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Three-Phase AC Power Electronics
Student Manual
Electric Power / Controls
Three-Phase AC Power Electronics
Student Manual
86362-00
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 a
nondisclosure 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 in
this 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.
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
A Three-Phase AC Power Electronics
v
SafetyandCommonSymbols
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
vi
Three-Phase AC Power Electronics A
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 A Three-Phase AC Power Electronics
vii
TableofContents
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 viii
Three-Phase AC Power Electronics A
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.
A Three-Phase AC Power Electronics
ix
Preface
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), largescale 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.
x
Three-Phase AC Power Electronics A
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
xi
Introduction
Three‐PhaseACPowerElectronics
DISCUSSIONOF
FUNDAMENTALS
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.
A Three-Phase AC Power Electronics
1
1
Exercise
PowerDiodeThree‐PhaseRectifiers
EXERCISEOBJECTIVE
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.
DISCUSSIONOUTLINE
The Discussion of this exercise covers the following points:




DISCUSSION
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 threephase 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.
Line terminals
1, 2, and 3
Three-phase
ac power source
Load
Neutral
terminal
Figure 2. Three-phase half-wave rectifier (positive-polarity output).
Figure 3 shows the waveforms of the circuit voltages and currents in the threeis the voltage measured
phase half-wave rectifier. The rectifier output voltage
at point X with respect to the neutral terminal N of the three-phase ac power
.
source. Therefore,
A Three-Phase AC Power Electronics
3
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
(V)
(
Phase voltages
,
,
)
90
210
30
150
30
150
90
330
270
210
30
150
30
150
330
Phase
angle (°)
270
(A)
Diode current
)
(
Phase
angle (°)
(A)
Diodecurrent
(
)
150
270
150
270
Phase
angle (°)
270
Phase
angle (°)
270
Phase
angle (°)
(A)
Diode current
)
(
30
270
30
270
30
(A)
Rectifier output
current )
(
30
150
(V)
Rectifier output
voltage
)
(
,
150
,
,
0.5
30
150
0.827
0.675
,
,
,
210
270
330
30
90
150
210
270
330
Phase
angle (°)
Figure 3. Waveforms of the voltages and currents in the three-phase half-wave rectifier
(positive-polarity output).
4
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
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
(phase voltage
) becomes higher than the voltage
anode of diode
). Therefore, diode
enters into
at its cathode (phase voltage
to stop conducting) and
conduction (this causes diode
current starts flowing from point X toward the neutral terminal N via
the load (resistor ). Consequently, the rectifier output voltage
follows
. This situation lasts until the
the positive peak of phase voltage
phase angle reaches 150°. Between phase angles 30° and 150°,
and
are blocked since the voltages (phase voltages
diodes
, respectively) present at their anodes are both lower than the
and
) present at their cathodes.
voltage (phase voltage

When the phase angle reaches 150°, the voltage at the anode of
(phase voltage
) becomes higher than the voltage at its
diode
). Therefore, diode
enters into
cathode (phase voltage
to stop conducting) and current
conduction (this causes diode
starts flowing from point X toward the neutral terminal N via the
load (resistor ). Consequently, the rectifier output voltage
follows the
. This situation lasts until the phase
positive peak of phase voltage
angle reaches 270°. Between phase angles 150° and 270°, diodes
are blocked since the voltages (phase voltages
and
,
and
respectively) present at their anodes are both lower than the
) present at their cathodes.
voltage (phase voltage

When the phase angle reaches 270°, the voltage at the anode of
(phase voltage
) becomes higher than the voltage at its
diode
). Therefore, diode
enters into
cathode (phase voltage
to stop conducting) and current
conduction (this causes diode
starts flowing from point X toward the neutral terminal N via the
load (resistor ). Consequently, the rectifier output voltage
follows the
. This situation lasts until the phase
positive peak of phase voltage
angle reaches 30° of the subsequent cycle. Between phase angles 270°
and
are blocked since the voltages (phase
and 30°, diodes
and
, respectively) present at their anodes are both
voltages
) present at their cathodes.
lower than the voltage (phase voltage
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
) and (0.5 ‐ ,
) This implies that the ripple (amplitude of the
voltage ( ‐ ,
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
A Three-Phase AC Power Electronics
5
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
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 threephase ac power source. The average value of the rectifier output voltage ,
is:
,
where
0.827
,
0.675
,
,
is the amplitude of the phase voltage.
,
is the rms value of the line-to-line voltage.
(1)
To conclude, the three-phase half-wave rectifier acts like three single-phase halfwave rectifiers (one for each phase) operating one after another. The phase
,
, and
delivered by the three-phase ac power source,
currents
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 threephase 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.
6
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
Line
terminals
Three-phase
ac power source
Neutral terminal
Figure 4. Three-phase half-wave rectifier (negative-polarity output).
a
The + and
signs next to voltage
in the figure indicate the convention of
is negative when the voltage at
measurement of this voltage. The value of
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 threephase 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
or
)
intervals of 120°. The waveform of the rectifier output voltage (
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 ‐ ,
.
A Three-Phase AC Power Electronics
7
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
(V)
(
Phase voltages
,
,
)
90
210
30
330
150
90
270
30
210
150
330
Phase
angle (°)
330
Phase
angle (°)
330
Phase
angle (°)
270
(A)
Diode current
(
)
210
330
210
(A)
Diode current
)
(
90
90
330
(A)
Diode current
)
(
90
(A)
Rectifier output
current )
(
210
Phase
angle (°)
Waveform obtained with a three-phase half-wave rectifier with a
positive-polarity output (shown for comparison)
90
210
30
330
150
90
30
270
330
210
150
270
Phase
angle (°)
Waveform obtained with a three-phase half-wave rectifier with a
positive-polarity output (shown for comparison)
(V)
Rectifier output
voltage
(
‐ )
90
210
90
330
210
30
0.5
150
,
90
30
270
,
,
330
210
150
270
,
Phase
angle (°)
0.827
0.675
,
,
Figure 5. Waveforms of the voltages and currents in the three-phase half-wave
rectifier (negative-polarity output).
8
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
The average value of the rectifier
to 0.827
or 0.675
.
,
,
output
voltage
,
, is
equal
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
and
) at the anodes of diodes
and .
than the voltages (
This current , flows from X toward N through resistor . On the other
and
are blocked since the voltages (
and
)
hand, diodes
) at their cathodes. The
at their anodes are lower than the voltage (
voltage between X and N (EX‐N) therefore follows the positive peak of
.
phase voltage

also conducts current since the voltage (
) at its
Meanwhile, diode
and
) at the cathodes of
cathode is lower than the voltages (
and . This current , flows from N towards Y through
diodes
A Three-Phase AC Power Electronics
9
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
resistor . On the other hand, diodes
and
are blocked since the
and
) at their cathodes are higher than the
voltages (
) at their anodes. The voltage between Y and N (
)
voltage (
.
therefore follows the negative peak of phase voltage
Line
terminals
Three-phase
ac power source
(a)
A three-phase full-wave rectifier can be viewed as a combination of a three-phase halfwave rectifier with a positive-polarity output and a three-phase half-wave rectifier with a
negative-polarity output.
Line
terminals
Three-phase
ac power source
(b)
Usual representation of a three-phase full-wave rectifier. Terminal N is the neutral
conductor of the source.
Figure 6. Three-phase full-wave rectifier.
10
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
(
V 90
Phase voltages
,
,
)
30
Order of
conduction of
the diodes
210
150
90
210
30
150
5
6
6
4
0.675
90
,
Rectifier output
current
210
150
30
330
270
Phase
angle (°)
A V
Voltages
and
330
270
30
330
0.675
90
150
,
270
A 210
270
Phase
angle (°)
4
90
30
,
330
210
150
330
Phase
angle (°)
330
Phase
angle (°)
270
s
30
90
150
210
270
1.65
1.35
210
270
,
V
,
,
Rectifier output
voltage
30
90
150
210
270
330
30
90
150
330
Phase
angle (°)
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
) to follow the
causes the waveform of the voltage between X and N (
positive peaks of the phase voltages of the three-phase source. The waveform of
is therefore identical to that produced by a three-phase half-wave
voltage
rectifier with a positive-polarity output.
resulting from the successive conduction of diodes ,
Similarly, the current
, and
causes the waveform of the voltage between Y and N (
) to follow
A Three-Phase AC Power Electronics
11
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
the negative peaks of the phase voltages of the three-phase ac power source.
is therefore identical to that produced by a threeThe waveform of voltage
phase half-wave rectifier with a negative-polarity output.
of the three-phase full-wave rectifier is equal to the sum
The output voltage
(or
). The rectifier output voltage waveform is,
of
therefore, a pulsating positive voltage made of six pulses per cycle. The average
value of the rectifier output voltage ,
is:
,
where
1.65
1.35
,
(2)
,
,
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 .
1
Line
terminals
Three-phase
ac power source
R
4
5
6
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.
12
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Discussion
I (A)
Current
30
150
30
Phase
angle (°)
150
I (A)
Current
150
270
150
Phase
angle (°)
270
(A)
Current
30
270
30
Phase
angle (°)
270
(A)
210
Current
330
210
330
Phase
angle (°)
330
Phase
angle (°)
(A)
90
Current
330
90
(A)
90
Current
210
90
Phase
angle (°)
210
(A)
Current
(
1
210
)
30
330
150
210
30
330
Phase
angle (°)
330
270
Phase
angle (°)
270
Phase
angle (°)
150
(A)
Current
(
2
90
)
330
150
90
270
150
(A)
Current
(
3
90
)
210
30
90
270
210
30
(A)
Rectifier output
current 30
90
150
210
270
330
30
90
150
210
270
330
Phase
angle (°)
Figure 9. Waveforms of the diode currents, source currents, and rectifier output current.
A Three-Phase AC Power Electronics
13
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) and
polarity dc power supply can be obtained by simply using the positive (
) voltages produced by the rectifier separately. This requires the
negative (
use of the neutral conductor of the ac power source as it serves as the common
and
point of the dual-polarity dc power supply. The voltage waveforms
, i.e., the voltage
are shown in Figure 7. The average value of voltage
between the positive terminal and common (neutral) terminal of the dual-polarity
, rms. On the other hand, the average
dc power supply, is equal to 0.675
, i.e., the voltage between the negative terminal and
value of voltage
common terminal of the dual-polarity dc power supply, is equal to
, rms. The average value of the total voltage (
) produced, i.e., the
-0.675
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
).
Positive
terminal
1
Line
terminals
Common
terminal
Three-phase
ac power source
4
5
6
Negative
terminal
Figure 10. Dual-polarity dc power supply.
PROCEDUREOUTLINE
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 PROCEDURE
High voltages are present in this laboratory exercise. Do not make of modify any
banana jack connections with the power on unless otherwise specified.
14
Three-Phase AC Power Electronics A
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).
A Three-Phase AC Power Electronics
15
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
1
1
2
N
3
Local ac power network
Voltage
(V)
Frequency
(Hz)
(Ω)
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 avoid
excessive 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.
16
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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
, and
, are asymmetrical, which means that they have
diode currents ,
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.
A Three-Phase AC Power Electronics
17
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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
, and . Record the conduction angle of the diodes. Then,
currents ,
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
and
measure the average (dc) values of the rectifier output voltage
rectifier output current , respectively. Record these values below.
18
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
Then, calculate the rectifier output power
and current .
voltage
Average rectifier output voltage
Average rectifier output current
Rectifier output power
from the average values of
V
,
A
,
,
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
value below.
V
‐ ,
Calculate the maximum positive value of phase voltage
value below.
,
,
Compare
,
previous step. Is
 Yes
√2
. Record this
V
to the rectifier output voltage
approximately equal to 0.827
,
,
,
measured in the
?
 No
Calculate the line-to-line voltage
,
‐ ,
Compare
,
previous step. Is
 Yes
. Record this
√3
,
and record your result.
V
to the rectifier output voltage
,
approximately equal to 0.675
,
,
measured in the
?
 No
15. On the Power Supply, turn the three-phase ac power source off by setting
the corresponding switch to O (off).
A Three-Phase AC Power Electronics
19
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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).
4
1
2
N
3
Local ac power network
Voltage
(V)
Frequency
(Hz)
(Ω)
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.
20
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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.
19. 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
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.
A Three-Phase AC Power Electronics
21
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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
22
 No
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
23. In the Metering window, make sure meters E4 and I4 are set to measure the
and rectifier output
average (dc) values of the rectifier output voltage
current respectively. Record these values below.
from the average values of
Then, calculate the rectifier output power
and current .
voltage
Average rectifier output voltage
Average rectifier output current
Rectifier output power
V
,
,
,
A
W
,
Compare the average output voltage of the three-phase half-wave
rectifier (negative-polarity output) to that previously obtained for a threephase 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
value below.
,
√2
,
Compare
,
previous step. Is
 Yes
,
. Record this
V
,
measured in the
,
?
 No
,
,
√3
,
and record your result.
V
to the rectifier output voltage measured in the previous
approximately equal to 0.675 ∙
?
,
,
,
 Yes
A Three-Phase AC Power Electronics
to the rectifier output voltage
approximately equal to 0.827 ∙
Calculate the line-to-line voltage
Compare
step. Is
.
 No
23
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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 positivepolarity 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 threephase half-wave rectifiers.
is the three-phase
27. Set up the circuit shown in Figure 13. In this circuit,
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
and
are implemented with the Resistive Load
module. Resistors
module. The resistance values to be used for these resistors depend on your
local ac power network voltage (see table in diagram).
a
24
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.
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
2
1
3
1
1
2
N
3
2
5
4
Local ac power network
6
,
(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).
28. On the Power Supply, turn the three-phase ac power source on.
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 threephase 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 Three-Phase AC Power Electronics
25
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
30. Describe the waveforms of the rectifier output current and rectifier output
voltage with respect to the waveforms of the source phase voltages, and
explain.
Observe the waveforms of the phase currents delivered by the source,
,
, and
. These currents are respectively equal to
,
i.e.,
, 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 fullwave 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 halfwave rectifier, resulting in a smoother voltage at the output of the threephase full-wave rectifier?
26
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
32. In the Metering window of LVDAC-EMS, make sure meters E4 and I4 are set
and
to measure the average (dc) values of the rectifier output voltage
rectifier output current respectively. Record these values below.
from the average value
Then, calculate the rectifier output power
current .
Average rectifier output voltage
Average rectifier output current
Rectifier output power
and
V
,
,
,
A
W
,
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 positiveor negative-polarity output (recorded in steps 13 and 23, respectively).
33. Set meter E1 to measure the rms value of phase voltage
value below.
,
V
Calculate the maximum positive value of phase voltage
value below.
,
,
Compare
,
previous step. Is
 Yes
√2
. Record this
V
to the rectifier output voltage
,
approximately equal to 1.65
,
,
measured in the
?
 No
Calculate the line-to-line voltage
,
Compare
step. Is
. Record this
,
√3
,
and record your result.
V
to the rectifier output voltage measured in the previous
approximately equal to 1.35
?
,
,
,
 Yes
 No
34. On the Power Supply, turn the three-phase ac power source off.
A Three-Phase AC Power Electronics
27
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
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.
is the three-phase
35. Set up the circuit shown in Figure 14. In this circuit,
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
and
are
Rectifier and Filtering Capacitors module. Resistors
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).
1
1
2
N
3
4
5
Local ac power network
6
,
(Ω)
Voltage
(V)
Frequency
(Hz)
120
60
300
220
50
1100
240
50
1200
220
60
1100
Figure 14. Dual-polarity dc power supply.
28
Three-Phase AC Power Electronics A
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
36. On the Power Supply, turn the three-phase ac power source on.
The power rating of resistors
and
in the circuit diagram of Figure 14 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.
37. In the Oscilloscope, make the necessary settings to display phase
(E1) and the voltages (E2 and E3) at the positive and negative
voltage
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:
Average voltage at the negative output:
A Three-Phase AC Power Electronics
V
V
29
Exercise 1 – Power Diode Three-Phase Rectifiers  Procedure
43. On the Power Supply, turn the three-phase ac power source off.
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.
1
1
2
N
3
4
6
5
Figure 15. Dual-polarity dc power supply with capacitors.
44. On the Power Supply, turn the three-phase ac power source on.
The power rating of resistors
and
in the circuit diagram of Figure 15 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.
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:
Average voltage at the negative output:
30
V
V
Three-Phase AC Power Electronics A
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.
CONCLUSION
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 halfwave 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.
A Three-Phase AC Power Electronics
31
Exercise 1 – Power Diode Three-Phase Rectifiers  Review Questions
REVIEWQUESTIONS
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 negativepolarity output to that of a three-phase half-wave rectifier with a positivepolarity output. Do these rectifiers have the same conduction angles, ripple
frequencies, and average output voltages?
32
Three-Phase AC Power Electronics A
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.
A Three-Phase AC Power Electronics
33
2
Exercise
TheSingle‐PhasePWMInverterwithDual‐PolarityDCBus
EXERCISEOBJECTIVE
When you have completed this exercise, you will be familiar with the singlephase PWM inverter with dual-polarity dc bus.
DISCUSSIONOUTLINE
The Discussion of this exercise covers the following points:

DISCUSSION
Operation of a single-phase PWM inverter implemented with a dualpolarity 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 fourquadrant 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
is alternately applied to the output of the four-quadrant chopper
voltage
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
and
are on, and negative when
on. It is positive when electronic switches
and
are on.
electronic switches
Four-quadrant chopper
Filter
+
Load
-
Switching control
signal generator
Figure 16. Four-quadrant chopper used as a single-phase inverter.
A Three-Phase AC Power Electronics
35
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 singleand ), two freephase PWM inverter consists of two electronic switches (
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 dcbus requires only two electronic switches to obtain ac voltage and current at the
inverter output.
Single-phase inverter
Dual-polarity dc power supply
1
/2
Filter
2
N
3
/2
+
Load
-
Switching control
signal generator
Figure 17. Single-phase inverter implemented with a dual-polarity dc bus.
36
Three-Phase AC Power Electronics A
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Discussion
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
⁄2) is applied to the load and current
is on, a positive voltage (
switch
flows in the load in the direction shown in Figure 18a. Conversely, when
⁄2) is applied to the load and
is on, a negative voltage (
electronic switch
current flows in the load in the direction shown in Figure 18b.
1
/2
2
N
3
+
Load
-
Load current flow
1
2
N
3
/2
+
Load
-
Load current flow
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
A Three-Phase AC Power Electronics
37
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Discussion
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 singlephase 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
of the voltage sine wave at the inverter output
index m. The amplitude
,
depends on both the input voltage , i.e.,
⁄2, and the modulation index m.
where
,
m
⁄2
∙
,
∙
(3)
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
⁄2), expressed in V
inverter input (
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 ∙
(4)
√2
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.
38
Three-Phase AC Power Electronics A
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Discussion
Duty-cycle
control input signal
∝
∝
Time
Output current
Output voltage
after filtering
Output voltage
before filtering
Average value
Time
Time
Time
Figure 19. Waveforms related to a single-phase PWM inverter implemented with a dual-polarity
dc power supply.
A Three-Phase AC Power Electronics
39
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Procedure Outline
PROCEDUREOUTLINE
The Procedure is divided into the following sections:




Setup and connections Operation of a single-phase PWM inverter implemented with a dualpolarity 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. PROCEDURE
High voltages are present in this laboratory exercise. Do not make of modify any
banana jack connections with the power on unless otherwise specified.
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.
Notice that the prefix IGBT
has been left out in this
manual when referring to
the IGBT Chopper/Inverter
module.
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.
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 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
40
Three-Phase AC Power Electronics A
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
and . Use the inductor and the capacitor of the Filtering
capacitors
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).
Dual-polarity dc power supply
Single-phase inverter
1
Filter
2
N
3
Switching control
signals from DACI
Local ac power network
Voltage
(V)
Frequency
(Hz)
(Ω)
(mH)
(µF)
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.
A Three-Phase AC Power Electronics
41
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Procedure
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:
42

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
,
,
, and
as required by singlefor electronic switches
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%
/2).
of the half dc bus voltage (50% of
Three-Phase AC Power Electronics A
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Procedure

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
(AI-1) and
display the switching control signals of electronic switches
(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
by channels 1 and 2 on the Oscilloscope screen complementary?
 Yes
displayed
 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
A Three-Phase AC Power Electronics
 No
43
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Procedure
Relationshipbetweenoutputvoltage,inputvoltage,andmodulation
index
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 singlephase 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
) using the
voltage at the output of the single-phase PWM inverter ( ,
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
) approximately equal to the calculated values,
phase PWM inverter ( ,
confirming that ,
/2
?
 Yes
44
 No
Three-Phase AC Power Electronics A
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Conclusion
Effectofavariationinfrequencyofthesignalthatmodulatestheduty
cycleoftheswitchingcontrolsignalsontheamplitudeandfrequency
oftheloadvoltage.
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.
CONCLUSION
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 sinewave 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.
A Three-Phase AC Power Electronics
45
Exercise 2 – The Single-Phase PWM Inverter with Dual-Polarity DC Bus  Review Questions
REVIEWQUESTIONS
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 singlephase 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?
46
Three-Phase AC Power Electronics A
3
Exercise
TheThree‐PhasePWMInverter
EXERCISEOBJECTIVE
When you have completed this exercise, you will be familiar with the three-phase
PWM inverter.
DISCUSSIONOUTLINE
The Discussion of this exercise covers the following points:


DISCUSSION
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
), six free-wheeling diodes (
to
), and a switching
and
control signal generator.
Three-phase
PWM inverter
Dual-polarity
dc power supply
Three-phase
filter
Phase 1
1
2
Phase 2
2
N
3
2
Phase 3
Switching control
signal generator
Neutral conductor
Three-phase load
Figure 21. The three-phase PWM inverter consists of three single-phase PWM inverters.
A Three-Phase AC Power Electronics
47
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
/2 is applied to the load when the upper electronic
way: a positive voltage (
or ) is on, whereas a negative voltage (/2 is applied to the
switch ( ,
or ) is on.
load when the lower electronic switch ( ,
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 threephase 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 threephase 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
is the same as
voltage at each output of the three-phase PWM inverter ,
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.
48
Three-Phase AC Power Electronics A
E (V)
90
30
210
150
330
270
90
30
210
330
150
Phase
angle (°)
270
Inverter output currents
(
,
,
)
(
Inverter output voltages
1 ,
2 ,
3 )
Exercise 3 – The Three-Phase PWM Inverter  Discussion
(A)
90
30
210
150
330
270
90
30
210
330
150
Phase
angle (°)
270
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.
90°
180°
0°
270°
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).
A Three-Phase AC Power Electronics
49
Exercise 3 – The Three-Phase PWM Inverter  Discussion
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.
Single-polarity
dc power supply
Three-phase PWM inverter
Three-phase filter
Phase 1
Phase 2
Phase 3
Switching control
signal generator
Three-phase load
Figure 24. Three-phase PWM inverter without neutral conductor and supplied by a singlepolarity dc power supply.
50
Three-Phase AC Power Electronics A
Exercise 3 – The Three-Phase PWM Inverter  Procedure Outline
PROCEDUREOUTLINE
The Procedure is divided into the following sections:



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 PROCEDURE
High voltages are present in this laboratory exercise. Do not make of modify any
banana jack connections with the power on unless otherwise specified.
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.
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.
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 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.
A Three-Phase AC Power Electronics
51
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
as well as capacitors , , , and .
implement inductors , , 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).
Dual-polarity
dc power supply
Three-phase inverter
Three-phase filter
1
2
N
3
Switching control
signals from DACI
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.
52
Three-Phase AC Power Electronics A
Exercise 3 – The Three-Phase PWM Inverter  Procedure
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‐polarity
dcpowersupply
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.
A Three-Phase AC Power Electronics
53
Exercise 3 – The Three-Phase PWM Inverter  Procedure
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
(Ch1).
switching control signal of electronic switch
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. 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).
54
Three-Phase AC Power Electronics A
Exercise 3 – The Three-Phase PWM Inverter  Procedure
15. 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.
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.
DC bus voltage (
):
V
20. Measure the amplitude of phase 1 voltage (Ch1) at the output of the threephase 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
V
inverter (after filtering):
A Three-Phase AC Power Electronics
55
Exercise 3 – The Three-Phase PWM Inverter  Procedure
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
Effectoftheneutralconductoronthevoltageandcurrentwaveforms
attheoutputofthethree‐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.
23. 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.
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.
56
Three-Phase AC Power Electronics A
Exercise 3 – The Three-Phase PWM Inverter  Procedure
26. 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.
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.
DC bus voltage (
):
V
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
V
PWM inverter (after filtering):
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
A Three-Phase AC Power Electronics
 No
57
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 threephase 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.
CONCLUSION
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.
REVIEWQUESTIONS
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)?
58
Three-Phase AC Power Electronics A
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.
A Three-Phase AC Power Electronics
59
Appendix A
EquipmentUtilizationChart
The following Lab-Volt equipment is required to perform the exercises in this manual.
Equipment
Model
Exercise
8131
Workstation
1
1
8311
Resistive Load
1
(1)
Description
8325-A
Filtering Inductors/Capacitors
8326
Three-Phase Filters
8823
Power Supply
8837(2)
IGBT Chopper/Inverter
2
1
3
1
1
1
1
1
1
1
1
1
1
1
1
8842-A
Rectifier and Filtering Capacitors
1
8951-L
Connection Leads
1
1
1
Personal Computer
1
1
1
8990
(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
9063-C
(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.
Model 9063-C consists of the Data Acquisition and Control Interface, Model 9063,
with functions 9069-1 and 9069-2.
(3)
A Three-Phase AC Power Electronics
3
61
Appendix B
ImpedanceTablefortheLoadModules
The following table gives impedance values which can be
the Resistive Load, Model 8311, the Inductive Load,
Capacitive Load, Model 8331. Figure 26 shows the
connections. Other parallel combinations can be used
impedance values listed.
obtained using either
Model 8321, or the
load elements and
to obtain the same
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
1200
4400
4800
I
600
2200
2400
300
1100
1200
400
1467
1600
I
240
880
960
I
200
733
800
171
629
686
I
150
550
600
I
133
489
533
120
440
480
I
109
400
436
I
100
367
400
92
338
369
86
314
343
I
80
293
320
I
75
275
300
71
259
282
I
67
244
267
I
63
232
253
60
220
240
57
210
229
A Three-Phase AC Power Electronics
2
3
4
5
6
7
8
9
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
63
AppendixB
ImpedanceTablefortheLoadModules
Figure 26. Location of the load elements on the Resistive Load, Inductive Load, and Capacitive
Load, Models 8311, 8321, and 8331, respectively.
64
Three-Phase AC Power Electronics A
Appendix C
CircuitDiagramSymbols
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.
Symbol
Equipment and Connections
Data Acquisition and
Control Interface (9063)
Voltage
inputs
Current
inputs
Isolated voltage and
current measurement inputs
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.
A Three-Phase AC Power Electronics
65
AppendixC
CircuitDiagramSymbols
Symbol
Equipment and Connections
Four-Pole Squirrel Cage Induction
Motor (8221-0)
Induction
machine
Three-phase induction
machine
Three-Phase Induction
Machine (8221-B)
Induction
machine
Three-phase induction
machine
Synchronous
Motor / Generator (8241-2)
Synchronous
motor
Three-phase synchronous
motor
66
Three-Phase AC Power Electronics A
AppendixC
CircuitDiagramSymbols
Symbol
Equipment and Connections
Synchronous
Motor / Generator (8241-2)
Synchronous
generator
Three-phase synchronous
generator
Three-Phase Wound-Rotor
Induction Machine (8231-B)
Woundrotor
induction
machine
Three-phase wound-rotor
induction machine
A Three-Phase AC Power Electronics
67
AppendixC
CircuitDiagramSymbols
Symbol
Equipment and Connections
Permanent Magnet
Synchronous Machine (8245)
U
PMSM
V
W
Permanent Magnet
Synchronous Machine
Rectifier and Filtering
Capacitors (8842-A)
Power diode three-phase
full-wave rectifier
Power Thyristors
(8841)
Power thyristor
three-phase bridge
68
Three-Phase AC Power Electronics A
AppendixC
Symbol
CircuitDiagramSymbols
Equipment and Connections
IGBT Chopper / Inverter
(8837-B)
Three-phase inverter
A Three-Phase AC Power Electronics
69
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
A Three-Phase AC Power Electronics
71
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.
A Three-Phase AC Power Electronics
73
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A Three-Phase AC Power Electronics
75