-7 to the sequence of polarity combinations obtained when reading Table 4-3 from bottom to top.

G 14. On the Power Supply, make sure that the voltage control knob is set to the 0 position then set the 24-V ac power switch and the main power switch to the I (ON) position. The POWER ON LED on the Chopper / Inverter module should light up to indicate that the module is correctly powered. Slowly set the voltage control knob of the Power Supply to the 20 position (20% of the ac network line voltage). The three-phase synchronous motor may start to rotate. If so, slightly vary the setting of the DC SOURCE 1 control knob on the Chopper / Inverter Control Unit so that the three-phase synchronous motor stops rotating. Note: Do not allow the three-phase synchronous motor to remain stationary too long, because a fairly high dc current may flow in the motor. This could eventually make the circuit breakers on the Power Diodes module trip.

On the Chopper / Inverter Control Unit, slightly turn the DC SOURCE 1 control knob clockwise so that the three-phase synchronous motor starts rotating at minimum speed. In which direction does the three-phase synchronous motor rotate?

4-14

Operation of a Synchronous Motor as a Stepper Motor Describe the way the three-phase synchronous motor rotates.

G 15. On the Chopper / Inverter Control Unit, slightly turn the DC SOURCE 1 control knob counterclockwise so that the three-phase synchronous motor stops rotating, then continue to turn it counterclockwise until the three-phase synchronous motor again starts rotating at minimum speed. In which direction does the three-phase synchronous motor rotate?

From the observations made so far, does the three-phase synchronous motor operate similarly to a stepper motor? Explain.

Varying the speed of rotation in a three-phase voltage-source inverter synchronous motor drive

G 16. On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob so that the three-phase synchronous motor rotates clockwise at a speed of approximately 150 r/min. On the Power Supply, set the voltage control knob to the 30 position. On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob clockwise until the period of the waveforms of the phase voltage and line current at the outputs of the voltage-source inverter decreases to approximately 30 ms. While doing this, observe the speed of the three-phase synchronous motor indicated by the Speed Sensor / Tachometer. On the Power Supply, set the main power switch to the O position. Describe what happens. Explain.

4-15

Operation of a Synchronous Motor as a Stepper Motor G 17. On the Power Supply, set the main power switch to the I position then slowly set the voltage control knob to the 70 position. Describe what happens. Explain.

G 18. On the Chopper / Inverter Control Unit, slowly turn the DC SOURCE 1 control knob clockwise until the speed indicated by the Speed Sensor / Tachometer increases to approximately 2500 r/min then slowly turn it in the opposite direction until the speed decreases to approximately 1250 r/min. While doing this observe the waveforms of the phase voltage and line current at the outputs of the voltage-source inverter. Describe the relationship between the speed of the three-phase synchronous motor and the operating frequency of the voltage-source inverter.

G 19. On the Chopper / Inverter Control Unit, set the DC SOURCE 1 control knob very rapidly to the MAX. position to rapidly increase the operating frequency of the voltage-source inverter. While doing this, observe the change in the speed of rotation on the Speed Sensor / Tachometer. On the Power Supply, set the main power switch and the 24-V ac power switch to the O position Describe what happens. Explain

G 20. Set the rocker switch on the Enclosure / Power Supply to the O position. Remove all leads, cables, and probes.

4-16

Operation of a Synchronous Motor as a Stepper Motor CONCLUSION In this exercise, you verified that a three-phase synchronous motor operates similarly to a stepper motor when dc voltages are applied to the stator windings according to a predetermined sequence of polarity combinations. You observed that the direction of rotation of the three-phase synchronous motor depends on the order in which the various polarity combinations are applied. You found that the sequence of polarity combinations determined from the waveforms of the line-to-neutral voltages (phase voltages) at the outputs of a 180°modulation three-phase voltage-source inverter is identical to that required to operate a three-phase synchronous motor as a stepper motor. You found that interchanging two of the phase voltages at the inverter outputs is the same as reversing the order of the various polarity combinations in the sequence, and that this reverses the direction of rotation of the three-phase synchronous motor. You observed that the speed of the synchronous motor increases as the operating frequency of the inverter is increased and vice versa. You observed that for a given phase voltage at the outputs of the voltage-source inverter, the three-phase synchronous motor virtually stops rotating when the operating frequency of the inverter reaches a certain value. You observed that the three-phase synchronous motor also virtually stops rotating when the operating frequency of the inverter is rapidly increased. In both cases, the strength of the revolving magnetic field was insufficient to maintain the N and S poles of the rotor aligned with those of the revolving field. REVIEW QUESTIONS 1. What is a stepper motor?

2. Briefly explain the operation of a stepper motor.

3. How is the direction of rotation of a stepper motor reversed?

4-17

Operation of a Synchronous Motor as a Stepper Motor 4. Briefly explain why a three-phase synchronous motor operates similarly to a stepper motor when it is driven by a 180°-modulation three-phase voltagesource inverter.

5. In a three-phase voltage-source inverter synchronous motor drive operating at a constant voltage, explain briefly why the synchronous motor virtually stops rotating when the operating frequency of the inverter reaches a high enough value.

4-18

Appendix

A

Circuit Diagram Symbols Introduction Various symbols are used in many of the circuit diagrams given in the DISCUSSION and PROCEDURE sections of this manual. Each symbol is a functional representation of a device used in power electronics. For example, different symbols represent a variable-voltage single-phase ac power supply, a three-phase thyristor bridge, and a synchronous motor/generator. The use of these symbols greatly simplifies the circuit diagrams, by reducing the number of interconnections shown, and makes it easier to understand operation. For each symbol used in this and other manuals of the Lab-Volt Power Electronics series, this appendix gives the name of the device which the symbol represents and a diagram showing the equipment, and in some cases the connections, required to obtain the device. Notice that the terminals of each symbol are identified using encircled numbers. Identical encircled numbers identify the corresponding terminals in the equipment and connections diagram.

SYMBOL

EQUIPMENT AND CONNECTIONS

A-1

Circuit Diagram Symbols

A-2

Circuit Diagram Symbols

A-3

Circuit Diagram Symbols

A-4

Circuit Diagram Symbols

A-5

Circuit Diagram Symbols

A-6

Circuit Diagram Symbols

A-7

Circuit Diagram Symbols

A-8

Circuit Diagram Symbols

A-9

Circuit Diagram Symbols

A-10

Circuit Diagram Symbols

A-11

Circuit Diagram Symbols

A-12

Appendix

B

Impedance Table for the Load Modules The following table gives impedance values which can be obtained using either the Resistive Load, Model 8311, the Inductive Load, Model 8321, or the Capacitive Load, Model 8331. Figure B-1 shows the load elements and connections. Other parallel combinations can be used to obtain the same impedance values listed.

IMPEDANCE (S)

SWITCH POSITIONS FOR LOAD ELEMENTS

120 V 60 Hz

220 V 50 Hz

240 V 50 Hz

1

2

3

4

5

6

1200

4400

4800

I

600

2200

2400

300

1100

1200

400

1467

1600

I

240

880

960

I

200

733

800

171

629

686

I

150

550

600

I

I

I

I

133

489

533

120

440

480

109

400

436

100

367

400

92

338

369

86

314

343

I

80

293

320

I

75

275

300

71

259

282

I

67

244

267

I

I

63

232

253

60

220

240

57

210

229

7

8

9

I I I I I

I

I

I

I

I I

I I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I I

I I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

Table B-1. Impedance table for the load modules.

B-1

Impedance Table for the Load Modules

Figure B-1. Location of the load elements.

B-2

Impedance Table for the Load Modules

The following table gives inductance values which can be obtained using the Inductive Load module, Model 8321. Figure B-1 shows the load elements and connections. Other parallel combinations can be used to obtain the same inductance values listed.

INDUCTANCE (H)

SWITCH POSITIONS FOR LOAD ELEMENTS

120 V

220 V

240 V

1

2

3

3.20

14.00

15.30

I

1.60

7.00

7.60

0.80

3.50

3.80

1.07

4.67

5.08

I

0.64

2.80

3.04

I

0.53

2.33

2.53

0.46

2.00

2.17

I

0.40

1.75

1.90

I

0.36

1.56

1.69

0.32

1.40

1.52

I

0.29

1.27

1.38

I

0.27

1.17

1.27

0.25

1.08

1.17

0.23

1.00

1.09

I

0.21

0.93

1.01

I

0.20

0.88

0.95

0.19

0.82

0.89

0.18

0.78

0.85

0.17

0.74

0.80

0.16

0.70

0.76

0.15

0.67

0.72

4

5

6

I

I

I

I

I

I

I

I

I

I

7

8

9

I

I

I

I I I I I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I I

I I

Table B-2. Inductance table for the Inductive Load module.

B-3

B-4

Appendix

C

Equipment Utilization Chart The following Lab-Volt equipment is required to perform the exercises in this manual.

EQUIPMENT MODEL

DESCRIPTION

EXERCISE 1

2

3

4

1

1

1

1

1

1

8110

Mobile Workstation

8221

Four-Pole Squirrel-Cage Induction Motor

8241

Three-Phase Synchronous Motor/Generator

8311

Resistive Load

8325

Smoothing Inductors

1

8331

Capacitive Load

2

8341

Single-Phase Transformer

1

8412-1X

DC Voltmeter/Ammeter

8426

AC Voltmeter

8821

1 1

1

1

1

1

1

1

1

1

1

1

1

1

Power Supply

1

1

1

1

8837

MOSFET Chopper / Inverter

1

1

1

1

8837-AX

IGBT Chopper / Inverter

1

1

1

1

8840

Enclosure / Power Supply

1

1

1

1

8842

Power Diodes

1

1

1

1

8931

Speed Sensor / Tachometer

1

1

1

8951

Connection Leads

1

1

1

1

9029

Chopper / Inverter Control Unit

1

1

1

1

9056

Current/Voltage Isolator

1

1

1

1

Additional Equipment Completion of the exercises in this manual requires a dual-trace oscilloscope, Lab-Volt Model 797 or equivalent.

C-1

Bibliography Bühler, H. Électronique de puissance, 2e édition, Paris: Éditions Georgi, 1981. ISBN 2-604-00017-2 Leonhard, W. Control of Electrical Drives, Berlin: Springer-Verlag, 1985. ISBN 3-540-13650-9 Séguier, Guy. L'électronique de puissance, 4e édition, Paris: Dunod, 1979. ISBN 2-04-010821-1 Various authors, SCR Manual Including Triacs and other Thyristors, 6th edition, New York: General Electric Company, 1979. Wildi, Theodore. Electrical Machines, Drives, and Power Systems, 2nd edition, New Jersey: Prentice Hall, 1991. ISBN 0-13-251547-4

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