Three-Phase AC Power Electronics - College of Engineering AC Pow… · Introduction Three-Phase AC Power Electronics ... Exercise 1 Power Diode Three-Phase Rectifiers ... (SI) followed

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Student Manual|3086362000000<~86362-00

Three-Phase AC Power Electronics

Electric Power / Controls

Electric Power / Controls

Three-Phase AC Power Electronics

Student Manual86362-00

A

First Edition Published April 2014

© 2011 by Lab-Volt Ltd. Printed in Canada All rights reserved

ISBN 978-2-89640-499-5 (Printed version) ISBN 978-2-89640-738-5 (CD-ROM)

Legal Deposit – Bibliothèque et Archives nationales du Québec, 2011 Legal Deposit – Library and Archives Canada, 2011

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form by any means, electronic, mechanical, photocopied, recorded, or otherwise, without prior written permission from Lab-Volt Ltd.

Information in this document is subject to change without notice and does not represent a commitment on the part of Lab-Volt. The Lab-Volt® materials described in this document are furnished under a license agreement or anondisclosure agreement.

The Lab-Volt® logo is a registered trademark of Lab-Volt Systems.

Lab-Volt recognizes product names as trademarks or registered trademarks of their respective holders.

All other trademarks are the property of their respective owners. Other trademarks and trade names may be used inthis document to refer to either the entity claiming the marks and names or their products. Lab-Volt disclaims any proprietary interest in trademarks and trade names other than its own.

A Three-Phase AC Power Electronics v

SafetyandCommonSymbols

The following safety and common symbols may be used in this manual and on the Lab-Volt equipment:

Symbol Description

DANGER indicates a hazard with a high level of risk which, if not avoided, will result in death or serious injury.

WARNING indicates a hazard with a medium level of risk which, if not avoided, could result in death or serious injury.

CAUTION indicates a hazard with a low level of risk which, if not avoided, could result in minor or moderate injury.

CAUTION used without the Caution, risk of danger sign , indicates a hazard with a potentially hazardous situation which, if not avoided, may result in property damage.

Caution, risk of electric shock

Caution, hot surface

Caution, risk of danger

Caution, lifting hazard

Caution, hand entanglement hazard

Notice, non-ionizing radiation

Direct current

Alternating current

Both direct and alternating current

Three-phase alternating current

Earth (ground) terminal

SafetyandCommonSymbols

vi Three-Phase AC Power Electronics A

Symbol Description

Protective conductor terminal

Frame or chassis terminal

Equipotentiality

On (supply)

Off (supply)

Equipment protected throughout by double insulation or reinforced insulation

In position of a bi-stable push control

Out position of a bi-stable push control

A Three-Phase AC Power Electronics vii

TableofContents

Preface ...................................................................................................................ix

About This Manual .................................................................................................xi

Introduction Three-Phase AC Power Electronics ........................................... 1

DISCUSSION OF FUNDAMENTALS ....................................................... 1

Exercise 1 Power Diode Three-Phase Rectifiers .......................................... 3

DISCUSSION ..................................................................................... 3 Three-phase half-wave rectifier (positive-polarity output) ........ 3 Three-phase half-wave rectifier (negative-polarity output) ...... 6 Three-phase full-wave rectifier ................................................. 9 Dual-polarity dc power supply ................................................ 14

PROCEDURE ................................................................................... 14 Setup and connections .......................................................... 15 Three-phase half-wave rectifier (positive-polarity output) ...... 15 Three-phase half-wave rectifier (negative-polarity output) .... 20 Three-phase full-wave (bridge) rectifier ................................. 24 Dual-polarity dc power supply ................................................ 28

Exercise 2 The Single-Phase PWM Inverter with Dual-Polarity DC Bus ............................................................................................... 35

DISCUSSION ................................................................................... 35 Operation of a single-phase PWM inverter implemented with a dual-polarity dc bus ..................................................... 36

PROCEDURE ................................................................................... 40 Setup and connections .......................................................... 40 Operation of a single-phase PWM inverter implemented with a dual-polarity dc bus ..................................................... 42 Relationship between output voltage, input voltage, and modulation index .................................................................... 44 Effect of a variation in frequency of the signal that modulates the duty cycle of the switching control signals on the amplitude and frequency of the load voltage. ............. 45

Exercise 3 The Three-Phase PWM Inverter ................................................ 47

DISCUSSION ................................................................................... 47 Operation of the three-phase PWM inverter .......................... 47 Current in the neutral conductor ............................................ 49

TableofContents

viii Three-Phase AC Power Electronics A

PROCEDURE ................................................................................... 51 Setup and connections .......................................................... 51 Operation of a three-phase PWM inverter powered by a dual-polarity dc power supply ................................................ 53 Effect of the neutral conductor on the voltage and current waveforms at the output of the three-phase PWM inverter ... 56

Appendix A Equipment Utilization Chart ...................................................... 61

Appendix B Impedance Table for the Load Modules ................................... 63

Appendix C Circuit Diagram Symbols ........................................................... 65

Acronyms ............................................................................................................. 71

Bibliography ......................................................................................................... 73

We Value Your Opinion!....................................................................................... 75

A Three-Phase AC Power Electronics ix

Preface

The production of energy using renewable natural resources such as wind, sunlight, rain, tides, geothermal heat, etc., has gained much importance in recent years as it is an effective means of reducing greenhouse gas (GHG) emissions. The need for innovative technologies to make the grid smarter has recently emerged as a major trend, as the increase in electrical power demand observed worldwide makes it harder for the actual grid in many countries to keep up with demand. Furthermore, electric vehicles (from bicycles to cars) are developed and marketed with more and more success in many countries all over the world.

To answer the increasingly diversified needs for training in the wide field of electrical energy, Lab-Volt developed the Electric Power Technology Training Program, a modular study program for technical institutes, colleges, and universities. The program is shown below as a flow chart, with each box in the flow chart representing a course.

The Lab-Volt Electric Power Technology Training Program.

Preface

x Three-Phase AC Power Electronics A

The program starts with a variety of courses providing in-depth coverage of basic topics related to the field of electrical energy such as ac and dc power circuits, power transformers, rotating machines, ac power transmission lines, and power electronics. The program then builds on the knowledge gained by the student through these basic courses to provide training in more advanced subjects such as home energy production from renewable resources (wind and sunlight), large-scale electricity production from hydropower, large-scale electricity production from wind power (doubly-fed induction generator [DFIG], synchronous generator, and asynchronous generator technologies), smart-grid technologies (SVC, STATCOM, HVDC transmission, etc.), storage of electrical energy in batteries, and drive systems for small electric vehicles and cars.

A Three-Phase AC Power Electronics xi

AboutThisManual

This manual, Three-Phase AC Power Electronics, introduces the student to the student to power electronic circuits (rectifiers and inverters) used to perform ac/dc power conversion in three-phase circuits. The course begins with the study of three-phase diode rectifiers. The student then becomes familiar with the operation of the single-phase PWM inverter built with a dual-polarity dc bus. The course continues with the operation of the three-phase PWM inverter built with a single-polarity or dual-polarity dc bus. The course concludes with the study of the three-phase PWM inverter.

The equipment for the course mainly consists of the IGBT Chopper/Inverter module. The operation of the IGBT Chopper/Inverter module is controlled by the Lab-Volt LVDAC-EMS software. The Resistive Load, Filtering Inductors/Capacitors, Three-Phase Filter, Power Supply, Rectifier and Filtering Capacitors, and the Data Acquisition and Control Interface are also used to perform the exercises in this manual

Safety considerations

Safety symbols that may be used in this manual and on the Lab-Volt equipment are listed in the Safety Symbols table at the beginning of the manual.

Safety procedures related to the tasks that you will be asked to perform are indicated in each exercise.

Make sure that you are wearing appropriate protective equipment when performing the tasks. You should never perform a task if you have any reason to think that a manipulation could be dangerous for you or your teammates.

Prerequisite

As a prerequisite to this course, you should have read the manuals titled DC Power Circuits, p.n. 86350, DC Power Electronics, p.n. 86356, Single-Phase AC Power Circuits, p.n. 86358, Single-Phase AC Power Electronics, p.n. 86359, and Three-Phase AC Power Circuits, p.n. 86360.

Systems of units

Units are expressed using the International System of Units (SI) followed by the units expressed in the U.S. customary system of units (between parentheses).

A Three-Phase AC Power Electronics 1

Power electronics circuits can be found wherever there is a need to modify a form of electrical energy (i.e., change in voltage, current, or frequency). In modern systems, the conversion is performed with semiconductor switching devices such as diodes, thyristors, and transistors. By contrast with microelectronics systems concerned with transmission and processing of signals and data, substantial amounts of electrical energy are processed in power electronics.

As efficiency is at a premium in power electronics, the losses caused by a power electronic device should be as low as possible. The instantaneous power dissipated in a device is equal to the product of the voltage across the device and the current through it. From this, one can see that the losses of a power device are at a minimum when the voltage across it is zero or when no current flows through it. Therefore, a power electronic converter is built around one or more devices operating in switching mode (either on or off). With such a structure, the energy is transferred from the input of the converter to its output by bursts and with minimal power losses.

For instance, uninterruptible power supplies (UPS) use a battery (dc power) and an inverter to supply ac power when main power is not available. When main power is restored, a rectifier is used to supply dc power to recharge the battery.

In three-phase, variable-speed induction motor drives, a three-phase rectifier and a three-phase inverter are used to convert three-phase fixed-frequency ac power into three-phase variable-frequency ac power that is used to power the drive motor.

Figure 1. Three-phase variable-frequency ac drive.

Three‐PhaseACPowerElectronics

Introduction

DISCUSSIONOFFUNDAMENTALS


When you have completed this exercise, you will be familiar with three-phase half-wave and full-wave rectifiers. You will be familiar with the waveforms of the voltages and currents present in these rectifiers. You will know how to calculate the average dc voltage provided by each type of rectifier. You will know the advantages of three-phase rectifiers over single-phase rectifiers. You will also be introduced to the dual-polarity dc power supply.

The Discussion of this exercise covers the following points:

Three-phase half-wave rectifier (positive-polarity output) Three-phase half-wave rectifier (negative-polarity output) Three-phase full-wave rectifier Dual-polarity dc power supply

Three‐phasehalf‐waverectifier(positive‐polarityoutput)

A three-phase half-wave rectifier with a positive-polarity output converts three-phase ac voltage into positive dc voltage. The rectifier consists of three diodes connected between a three-phase ac power source and a load (resistor ), as Figure 2 shows.

Figure 2. Three-phase half-wave rectifier (positive-polarity output).

Figure 3 shows the waveforms of the circuit voltages and currents in the three-phase half-wave rectifier. The rectifier output voltage is the voltage measured at point X with respect to the neutral terminal N of the three-phase ac power source. Therefore, .

PowerDiodeThree‐PhaseRectifiers

Exercise1

EXERCISEOBJECTIVE

DISCUSSIONOUTLINE

DISCUSSION

Line terminals 1, 2, and 3

Neutral terminal

Three-phaseac power source

Load

Exercise 1 – Power Diode Three-Phase Rectifiers Discussion

4 Three-Phase AC Power Electronics A

Figure 3. Waveforms of the voltages and currents in the three-phase half-wave rectifier (positive-polarity output).

Phase voltages ( , , )

Diode current ( )

Diodecurrent ( )

Diode current ( )

Rectifier output current ( )

Rectifier output

voltage ( )

30

90 210 Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

150 270

330

30

90 210

150 270

330

30 150 30 150

150 270 150 270

30 270 30 270

30 150 270 30 150 270

30 210150 270 330 30 90 210 150 270 330

, 0.827 ,

0.675 ,, ,

(V)

(A)

(A)

(A)

(A)

(V)

0.5 ,



Each diode conducts current when the voltage at its anode is higher than the voltage at its cathode. Whenever a diode stops conducting, another diode immediately starts conducting. Thus, the forward current is interrupted and transferred from one diode to another. The sudden switchover from one diode to another is called natural commutation. Natural commutation occurs at the following phase angles: 30°, 150°, and 270°.

The circuit operates as described below.

Initially, at phase angle 0°, diode is conducting and diodes and

are blocked. When the phase angle reaches 30°, the voltage at the anode of diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction (this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ). Consequently, the rectifier output voltage follows the positive peak of phase voltage . This situation lasts until the phase angle reaches 150°. Between phase angles 30° and 150°, diodes and are blocked since the voltages (phase voltages and , respectively) present at their anodes are both lower than the voltage (phase voltage ) present at their cathodes.

When the phase angle reaches 150°, the voltage at the anode of diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction (this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ). Consequently, the rectifier output voltage follows the positive peak of phase voltage . This situation lasts until the phase angle reaches 270°. Between phase angles 150° and 270°, diodes and are blocked since the voltages (phase voltages and , respectively) present at their anodes are both lower than the voltage (phase voltage ) present at their cathodes.

When the phase angle reaches 270°, the voltage at the anode of diode (phase voltage ) becomes higher than the voltage at its cathode (phase voltage ). Therefore, diode enters into conduction (this causes diode to stop conducting) and current starts flowing from point X toward the neutral terminal N via the load (resistor ). Consequently, the rectifier output voltage follows the positive peak of phase voltage . This situation lasts until the phase angle reaches 30° of the subsequent cycle. Between phase angles 270° and 30°, diodes and are blocked since the voltages (phase voltages and , respectively) present at their anodes are both lower than the voltage (phase voltage ) present at their cathodes.

Each diode allows current to flow through resistor during equal intervals of 120°. Therefore, the waveform of the rectifier output current and voltage ( and ) are composed of three positive pulses of equal duration (120° phase interval each) per cycle of the ac source voltage. The rectifier output voltage varies between the maximum positive value of the phase voltage ( ‐ , ) and (0.5 ‐ , ) This implies that the ripple (amplitude of the pulses) in the output voltage of a three-phase half-wave rectifier is 50% lower than the ripple in the output voltage of single-phase rectifiers. Furthermore, the



ripple frequency of the three-phase half-wave rectifier output voltage is 180 Hz (150 Hz in a 50-Hz ac power network), compared to 60 Hz (50 Hz in a 50-Hz ac power network) for a single-phase half-wave rectifier, and 120 Hz (100 Hz in a 50-Hz ac power network) for a single-phase full-wave rectifier. The lower ripple amplitude and higher ripple frequency result in a smoother voltage at the output of the three-phase rectifier. A smoother voltage is an important advantage in high-power rectifier circuits because this permits the use of smaller semiconductor devices with lower power ratings.

Neglecting the voltage drops across the diodes in the three-phase half-wave rectifier, the amplitude of the rectifier output voltage , is equal to the

amplitude (positive maximum value) of the phase voltage ‐ , of the three-phase ac power source. The average value of the rectifier output voltage , is:

, 0.827 , 0.675 , (1)

where , is the amplitude of the phase voltage.

, is the rms value of the line-to-line voltage.

To conclude, the three-phase half-wave rectifier acts like three single-phase half-wave rectifiers (one for each phase) operating one after another. The phase currents , , and delivered by the three-phase ac power source, which are respectively equal to currents , , and , are asymmetrical, which means that they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.

Three‐phasehalf‐waverectifier(negative‐polarityoutput)

A three-phase half-wave rectifier with a negative-polarity output converts three-phase ac voltage into negative dc voltage. Figure 4 shows the circuit diagram of a three-phase half-wave rectifier with a negative-polarity output. The circuit is identical to that studied in the previous section of this discussion, except that the diodes are connected in the opposite direction. Its operation is thus very similar to that of the three-phase half-wave rectifier with a positive-polarity output.



Figure 4. Three-phase half-wave rectifier (negative-polarity output).

a The + and signs next to voltage in the figure indicate the convention of measurement of this voltage. The value of is negative when the voltage at the positive terminal of load resistor is lower than the voltage at the negative terminal of this resistor (e.g., when 120 ).

Figure 5 shows the waveforms of the circuit voltages and currents in the three-phase half-wave rectifier of Figure 4. Each diode allows current ( ) to flow from the neutral terminal N toward point Y through the load resistor during equal intervals of 120°. The waveform of the rectifier output voltage ( or ) therefore follows the negative peaks of the phase voltages of the three-phase source. This voltage waveform is thus composed of three negative pulses (120° interval each) per cycle of the ac source voltage, the voltage varying between the maximum negative value of the phase voltage ( ‐ , )

and 0.5 ‐ , .

Line terminals


Neutral terminal



Figure 5. Waveforms of the voltages and currents in the three-phase half-wave rectifier (negative-polarity output).

Phase voltages ( , , )

30

90 210 Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

150 270

330

30

90 210

150 270

330

30 150 270 30 150 270

, 0.827 ,

0.675 ,

Waveform obtained with a three-phase half-wave rectifier with a positive-polarity output (shown for comparison)

210 330 210 330

90 330 90 330

90 210 90 210

90 210 330 90 210 330

Waveform obtained with a three-phase half-wave rectifier with a positive-polarity output (shown for comparison)

30 150 270 30 150 270

90 210 330 90 210 330

, ,

(V)

(A)

(A)

(A)

(A)

(V)

0.5 ,

Diode current ( )

Diode current ( )

Diode current ( )

Rectifier output

current ( )

Rectifier output

voltage ( ‐ )



The average value of the rectifier output voltage , , is equal

to 0.827 , or 0.675 , .

Note that the phase angle intervals during which the diodes conduct current in the three-phase half-wave rectifier with a negative-polarity output differ from the phase angle intervals during which the diodes conduct in the three-phase rectifier with a positive-polarity output. This causes the pulses in the output voltage of the three-phase half-wave rectifier with a negative-polarity output to be offset 60° with respect to the pulses in the output voltage of the three-phase half-wave rectifier with a positive-polarity output.

Three‐phasefull‐waverectifier

The three-phase full-wave rectifier, also called a three-phase bridge rectifier, is the most commonly used in industrial applications. The circuit can be viewed as a combination of a three-phase half-wave rectifier with a positive-polarity output and a three-phase half-wave rectifier with a negative-polarity output, as Figure 6a shows. This circuit can be redrawn as shown in Figure 6b (usual representation of a three-phase full-wave rectifier). Terminal N is the neutral conductor of the source. Figure 7 shows the waveforms of the circuit voltages and currents.

The diodes successively conduct current by pairs, one pair after another during equal intervals of 60°, as indicated in Table 1. During each interval, a diode ( ,

, or ) conducts current from X towards the neutral point N through resistor , while another diode ( , , or ) conducts current from the neutral point N towards Y through resistor .

Table 1. Conducting diodes for each 60° interval.

Angular interval Conducting diodes

30° - 90° and

90° - 150° and

150° - 210° and

210° - 270° and

270° - 330° and

330° - 30° and

For example, when the phase angle is between 30°and 90°:

Diode conducts current since the voltage ( ) at its anode is higher than the voltages ( and ) at the anodes of diodes and . This current , flows from X toward N through resistor . On the other hand, diodes and are blocked since the voltages ( and ) at their anodes are lower than the voltage ( ) at their cathodes. The voltage between X and N (EX‐N) therefore follows the positive peak of phase voltage .

Meanwhile, diode also conducts current since the voltage ( ) at its cathode is lower than the voltages ( and ) at the cathodes of diodes and . This current , flows from N towards Y through



resistor . On the other hand, diodes and are blocked since the voltages ( and ) at their cathodes are higher than the voltage ( ) at their anodes. The voltage between Y and N ( ) therefore follows the negative peak of phase voltage .

Figure 6. Three-phase full-wave rectifier.


Line terminals


Line terminals

(a) A three-phase full-wave rectifier can be viewed as a combination of a three-phase half-wave rectifier with a positive-polarity output and a three-phase half-wave rectifier with a negative-polarity output.

(b) Usual representation of a three-phase full-wave rectifier. Terminal N is the neutral conductor of the source.



Figure 7. Waveforms of the voltages and currents in the three-phase full-wave rectifier.

The current resulting from the successive conduction of diodes , , and causes the waveform of the voltage between X and N ( ) to follow the positive peaks of the phase voltages of the three-phase source. The waveform of voltage is therefore identical to that produced by a three-phase half-wave rectifier with a positive-polarity output.

Similarly, the current resulting from the successive conduction of diodes , , and causes the waveform of the voltage between Y and N ( ) to follow

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase voltages ( , , ) 30

90 210

150 270

330

30

90 210

150 270

330

Order of conduction of

the diodes

Voltages ,

and 30

90 210

150 270

330

30

90 210

150 270

330

6 4 5 6 4

V

A

0.675 ,

0.675 , s

30 90 210150 270 330 30 90 210 150 270 330

30 90 210150 270 330 30 90 210 150 270 330

Rectifier output current

Rectifier output

voltage

V

A

V , 1.65 ,

1.35 ,



the negative peaks of the phase voltages of the three-phase ac power source. The waveform of voltage is therefore identical to that produced by a three-phase half-wave rectifier with a negative-polarity output.

The output voltage of the three-phase full-wave rectifier is equal to the sum of (or ). The rectifier output voltage waveform is, therefore, a pulsating positive voltage made of six pulses per cycle. The average value of the rectifier output voltage , is:

, 1.65 , 1.35 , (2)

where , is the amplitude of the phase voltage.

, is the rms value of the line-to-line voltage.

The average current flowing to or from the neutral terminal N is null. Thus, , , , . Therefore, the neutral (N) conductor of the three-phase source is not necessary for proper operation of the three-phase full-wave rectifier. This conductor is shown in Figure 6 to assist in the explanation of circuit operation. Figure 8 shows the rectifier circuit diagram without the neutral conductor. The two load resistors ( in Figure 6) have been replaced by a single resistor .

Figure 8. Three-phase full-wave rectifier without the neutral conductor.

The ripple amplitude in the output voltage and current of a three-phase full-wave rectifier is lower than that observed in a three-phase half-wave rectifier. Also, the ripple frequency is twice that observed in a three-phase half-wave rectifier. Therefore, three-phase full-wave rectifiers are preferred to three-phase half-wave rectifiers because they provide a smoother output voltage and current.

Note that in the three-phase full-wave rectifier, the currents delivered by the three-phase source are symmetrical, i.e., they have a null average (dc) value, which is the normally desired condition. Figure 9 shows the currents flowing through the diodes, the currents delivered by the source, and the rectifier output current. The average (dc) value of each of the currents delivered by the source ( , , and ) is null.

1

4 5 6

RThree-phase ac power source

Line terminals



Figure 9. Waveforms of the diode currents, source currents, and rectifier output current.

Phase angle (°) 30 90 210150 270 330 30 90 210 150 270 330

Rectifier output current

Current 3 ( )

Current 2 ( )

Current 1 ( )

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

Phase angle (°)

30

90 210

270 30

90 210

270

90

150 270

330 90

150 270

330

30

210

150

330

30

210

150

330

90 210 90 210

90 330 90 330

210 330 210 330

30 270 30 270

150 270 150 270

30 150 30 150

Current

Current

Current

Current

Current

Current

(A)

(A)

(A)

(A)

(A)

(A)

(A)

(A)

I (A)

I (A)

Exercise 1 – Power Diode Three-Phase Rectifiers Procedure Outline


Dual‐polaritydcpowersupply

The three-phase full-wave rectifier configuration in Figure 6b shows that a dual-polarity dc power supply can be obtained by simply using the positive ( ) and negative ( ) voltages produced by the rectifier separately. This requires the use of the neutral conductor of the ac power source as it serves as the common point of the dual-polarity dc power supply. The voltage waveforms and are shown in Figure 7. The average value of voltage , i.e., the voltage between the positive terminal and common (neutral) terminal of the dual-polarity dc power supply, is equal to 0.675 , rms. On the other hand, the average value of voltage , i.e., the voltage between the negative terminal and common terminal of the dual-polarity dc power supply, is equal to -0.675 , rms. The average value of the total voltage ( ) produced, i.e., the voltage between the positive and negative terminals of the dual-polarity dc power supply is equal to 1.35 , rms (twice the value of or ).

Figure 10. Dual-polarity dc power supply.

The Procedure is divided into the following sections:

Setup and connections Three-phase half-wave rectifier (positive-polarity output) Three-phase half-wave rectifier (negative-polarity output) Three-phase full-wave (bridge) rectifier Dual-polarity dc power supply

High voltages are present in this laboratory exercise. Do not make of modify anybanana jack connections with the power on unless otherwise specified.

PROCEDUREOUTLINE

PROCEDURE


1

4 5 6

Line terminals

Commonterminal

Negativeterminal

Positive terminal

Exercise 1 – Power Diode Three-Phase Rectifiers Procedure


Setupandconnections

In this part of the exercise, you will set up and connect the equipment.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the exercise.

Install the equipment in the Workstation.

2. Make sure that the ac and dc power switches on the Power Supply are set to the O (off) position, then connect the Power Supply to a three-phase ac power outlet.

3. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Turn the 24 V ac power supply on.

4. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer.

5. Turn the host computer on, then start the LVDAC-EMS software.

In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface is detected. Make sure that the Computer-Based Instrumentation function for the Data Acquisition and Control Interface is available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window.

Three‐phasehalf‐waverectifier(positive‐polarityoutput)

In this part of the exercise, you will set up a three-phase half-wave rectifier with a positive-polarity output. You will observe the waveforms of the voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power. You will compare your results to those that are obtained with a single-phase full-wave rectifier.

6. Set up the circuit shown in Figure 11. In this circuit, is the three-phase ac power source of the Power Supply, Model 8823. E1 through E4 and I1 through I4 are voltage and current inputs of the Data Acquisition and Control Interface. The three diodes are those in the Rectifier and Filtering Capacitors module. Resistor is implemented with the Resistive Load module. The resistance value to be used for this resistor depends on your local ac power network voltage (see table in diagram).



Local ac power network

(Ω) Voltage (V)

Frequency (Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 11. Three-phase half-wave rectifier with a positive-polarity output (observation of voltage and current waveforms and measurement of parameters).

7. Turn the Power Supply on by setting the ac power switch to I (on).

The power rating of resistor in the circuit diagram of Figure 11 is exceeded significantly. Do not leave the ac power source on for periods longer than 10 minutes to avoidexcessive heating of the resistors in the Resistive Load module.

8. In LVDAC-EMS, start the Oscilloscope and make the necessary settings to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Make sure that the time base is set to display at least two cycles of the sine waves.

1

N

1

2

3



9. Describe the waveforms of the rectifier output current and rectifier output voltage with respect to the waveforms of the source phase voltages, and explain.

a The phase currents delivered by the source, which are respectively equal to diode currents , , and , are asymmetrical, which means that they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.

10. During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.



During the positive peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

11. Evaluate the conduction angle of the diodes from the waveforms of currents , , and . Record the conduction angle of the diodes. Then, compare this angle to the conduction angle of the diodes in a single-phase full-wave rectifier.

a The operation of the single-phase full-wave rectifier is covered in the manual Single-Phase AC Power Electronics (part number 86359). You can refer to this manual if necessary.

Conduction angle of the diodes °

12. Measure and record the ripple frequency at the output of the three-phase half-wave rectifier (positive-polarity output). Then, compare this ripple frequency to the ripple frequency at the output of a single-phase full-wave rectifier.

Ripple frequency Hz

13. In LVDAC-EMS, open the Metering window. Set meters E4 and I4 to measure the average (dc) values of the rectifier output voltage and rectifier output current , respectively. Record these values below.



Then, calculate the rectifier output power from the average values of voltage and current .

Average rectifier output voltage , V

Average rectifier output current , A

Rectifier output power , , W

Compare the average output voltage of the three-phase half-wave rectifier (positive-polarity output) to that of a single-phase full-wave rectifier.

14. Set meter E1 to measure the rms value of phase voltage . Record this value below.

‐ , V

Calculate the maximum positive value of phase voltage . Record this value below.

, , √2 V

Compare , to the rectifier output voltage , measured in the

previous step. Is , approximately equal to 0.827 , ?

Yes No

Calculate the line-to-line voltage , and record your result.

, ‐ , √3 V



Yes No

15. On the Power Supply, turn the three-phase ac power source off by setting the corresponding switch to O (off).



Three‐phasehalf‐waverectifier(negative‐polarityoutput)

In this part of the exercise, you will set up a three-phase half-wave rectifier with a negative-polarity output. You will observe the waveforms of the voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power.

16. Set up the circuit shown in Figure 12. The circuit is identical to that studied in the previous section of the procedure, except that the diodes are connected in the opposite direction (i.e., the other three diodes in the Rectifier and Filtering Capacitors module are used, hence the different numbering of the diodes in the diagrams of Figure 11 and Figure 12). Resistor is implemented with the Resistive Load module. The resistance value to be used for resistor depends on your local ac power network voltage (see table in diagram).


(Ω) Voltage (V)

Frequency (Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 12. Three-phase half-wave rectifier with a negative-polarity output (observation of voltage and current waveforms and measurement of parameters).

17. On the Power Supply, turn the three-phase ac power source on.

N

4

1

2

3



The power rating of resistor in the circuit diagram of Figure 12 is exceeded significantly. Do not leave the ac power source on for periods longer than 10 minutes to avoid excessive heating of the resistors in the Resistive Load module.

18. In the Oscilloscope, make sure that the proper settings are made to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Make sure that the time base is set to display at least two cycles of the sine waves.


a Note that the phase currents delivered by the source, which are respectively equal to diode currents , , and , are asymmetrical, i.e., they have a non-null average (dc) value. This results in dc current flow through the ac power source, i.e., through the electrical power network, which is highly undesirable.

20. During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.



During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

During the negative peak of phase voltage , which diode is in the conducting state? Which diodes are blocked? Explain by referring to the observed waveforms.

21. Evaluate the conduction angle of the diodes from the waveforms of currents , , and . Record the conduction angle of the diodes.

Conduction angle of the diodes °

Compare this angle to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 11). Are they the same?

Yes No

22. Measure and record the ripple frequency at the output of the three-phase half-wave rectifier (negative-polarity output).

Ripple frequency Hz

Compare this ripple frequency to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 12). Are they the same?

Yes No



23. In the Metering window, make sure meters E4 and I4 are set to measure the average (dc) values of the rectifier output voltage and rectifier output current respectively. Record these values below.

Then, calculate the rectifier output power from the average values of voltage and current .




Compare the average output voltage of the three-phase half-wave rectifier (negative-polarity output) to that previously obtained for a three-phase half-wave rectifier with a positive-polarity output (recorded in step 13). Are they approximately equal (neglect the voltage polarity)?

Yes No

24. Make sure meter E1 is set to measure the rms value of phase voltage . Record this value below.

, V

Calculate the maximum negative value of phase voltage . Record this value below.

, , √2 V


previous step. Is , approximately equal to 0.827 ∙ , ?

Yes No


, , √3 V

Compare , to the rectifier output voltage measured in the previous

step. Is , approximately equal to 0.675 ∙ , ?

Yes No



25. Is the operation of the three-phase half-wave rectifier with a negative-polarity output similar to that of the three-phase half-wave rectifier with a positive-polarity output? Do these rectifiers have the same conduction angles, ripple frequencies, and average output voltages? Explain.

26. On the Power Supply, turn the three-phase ac power source off.

Three‐phasefull‐wave(bridge)rectifier

In this part of the exercise, you will set up a three-phase full-wave rectifier. You will observe the waveforms of the voltages and currents in the rectifier. You will measure the frequency (ripple) of the rectified voltage, the conduction angle of the diodes, as well as the average values of the rectified voltage, current, and power. You will compare your results to those previously obtained with three-phase half-wave rectifiers.

27. Set up the circuit shown in Figure 13. In this circuit, is the three-phase ac power source of the Power Supply (Model 8823). E1 through E4 and I1 through I4 are voltage and current inputs of the Data Acquisition and Control Interface. The six diodes are those in the Rectifier and Filtering Capacitors module. Resistors and are implemented with the Resistive Load module. The resistance values to be used for these resistors depend on your local ac power network voltage (see table in diagram).

a Use two resistors in series for the rectifier output load. (The resistance value to be used for each resistor is indicated in the table). If a single resistor is used, the nominal voltage of the resistor will be greatly exceeded.



Local ac power network ,

(Ω) Voltage (V)

Frequency (Hz)

120 60 171

220 50 629

240 50 686

220 60 629

Figure 13. Three-phase full-wave rectifier (observation of voltage and current waveforms and measurement of parameters).


The power rating of resistors and in the circuit diagram of Figure 13 is exceeded significantly. Do not leave the ac power source on for periods longer than 10 minutes to avoid excessive heating of the resistors in the Resistive Load module.

29. In the Oscilloscope, make the necessary settings to display the phase voltages (E1, E2, and E3) and phase currents (I1, I2, and I3) of the three-phase ac power source on channels 1, 2, 3, 4, 5, and 6, respectively. Also, display the rectifier output current (I4) and rectifier output voltage (E4) on channels 7 and 8, respectively. Make sure that the time base is set to display at least two cycles of the sine waves.

N

1 2 3

4 5 6

1

2

1

2

3




Observe the waveforms of the phase currents delivered by the source, i.e., , , and . These currents are respectively equal to ,

, and . These currents are symmetrical, i.e., they have a null average (dc) value, which is the normal operating condition desired.

31. Measure and record the ripple frequency at the output of the three-phase full-wave rectifier.

Ripple frequency Hz

Compare this ripple frequency to that previously obtained for a three-phase half-wave rectifier with a positive- or negative-polarity output. Is the ripple frequency of a three-phase full-wave rectifier twice that of a three-phase half-wave rectifier, resulting in a smoother voltage at the output of the three-phase full-wave rectifier?



32. In the Metering window of LVDAC-EMS, make sure meters E4 and I4 are set to measure the average (dc) values of the rectifier output voltage and rectifier output current respectively. Record these values below.

Then, calculate the rectifier output power from the average value and current .




Compare the average output voltage of the three-phase full-wave rectifier to that previously obtained for a three-phase half-wave rectifier with a positive- or negative-polarity output (recorded in steps 13 and 23, respectively).

33. Set meter E1 to measure the rms value of phase voltage . Record this value below.

, V

Calculate the maximum positive value of phase voltage . Record this value below.

, , √2 V



Yes No


, , √3 V

Compare , to the rectifier output voltage measured in the previous

step. Is , approximately equal to 1.35 , ?

Yes No




Dual‐polaritydcpowersupply

In this part of the exercise, you will modify the three-phase full-wave rectifier circuit to obtain a dual-polarity dc power supply. You will determine the voltages at the output of the dual-polarity dc power supply.

35. Set up the circuit shown in Figure 14. In this circuit, is the three-phase ac power source of the Power Supply. E1 through E3 are voltage inputs of the Data Acquisition and Control Interface. The diodes are those in the Rectifier and Filtering Capacitors module. Resistors and are implemented with the Resistive Load module. The resistance values to be used for these resistors depend on your local ac power network voltage (see table in diagram).

Local ac power network ,

(Ω) Voltage (V)

Frequency (Hz)

120 60 300

220 50 1100

240 50 1200

220 60 1100

Figure 14. Dual-polarity dc power supply.

1

4 5 6

1

2

3

N





37. In the Oscilloscope, make the necessary settings to display phase voltage (E1) and the voltages (E2 and E3) at the positive and negative outputs of the dual-polarity dc power supply on channels 1, 2, and 3, respectively. Make sure that the time base is set to display at least two cycles of the sine waves.

38. Describe the waveform of the voltage (input E2) at the positive output of the dual-polarity dc power supply.

39. Is it identical to the waveform of the voltage at the output of a three-phase half-wave rectifier with a positive-polarity output?

Yes No

40. Describe the waveform of the voltage (input E3) at the negative output of the dual-polarity dc power supply.

41. Is it identical to the waveform of the voltage at the output of a three-phase half-wave rectifier with a negative-polarity output?

Yes No

42. In the Metering window of LVDAC-EMS, set meters E2 and E3 to measure the average (dc) values of the voltage at the positive and negative outputs of the dual-polarity dc power supply. Record these values below.

Average voltage at the positive output: V

Average voltage at the negative output: V




Modify the circuit by adding capacitors in parallel with the resistors as shown in Figure 15. The capacitors are those in the Rectifier and Filtering Capacitors module.

Figure 15. Dual-polarity dc power supply with capacitors.



Display the voltage waveforms on the Oscilloscope screen. Describe how the voltage waveforms at the outputs of the dual-polarity dc power supply are affected by the insertion of the filtering capacitors.

45. In the Metering window of LVDAC-EMS, set meters E2 and E3 to measure the average (dc) values of the voltage at the positive and negative outputs of the dual-polarity dc power supply. Record these values below.

Average voltage at the positive output: V

Average voltage at the negative output: V

1

4 5 6

1

2

3

N

Exercise 1 – Power Diode Three-Phase Rectifiers Conclusion


46. Compare the average (dc) values of the voltage measured at the positive and negative outputs of the dual-polarity dc power supply with and without filtering capacitors. What is the effect of adding filtering capacitors?

47. Do your results confirm that splitting the output of a three-phase full-wave rectifier allows a dual-polarity dc power supply to be obtained?

Yes No

48. On the Power Supply, turn the three-phase ac power source off. Close LVDAC-EMS. Disconnect all leads and return them to their storage location.

In this exercise, you studied the operation of three-phase half-wave and full-wave rectifiers. You learned that a three-phase half-wave rectifier uses three diodes to provide a dc voltage composed of three pulses of equal duration per cycle of the ac source voltage. This voltage can be either positive or negative, depending on the direction in which the diodes are connected. This rectifier has a narrower conduction angle (120°) and a higher ripple frequency than single-phase half-wave and full-wave rectifiers, and thus, provides a smoother output voltage. However, the three-phase half-wave rectifier has the following drawback: the phase currents delivered by the source have a non-null average (dc) value, which results in a flow of dc current through the electrical load, but also through the electrical power network, which is highly undesirable. This drawback is eliminated with the use of a three-phase full-wave rectifier. This rectifier uses six diodes to provide a dc voltage composed of six pulses of equal duration per cycle of the ac source voltage. This rectifier provides twice the average voltage of a three-phase half-wave rectifier. Furthermore, this voltage is smoother than that of a three-phase half-wave rectifier and the phase currents delivered by the source have a null average (dc) value, which is the normal operating condition desired. You also learned that a dual-polarity dc power supply can be obtained by simply using the positive and negative voltages produced by a three-phase full-wave rectifier separately. This requires the use of the neutral conductor of the ac power source as it serves as the common point of the dual-polarity dc power supply.

CONCLUSION

Exercise 1 – Power Diode Three-Phase Rectifiers Review Questions


1. What is a three-phase half-wave rectifier with a positive-polarity output? How does it work? Describe the waveform of the rectifier output voltage with respect to the waveform of the source voltage waveform.

2. Is the output voltage of three-phase half-wave rectifiers smoother than the output voltage of single-phase rectifiers? Explain by comparing the amplitude of the ripple in these voltages and the ripple frequency of these voltages.

3. Compare the operation of a three-phase half-wave rectifier with a negative-polarity output to that of a three-phase half-wave rectifier with a positive-polarity output. Do these rectifiers have the same conduction angles, ripple frequencies, and average output voltages?

REVIEWQUESTIONS

Exercise 1 – Power Diode Three-Phase Rectifiers Review Questions


4. What is a three-phase full-wave rectifier? How does it work? Describe the waveform of the rectifier output voltage with respect to the waveform of the source voltage waveform and explain.

5. Give three advantages of three-phase full-wave rectifiers over three-phase half-wave rectifiers.


When you have completed this exercise, you will be familiar with the single-phase PWM inverter with dual-polarity dc bus.


Operation of a single-phase PWM inverter implemented with a dual-polarity dc bus

You have learned previously (in the manual Single-Phase AC Power Electronics, part number 86359) that a single-phase inverter can be implemented using a four-quadrant chopper (see Figure 16). The four electronic switches of the four-quadrant chopper are switched in pairs, that is, with and with . When one pair of electronic switches is on, the other pair is off. Therefore, the input voltage is alternately applied to the output of the four-quadrant chopper through either one of the two pairs of electronic switches. The instantaneous polarity of the output voltage depends on which pair of electronic switches is on. It is positive when electronic switches and are on, and negative when electronic switches and are on.

Figure 16. Four-quadrant chopper used as a single-phase inverter.

TheSingle‐PhasePWMInverterwithDual‐PolarityDCBus

Exercise2

EXERCISEOBJECTIVE

DISCUSSIONOUTLINE

DISCUSSION

Load

Switching control signal generator

Four-quadrant chopper

Filter

+

-

Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus Discussion


The average (dc) output voltage of the four-quadrant chopper during an on-off cycle depends on the time each pair of electronic switches is on during this cycle. This, in turn, depends on the duty cycle of the switching control signals. Modulating the duty-cycle using a sine-wave signal allows a sine wave to be produced at the output of the four-quadrant chopper. Naturally, some filtering is required to obtain a sine wave at the chopper output (see Figure 19).

Operationofasingle‐phasePWMinverterimplementedwithadual‐polaritydcbus

Figure 17 shows a simplified diagram of a single-phase inverter implemented with a dual-polarity dc bus (i.e., a dual-polarity dc power supply). The single-phase PWM inverter consists of two electronic switches ( and ), two free-wheeling diodes ( and ), and a switching control signal generator. The operation of the single-phase inverter implemented with a dual-polarity dc bus is quite similar to that of a single-phase inverter implemented using a four-quadrant chopper. However, the single-phase inverter implemented with a dual-polarity dc-bus requires only two electronic switches to obtain ac voltage and current at the inverter output.

Figure 17. Single-phase inverter implemented with a dual-polarity dc bus.

Dual-polarity dc power supply Single-phase inverter


Load

/2

/2

N

1

2

3

+

-

Filter



In single-phase inverters implemented with a dual-polarity dc bus, the two switching control signals are complementary to ensure that when one electronic switch is on, the other is off, and vice versa. For instance, when electronic switch is on, a positive voltage ( 2⁄ ) is applied to the load and current flows in the load in the direction shown in Figure 18a. Conversely, when electronic switch is on, a negative voltage ( 2⁄ ) is applied to the load and current flows in the load in the direction shown in Figure 18b.

Figure 18. Current flow in a single-phase inverter implemented with two electronic switches.

The voltage waveform at the output of the single-phase inverter depends on the signal that modulates the duty cycle of the switching control signals. When a

Load

/2

N

1

2

3

+

-

Load

/2

N

1

2

3

+

-

Load current flow

Load current flow



sine-wave signal is used to modulate the duty cycle (i.e., and ) of the switching control signals, the voltage waveform at the single-phase inverter output is a train of rectangular bipolar pulses whose width varies in accordance with the instantaneous voltage of the sine-wave signal. The dashed line drawn over the train of rectangular bipolar pulses in Figure 19 shows the average voltage over each cycle of the rectangular bipolar pulse train at the inverter output. This voltage is an ac voltage having the same form (sinusoidal) as the signal applied to the duty-cycle control input of the single-phase inverter.

The range over which the width of the rectangular bipolar pulses at the single-phase inverter output varies, depends on the sine-wave signal (duty-cycle control signal) amplitude. Increasing the sine-wave signal amplitude increases the range of variation of the pulse width, and therefore, the amplitude of the ac voltage at the single-phase inverter output. In other words, the amplitude of the ac voltage at the output of the single-phase inverter is proportional to the modulation index m. The amplitude , of the voltage sine wave at the inverter output depends on both the input voltage , i.e., 2⁄ , and the modulation index m.

, ∙ 2⁄ ∙ (3)

where , is the amplitude of the voltage sine wave at the single-phase

PWM inverter output, expressed in V

is half the average (dc) voltage at the single-phase PWM inverter input ( 2⁄ ), expressed in V

m is the modulation index (pure number)

is the average (dc) voltage at the single-phase PWM inverter input, expressed in V

Equation (4) shows how the above equation can be modified to calculate the rms value of the voltage sine wave at the single-phase PWM inverter output.

,∙

√2

2⁄ ∙

√2 (4)

The rate at which the pulse width varies at the single-phase inverter output depends on the frequency of the modulating sine-wave signal. Increasing the sine-wave signal frequency increases the rate at which the pulse width varies, and therefore, the frequency of the ac voltage at the single-phase inverter output.

In many applications, a voltage whose waveform is a train of rectangular bipolar pulses (instead of a sine wave) can affect the operation of devices sensitive to electromagnetic interference (EMI). For this reason, a filter made of an inductor and a capacitor is usually added at the output of the single-phase inverter to smooth the current and voltage waveforms. This results in a sinusoidal voltage waveform (see Figure 19). The current waveform is similar to the voltage waveform when the load is purely resistive as shown in Figure 19.



Figure 19. Waveforms related to a single-phase PWM inverter implemented with a dual-polarity dc power supply.

Du

ty-c

ycle

co

ntro

l inp

ut s

igna

l O

utpu

t vol

tage

be

fore

filte

ring

O

utpu

t vol

tage

af

ter

filte

ring

Out

put c

urr

ent

Average value

∝ ∝

Time

Time

Time

Time

Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure Outline



Setup and connections Operation of a single-phase PWM inverter implemented with a dual-

polarity dc bus Relationship between output voltage, input voltage, and modulation

index Effect of a variation in frequency of the signal that modulates the duty

cycle of the switching control signals on the amplitude and frequency of the load voltage.


Setupandconnections





3. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply.

Connect the Low Power Input of the Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on.



In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface is detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the Data Acquisition and Control Interface are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac

PROCEDUREOUTLINE

PROCEDURE

Notice that the prefix IGBT has been left out in this manual when referring to the IGBT Chopper/Inverter module.

Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus Procedure


power network, then click the OK button to close the LVDAC-EMS Start-Up window.

6. Set up the circuit shown in Figure 20. Use the diodes and the capacitors in the Rectifier and Filtering Capacitors to implement the diode rectifier and capacitors and . Use the inductor and the capacitor of the Filtering Inductors/Capacitors module to implement inductor , and capacitor . Resistor is implemented with the Resistive Load module. The resistance value to be used for this resistor depends on your local ac power network voltage (see table in diagram).


(Ω)

(mH)

(µF) Voltage (V)

Frequency (Hz)

120 60 171 2 5

220 50 629 8 1.5

240 50 686 8 1.5

220 60 629 8 1.5

Figure 20. Single-phase PWM inverter implemented with a dual-polarity dc bus.

7. Connect the Digital Outputs of the Data Acquisition and Control Interface (DACI) to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable.

Single-phase inverterDual-polarity dc power supply

Filter

Switching control signals from DACI

1

2

3 N



Connect Switching Control Inputs 1 and 4 of the Chopper/Inverter to Analog Inputs 1 and 2 of the Data Acquisition and Control Interface using miniature banana plug leads. These connections allow the observation of the switching control signals of the electronic switches in the Chopper/Inverter.

Connect the common (white) terminal of the Switching Control Inputs on the Chopper/Inverter to one of the two analog common (white) terminals on the Data Acquisition and Control Interface using a miniature banana plug lead.

On the Chopper/Inverter, set the Dumping switch to the O (off) position. The Dumping switch is used to prevent overvoltage on the dc bus of the Chopper/Inverter. It is not required in this exercise.

Operationofasingle‐phasePWMinverterimplementedwithadual‐polaritydcbus

In this part of the exercise, you will use the circuit shown in Figure 20 to observe the operation of a single-phase PWM inverter implemented with a dual-polarity dc bus. You will first observe the switching control signals. Then, you will vary the frequency and the amplitude of the sine-wave signal modulating the duty cycle of the switching control signals to observe the effects they have on the switching control signals as well as the waveforms of the voltage and current at the output of the single-phase PWM inverter.

8. In LVDAC-EMS, open the Chopper/Inverter Control window and make the following settings:

Set the Function parameter to Single-Phase, PWM Inverter. This setting allows the Data Acquisition and Control Interface to generate the switching control signals required by a single-phase PWM inverter implemented with a dual-polarity dc bus.

Set the DC Bus parameter to Bipolar. This setting causes the switching control signal generator to produce signals for electronic switches and only. When the DC Bus parameter is set to Unipolar, the switching control signal generator produces signals for electronic switches , , , and as required by single-phase inverters implemented with a single-polarity dc bus.

Set the Switching Frequency parameter to 4000 Hz.

Set the Frequency parameter to the frequency of your local ac power network. This parameter sets the frequency of the signal that modulates the duty cycle of the switching control signals.

Set the Peak Voltage parameter to 50%. This parameter sets the modulation index m, i.e., it sets the amplitude of the signal that modulates the duty cycle of the switching control signals. When the Peak Voltage parameter is set to 50%, the amplitude of the modulating signal is set to make the duty cycle vary over the span (25% to 75%) to obtain a peak output voltage corresponding to 50% of the half dc bus voltage (50% of /2).



Start the Single-Phase, PWM Inverter.

9. Turn the Power Supply on.

10. In LVDAC-EMS, open the Oscilloscope window and use channels 1 and 2 to display the switching control signals of electronic switches (AI-1) and

(AI-2), channel 3 to display the dc bus voltage (E1), channel 4 to display the voltage at the output of the single-phase inverter before filtering (E2), channel 5 to display the voltage at the output of the single-phase inverter after filtering (E3), and channel 6 to display the current flowing through the load (I1).

Select the Continuous Refresh mode, set the time base to 2 ms/div, and set the trigger controls so that the Oscilloscope triggers when the load voltage waveform (Ch5) passes through 0 V with a positive (rising) slope.

Select convenient vertical scale and position settings to facilitate observation of the waveforms.

Finally, set the Oscilloscope so that the waveforms are displayed on the screen using staircase steps (squared display mode).

11. Print or save the waveforms displayed on the Oscilloscope screen for future reference. It is suggested that you include these waveforms in your lab report.

12. Are the switching control signals of electronic switches and displayed by channels 1 and 2 on the Oscilloscope screen complementary?

Yes No

13. Is the voltage at the output of the single-phase inverter before filtering (E2), displayed by channel 4 on the Oscilloscope screen, pulse-width modulated as shown in Figure 19?

Yes No

14. Is the voltage at the output of the single-phase inverter after filtering (load voltage), displayed on channel 5, sinusoidal as shown in Figure 19?

Yes No



Relationshipbetweenoutputvoltage,inputvoltage,andmodulationindex

In this step you will vary the modulation index (Peak Voltage parameter) and observe the effect on the amplitude of the voltage at the output of the single-phase PWM inverter.

15. Measure the average (dc) value of the dc bus voltage, i.e., the voltage between the positive and negative terminals of the dual-polarity dc power supply.

DC bus voltage ( ): V

16. For each modulation index m in Table 2, calculate the amplitude of the voltage at the output of the single-phase PWM inverter ( , ) using the dc bus voltage you measured in the previous step. Record your results in Table 2.

Table 2. Relationship between the output voltage, input voltage, and modulation index.

DC bus voltage

(V)

Modulation index m

Amplitude of the inverter output voltage

[calculated] (V)

Amplitude of the inverter output voltage

[measured] (V)

0

0.2

0.4

0.6

0.8

1.0

17. In the Chopper/Inverter Control window, successively set the modulation index (Peak Voltage parameter) to each value shown in Table 2. For each value, measure the amplitude of the voltage at the output of the single-phase PWM inverter (after filtering) and record the values in the table.

Are the amplitude values of the voltage measured at the output of the single phase PWM inverter ( , ) approximately equal to the calculated values, confirming that , /2 ?

Yes No

Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus Conclusion


Effectofavariationinfrequencyofthesignalthatmodulatesthedutycycleoftheswitchingcontrolsignalsontheamplitudeandfrequencyoftheloadvoltage.

In this step, you will vary the Frequency parameter in the Chopper/Inverter Control window and observe the effect on the amplitude and frequency of the voltage at the output of the single-phase PWM inverter.

18. In the Chopper/Inverter Control window set the Peak Voltage parameter to 50%.

Vary slowly the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) from 10 Hz to 120 Hz and from 120 Hz to 10 Hz while observing the amplitude and frequency of the voltage at the output of the single-phase PWM inverter (after filtering).

Describe the relationship between the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) and the frequency of the voltage at the output of the single-phase PWM inverter (after filtering).

19. Does the frequency of the sine-wave signal modulating the duty cycle of the switching control signals (Frequency parameter) affect the amplitude of the voltage at the output of the single-phase PWM inverter (after filtering)?

Yes No

20. From your observations, is it possible to convert dc power into ac power using a single-phase PWM inverter implemented with a dual-polarity dc bus and two electronic switches?

Yes No

21. Stop the voltage source and the Single-Phase, PWM Inverter.

Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.

In this exercise, you saw that a single-phase PWM inverter can be implemented with a dual-polarity dc bus and only two electronic switches. You observed that the frequency and amplitude of the voltage at the output of a single-phase PWM inverter are respectively proportional to the frequency and amplitude of the sine-wave signal modulating the duty cycle. You verified the relationship between the amplitude of the voltage at the inverter output and the average voltage of the dual-polarity dc bus.

CONCLUSION

Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus Review Questions


1. What type of power supply is required to implement a single-phase PWM inverter with two electronic switches?

2. How can the amplitude and frequency of the voltage at the output of a single-phase PWM inverter be varied?

3. What is the voltage waveform at the output of a single-phase PWM inverter when a sine-wave signal is used to modulate the duty cycle of the switching control signals?

4. What is the use of the filter made of an inductor and a capacitor that is usually added at the output of a single-phase PWM inverter?

5. A single-phase PWM inverter with a dual-polarity dc bus supplies power to a 230 V load. What modulation index is required knowing the voltage between the positive and negative terminals of the dual-polarity dc bus is 800 V?

REVIEWQUESTIONS


When you have completed this exercise, you will be familiar with the three-phase PWM inverter.


Operation of the three-phase PWM inverter Current in the neutral conductor

Operationofthethree‐phasePWMinverter

Basically, the three-phase PWM inverter consists of three single-phase PWM inverters powered by a dual-polarity dc bus. As Figure 21 shows, the three-phase PWM inverter contains three pairs of electronic switches ( and , and , and and ), six free-wheeling diodes ( to ), and a switching control signal generator.

Figure 21. The three-phase PWM inverter consists of three single-phase PWM inverters.

TheThree‐PhasePWMInverter

Exercise3

EXERCISEOBJECTIVE

DISCUSSIONOUTLINE

DISCUSSION

Dual-polarity dc power supply

Three-phase PWM inverter


Phase 1

Phase 2

Phase 3

N

Three-phase load

1

2

3

Neutral conductor

Three-phase filter

2

2

Exercise 3 – The Three-Phase PWM Inverter Discussion


The two signals that the switching control signal generator produces for each pair of electronic switches are complementary rectangular pulses to ensure that when one electronic switch in a pair is on, the other electronic switch in this pair is off, and vice versa. Each of the three pairs of electronic switches operates the same way: a positive voltage ( /2 is applied to the load when the upper electronic switch ( , or ) is on, whereas a negative voltage (- /2 is applied to the load when the lower electronic switch ( , or ) is on.

The voltage waveform at the outputs of the three-phase PWM inverter depends on the waveform of the signal that modulates the duty cycle of the switching control signals. When a sine-wave signal is used to modulate the duty cycle of the switching control signals, the voltage waveform at the three-phase PWM inverter outputs consists of three trains of rectangular bipolar pulses whose width varies in accordance with the instantaneous voltage of the modulating sine-wave signal. The average voltage of each on-off cycle of the rectangular bipolar pulse trains at the inverter outputs thus also varies sinusoidally.

As for the single-phase inverter, a filter made of inductors and capacitors (see example in Figure 25) is usually added at the outputs of the three-phase-PWM inverter to smooth the voltage and current waveforms. This results in sinusoidal voltage waveforms (when a sine-wave signal is used to modulate the duty cycle of the switching control signals and ideal filtering is assumed) at the outputs of the three-phase PWM inverter. The three-pairs of complementary switching control signals used with the three pairs of electronic switches in the thee-phase PWM inverter are phase shifted by 120° with respect to each other. Consequently, the sinusoidal voltage waveforms at the outputs of the three-phase PWM inverter are also phase shifted by 120° with respect to each other as shown by voltage waveforms , , and in Figure 22. Note that ideal filtering at the PWM inverter outputs is assumed in Figure 22 as the voltage waveforms shown are pure sine waves. The current waveforms are similar to the voltage waveforms when the load is purely resistive as shown by current waveforms , , and in Figure 22. The outputs of the three-phase PWM inverter are usually connected to loads that are both balanced and similar in nature.

The amplitude of the sinusoidal voltages at the outputs of the three-phase PWM inverter can be varied by varying the amplitude of the sine-wave signal modulating the duty cycle of the switching control signals. The amplitude of the voltage at each output of the three-phase PWM inverter , is the same as with a single-phase PWM inverter with dual-polarity dc bus, it is proportional to the modulation index.

The frequency of the voltages at the output of the three-phase PWM inverter can be varied by varying the frequency of the sine-wave signal modulating the duty cycle of the switching control signals.



Figure 22. Voltage and current waveforms at the outputs of a three-phase PWM inverter (ideal filtering is assumed).

Currentintheneutralconductor

When ideal filtering at the outputs of a three-phase PWM inverter is assumed, the line currents are pure sine waves that have the same amplitude, same frequency, and are phase-shifted by 120° with respect to each other, and their sum is null as shown in the phasor diagram of Figure 23. The neutral conductor can thus be removed without disturbing the operation of the three-phase PWM inverter.

Figure 23. Phasor diagram showing that the sum of the line currents in a three-phase PWM inverter is null (ideal filtering assumed at the PWM inverter outputs).

30

90 210

Phase angle (°)

150 270

330

30

90 210

150 270

330

E (V)

30

90 210

Phase angle (°)

150 270

330

30

90 210

150 270

330

(A) In

vert

er o

utpu

t vol

tage

s

(1,

2,

3)

Inve

rter

out

put c

urre

nts

(,

,

)

0° 180°

270°

90°



Actual filters used in the three-phase PWM inverters, however, are not ideal. Consequently, the voltage and current waveforms at the three-phase PWM inverter outputs are slightly distorted sine waves. This also causes a residual current to flow into the neutral conductor. However, since the residual current in the neutral conductor is useless from an operational point of view, this does not prevent the neutral conductor from being removed. In fact, removing the neutral conductor eliminates the residual current and improves the voltage and current waveforms at the three-phase PWM inverter outputs.

Removing the neutral conductor also eliminates the need for a dual-polarity dc power supply. Figure 24 shows a diagram of a three-phase PWM inverter without a neutral conductor and powered by a single-polarity dc power supply.

Figure 24. Three-phase PWM inverter without neutral conductor and supplied by a single-polarity dc power supply.

Three-phase PWM inverter

Phase 1

Phase 2

Phase 3

Three-phase load


Single-polarity dc power supply

Three-phase filter

Exercise 3 – The Three-Phase PWM Inverter Procedure Outline



Setup and connections Operation of a three-phase PWM inverter powered by a dual-polarity dc

power supply Effect of the neutral conductor on the voltage and current waveforms at

the output of the three-phase PWM inverter


Setupandconnections





3. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply.

Connect the Low Power Input of the Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on.



In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface is detected. Make sure that the Computer-Based Instrumentation and Chopper/Inverter Control functions for the Data Acquisition and Control Interface are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window.

PROCEDUREOUTLINE

PROCEDURE

Exercise 3 – The Three-Phase PWM Inverter Procedure


6. Set up the circuit shown in Figure 25. Use the diodes and capacitors in the Rectifier and Filtering Capacitors to implement rectifier and capacitors and . Use the inductors and capacitors of the Three-Phase Filter to implement inductors , , and as well as capacitors , , , and . Resistors , , and are implemented with the Resistive Load module. The resistance value to be used for these resistors depends on your local ac power network voltage (see table in diagram).

Local ac power network , ,

(Ω) , , (mH)

, , ,

(µF) Voltage (V)

Frequency (Hz)

120 60 171 2 5

220 50 629 8 1.5

240 50 686 8 1.5

220 60 629 8 1.5

Figure 25. Three-phase PWM inverter.

7. Connect the Digital Outputs of the Data Acquisition and Control Interface (DACI) to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable.

Connect Switching Control Inputs 1 to 6 of the Chopper/Inverter to Analog Inputs 1 to 6 of the Data Acquisition and Control Interface using miniature banana plug leads. These connections allow the observation of the switching control signals of the electronic switches in the Chopper/Inverter.

N

Switching control signals from DACI

Dual-polarity dc power supply Three-phase inverter Three-phase filter

1

2

3



Connect the common (white) terminal of the Switching Control Inputs on the Chopper/Inverter to one of the two analog common (white) terminals on the Data Acquisition and Control Interface using a miniature banana plug lead.

On the Chopper/Inverter, set the Dumping switch to the O (off) position. The Dumping switch is used to prevent overvoltage on the dc bus of the Chopper/Inverter. It is not required in this exercise.

Operationofathree‐phasePWMinverterpoweredbyadual‐polaritydcpowersupply

In this part of the exercise, you will use the circuit shown in Figure 25 to observe the operation of a three-phase PWM inverter powered by a dual-polarity dc power supply. You will observe the switching control signals of the electronic switches, and the waveforms of the voltages and currents at the output of the inverter with and without the neutral conductor.

8. In the Chopper/Inverter Control window of LVDAC-EMS, make the following settings:

Set the Function parameter to Three-Phase, PWM Inverter. This setting allows the Data Acquisition and Control Interface to generate the switching control signals required by a three-phase PWM inverter.

Set the Switching Frequency parameter to 400 Hz. This will allow observation of the switching control signals in the three-phase PWM inverter, using the Oscilloscope.

Set the Phase Sequence parameter to Fwd (1-2-3). This parameter sets the phase sequence [Fwd (1-2-3) or Rev (1-3-2)]. The phase sequence Fwd (1-2-3) causes a three-phase motor supplied by the three-phase PWM inverter to rotate in the forward direction.

Set the Frequency parameter to the frequency of your local ac power network.

Set the Peak Voltage parameter to 90%. This parameter sets the modulation index, i.e., it sets the amplitude of the signal that modulates the duty cycle of the switching control signals. When the Peak Voltage parameter is set to 90%, the amplitude of the modulating signal is set to make the duty cycle vary to obtain a peak output voltage corresponding to 90% of the half dc bus voltage (45% of ).

Note that parameters Q1 to Q6 are set to PWM.

Start the Three-Phase, PWM Inverter.

9. Turn the Power Supply on.



10. In LVDAC-EMS, open the Oscilloscope window and use channels 1 through 6 to display the switching control signals of electronic switches

(AI-1) to (AI-6).

Select the Continuous Refresh mode, set the time base to 5 ms/div, and set the trigger controls so that the Oscilloscope triggers on the rising edge of the switching control signal of electronic switch (Ch1).

Select convenient vertical scale and position settings to facilitate observation of the waveforms.

Finally, set the Oscilloscope so that the waveforms are displayed on the screen using staircase steps (squared display mode).


12. Do your observations confirm that the switching control signals in each pair of electronic switches - , - , and - are complementary?

Yes No

Do your observations confirm that the switching control signals of consecutive pairs of electronic switches seem to be phase shifted by 120°?

Yes No

13. In the Chopper/Inverter Control window of LVDAC-EMS, make the following settings:

Set the Switching Frequency parameter to 20 000 Hz

Set the Peak Voltage parameter to 100%.

14. In the Oscilloscope window, set channels 1 through 3 to display the phase voltages at the three-phase PWM inverter outputs (E1, E2, and E3), channel 4 to display the dc bus voltage (E4), channels 5 through 7 to display the line currents at the inverter outputs (I1, I2, and I3), and channel 8 to display the current in the neutral conductor (I4).

Set the trigger controls so that the Oscilloscope triggers when the load voltage waveform (Ch1) passes through 0 V with a positive slope.

Make sure that the Continuous Refresh mode is selected. Select convenient vertical scale and position settings to facilitate observation of the waveforms.

Finally, set the Oscilloscope so that the waveforms are displayed as usual (normal display mode).




16. Are the waveforms of the phase voltages at the output of the three-phase PWM inverter sinusoidal, balanced, and phase shifted by 120° with respect to each other?

Yes No

17. In the Chopper/Inverter Control window, use the Peak Voltage control knob to slowly vary the Peak Voltage parameter from 10% to 100% while observing the phase voltages at the three-phase PWM inverter output.

Do your observations confirm that the Peak Voltage parameter (i.e., the modulation index m) controls the amplitude of the phase voltages at the output of the three-phase PWM inverter?

Yes No

18. In the Chopper/Inverter Control window, set the Peak Voltage parameter to 40% of half the dc bus voltage (modulation index 0.4).

Using the Frequency control knob, slowly vary the Frequency parameter from 10 Hz to 120 Hz while observing the phase voltages at the output of the inverter.

Do your observations confirm that the Frequency parameter controls the frequency of the phase voltages at the output of the three-phase PWM inverter?

Yes No

19. Set the Frequency parameter to the frequency of your local ac power network.

Measure the average (dc) value of the dc bus voltage, i.e., the voltage between the positive and negative terminals of the dc power supply.


20. Measure the amplitude of phase 1 voltage (Ch1) at the output of the three-phase PWM inverter (after filtering) and record the value.

a Since the phase voltages at the output of the three-phase PWM inverter are identical, this observation can also be done using phase 2 and phase 3 voltages.

Amplitude of phase 1 voltage at the output of the three-phase PWM inverter (after filtering): V



21. Compare the amplitude of phase 1 voltage at the output of the three-phase PWM inverter (after filtering) to the amplitude of the voltage at the output of the single-phase PWM inverter (after filtering) measured in the previous exercise when the modulation index is set to 0.4 (see Table 2). Do the values confirm that the amplitude of the voltage at each output of the three-phase PWM inverter is the same as the amplitude of the voltage at the output of a single-phase PWM inverter with dual-polarity dc bus?

Yes No

Effectoftheneutralconductoronthevoltageandcurrentwaveformsattheoutputofthethree‐phasePWMinverter

In the next steps, you will compare the voltage and current waveforms at the output of the three-phase PWM inverter with and without neutral conductor between the load and the power supply.

22. In the Oscilloscope window, close channels 2, 3, 4, 6, and 7 to display one of the phase voltage at the three-phase PWM inverter output and the corresponding line current as well as the current in the neutral conductor.

Position and set the scale of channels 1 and 5 so that each waveform covers approximately half of the Oscilloscope screen. Position channel 8 at the bottom of the Oscilloscope screen. These settings facilitate the observation of the waveforms at the inverter output in order to compare the effect the neutral conductor may have on the phase voltages and line currents.


24. Are the voltage and current at the three-phase PWM inverter output pure sine waves? Explain why some current flows through the neutral conductor?

25. Turn the Power Supply off.

Remove the neutral conductor (represented by a red line in Figure 25) between the load and the positive terminal of input I4 on the Data Acquisition and Control Interface.

Turn the Power Supply on.




27. Compare the amplitude and frequency of the phase voltage and line current at the output of the three-phase PWM inverter with and without the neutral conductor. Do your observations confirm that the neutral conductor between the load and the power supply can be removed without affecting the amplitude and frequency of the phase voltages and line currents at the output of the inverter?

Yes No

28. Compare the waveforms of the phase voltages and line currents at the output of the inverter with and without the neutral conductor. Describe how the waveforms are affected when the neutral conductor is removed.

29. Does removing the neutral conductor improve the waveforms of the phase voltage and line current at the three-phase PWM inverter output?

Yes No

30. Measure the average (dc) value of the dc bus voltage, i.e., the voltage between the positive and negative terminals of the dc power supply.


31. Measure the amplitude of phase 1 voltage at the output of the three-phase PWM inverter (after filtering) and record the value.

Amplitude of phase 1 voltage at the output of the three-phase PWM inverter (after filtering): V

32. Compare the amplitude of phase 1 voltage at the output of the three-phase PWM inverter (after filtering) measured with and without neutral conductor (step 20 and step 31 respectively). Do the measured values confirm that the neutral conductor can be removed without negatively affecting the operation of the three-phase PWM inverter?

Yes No

Exercise 3 – The Three-Phase PWM Inverter Conclusion


33. From the observations you made in this section of the exercise, can you conclude that a dual-polarity power supply is not required to supply a three-phase PWM inverter? Explain.

Yes No

34. Stop the voltage source and the Three-Phase, PWM Inverter.

Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.

In this exercise, you observed that the switching control signals of each pair of electronic switches in a three-phase PWM inverter are complementary and that the switching control signals of one pair is phase shifted by 120° with respect to those of the other pairs.

You observed that the waveforms of the phase (line-to-neutral) voltages at the outputs of the three-phase PWM inverter are phase shifted by 120° from one another. You also observed that when the load is purely resistive, the waveforms of the line currents at the outputs of the three-phase PWM inverter are similar to the phase voltage waveforms. You saw that the amplitude and frequency of the voltages at the output of the three-phase PWM inverter can be varied by respectively varying the amplitude (i.e., the modulation index) and frequency of the sine-wave signal modulating the duty cycle of the switching control signals.

You saw that residual current flows in the neutral conductor when a three-phase inverter is powered with a dual-polarity dc power supply. You also observed that removing the neutral conductor improves the waveforms of the phase voltages and line currents at the three-phase PWM inverter outputs (they become almost pure sine waves) without affecting the operation of the three-phase PWM inverter.

1. What is the phase shift between the waveforms of the phase voltages at the outputs of a three-phase PWM inverter?

2. What determines the voltage waveform at the outputs of a three-phase PWM inverter (after filtration)?

CONCLUSION

REVIEWQUESTIONS

Exercise 3 – The Three-Phase PWM Inverter Review Questions


3. How can the amplitude and frequency of the voltages at the output of a three-phase PWM inverter be varied?

4. Why is the sum of the line currents at the outputs of a three-phase PWM inverter not null even when the load is balanced?

5. Is it possible to remove the neutral conductor in a three-phase PWM inverter powered by a dual-polarity dc power supply? If so, does this have any effect on the waveforms of the phase voltages and line currents at the three-phase PWM inverter outputs? Briefly explain.


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

Equipment Exercise

Model Description 1 2 3

8131(1) Workstation 1 1 1

8311 Resistive Load 1 1 1

8325-A Filtering Inductors/Capacitors 1

8326 Three-Phase Filters 1

8823 Power Supply 1 1 1

8837(2) IGBT Chopper/Inverter 1 1

8842-A Rectifier and Filtering Capacitors 1 1 1

8951-L Connection Leads 1 1 1

8990 Personal Computer 1 1 1

9063-C(3) Data Acquisition and Control Interface 1 1 1

30004-2 24 V AC Power Supply 1 1 1

30011-4 4 mm Safety Banana Plug Leads, 300 mm long 3 (1) The Mobile Workstation, Model 8110, or the Workstation, Model 8134, can also be used. (2) The prefix IGBT has been left out in this manual when referring to this module.

(3) Model 9063-C consists of the Data Acquisition and Control Interface, Model 9063, with functions 9069-1 and 9069-2.

EquipmentUtilizationChart

Appendix A


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 26 shows the load elements and connections. Other parallel combinations can be used to obtain the same impedance values listed.

Table 3. Impedance table for the load modules.

Impedance (Ω) Position of the switches

120 V 60 Hz

220 V 50 Hz/60 Hz

240 V 50 Hz

1 2 3 4 5 6 7 8 9

1200 4400 4800 I

600 2200 2400 I

300 1100 1200 I

400 1467 1600 I I

240 880 960 I I

200 733 800 I I

171 629 686 I I I

150 550 600 I I I I

133 489 533 I I I I

120 440 480 I I I

109 400 436 I I I I

100 367 400 I I I I I

92 338 369 I I I I I

86 314 343 I I I I I I

80 293 320 I I I I I I I

75 275 300 I I I I I I I

71 259 282 I I I I I I

67 244 267 I I I I I I I

63 232 253 I I I I I I I I

60 220 240 I I I I I I I I

57 210 229 I I I I I I I I I

ImpedanceTablefortheLoadModules

Appendix B

AppendixB ImpedanceTablefortheLoadModules


Figure 26. Location of the load elements on the Resistive Load, Inductive Load, and Capacitive Load, Models 8311, 8321, and 8331, respectively.


Various symbols are used in the circuit diagrams of this manual. Each symbol is a functional representation of a particular electrical device that can be implemented using Lab-Volt equipment. The use of these symbols greatly simplifies the number of interconnections that need to be shown on the circuit diagram, and thus, makes it easier to understand the circuit operation.

For each symbol other than those of power sources, resistors, inductors, and capacitors, this appendix gives the name of the device which the symbol represents, as well as the equipment and the connections required to properly connect the device to a circuit. Notice that the terminals of each symbol are identified using circled letters. The same circled letters identify the corresponding terminals in the Equipment and Connections diagram. Also notice that the numbers (when present) in the Equipment and Connections diagrams correspond to terminal numbering used on the actual equipment.

a When a current at inputs I1, I2, I3, or I4 exceeds 4 A (either permanently or momentarily), use the corresponding 40 A input terminal and set the Range parameter of the corresponding input to High in the Data Acquisition and Control Settings window of LVDAC-EMS.

CircuitDiagramSymbols

Appendix C

Equipment and Connections Symbol

Isolated voltage and current measurement inputs

Data Acquisition and Control Interface (9063)

Voltage inputs

Current inputs

AppendixC CircuitDiagramSymbols


Three-phase induction machine

Four-Pole Squirrel Cage Induction Motor (8221-0)

Induction machine

Three-phase synchronous motor


Synchronous Motor / Generator (8241-2)

Synchronous motor

Three-phase induction machine

Three-Phase Induction Machine (8221-B)

Induction machine



Three-phase synchronous generator

Synchronous generator

Synchronous Motor / Generator (8241-2)

Wound-rotor

induction machine

Three-phase wound-rotor induction machine

Three-Phase Wound-Rotor Induction Machine (8231-B)




Power diode three-phase full-wave rectifier

Rectifier and Filtering Capacitors (8842-A)

Power thyristor three-phase bridge

Power Thyristors (8841)


Permanent Magnet Synchronous Machine (8245)

Permanent Magnet Synchronous Machine

PMSM

U

V

W



Three-phase inverter

IGBT Chopper / Inverter (8837-B)



Acronyms

The following acronyms are used in this manual:

AVG average

DACI Data Acquisition and Control Interface

DB9 common type of electrical connector having 9 pins

EMS Electromechanical System

EMI electromagnetic interference

IGBT insulated-gate bipolar transistor

LVDAC Lab-Volt Data Acquisition and Control

PWM pulse-width modulation

USB universal serial bus

UPS uninterruptible power supply


Bibliography

Jackson, Herbert W, Introduction to Electric Circuits, 8th ed. Oxford: Oxford University Press, 2008, ISBN 0-19-542310-0.

Wildi, Theodore, Electrical Machines, Drives, and Power Systems, 6th ed. New Jersey: Pearson Prentice Hall, 2006, ISBN 0-13-177691-6.


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