CONCORDIA UNIVERSITY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

ELEC 440

CONTROLLED ELECTRIC DRIVES

LABORATORY MANUAL

Prepared by

Dr. S. Williamson

And

Mr. J. Woods

January - 2013

ii

TABLE OF CONTENTS

TABLE OF CONTENTS ...................................................................................................... i I. INTRODUCTION Safety .............................................................................................................................. ii General safety rules ......................................................................................................... ii Laboratory rules .............................................................................................................. v Scope of electronic laboratory ....................................................................................... vi Organization of the manual ............................................................................................ vi Experiments ................................................................................................................... vi The lab report ................................................................................................................ vii Grading scheme ........................................................................................................... viii II. EXPERIMENT 1 DC MACHINE PARAMETERS .......................................................................................... 1 III. EXPERIMENT 2 PHASE CONTROLLED DC MOTOR DRIVES .................................................................... 4 IV. EXPERIMENT 3 DC/DC CHOPPER CONTROLLED DC MOTOR DRIVES ................................................. 9 V. EXPERIMENT 4 INDUCTION MOTOR DRIVES ........................................................................................ 14 VI. EXPERIMENT 5 VOLTS-HERTZ CONTROL OF INDUCTION MOTOR DRIVES ......................................... 19

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INTRODUCTION

SAFETY

Engineers are often required to use hand and power tools in constructing prototypes or in setting up experiments. Specifically, electrical engineers use test instruments to measure the electrical characteristics of components, devices, and electronics systems. These tasks are interesting and challenging, but they may also involve certain hazards if one is careless in his/her work habits. It is therefore essential that students learn the principles of safety at the very beginning of their career and that they practice these principles. Safe work requires a careful and deliberate approach to each task. Before undertaking an experiment, students must understand what to do and how to do it. They must plan everything, setting out tools, equipment, and instruments on the workbench in a neat and orderly fashion, . Extraneous items should be removed, and all cables should be securely fastened.

GENERAL SAFETY RULES

The first rule of personal safety is always:

Think First! This rule applies to all industrial workers as well as to those working with electricity. Develop good habits of workmanship. Learn to use tools correctly and safely. Always study the job at hand and think through your procedures, your methods, and the applications of tools, instruments, and machines before searching. Never permit yourself to be distracted from your work, and never distract another worker engaged in hazardous work. Don't be a clown! Jokes are fun and so is "horsing around", but not near moving machinery or electricity. There are generally three kinds of accidents which may occur to electrical students and technicians- electric shock, burns, and equipment-related injuries. Your knowing and studying about them, and observing simple rules will make you a safe person to work with. You could personally be saved from painful and expensive experiences.

iv

Electric shocks What about electric shocks? Are they fatal? The physiological effects of electric currents can generally be predicted with the chart shown in Fig. 1:

0.2

0.1DEATH

EXTREME BREATHINGDIFFICULTIES

SEVERE SHOCK

CANNOT LET GO

LABOREDBREATHING

MUSCULARPARALYSIS

PAINFUL0.01

0.001

MILD SENSATION

SENSATIONTHRESHOLD

AMPERES

Fig. 1 Physiological effects of electrical currents. Notice that it is the current that does the damage. Currents above 100 mA, or only one tenth of an ampere, are fatal. A workman who has contacted currents greater than 200 mA may live to see another day if given rapid treatment. Currents less than 100 mA can be serious and painful. A safe rule: Do not place yourself in a position to get any kind of shock. Nine rules for safe practice and avoiding electric shocks:

1. Work with one hand behind you or in your pocket. A current between two hands crosses your heart and can be more lethal than a current form hand to foot. A wise technician always works with one hand. Watch your TV serviceman.

v

2. Be sure of the condition of the equipment and the dangers it can present before working on it. Many sportsmen are killed by supposedly unloaded guns; many technicians are killed by supposedly "dead" circuits.

3. Never rely on safety devices such as fuses, relays, and interlock systems to protect you. They may not be working and may fail to protect you when most needed.

4. Never remove the grounding prong of a three-wire plug. This eliminates the grounding feature of the equipment making it a potential shock hazard.

5. Do not work on a cluttered bench. A disorganized mess of connecting leads, components and tools only leads to careless thinking, short circuits, shocks, and accidents. Develop systematized and organized work habits.

6. Do not work on wet floors. Your contact resistance to ground is greatly reduced on a wet floor. Work on a rubber mat or an insulated floor.

7. Do not work alone. It is just good sense to have someone around to shut off the power, to give artificial respiration, or to call a doctor.

8. Never talk to anyone while working. Do not let yourself be distracted. Also, don't talk to someone who is working on dangerous equipment. Do not be the cause of an accident.

9. Always move slowly working around electrical circuits. Violent and rapid movements lead to accidental short circuits and shocks.

Burns Accidents caused by burns, although usually not fatal, can be painfully serious. The dissipation of electrical energy produces heat. Four rules for safe practice and avoiding burns: 1. Resistors get very hot, especially those that carry high currents such as the ones

in this lab. Watch those five- and ten-watt resistors. They will burn the skin of your fingers. Stay away from them until they cool down.

2. Be on guard for all capacitors which may still retain charges. Not only can you get a dangerous and sometimes fatal shock, you may also get a burn from an electrical discharge. If the rated voltage of electrolytic capacitors is exceeded or their polarities reversed they may get very hot and may actually burst.

vi

LABORATORY RULES

Considering the number of students attending the labs and in order for the lab to operate properly, the students are asked to abide by the following rules:

1. No smoking, eating, or drinking is permitted in the laboratory. 2. Overcoats, lose clothes (i.e. ties) and briefcases are not permitted in the

laboratory, however, a table will allocated for those students that must bring these items.

3. All damaged or missing equipment and cables must be reported immediately to the demonstrator. Failure to do so will result in students being charged for damages and losses or extra lab work can be assigned.

4. Writing on work benches will result in ejection from the laboratory. 5. Student are required to have their preliminary calculation completed before being

admitted to the corresponding lab session (preparation and participation evaluation).

6. All data must be recorded on the laboratory sheets in ink and must be signed by the demonstrator before students leave.

7. No more than three students are allowed to occupy one laboratory workbench 8. Any student who is more than 30 minutes late will not be permitted into the

laboratory room. Furthermore, repeated tardiness will not be tolerated. 9. Do not connect or use any alligator clip leads on equipment binding posts. These

posts are meant to be used with banana plugs or straight wires only. 10. Demonstrators must verify all setups before any power switches are switched on. 11. Any changes even minor ones to the setup must be done when the power is off. 12. Any unusual equipment and machine operating conditions such smoke, sparking,

loud noise, or burning smell must be immediately reported to the lab demonstrator and all power supplies should be switched off.

13. Always try to maintain low stress conditions during power up and power down. For example, start the variac at zero during a power-up or reduce the armature voltage to zero before power-down. Not only will this increase the life span of expensive equipment in this laboratory but usually fault conditions can be found before harm or serious damage is incurred.

14. Student's complains concerning lab demonstrators should be presented to the full time lab instructor.

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SCOPE OF THE ELECTROMECHANICS LABORATORY

The main objectives of laboratory work are as follows: - To provide practical experience in electromechanical devices. - To provide experience in electrical measurements. - To provide experience in report-writing.

All three aspects are very important since an engineer spends most of his/her career designing, measuring, and testing his/her designs and reporting on his/her results.

ORGANIZATION OF THIS MANUAL

This manual is divided into 5 sections, each section describing one experiment. Each section is broken down into parts as follows:

I. Objectives II. Introduction III. Calculations IV. Experimental procedure V. Questions

The first part gives the objectives of the experiment. The second part provides a brief introduction to the experiment. Relevant theory is often included in this part for the convenience of the student. The third part describes the experimental procedure to be adopted and is itself broken down into subsections. Some of these subsections indicate to the student how to connect and test a particular circuit. Other subsections require the student to carry out a number of preliminary calculations. The fourth part gives a list of questions which should be answered by the student when the experiment has been completed and be included in the lab report.

EXPERIMENTS

Each experiment must be studied in advance and required preliminary calculations completed. If the theory is understood, the student knows exactly what to expect in an experiment and accurate measurements can be obtained very quickly. The procedure section may often dictate that graphs be plotted. It is a very good engineering practice to plot such graphs as the readings are taken. In this way

viii

discrepancies can be immediately detected and checked. Often sketches of various waveforms are required. These should be drawn clearly and relevant quantities, such as peak values, should be given. Devices are invariably characterized with maximum voltage, current, and power ratings. These should never be exceeded. Otherwise, the properties of a device may be impaired, or it may be damaged (motor, transformer) If in doubt about the use of a particular instrument, the operating instructions provided by the manufacturer should be read. Defective equipment must be reported immediately to the demonstrator or technical support. This is justified also by the fact that some equipment may be used in more than one experiment and knowing the exact characteristics of this equipment may be important. Each group is required to work at the same bench location each week. Equipment and components must be returned to their places. The benches must be left clear at the end of the experiment. Since the laboratory represents a significant portion of the student's practical training, it is imperative that the students perform all the experiments. If a student has missed an experiment due to circumstances entirely beyond his/her control, that student will have the opportunity to perform it at the end of the term. However, it is most unlikely that arrangements can be made for any individual to perform more than one experiment at this time. Any student who misses more than one experiment will not be eligible for any form of passing grade. That is, should a student miss more than one experiment, the student will earn the grade "R" (REPEAT)! Information concerning these arrangements will be provide by the full time lab instructor.

LAB REPORTS

For each experiment, a lab report must be written which can be regarded as a record of all activities, observations, and discussions pertaining to the experiment. Lab reports should above all be legible and should contain as much relevant information as possible. A lab report should consist of papers stapled together with a title page identifying the course, lab section, experiment, date, student's name, student's ID number, and demonstrator's name. Any reports without a proper title page will be rejected. Each lab report should be divided into five parts as follows:

ix

Objectives: they have to be stated clearly and can be copied from the lab

manual.

Preliminary Calculations:

results and a summary of computations should be given.

Experimental procedure and results:

should be broken down into items 1, 2, 3, etc., as in the lab manual. Each item should briefly contain the conditions of the experiment and the results.

Questions and discussions:

answer all the questions (if any) posed in the lab manual or by the lab demonstrator. Discuss any problems encountered during the experiment and any important observations made during the report write-up.

Conclusions: should be brief

GRADING SCHEME

Each lab report will be marked out of seven. Late lab reports will be marked out of three and no lab will be accepted after the last day of classes. There will be a final lab test based on experiments performed during the term. The grading scheme is as follows:

A. Lab reports: 1. Objectives and preliminary calculations 15% 2. Experimental results 25% 3. Questions, discussion and conclusions 25% 4. Preparation 15%

B. Experimental setup and participation 20%

It is important that the student prepares for each experiment by reading the instructions before the student goes to the laboratory. Therefore, both the preparation and the participation will be evaluated during the laboratory.

1

EXPERIMENT 1

DC MACHINE PARAMETERS

DC Machine Modeling and Simulation

I. DC Machine Parameters: Lab-Volt DC Machine 8211-00

The study of electrical motor drives begins with the determination of a model of the both

the power conversion circuits and the machines that are to be controlled. These models

are used for analysis and simulation of the drive systems. This laboratory will examine a

dc machine supplied by two different dc power conversion circuits. Thus, to start, the

circuit components of the dc machine-model must be identified and measured.

The open-loop model of the dc machine is with a load torque modeled as a disturbance

can be represented by two equations.

·

The Laplace transform representation results in

This standard formulation is found in the ELEC 440 Course Notes, lecture - 4, the text by

Krishnan. Chap. 2 and Chap. 3.6.1, and most other reference texts for motors and drives .

2

Note that depending on the author, the torque constant, Kt, and the back-emf constant,

Kφ, may be combined as Kb. Also, the back-emf voltage, E or Eemf may be designated

Ea, the armature voltage, when Ia = 0.

In order to begin the process of simulating a dc machine and we must find values for six

variables used by the equations describing the open-loop model: 1) Ra, the armature

resistance, 2) La, the armature inductance, 3) Kφ, the back-emf constant, 4) Kt, the

torque constant, 5) J, the moment of inertia and 6) B, the damping constant. Note that

for transient simulation Ra (and La) are found from ac tests.

A. Armature Resistance from DC Test. Analysis of the dc machine in steady-state

operation requires a dc value for the armature resistance, Ra. The armature resistance is

first found from a dc voltage test. A dc voltage is applied to the terminals of the

armature (terminals 1 and 2 on the Lab-Volt dc machine), Fig. 1-1. The voltage is raised

until the rated current is reached, 3 Adc. The voltage, E, and the current, I1, are

measured. The results are given below. This value of Ra,dc is required for the

determination of the moment of inertia, J, to be found later in this experiment.

shuntfield

150Vdc DMM A

V

+ -

+

-

+

-

0-120 Vdc

7

N

1

2

7

8

56

I1

To DAQ

To DAQE1

Dynamometer

Figure 1-1. Test circuit for Ra, dc.

3

The armature resistance (dc test) is given by,

, /

Table 1. DC Armature Test Values. Va (Vdc) Ia (Adc) Power (W) Ra,dc (Ohms)

Some sources, suggest applying the rated voltage to the armature terminals. In most cases

this cannot be done because the current would be too great. The belt should be attached

connecting the dc machine to the unpowered prime-mover-dynamometer at standstill.

This will prevent the motor from rotating and allowed the measurements to be taken. The

rotor was shifted 120o , and 240o and 2 more sets of measurements were taken

B. Armature Inductance, La (H), and Armature Resistance (Ohms) from AC Test,

Ra,ac (Ohms). The parameters derived from ac tests are used for modeling and

simulation the dc machine in transient modes of operation. The armature is connected to

an ac voltage source as shown in Fig. 1-2. While the motor is at standstill, a low ac

voltage is applied to the terminals (f = 60 Hz, ω = 377). Since the belt is connected to the

unpowered dynamometer, the dc machine will not rotate. The voltage (E1) and the

current (I1) at the armature terminals are measured. This is an ac measurement, hence

the oscilloscope or the Lab-Volt phasor-analyzer are used to determine the angle,θ,

between the current and the voltage. The circuit and the test results are given below

, ·

· θ

2

4

Table 2. Test Results for Armature AC Parameters. V(Vac) I(Aac) Power (W) Angle θ

Oscilloscope (Deg)

The values for Ra,ac and La were found to be:

Table 3. Armature AC values. Ra,ac (Ohms)

Xa (Ohms) La (H)

C. Values of the Shunt and Series Field Impedances (optional). The values of the Shunt field and Series field are found from the Lab-Volt dc machine

using the same test procedure that was used for the armature. The sources (Vac and Vdc)

and metering are transferred from the armature circuit to the appropriate field circuit –

terminals 5 and 6 for the shunt field and terminals 3 and 4 for the series field.

shuntfield

DMM A

V

+ -

+

-

+

-

0-120 Vac

4

N

1

2

7

8

56

I1

To DAQ

To DAQ

E1

i To Osc.

e

To Osc.Dynamometer

Figure 1-2. The Test circuit for the armature ac tests.

5

Table 4. Shunt Field DC Test Values. Va (Vdc) Ia (Adc) Power (W) Ra (Ohms)

Table 5. Results of AC Tests on Shunt Field. V(Vac) I(Aac) Power (W) Angle θ

Oscilloscope (Deg)

After calculation the Shunt Field Parameters noted.

Table 6. Shunt Field AC Parameters. Shunt Rdc,Ω Shunt Rac, Ω Shunt Xl, Ω The values of the test on the Series field are taken from the Lab-Volt circuits using the

same procedures described above.

Table 7. Series Field DC Test Values. Va (Vdc) Ia (Adc) Power (W) Ra (Ohms)

Table 8. Series Field AC Test Results. V(Vac) I(Aac) Power (W) Angle θ

Oscilloscope (Deg)

6

Table 9. Shunt Field Parameters. Series Rdc,Ω, Rs Series Rac, Ω Series Xl, Ω, Xs Figure 1-3, below, is a summary of the configurations of the equivalent circuits of the

armature winding and the series and shunt field windings.

D. Back-EMF Constant, Kφ

The Lab-Volt dc machine is connected as shown in the Fig. 1-4 below. The Lab-Volt

power supply is off. The variable transformer (autotransformer) dial is turned to the

minimum position, full counter-clockwise. The belt connects the dc machine to the prime

mover – dynamometer. The armature of the dc machine is open circuited. The large

rheostat wiper is at mid-position. The small field potentiometer on the dc machine is

at a minimum position (full clockwise, 0Ω)

Turn on the Lab-Volt power supply. Turn dial of the Lab-Volt power supply clockwise

slowly. The dc voltage to the input of the prime-mover is raised until the dc machine is

Ea

A

1

2

DC

Ra

Ra,ac Xa, La

Armature

AC

5

6

DC

Rp

Rp,ac Xp, Lp

Shunt Field

7

Rpot

8

From AC Test

From AC Test

From DC Test

From DC Test

3

4 DC

1.7 Ω

Rs,ac Xs, Ls

Series Field

From AC Test

From DC Test Rs

From DC Test or Nameplate

Figure 1-3. Summary of dc machine parameters.

7

rotating at 2000 rpm. Next, the voltage to the field of the dc machine (E1) is raised until

the maximum field current (I1, DMM) is obtained or the maximum voltage is applied

(120Vdc). The back-emf or the open-circuit armature voltage (Ea) is measured at test

point E2. At 2000 rpm measure the voltage, Ea, the field current, I1, the field voltage,

E1, and the speed, N. Repeat these measurements at descending intervals of 100 rpm,

i.e. 1900 rpm, 1800 rpm until the machine stalls (100 rpm). Turn off the power.

The data recorded is given below Table 10.

Table 10. Results of Test for Kφ. ω rads/sec Ea Vdc n rpm

Display

PRIME-MOVER DYNAMOMETER

MODE PRIME MOVER M

DYN

TorqueOutput

Speed Output

SPEED

N T To DAQ

TORQUE

DISPLAY

0-120 Vdc

+

-

7

N

INPUT 24VacEXT

MODEMAN

MANUAL

MIN MAX 1 2

I3

To DAQ

G

To DAQ

Timing Belt

+ 8

-N

120 VdcE1

E2

Ea

To DAQ

A

V

DMM

Ohmite RheostatRh

DC Machine 1

2

To DAQ

shunt field

7

8 5 6

I1

To DAQ

+

+

++

+

24 Vac

Figure 1-4. Circuit for determining Kφ.

8

The collected data is used to produce a curve with ω as the x-axis and Ea as the y-axis.

The values of Ea and speed are entered into an Excel spreadsheet and plotted using th

scatter plot function. Note that the armature voltage, Ea, is actually the back-emf

voltage since there is no current, Ia = 0. Plot the curve.

The slope of this curve is the back-emf constant that satisfies the relation,

or

Where Ea is in volts dc and ω is rads/sec. From the given plot

∆ ∆ ω

Table 7. Results of Tests for Kφ. ∆ ω (rads/s) ∆ Ea (Vdc) K φ

9

Method of Identifying Kφ and Kt Simultaneously. The values of Ea, torque, armature

current, Ia, (I2) and speed (n, ω) can be found by using the two circuits as shown in the

drawings below (Fig.5 , Fig. 6) .

D. Determination of Kφ (Open Circuit Characteristic, OCC) The armature

connections are open-circuited and the armature current is at zero. The machine is

operating at rated speed, rated field voltage and rated field current. The speed is reduced

in steps of 1000 rpm (104.6 rads/s). The speed and the back-emf, Ea, (E2) are measured.

In this case Ea is given explicitly and Kφ is the slope of

, ,, max ,

This satisfies the general relation since Ia = 0. From Chapman, Chap. 9.4, 3 ed:

· or

· And

· From El-Sharawi, Chap. 5.1.1, 1st ed.:

·

· · And

· Note that El-Sharkawi uses the variable Kφ for both Kφ and Kt.

Now since Ia (I2) and the torque Tind from the dynamometer are available, the torque

constant can be found (from Chapman, Chap. 9.4, 3 ed)

·

* Tind = Pa/ω = Ea·Ia/ω

El-Sharkawi uses Td = (Ea·Ia) /ω .

10

Note that Kφ and Kt have the same numerical value when derived using mks units.

Although the units are different, the actual value is the same – they have the same slope

when plotted. Thus

·

· Where Ea is given in V, Ia in amps, Tind in N-m, ω in rads/s, and Kφ in V/ rad/s and Kt

is in N-m/A.

Procedure. The Lab-Volt dc machine is connected as shown in the Fig. 5 below. The

Lab-Volt power supply is off. The variable transformer (autotransformer) dial is turned

to the minimum position, full counter-clockwise. The belt connects the dc machine to the

prime mover – dynamometer. The armature of the dc machine is open circuited. The

large Ohmite rheostat wipers are at mid-position. The small field potentiometer on the dc

machine is at a minimum position (full clockwise, 0Ω).

Display

PRIME-MOVER DYNAMOMETER

MODE PRIME MOVER M

DYN

TorqueOutput

Speed Output

SPEED

N T

To DAQ

TORQUE

DISPLAY

0-120 Vdc

+

-

7

N

INPUT 24VacEXT

MODEMAN

MANUAL

MIN MAX 1 2

I3

To DAQ

G

To DAQ

Timing Belt

+ 8

-N

120 VdcE1

E2

Ea

To DAQ

A

DMM

Ohmite RheostatRh

DC Machine 1

2

To DAQ

shunt field

7

8 5 6

I1

To DAQ

Ra I2

+

+

++

+

To DAQ

Field Rheostat

+

Ia

24 Vac

Eemf

Figure 5. The circuit for determining the open-circuit machine constant (OCC) or the speed constant, Kφ.

11

The PC, the oscilloscope and the Lab-Volt data acquisition module are powered-on. The Lab-Volt LVDAM software, the Excel software and the oscilloscope software are

working normally.

Turn on the Lab-Volt power supply. Turn dial of the Lab-Volt power supply clockwise

slowly. The dc voltage to the input of the prime-mover is raised until the dc machine is

rotating at 2000 rpm. First, slowly turn the field rheostat on the dc machine full

clockwise (0Ω). Then adjust the large Ohmite rheostat slowly from the mid-position.

This will raise the voltage to the field of the dc machine. (E1) is raised until the

maximum field current (I1, DMM) is obtained or the maximum voltage is applied

(120Vdc). The rated field voltage, E1, is kept constant.

The back-emf or the open-circuit armature voltage (Ea) is measured at test point E2.

At 2000 rpm record the values of the field voltage (E1), field current (I1), Vt (E2),

armature current (I2), n,(dynamometer), and Torque (dynamometer) in the Lab-Volt data

table. Rs can be measured at the end of the experiment. Ra,dc was determined earlier.

After the first set of measurements, turn down the voltage on the variac (counter-

clokwise) and slow the prime-mover-dynamometer to 1900 rpm. Repeat the

measurements. Repeat these measurements at descending intervals of 100 rpm, i.e. 1800

rpm, 1700 rpm until the machine stalls (100 rpm). Turn off the power. Transfer the Lab-

Volt data table to an Excel spreadsheet for computation and analysis (plotting).

Table 8. Results of Tests for Kφ. ∆ ω (rads/s) ∆ Ea (Vdc) K φ

E. Determination of Kt (Torque Characteristic) The armature is reconnected as

shown in Fig. 6 below. The variable dc supply is removed from the dynamometer-

prime-mover and connected in series to a second Ohmite rheostat, Rs, and then to the

terminals of the armature. The second resistance, Rs, may be set to the minimum

position and used as a safety element since it is equipped with a fuse. The machine will

12

be tested by changing the value of Ia (I2) and recording the speed and armature voltage,

Vt (E2). The field voltage and current are held constant at their rated values The

armature current is initially set to the minimum value and is raised until the maximum

valued (I2 = 3Adc) or the maximum rated voltage (E2 = 120 Vdc) is observed. The

speed and the armature terminal voltage, Vt, (E2) are measured. In this case Ea must be

calculated. Ea will be required to compute the induced torque, τind or Tind . After Ea is

found for each step, it is used with Ia and the observed speed, ω, to calculate the torque

constant, Kt.

, ,, max ,

Display

PRIME-MOVER DYNAMOMETER

MODE PRIME MOVER M

DYN

TorqueOutput

Speed Output

SPEED

N T To DAQ

TORQUE

DISPLAY

0-120 Vdc

+

-

7

N

INPUT 24VacEXT

MODEMAN

MANUAL

MIN MAX 1 2

To DAQ

G

To DAQ

Timing Belt

+ 8

-N

120 VdcE1

E2

Ea

To DAQ

A

DMM

Ohmite RheostatRh

DC Machine 1

2

To DAQ

shunt field

7

8 5 6

I1

To DAQ

Ra I2

+

+

+

++

To DAQ

Field Rheostat

+

Rs

Ia

24 Vac

Vt

Eemf

Figure 6. The circuit for determining the torque constant, Kt.

13

To start the evaluation of the torque constant, Kt, Ea is derived (From Chapman, Chap. 9.4, 3 ed.):

· or

· If Rs is used and has a non-zero value then the equation becomes:

· · And (From El-Sharawi, Chap. 5.1.1, 1st ed.):

· · or

· · And

· Note that El-Sharkawi uses the variable Kφ for both Kφ and Kt. Also, the induced torque

is represented by Td, Tind or τind depending on the author and reference text.

The torque from the dynamometer is available but not used since it represents the applied

torque, τapp , which includes the rotational losses of the machine. the torque constant can

be found (from Chapman, Chap. 9.4, 3 ed)

where Tind = Td= Pa/ω = Ea·Ia/ω. Procedure. The Lab-Volt dc machine is connected as shown in the Fig. 6 The minimum

position and value of Rs (approximately 2 - 10Ω) and the corresponding terminals are

determined. This must be determined prior to the start of the experiment. The Lab-Volt

power supply is off. The variable transformer (autotransformer) dial is turned to the

minimum position, full counter-clockwise. The belt connects the dc machine to the prime

mover – dynamometer. The armature of the dc machine is connected to the variable dc

supply (7, N) in series with the large Ohmite rheostat, Rs. This large Ohmite rheostat has

the wiper are at a mid-position or approximately 100Ω. The small field potentiometer

on the dc machine is at a maximum position (full counter-clockwise). The PC, the

oscilloscope and the Lab-Volt data acquisition module are powered-on. The Lab-Volt

14

LVDAM software, the Excel software and the oscilloscope software are working

normally.

Turn on the Lab-Volt power supply. First, slowly turn the field rheostat on the dc

machine full clockwise (0Ω). Then adjust the large Ohmite rheostat on the field circuit

slowly from the mid-position. This will raise the voltage to the field of the dc machine.

Vf (E1) is raised until the maximum field current (I1, DMM) is obtained or the maximum

voltage is applied (120- 140Vdc). The rated field voltage, E1, is kept constant.

Turn the dial of the Lab-Volt dc power supply clockwise slowly. Adjust the rheostat and

the Lab-Volt dc supply until Vt (E2) and Ia (I2) are at rated values (120 - 140 Vdc,

maximum 3.0 Adc – 0.6 Adc is the likely maximum). Note the position of the rheostat

and do not change it. The value of Rs must be determined at the end of the

measurements (approximately 3 – 10 Ω at end position).

Now lower the Lab-Volt main power supply in steps of 0.01 Adc (I2) until the lowest value of Ia (I2) is observed. At each step record the values of the field voltage (E1), field current (I1), Vt (E2),

armature current (I2), n,(dynamometer), and Torque (dynamometer) in the Lab-Volt data

table. Rs is measured at the end of the experiment. Ra,dc was determined earlier. After

the first set of measurements, turn down the voltage on the variac (counter-clockwise)

and turn off the power. Transfer the Lab-Volt data table to an Excel spreadsheet for

computation and analysis (plotting).

Table 9. Results of Tests for Kt. ∆Torque (N-m) ∆ Ia (Vdc) K τ

F. Determination of Moment of Inertia, J, and Viscous Friction, B The moment of inertia, J, and the friction coefficient, B, can be determined from a spin-

down test. At the start of the spin-down test, the dc machine is shunt connected and is

rotating at a speed that correspond to the full input voltage, Vt, of 120 – 140 V. The

voltage source is turned-off quickly at a given instant and the machine decelerates to a

15

full stop. At the moment that the power is removed Vt = Ea or the back-emf since the

input current ceases to flow. There are several methods of determining the value of J and

B. The method given below uses a partial graphical analysis.

Comparison of the speed curve and the back-emf response. Initially, the deceleration

curve of the terminal voltage, Vt, and the speed, n (rpm), are observed and the

oscilloscope trace is taken and analyzed. The circuit is connected as shown in Fig. 7

below.

Procedure. The Lab-Volt data table is active and ready to record, E1, I1, I2, I3 AI-8 (n),

and P1. The oscilloscope is ready to record Vt or Ea (e1), and speed, (e2, n, rpm) at a

slow time-base – 500 m sec/div. CH1 (Vt or Ea) and CH2 (speed) are set to 1 V/div. The

Lab-Volt tachogenerator is ready to record the initial speed of the system at the start of

the test. The shunt field rheostat is set to the minimum resistance, 0 Ω. Thus, the

maximum field current will flow. The machine speed is adjusted to the rated value or a

sufficiently high speed (1500 -2000 rpm, 157 -209 rad/s).

+ 7

-N

0 -120 Vdc E1

Lab-Volt Tacho

Ea

DC Machine

1

2

To DAQ

shunt field

7

85 6To Osc.

I1

n

+

0 -120 Vdce1

To DAQ

La

Ra

I2 I3+ +

++

To DAQ

To DAQ

If

Ia

+

e2

AI8 To DAQ

To Osc.

Dynamometer Belt is Disconnected

Figure 7. The circuit used to compare the speed and back-emf curves.

16

Figure 8. Comparison of speed and back-emf curves.

While the machine is running a set of data is taken using the Lab-Volt data table.

The oscilloscope is active and a set of curves begin to print on the screen. At a given

point, (t1), the power to the dc machine is shut-off and the machine decelerates. When

the curves are at zero the stop button on the oscilloscope is pressed. This will freeze the

display. The trace associated with Ea will be a smooth first-order curve. The trace

associated with the speed will have 2 sections. The first part is a smooth first-order

exponential until the break point after which the second portion becomes a linear function

with a negative slope. The cursors should be used to record the voltages associated with

the power-off points a and c (E1, t1, Vn1) and the break-points b and d (E2, t2, Vn2) as well

as the time interval between, ∆t. The oscilloscope traces can now be printed.

Table 10. The Speed and Back-EMF Curve Estimation. Vn1 (c)

(Osc.

at t1)

n1

(Lab‐

Volt)

VE1 (a)

(Osc.

at t1)

Ea1

(Lab‐

Volt)

Vn2 (d)

(Osc.

at t2)

n2

VE2 (b)

(Osc.

at t2)

Ea2 ∆t

(Osc.

at ∆t)

Trial 1

Trial 2

17

The value of the voltage that corresponds to the initial speed and the value of the initial

voltage are taken from the Lab-Volt data table. The values of Ea2 and n2 (ω2) can be

calculated by using a simple proportion. Have the demonstrator verify the recorded

values.

G. Determination of the moment of inertia, J, without direct speed measurement.

This method determines the values of the inertial constant, J, and the viscous friction, B,

without the use of the dynamometer (A. N. Popov) The circuit of Fig. 9 will provide the

curves shown in Fig. 10 and the data for Table 11. The circuit is connected as in Fig. 9,

so that Ea – the back-emf, is sent to the oscilloscope as the variable, e1. The armature

current Ia is observed on the oscilloscope, i2, and on the Lab-Volt data table as I2. The

relation between Ea and speed can be verified using the machine constant.

·

where ω is in radians /sec. The test is as follows.

Reconnect the circuit as shown in Fig. 9 below. The Lab-Volt data table should be active

and ready to record, E1, I1, I2, I3, AI8 and P1. The oscilloscope is ready to record Ea

and Ia at a slow time-base – 500 m sec/div. CH1 is at 1 V/div.. Channel 2 is at 5 V/div..

The shunt field rheostat is set to the minimum resistance, 0 Ω. Thus, the maximum field

current will flow. The machine speed is adjusted to the rated value or a sufficiently high

speed or the same value as the previous section (1500 -2000 rpm, 157 -209 rad/s).

The initial Lab-Volt values are taken. The oscilloscope is active and a set of curves begin

to print on the screen. At a given point, (t1), the power to the dc machine is shut-off and

the machine decelerates. When the curves are at zero the stop button on the oscilloscope

is pressed. This will freeze the display. The traces associated with Ea and Ia will be

smooth first-order curves similar to those in Fig 10 below. Print the oscilloscope

display. The cursors should be used to record the voltages associated with the power-off

points a and c (E1, t1, Ia1) and the break-points b and d (E2, t2, Ia2). Set the cursors so

that the measurements are taken for the same time interval, ∆t, as the first test. The

oscilloscope voltages for the trace representing Ea should be the same (points a and b

Figs. 8 and 10). Referring to Fig. 10, the initial values and voltages on the oscilloscope

(t1, points a and c) correspond to the initial values of the data table. The values at the

18

break-point (t2, points b and d) are calculated using the interpolation method used in the

previous section. Be sure to print these curves.

Show your results to the demonstrator for verification. Note that ω1 and ω2 can be checked by calculating Ea and using the relation Ea = Kφ · ω. (Kφ was found earlier).

Figure 10. The oscilloscope trace of Ea (Speed) and Ia.

+ 7

-N

0 -120 Vdc E1

Lab-Volt Tacho

Ea

DC Machine

1

2

To DAQ

shunt field

7

85 6To Osc.

I1

n

+

0 -120 Vdce1

To DAQ

La

Ra

I2 I3+ +

++

To DAQ

To DAQ

If

Ia

+

i2

AI8 To DAQ

To Osc.

Dynamometer Belt is Disconnected

Figure 9. Circuit for testing for the inertia, J, and friction constant, B.

19

By using the initial values for Vt or Ea (E1 data table, Ea1, point a), the initial value for the armature current , Ia, (I2 data table, Ia1, point c) and the interpolated values and the previously determined value for Ra,dc, Ea is given by

· or

Since the speed, n, is in rpm it must be converted to ω, rad/s (ω1,ω2, Fig. 10). The power is given by

The torques T1 and T2 shown in Fig. 10 are found from the relation

ω

The moment of inertia is found from

1 2 · ∆

1 2 · ln 12

These values should yield a value of J on the order of 10 -3. Finally, the viscous friction, B, is found from

And τ is the time constant of the speed deceleration curve, Fig. 8. Good results can be

obtained by assuming that the initial speed falls to zero at the point indicated by ttangent. Thus, if the initial speed is 1500 rpm (157 rad/s) at point c (Fig. 8) and the tangent

crosses the x axis at 1.0 sec. then τ can be taken as τ = ωinitial/ ttangent = 157/1 =157.

Similarly, if the speed difference between points c and d is 1500 rpm – 1000 rpm or ∆ω =

53 rad/s and the time difference between c and d is ∆t = .365 sec then τ is found to be

∆ω

∆142

These values of τ and J should yield a value of B on the order of 10 -5. Note that he interval, ∆t, is taken from Fig. 8 and carried over to Fig. 10. The results of the test should be put in an Excel spreadsheet and the values of T1, T2, ω1, ω2, J and B can be calculated.

Table 11. First Trial Results of Test for Inertia, J, and Friction, B. Trial 1 Ea (V) Ia (A) P (W) n ω Torque

20

(rpm) (rad/s) (N‐m) Initial Measurement, at t1

(Lab‐Volt.) (Lab‐Volt) (Lab‐Volt) (Lab‐Volt) (Calculate) (Calculate)

Second Measurement, at ∆t, t2

(Calculate) (Calculate) (Calculate) (Calculate) (Calculate) (Calculate)

Table 12. Second Trial Results of Test for Inertia, J, and Friction, B. Trial 2

E1 (V) I1 (A) P1 (W) n (rpm)

ω(rad/s)

Torque (N‐m)

Initial Measurement, at t1

(Lab‐Volt) (Lab‐Volt) (Lab‐Volt) (Lab‐Volt) (Calculate) (Calculate)

Second Measurement, at ∆t, t2

(Calculate) (Calculate) (Calculate) (Calculate) (Calculate) (Calculate)

`

Table 13. Results of Calculations for Inertia, J. T1

(N‐m) T2 (N‐m)

ω1 (rad/s)

ω2 (rad/s)

∆ t (sec)

τ(sec)

J B

Trial 1 Trial 2 The moment of inertia, J, can also be found from the method of the U. of Minn. Drives lab.

,

,

From the oscilloscope traces of the spin-down tests (Fig. 8, Fig. 10) the results are Kt =

0.8 and Ia = .59 Adc. The difference in the speeds is dω = 157 rad/s – 104 rads/s (1500

rpm – 1000 rpm) and dt = .365 sec.. The result is of the magnitude 10-3 which agrees

with the previous calculations.

II Simulation

21

PSIM Simulation The validity dc motor parameters determined previously can be tested by creating a

circuit in PSIM. The PSIM representation is based on the state-space diagram shown in

Fig. 11. A load torque, Tl can be applied as a step input as shown in Fig. 12. The speed

output can be plotted in the PSIM package. The required parameters are listed in Table

14.

Table 14. Parameters for PSIM simulation Circuit. Parameter Value Ra, Armature Resistance, Lab-Volt and Tests

- Ω

La, Armature Inductance, Lab-Volt and Test

- H

J, Moment of Inertia, From Test - kg m2 B, Viscous Friction, From Test - N-m/(rads/sec) Kφ, or Km, From Test - V/rads/s Kt, From Test - N-m/A Note that Kφ and Kt have the same numerical value when derived using mks units.

Although the units are different, the actual value is the same. Thus

·

·

Where Ea is given in V, Ia in amps, Tind in N-m, ω in rads/s, and Kφ in V/ rad/s and Kt

is in N-m/A.

The open-loop model of the dc machine is with a load torque modeled as a disturbance

can be represented by two equations.

·

The Laplace transform representation results in

1

22

The circuit is from ELEC 440 Course Notes, lecture 4, Krishnan. Chap. 3.6.1 . A PSIM

version is shown below with arbitrary values. This circuit can be tested by applying a

step load (voltage source Vstep), to the summation point. This represents a step change

in the load torque on the motor. The speed response is given by the voltmeter Vω. On the

Lab-Volt test bench the same result can be accomplished by changing the torque setting

on the dynamometer.

V(s)

Eemf (s)

1(B + sJ)

Ia(s)1

(Ra + sL)+- +-Te(s)

m(s)KT

Tl(s)

K

Te(s) = Tind(s) depending on the source.Note: K = KT = Kb depending on the source.

The transfer function TFCN2 represents the relation

1

The block TFCN3 represents the relation

1

The values of La, Ra , J and B were found earlier by experimentation. The system can

be loaded by a step change of 1Vdc which corresponds to 1.0 N-m on the lab-Volt

dynamometer.

Figure 11. The block diagram of the state-space model of the dc motor.

23

Verification of DC Machine Constants by the Application of a Load Torque

(Transient Test, State-Space Block Diagram Model). To verify the results of the tests

done to determine the machine parameters connect the circuit shown in Fig. 12 below.

Start the dc machine with T = 1.0 N-m. The traces to be shown on the oscilloscope will

represent the speed of the machine and the load (torque) applied by means of the

dynamometer. The oscilloscope time-base should be adjusted to 100 ms/div or 200

ms/division. Channel 1 on the oscilloscope is connected to the meter of the tacho-

generator. Channel 1 is set to 1 V/division. Channel 2 on the oscilloscope is set to 5 V/div

and is connected to the torque output of the dynamometer.

Change the MODE toggle switch on the prime mover-dynamometer from the DYN

(dynamometer) position to the PRIME MOVER position. This will unload the machine.

Since there is no power supplied to the unit (120 Vdc), the dynamometer is coasting.

With the Lab-Volt data table, take an initial set of readings to record the speed and

torque.

Figure 12. A PSIM circuit for the simulation of a dc machine.

24

When the oscilloscope begins a sweep, apply the 1.0 N-m torque to the machine by

putting the toggle-switch back in the DYN position. With the Lab-Volt data table, take a

set of readings to record the speed and torque. When the motor speed is stable push the

run/stop button on the oscilloscope to freeze the speed and torque traces. Use the cursors

to find the total time required for the motor to stop. Print the oscilloscope display. The

lab-Volt Tacho-generator and meter should be used to find the speed. Note that the

Lab-Volt power supply is used as the dc voltage source – not the thyristor converter.

When the results of the load test with the dc machine and dynamometer are complete

they can be compared with traces obtained from th PSIM simulation. Compare the initial

and final speeds, torques and the total stop time with the results of the PSIm simulation.

Gnd.

Shunt Field

2

1

Timing Belt

N T

Speed Output

Torque Output

Prime Mover

Manual ModeMin.

Display

Dyn. Max.

Display

1

Mode

To DAQ and Osc. CH2

Torque

2

Man.

Prime Mover - Dynamometer

Speed

Input

Ext.

To DAQ

Vs, field

5

6

7

8

+Vf

N

ArmatureDC Machine

La

Ra

Lf

Field Rheostat

Rf

IfIa

+

7

N

Va

Vs, armature

Ea+

120 Vdc 0-120 Vdc

8

Tacho-generator

To Osc. CH1

Meter

Figure 13. Circuit used to test simulation results of machine parameter calculations.

25

EXPERIMENT 2

PHASE-CONTROLLED DC MOTOR DRIVES

I. OBJECTIVES There are three objectives to this experiment:

- To learn to use the oscilloscope, the Lab-Volt test equipment and the associated

software, LV-EMS and MS Excel;

- To observe the performance of the single-phase thyristor full-bridge circuit;

- To investigate the characteristics of a dc motor that is driven by a single-phase

thyristor controlled-converter.

II. INTRODUCTION The phase-controlled dc converter is a circuit that transforms the input from an ac power

source to variable dc power at the output. This can be accomplished by using the

thyristor as the switching component in a single-phase full-bridge configuration. Fig. 1.1

shows the converter connected to a resistive load. Note that the circuit required to initiate

conduction in the thyristors is not shown.

The firing angle of the thyristors in Fig. 1.1 is α = 60o (π/3 radians). With the resistive

load, the current through the thyristors and the load is discontinuous (α > 0). Hence,

there is no current overlap angle, µ. The dc voltage (Vdc, Krishnan, Vave, El-Sharkawi)

across the resistive load is given by

sin

– cos

1 cos

26

The value of Idc (Idc, Krishnan, Iave, El-Sharkawi), the dc current through the resistor, is

calculated from the value of Vdc.

Figure 2.1 The Single-phase full-bridge controlled rectifier with resistive load (α = 60o). (a) The circuit with a resistive load. (b) The input voltage waveform. (c) The input

current waveform. (d) The thyristor voltage. (e) The thyristor current. (f) The voltage across the resistor. (g) The current through the resistor.

With the addition of an inductance, the load becomes a series combination of a resistance

and inductor as shown in Fig. 1.2. The value of the counter dc source is assumed to be

27

zero. If the value XL is small, the load current may still be discontinuous but the current

conduction angle, γ, is extended past π radians to β radians.

Fig. 2.2 The Single-phase full-bridge controlled rectifier with resistive-inductive load (α = 60o). (a) The circuit with an RL load (E = 0 V). (b) The input voltage waveform. (c)

The input current waveform. (d) The thyristor voltage. (e) The thyristor current. (f) The voltage across the RL load. (g) The current through the resistor.

Thus β is the current extinction angle and γ = β - α. The value of the output voltage is

given by:

28

cos – cos

2 cos cos

See: Krishnan, Chap. 3.3.1, El-Sharkawi, Chap 3.3. As more inductance is added to the load circuit the current becomes continuous and the

value of the output voltage is given by:

2

πcosα

See: Krishnan, Chap. 3.3.1, El-Sharkawi, Chap 3.3.

When the current is discontinuous the average or dc voltage is related to the firing angle,

α, and the extinction angle β.

cos

When a dc machine is connected to the converter then the effect of the counter-emf, Ea,

must be included. Figure 1.3 (f), (g), show that if Vin is less than Ea when the thyristors

are triggered (at angle α) then the current will not flow until Vin is greater than Ea. Thus

the original firing angle is effectively displaced to α’. In this case the conduction angle,

γ, becomes γ = (α’ - β). The extinction angle remains the same (El-Sharkawi, Chap.

6.1.4.2). The calculation of the voltage at the terminals of the dc machine is

cos ·

29

Figure 2.3 The Single-phase full-bridge controlled rectifier with dc machine load (α = 60o). (a) The circuit with an RL load and counter emf source. (b) The input voltage

waveform. (c) The input current waveform. (d) The thyristor voltage. (e) The thyristor current. (f) The voltage across the dc machine terminals. (g) The current through the dc

machine.

Where K is the proportion of α’/180 with α’ measured in degrees or α’/π with α’

measured in radians. The dc voltage, Ea, is measured directly from the circuit using the

oscilloscope.

30

The radial speed can be found from

And

· 2π

60

If the motor is loaded with a given torque, the radial speed, ω, can be found from

ω φ

·

Where the motor constant, Kφ, and the armature resistance, Ra, are found by testing the

dc motor separately. Td is the induced or developed torque.

See: Krishnan, Chap. 3.3.1, El-Sharkawi, Chap 6

.

Note that the current phase angle is φ where

Three relationships exist between the firing angle, α, and φ:

The firing angle α is less than φ. Current will conduct at angle φ.

The firing angle α is equal to φ. Current will conduct at angle φ = α.

The firing angle α is greater than φ. Current will conduct at α.

Refer to: 1) Krishnan, Ramu, Electric Motor Drives, Upper Saddle River, N. J., Prentice

Hall, 2001, Chapter 3. 2) El-Sharkawi, M., Fundamentals of Electric Drives, Toronto,

Ontaio, Cengage Learning,

31

III. PRE-LAB CALCULATIONS

The values of Kφ, Ra, and La will be required from the experiment done previously for

ELEC 331 (Experiment 3, DC Machines) or from the preliminary exercise: DC Machine

Parameters. If the values are not available then they will have to be derived again. Refer

to the handout or the Appendix that details the methods of determining the machine

parameters.

IV. PROCEDURE

Warning:

High voltages are present in this experiment! DO NOT make any connection while the power is on.

A. DC Shunt Motor Drive using 1-Phase Full-Wave Rectifier

1. Connect the circuit as shown in Fig. 1.3. Do not connect the input smoothing reactors

at the start of the session. Do not connect the freewheeling diode at the start of the

session. Do not connect the output smoothing inductors at the start of the session. Do

not connect i1, to measure the input ac current at the start of the experiment – use it to

measure the thyristor current initially. Note that the smoothing reactors at the input and

output of the converter and the freewheeling diode will be disconnected and

reconnected to the circuit later during the experiment. Connect the tacho-generator and

meter to the DAQ at input 8/N. When the connections are complete make the

following initial setting on the equipment.

2. PC and Oscilloscope

32

Power-on the Lab-Volt data acquisition module on the bench ( green LED is on). Turn

on the oscilloscope (Fluke Combiscope) and the computer.

Start the Lab-Volt data acquisition software, LVDAC-EMS. On the right-hand (RHS)

panel set all voltages to HIGH. Set all currents to LOW. Make sure that the software is in

the connected mode – not stand-alone. Activate the metering: V1ac, I1ac, P1, V2dc, I2dc,

p2, I3dc, I4dc, Nrpm, TN-m and Pdynamometer. When the motor is running make sure that the

Figure 2.4 Single-phase full-bridge controlled rectifier and shunt connected dc machine.

33

speed, n, and torque, T, correspond approximately with the dynamometer display. To

measure α, β, and, if necessary, γ, use the synch. output from the thyristor module to find

0˚. The synch output (SYNCH. OUT) is a square-wave that is Hi from 0˚ -180˚. Connect

the synch. output to the external trigger (EXT TRIG) of the oscilloscope.

Start the Lab-Volt data table. Activate the values corresponding to the meter

display.

Start the Excel software. Label the columns corresponding to the LVDAC table.

Add a column for the thyristor firing angle, α and the extinction angle, β.

3. Thyristor Rectifier a) Connect the modules as shown in Fig. 1.3 without the input smoothing reactors. Note: The dc source is a part of the Thyristor Firing Unit. b) Choose the following settings: On the Power Supply: Mains Power Switch ........................................................................................ Off Voltage Meter Selector ................................................................................... 4-N Variac .................................................................... 0 or Full Counter Clock-Wise

On the Thyristor Firing Unit: FIRING CONTROL MODE ...............................................................................1

ANGLE CONTROL COMPLEMENT .............................................................. O ANGLE CONTROL ARC COSINE .................................................................. O DC SOURCE ............................................................................................... MAX

On the Power Thyristors module: Interconnection Switch S1 ...................................................................................0 Interconnection Switch S2 ...................................................................................0

On the Oscilloscope: Channel-1 Sensitivity ....................................................... 5 V/DIV (DC coupled) Channel-2 Sensitivity ....................................................... 1 V/DIV (DC coupled) Vertical Mode ..................................................................................... CHOPPED Time Base .............................................................................................. 2 ms/DIV Trigger Source ........................ EXT TRIG (Synch Output on Thyristor Module) Trigger Coupling ............................................................................................. DC

On the Prime Mover - Dynamometer:

34

Display Toggle Switch ............................................ Torque or Speed as required Mode Toggle Switch ...................................................................................... Dyn Operating Mode Toggle Switch .................................................................... Man Manual Mode Potentiometer ................................ Min (Full Counter-Clockwise)

Set the field resistance of the dc machine at its minimum resistance (full clockwise, 0Ω).

The Variac on the main power supply is set to 0, full counter-clockwise. Monitor the

analog ammeter, A, as you turn on the power and raise the voltage on the main power

supply until Vs (E1) is at 120 Vac.

Slowly turn the dc source on the thyristor controller from the max. to the min. position (α

= 180˚ to 0˚). Note the effect on the output dc voltage (e2 - Ch1, E2) and dc current (i2 -

Ch2, I2) waveforms.

On the oscilloscope, connect the external trigger (EXT TRIG) to the synchronized output

(SYNCH. OUT) jack on the thyristor module. (Normally, this trigger is set to the rising-

edge). Calibrate the cursors (1 cycle = 360˚) so that the firing angle, α, and the extinction

angle, β, can be identified. This is done by setting the vertical reference cursor (TRACK)

to the desired angle and then adjusting the voltage control of the thyristor controller to

coincide with the differential (∆) cursor.

Set the mechanical load at 0.0 N-m on the dynamometer. Set the firing angle at 0° on the

thyristor control unit. Use the LAB-VOLT data table and the MEASURE and phase-

angle cursor function on the oscilloscope and observe the performance characteristics as

given in Table 1. Collect the data in the LVDAC table and oscilloscope. Transfer it to

the Excel table for calculation and plotting the results. Remember to convert speed, n,

from rpm to ω in rad/sec as required. This can be done in the Excel spreadsheet.

For data set 1, set T = 0.0 N-m, α = 0˚ and the field resistance to the min. position – full

CCW. Vary the torque, T, on the dynamometer from 0.0 to 0.1 N-m in 4 steps: 0.0 N-m,

0.1 N-m, 0.15 N-m and 0.2 N-m. Record all the current and voltage values in the

LVDAC data table. For 1 setting only, record α, β, and, if necessary, γ from the

oscilloscope and plot the oscilloscope trace. To measure α, β, and, if necessary, γ, use

35

the synch. output from the thyristor module to find 0˚. The synch output is a square-wave

that is Hi from 0˚ -180˚.

For data set 2, reset the parameters and vary the field potentiometer from the min.

position (0 Ω) to the max. position (500 Ω) in 4 steps. Record all values.

For data set 3, reset the parameters and vary the firing angle α, from 0˚ to 90˚ in 4 steps –

0˚, 40˚, 60˚ and 90˚. Record the values of α separately. For 1 setting only, record α, β,

and, if necessary, γ from the oscilloscope and plot the oscilloscope trace. To measure α,

β, and, if necessary, γ, use the synch. output from the thyristor module to find 0˚. The

synch output is a square-wave that is Hi from 0˚ -180˚. Connect the synch. output to the

external trigger of the oscilloscope. Record all values as before.

Table 1. Converter Performance Data Parameter Torque, N-m If, Adc (I4) α, β, or γ

degrees

(From

Oscilloscope)

Plot in Excel

Data Set 1 Vary: 0, 0.1,

0.15, 0.2

Min. R, Max. If,

full CW

α = 0˚ ω versus T (T on

x-axis)

Data Set 2 0.0 Vary - CCW:

from Min R to

max. R in 4 steps

α = 0˚ ω versus If (If on

x-axis)

Data Set 3 0.0 Min. R, Max. If,

full CW

Vary α: 0˚, 40˚,

60˚, 90˚

ω versus α (α on

x-axis)

After the data has been collected as per the scheme in Table 1, continue to the next

section.

36

Output Smoothing Reactor. Set α=40˚and T = 0.1 N-m. Record the values of the

circuit voltages and currents. Set the oscilloscope display to record the maximum values

of the waveforms (MEAS. Function). Set the cursors to the # mode and find amplitude of

the voltage waveform components (= control) as well as the voltage and current angles,

α, β, and γ (|| control). Print the oscilloscope display. Write the angles on the printout if

necessary.

Set the angle control to full clockwise and Rf to 0Ω, full CW. Turn off the power (the

variac is full counter clockwise and the power switch is off). Connect the output

smoothing reactor. Restart the motor and set the controls so that α = 40˚, T = 0.1 N-m.

Note the differences in the waveforms, record the data and plot the oscilloscope

waveforms, Vdc and Idc. Turn-off the motor and remove the smoothing inductor.

Free-Wheeling Diode (FWD). Start the motor and adjust the firing angle, α, to 90˚ and

T = 0.1 N-m. Record the values of the circuit voltages and currents. Set the oscilloscope

display to record the maximum values of the waveforms (MEAS. Function). Set the

cursors to the # mode and find amplitude of the voltage waveform components ( =

control) as well as the voltage and current angles, α, β, and γ (|| control). Print the

oscilloscope display. Write the angles on the printout if necessary.

Set the angle control to full clockwise and turn off the power (the variac is full counter

clockwise and the power switch is off). Connect the free-wheeling diode. Restart the

motor and set the controls so that α = 90˚, T = 0.1 N-m. Note the difference in the output

waveforms and plot the oscilloscope traces, Vdc and Idc. Record the circuit data as

before. Turn off the power (the variac is full counter clockwise and the power switch is

off). Disconnect the free-wheeling diode.

Thyristor Voltage and Current. Set the mechanical load at 0.1 N-m on the

dynamometer. Set the firing angle at 40° on the thyristor control unit. Use the LAB-

VOLT data table and the MEASURE and CURSOR functions on the oscilloscope to

obtain values for the average armature voltage, armature current, thyristor current and

37

thyristor voltage. Using the oscilloscope, obtain traces of the waveforms of the armature

voltage, armature current, and thyristor voltage and thyristor current waveforms. Use the

oscilloscope to find a value for Ea, the counter emf.. If necessary, use the oscilloscope

MEASURE function to record the dc values of the voltage and current. After the data is

recorded in the Lab-Volt data-table it can be highlighted, copied and pasted into an Excel

spreadsheet for analysis. Be sure to make a plot of the oscilloscope waveforms. When

the measurements have been taken, set the voltage control (α control) of the thyristor

controller to the max. position (full clockwise). Turn down the main Lab-Volt power

supply (full counter-clockwise) and turn off the switch.

Verification of DC Machine Constants by the Application of a Load Torque

(Transient Test, State-Space Block Diagram Model). To verify the results of the tests

done to determine the machine parameters connect the circuit shown in Fig. 1-5 below.

Start the dc machine with T = 1.0 N-m. The traces to be shown on the oscilloscope will

represent the speed of the machine and the load (torque) applied by means of the

dynamometer. The oscilloscope time-base should be adjusted to 100 ms/div or 200

ms/division. Channel 1 on the oscilloscope is connected to the meter of the tacho-

generator. Channel 1 is set to 1 V/division. Channel 2 on the oscilloscope is set to 5 V/div

and is connected to the torque output of the dynamometer.

Change the MODE toggle switch on the prime mover-dynamometer from the DYN

(dynamometer) position to the PRIME MOVER position. This will unload the machine.

Since there is no power supplied to the unit (120 Vdc), the dynamometer is coasting.

With the Lab-Volt data table, take an initial set of readings to record the speed and

torque.

When the oscilloscope begins a sweep, apply the 1.0 N-m torque to the machine by

putting the toggle-switch back in the DYN position. With the Lab-Volt data table, take a

set of readings to record the speed and torque. When the motor speed is stable push the

run/stop button on the oscilloscope to freeze the speed and torque traces. Use the cursors

to find the total time required for the motor to stop. Print the oscilloscope display.

38

Figure 2-5 Circuit used to test simulation results of machine parameter calculations.

Compare the initial and final speeds, torques and the total stop time with the results of the

PSIm simulation. Note that the dc source is the Lab-Volt power supply and not the

single-phase full-bridge thyristor converter.

V. QUESTIONS

1. For dataset 1, use Excel to plot the radial speed, ω, versus the torque, T. Calculate ω

based on one value of the measured torque. Use corresponding values of Vdc, Kφ, and

Ra. How does a change in α effect Vdc and ω?

2. For dataset 2, plot ω versus If (If on the x-axis). How does a change in If effect the

speed ω? Is the result linear? Find an approximate constant Kf such that ω ≈ Kf/If

3. For dataset 3, use Excel to plot speed, ω, versus α (α on x-axis). For one value of α

(and the corresponding β or γ) calculate ω. How does a change in α effect Vdc and ω?

39

4. What is the effect of adding and removing the freewheeling diode on the load side?

Refer to the oscilloscope plots.

5. What is the effect of adding or removing the smoothing reactor at the output of the

rectifier?

6. When the current is discontinuous, what is the voltage that appears in the output

waveform, Vdc, before the thyristor is triggered and conduction starts?

7. Use PSIM to simulate the steady-state performance of the single-phase full-bridge

thyristor converter and motor when α = 0˚, field current, If, was at a maximum value (Rf

at min. position, full-CW), and T = 1.0 N-m.. Simulate the motor using Ra, La, and Ea.

Ea is found from the oscilloscope trace of Vdc or from Kφ. Note: this PSIM circuit will

require a bridge consisting of individual thyristors, a voltage sensor, an α-controller

(alpha controller) as well as the dc machine model. Check with your demonstrator if you

are not sure of the parameters derived previously.

8. Use PSIM to verify the transient performance of the state-space block diagram model

of the machine. This test is described in the procedure above and in the machine

parameter testing procedure.

B. Full-Wave Voltage Control of the Separately-Excited DC Machine The separately-excited dc machine is shown in Fig. 1-6 below. This configuration allows

for independent control of the armature and field voltages. The shunt field is normally

used when the dc machine is connected in this manner and the field rheostat is at a

minimum resistance (0Ω). Thus the maximum field current, If, will flow. The total

electrical power input to the machine in Fig. 1-4 is

, · ·

40

Figure 2.6 The separately-excited dc machine circuit.

If the field voltage, Vf, and the armature voltage, Va, have the same voltage source, Vs,

then

, ·

The mechanical power at the output of the machine is the product of the speed, ω (rads/s)

and the machine shaft-torque, Tm (N-m).

, ·

The values of Va, Ia, Vf, If, Tm and n (rpm) can be measured directly from the

experimental test circuit. The radial speed, ω, (rads/s) must be calculated.

Set the field regulator at its minimum resistance (full clockwise, 0Ω). By changing the

firing angle of the thyristors, the armature voltage can be controlled. As a result, the

speed can be controlled.

Procedure Connect the circuit as shown in Fig. 1.7. Do not connect the input

smoothing reactors. Do not connect the freewheeling diode. Do not connect the output

smoothing inductors. Do not connect i1, to measure the input ac current – use it to

measure the thyristor current. When the connections are complete make the following

initial setting on the equipment, the PC and Oscilloscope.

Figure 2.7 Single-phase full-bridge controlled rectifier and separately-excited shunt-field dc machine.

Shunt Field

Vs, field

+Vf

Armature

DC Machine

La

Ra

Lf

Field Rheostat

Rf

If Ia

+

VaVs,

armature

Ea+

41

Power-on the Lab-Volt data acquisition module on the bench ( green LED is on). Turn

on the oscilloscope (Fluke Combiscope) and the computer.

Start the Lab-Volt data acquisition software, LVDAC-EMS. On the right-hand (RHS)

panel set all voltages to HIGH. Set all currents to LOW. Make sure that the software is in

the connected mode – not stand-alone. Activate the metering: V1ac, I1ac, P1, V2dc, I2dc,

p2, I3dc, I4dc, Nrpm, TN-m and Pdynamometer. When the motor is running make sure that the

speed, n, and torque, T, correspond approximately with the dynamometer display. To

measure α, β, and, if necessary, γ, use the synch. output from the thyristor module to find

0˚.

A

4

Gnd.

N

Thyristor

O

I

O

I

S 1

S 2

2

1

Shunt Field

2

1

Power Thyristors

DC Machine

Timing Belt

N T

Speed Output

Torque Output

Prime Mover

Manual ModeMin.

Display

Dyn. Max.

Display

1

Mode

To DAQ

Torque

2

Man.

Prime Mover - Dynamometer

Speed

Input

Ext.

Smoothing Inductors

I 2 i 2

E 2

Diode from Power Diodes Module

24 Vac Firing Unit

Angle Control

DC Source

To DAQ To DAQ

To DAQ

0 - Vac

120

+

+

+

+

+

+

+

9

Conductor cable Control

Firing Inputs

To Osc.

e 2

To Osc.

To Osc.To DAQ

I 4

To DAQ Analog

Gnd

To DAQ

1 2

3 4

E 1

I1

i 1

i 1

e 1

5

6

7 8Smoothing Inductors

To Osc.

Armature5

6

7 8

To DAQ

5

6 7

8

I 3

+

0 - Vdc

120

7

N

+

+

+ +

+

+

Gnd.Synch Output

To DAQTacho Gen.

42

Start the Lab-Volt data table. Activate the values corresponding to the meter

display.

Start the Excel software. Label the columns corresponding to the LVDAC table.

Add a column for the thyristor firing angle, α and the extinction angle, β.

4. Thyristor Rectifier a) Connect the modules as shown in Fig. 1.3 without the input smoothing reactors. Note: The dc source is a part of the Thyristor Firing Unit. b) Choose the following settings: On the Power Supply: Mains Power Switch ........................................................................................ Off Voltage Meter Selector ................................................................................... 4-N Variac .................................................................... 0 or Full Counter Clock-Wise

On the Thyristor Firing Unit: FIRING CONTROL MODE ...............................................................................1

ANGLE CONTROL COMPLEMENT .............................................................. O ANGLE CONTROL ARC COSINE .................................................................. O DC SOURCE ............................................................................................... MAX

On the Power Thyristors module: Interconnection Switch S1 ...................................................................................0 Interconnection Switch S2 ...................................................................................0

On the Oscilloscope: Channel-1 Sensitivity ....................................................... 5 V/DIV (DC coupled) Channel-2 Sensitivity ....................................................... 1 V/DIV (DC coupled) Vertical Mode ..................................................................................... CHOPPED Time Base .............................................................................................. 2 ms/DIV Trigger Source ........................ EXT TRIG (Synch Output on Thyristor Module) Trigger Coupling ............................................................................................. DC

On the Prime Mover - Dynamometer: Display Toggle Switch ............................................ Torque or Speed as required Mode Toggle Switch ...................................................................................... Dyn Operating Mode Toggle Switch .................................................................... Man Manual Mode Potentiometer ................................ Min (Full Counter-Clockwise)

43

Set the field resistance of the dc machine at its minimum resistance (full clockwise, 0Ω).

The Variac on the main power supply is set to 0, full counter-clockwise. Monitor the

analog ammeter, A, as you turn on the power and raise the voltage on the main power

supply until Vs (E1) is at 120 Vac.

Set the mechanical load, using the dynamometer, at 0.1 N-m. Change the armature

voltage and measure the armature current, field current, and speed. Enter the measured

values in the Lab-Volt data table and transfer them to an Excel spreadsheet for

calculation.

Table 2. Data for Separately-Excited DC Machine with Thyristor Controller.

Va(V)

Ia(A)

IF(A)

ω (rad/s)

PElec.,in(W)

PMech.,out(W)

(%)η

Questions

1. Calculate the input electrical power for each measured point of operation.

2. Calculate the output mechanical power for each measured point of operation.

3. Calculate the efficiency, for each measured point of operation.

4. Plot the speed vs armature voltage.

5. Plot the efficiency vs armature current.

44

Gnd.2

1

Power Thyristors 2

DC Machine

Timing Belt

N T

Speed Output

Torque Output

Prime Mover

Manual ModeMin.

Display

Dyn. Max.

Display

1

Mode

To DAQ

Torque

2

Man.

Prime Mover - Dynamometer

Speed

Input

Ext.

+I4

To DAQ Analog Gnd

To DAQ

1 2

3 4

5 6

7 8

Armature

5 6

7 8

+

4

Vc

Thyristor

O

I

O

I

S1

S2

2

1

Power Thyristors 1

24 Vac

Firing Unit

Angle Control

DC Source

Va

9 Conductor cable

Firing Control Inputs

e1

1 2

3 4

To Osc.+

+

E4

I1+

Shunt Field

+To DAQ

5

67

8

I3

+

0 -Vdc

120

7

N

E2+

i2

+

I2+

E2+

0 – 120/208 Vac

5

6

Vb

4

Vc

Thyristor

O

I

O

I

S1

S2

2

1

24 Vac

Firing Unit

Angle Control

DC Source

Va

9 Conductor cable

Control Firing

Inputs

i1e1

To Osc.+

+

5

6

Vb

0 – 120/208 Vac

A+

e2+

+A

+

e2+

Smoothing Inductors 2

Smoothing Inductors 1

Gnd.Synch Output

Gnd.Synch Output

i1

Figure 2-8. Four-Quadrant Thyristor Drive.

45

EXPERIMENT 3

DC/DC Chopper Controlled DC Motor Drives

I. OBJECTIVES

There are two objectives to this experiment:

- To evaluate the performance of the single-phase 4-quadrant chopper when connected

to a separately-excited dc machine.

- To extend the state-space based model of the dc machine to include a speed control

loop with proportional-integral (PI) control. This model will be used for the design

and simulation of the circuit using the PSIM software. The results of the simulation

will be compared with the performance of the dc machine. II. INTRODUCTION The Fig. 2-1 shows the circuit model of a 4-quadrant dc chopper. The chopper is

connected to a separately-excited dc motor. Note that the switches are numbered to

match the panel of the Lab-Volt MOSFET module. The switching pattern shown is the

bipolar voltage scheme. In this example a bipolar triangular carrier wave is compared to

a reference voltage. The carrier voltage varies between Vmax = +10 Vdc and Vmin = -10

Vdc. This is a fixed pattern. The reference voltage is raised or lowered as the voltage

and the current across the armature of the machine are required to change. The switches

operate in complementary pairs. A positive voltage and current flow is observed when

Q1 and Q5 are switched-on and Q2 and Q4 are off. Since this is a voltage-controlled

pattern, switches in the same leg (Q1 and Q4 or Q2 and Q5) cannot turn-on at the same

time as this would result in a short-circuit. The switching action occurs within a given

period, T, which is determined by the switching frequency, fsw ( T = 1/fsw).

The modulation index is

10

46

+

+

+

Fig. 2-1. The 4 Quadrant dc chopper (a) circuit, (b) Vdc > 0, (c) Vdc < 0.

47

Thus, when the reference voltage equals 0, the modulation is 1/2. The duty cycle, D or δ,

is a measure of the ratio between the time a switch is on, ton, and the period, T, of the

switching pattern. The duty cycle for a switch or a pair of complementary switches is

If D is multiplied by 100 then the duty cycle is given as a percentage. The design of the

PWM hardware on this test-bench means that D = M.

The average output voltage as seen by the dc machine is

1

By substituting D = M = ton / T the armature voltage is

2 1

Since the rms voltage is independent of fsw then Vrms = Vs. In theory, there is no ripple

voltage present when M is equal to 0 or 1. Since these values are not realizable on the

test equipment, the voltage ripple, Vr, is given by

2 1

The voltage ripple factor (RF) is

2 12 1

The positive current for switches Q1 and Q5 for the period determined by M (Vdc = Vs) is

The average current is

1

48

for 0 < t < ton.

The value for the initial current, I-init is

·2 1

1

When switches Q2 and Q4 are turned-on (Vdc = -Vs)

and by starting the calculation time at ton , the dc current is

1

for 0 < t < T - ton. The initial condition is

·1 2

1

The average current per-cycle is

,

Note that in the circuit that will be used for the experiment, an additional smoothing

inductor has been added. The added reactance reduces the ripple of the output current

significantly and should be added to the value of La if a calculation of the current of the

test circuit is required.

III. CALCULATIONS A 4-quadrant single-phase chopper depicted in Fig. 2-1, has a nominal supply voltage,

Vs, of 120 Vdc. The actual value of the dc source is closer to 140 Vdc. The chopper is

required to drive a dc machine at a speed, n, of 1000 rpm. Initially, the machine is

unloaded, TL = 0.0 Nm, and an additional source of inertia is added to the shaft of the

49

motor with the addition of the prime mover - dynamometer. The speed and torque

constants of the machine (Kφ, KT) are the same as those determined in experiment 1.

1. If Vmax of the sawtooth carrier wave is 10 Vdc and Vmin is -10 Vdc (Fig. 1-1),

compute the modulation index, M, of the chopper when the reference voltage is

at 0 Vdc, +5 Vdc, -5 Vdc, +10 Vdc and -10 Vdc. What is the duty cycle, D,

when M = 0.5?

2. Compute the chopper output voltage, Vdc, when the reference voltage is at 0

Vdc, +5 Vdc, -5 Vdc, +10 Vdc and -10 Vdc.

3. What is the average armature current, Ia, when n = 1000 rpm? (Use the value of

the speed constant determined by earlier experiment.)

IV. PROCEDURE

Warning:

High voltages are present in this experiment! DO NOT make any connection while the power is ON.

1. Four Quadrant Chopper Drive for DC Motors – Circuit Connection 1. Connect the circuit shown in Fig. 2-2. (Bipolar SPWM) Do not connect the speed

feedback circuit. The PID module will be connected later. Connect the Lab-Volt prime mover – dynamometer to the shaft of the dc motor with the timing belt.

2. Choose the following settings:

On the Power Supply: Voltage Selector ..................................................................................................... 7-N

On the Chopper/Inverter Control Unit: MODE ..................................................................................................... CHOP.PWM DC SOURCE 1 ............................................. 3/4 position, clock-wise, CW, 3 o’clock

50

DC SOURCE 2 ........................................................................................ mid position

On the Power MOSFETS module: Interconnection Switch S1 ...........................................................................................1

On the Oscilloscope: Channel-1 Sensitivity ................................................................ 2 V/DIV (dc coupled) Channel-2 Sensitivity ................................................................ 2 V/DIV (dc coupled) Vertical Mode .............................................................................................. CHOPped Time Base ..................................................................................................... 2 ms/DIV Trigger Source .......................................... Ext, from Synch Out of MOSFET module Trigger Coupling ..................................................................................................... DC

2. Connect channel 1 of the oscilloscope to input 1 and the common of the MOSFET module. Connect channel 2 to input 4. Connect the synchronizing output of the MOSFET module (Synch Out) to the external trigger of the oscilloscope. (Fluke model, Refer to the Appendix for setting the oscilloscope).

3. Connect the shunt field of the dc motor to the fixed 120 Vdc source. Put the shunt field rheostat in the minimum position (full clockwise, 0 Ω).

4 The prime mover – dynamometer should be in the dynamometer mode under manual control with the manual potentiometer in the minimum position (full counterclockwise).

5. The tacho-generator should be connected to the dc motor. The output from the meter box should be connected to the Lab-Volt data acquisition module (8, N). A DMM should be connected across DC source 1 and the common to measure the control voltage, Vdc. Note the gain of the tacho-generator, n, rpm to Volts, dc. An Excel spreadsheet should be opened to store the observed data.

2. The Chopper-Inverter Control Unit

1. After the circuit is checked by your demonstrator, turn on the power supply and adjust to 120 Vac or full scale on the autotransformer (Variac) dial.

2. Using the oscilloscope, observe the waveform of the PWM by viewing the signal

from the gate of switch 1 (Input 1). The cursors can be adjusted to display t, 1/t (frequency) and ∆t as required. Slowly adjust the potentiometer of DC Source 2. What effect does the potentiometer for DC Source 2 have on the signal? Record the effect at the minimum, midpoint and maximum position of the trimpot. Return the potentiometer to the mid-position. Note the switching frequency, fsw, (Hz) and speed ,n (rpm).

51

Fig. 2-2. The Lab-Volt circuit.

3. Using the oscilloscope, observe the waveform of the PWM by viewing the signal from the gates of switches 1 (Input 1) and 4 (Input 4). Slowly adjust the

52

potentiometer of DC Source 1. Verify the switching action that is shown in Fig. 2-1 with respect to switches on the same leg.

4 Change channel 2 of the oscilloscope from the gate of switch 4 (Input 4) to the output dc voltage, e2. What effect does the potentiometer for DC Source 1 have on the PWM signal, duty-cycle (D), the chopper output voltage (E2), and the direction of motor rotation? Record the effect at the minimum, 25%, midpoint, 75%, and maximum position of the trimpot. Complete the data shown in Table 1 below. The data can be stored in an Excel file. Return the potentiometer to the 3/4 position, clock-wise, CW, 3 o’clock.

5 Change channel 1 of the oscilloscope from the gate of switch 1 (Input 1) to the voltage across the switch, e1. Change channel 2 of the oscilloscope from the dc output voltage to the current through the switch, i1. (CH1, 5 V/div., CH2, 0.1 V/div., MTB=200 us, EXT trig.). What effect does the potentiometer for DC Source 1 have on the PWM signal, and duty-cycle (D). Plot these waveforms and compare them to Fig. 2-1. Return the potentiometer to the 3/4 position, clock-wise, CW, 3 o’clock.

6 Make a plot of the speed versus the voltage of DC Source 1 (DC Source 1 on x

axis, speed, rpm, on y axis) to check your results. The data to be collected is summarized in Table 1. Make a plot of the switching frequency, fsw, versus the voltage of DC Source 2 (DC Source 1 on x axis and fsw, Hz, on y axis). Plot 3 points (min., mid., max.) to check your results.

Table 1. DC Source 1 DC Source 1, Potentiometer Position

Minimum 25% Midpoint 75% Maximum

DC Source 1, Vdc, DMM

Duty Cycle, D, %

Modulation Index, M

Output Voltage, E2

Motor Rotation, CW, CCW

Motor Speed, n, rpm

53

7 Start the dc motor and adjust the switching frequency to 1 kHz. Adjust the speed to 1000 rpm. Set the prime mover to the manual dynamometer mode and set the load on the motor to 1.0 N-m. On the oscilloscope set the time-base to 500 ms/div and record the shaft-torque (CH1) and the speed (CH2) when the load is applied as a step input. Plot the oscilloscope results and note the changes in the motor performance.

2. The Implementation of a PI Speed Control Loop

The speed control-loop is shown in Fig. 2-3. The speed loop has a nominal current

protection built into the bipolar-limiter section. The transfer function of the converter-

motor block was developed in a previous experiment. The gains of the individual blocks

can be determined experimentally by examining the input-output requirements of each

component and the matching them. The tuning of the proportional-integral controller

section requires a more sophisticated approach. The converter-motor is turned on and

testing can be done on an unloaded system. Initally, the integral control section, KI , is set

to zero and the proportional gain is increased until the output, ωout , begins to oscillate.

The gain associated with the oscillation is Kosc and the period of the oscillation is Posc.

Figure 2-3. The speed control loop.

The gains of the proportional and integral blocks can be determined from the Ziegler-

Nichols method where,

54

0.45

And 1.2 ·

This method can be used on the system under test as well as on the simulation circuit. As

well, if a particular gain is not possible, KI for example, then the equations can be

calculated in reverse to find a matching value of KP. This assumes that the period of

oscillation remains fixed.

A common representation of the PI controller (PSIM) is

1

Where

3. PID Controller Connection 1. The speed control circuit with the PI control section is shown in Fig. 1-2. Connect the

speed control loop as shown in Fig. 2-4(a). When the components have been connected

check with the demonstrator. Make the following settings on the Chopper-Inverter

Control Unit:

MODE. Set the mode to CHOP. PWM

DC SOURCE 1. Leave in the same position from the previous section. It should be set at

1000 rpm (5 o’clock).

DC SOURCE 2. This should be in the same position as the previous section. It should be

set for fsw = 1 kHz (mid-position or 12 o’clock).

On the PID controller make the following settings:

55

FEEDBACK INPUT AMPLIFIER A1. Set this gain to 1/3 of the range (10 – 11

o’clock).

LOW-PASS FILTER CUTOFF FREQUENCY. Put the dial in the 12 0’clock or mid-

position.

PROPORTIONAL GAIN. Disconnect (Point X) and set this gain to the minimum

position.

INTEGRAL, DERIVATIVE GAIN. Set this to the minimum position.

UPPER LIMIT. Adjust the upper limit to 2/3 of the range (2 - 3 o’clock).

LOWER LIMIT. Set this potentiometer to the maximum position.

Figure 2-4. The PID circuits: (a) set-up and (b) analysis.

56

OSCILLOSCOPE CH1. Output of the proportional control, 50 mV/div., MTB = 500

msec/ div. AC mode.

OSCILLOSCOPE CH2. Output of the tacho-generator, 50 mV/div., MTB = 500 msec/

div., AC mode.

OSCILLOSCOPE (Fluke-Philips). Math1 Filter is on at m1 = filt acq, PARAM = 41

samples.

Start the motor and raise the proportional gain potentiometer slowly until there is an

oscillation of the waveform of CH1 or CH2. This should be approximately the max.

position of the P trimpot. Reduce this approximately ½ rotation or until the oscillation is

minimized. Note the position of the P control dial. Turn the power supply down, stop the

motor and reconnect the output from the integral amplifier.

Start the motor and adjust the speed to 1000 rpm when the system is unloaded. The

MODE switch on the Prime-Mover Dynamometer is in the Prime-Mover position. Since

there is no dc power to this unit, the prime-mover is inactive. The feedback amplifier, A1,

may have to be raised slightly as well as the upper limit. The oscilloscope should have

the following settings:

OSCILLOSCOPE CH1. Output of the proportional control, 50 mV/div., MTB = 500

msec/ div. DC mode.

OSCILLOSCOPE CH2. Output of the tacho-generator, 0.2 V/div., MTB = 500 msec/

div., DC mode. The x-axis of CH2 should be displaced below the screen at the bottom.

OSCILLOSCOPE (Fluke-Philips). Math1 Filter is on at m1 = filt acq, PARAM = 41

samples.

Have the demonstrator check the circuit and make a few trial tests with the step load to

determine the stability of the circuit.

2 . While the motor is running, throw the MODE switch on the dynamometer to load the

motor with a step torque of 1.0 N-m. Observe the response on the oscilloscope. Raise the

integral gain by turning the trimpot slightly clock-wise. Remove and re-apply the load.

57

Note the change in the speed response. If the loaded and unloaded speeds are not the

same, the gain of amp A1 may have to be adjusted. Repeat until a satisfactory transient

response is obtained. Record and print the result on the oscilloscope. Note the changes in

the motor performance. Record all voltages and currents using the Lab-Volt data sheet

and transfer them to an Excel file. Repeat this exercise by recording the armature current

response, i2, and the speed response on the same plot. Before stopping the motor record

the tacho-generator speed and output voltage. Write the results in TABLE 2 below.

When a set of satisfactory results are recorded and plotted turn off the motor but do not

change any of the settings.

Use the DMM in order to identify the gains of the individual components: first measure

the output from DC Source 1 of the chopper control unit using the DMM. This is the

variable ωf in Fig. 2-3. Now set the DC SOURCE 1 to 1.0 Vdc. This voltage will be

injected into each block of the control loop individually and the output will be recorded.

Component Vin Vout Gain Tacho-generator Output, (Input is speed, rpm)

( RPM) na

Feedback Amp, A1 Low-Pass Filter Error Detector, A2, +ve input, Vref DC Source 1

Error Detector, A2, -ve input, Vfeedback

Proportional Amp, KP

Integral Amp, KI 1 Vdc Step Function to PI

* * *

Summing Amp Limiter Voltage at Control Input 1

na

58

The gain of each block is calculated from these values. Turn off the power (+15 Vdc, -15

Vdc ) on the power

supply before reconnecting the wires on the PI module.

Use the DMM to record the input and output voltages of each block of the PI control

loop. The gain of each component can be computed and recorded in the Excel spread

sheet. Refer to Fig. 2-2, Fig. 2-3 and Fig. 2-4. Table 2 lists the required measurements. In order to test the PI control block construct the circuit of Fig. 2-4 (b). Set the

oscilloscope channels to

CH1 DC SOURCE 1, 2 V/div., MTB = 500 ms/div.

CH2 Output of PI, 5 V/div., MTB = 500 ms/div.

The test is done by turning off the power to DC SOURCE 1 or by disconnecting the wire

at the output. The oscilloscope trace is then started and the power is applied or the wire

re-connected. This is a 1.0 Vdc step-input to the PI section. When the PI action is

complete, the oscilloscope trace is stopped. Fig. 2-5 shows the form that the response

should take.

Figure 2-5. Response of the PI block.

59

The proportional gain is given by the voltage level of KP. The value of KI is given by the

ratio of ∆V/∆t.

Figure 2-6. The PSIM model of the dc motor and speed control circuit.

60

V. QUESTIONS 1. The dc motor and the speed control circuit are shown in Fig. 2-3 and in the PSIM

circuit format in Fig.2-6. Use the information collected from the load test and the

measurements of the speed control loop block gains to complete the missing PSIM

parameters. Run the simulation with the gain values that were observed on the Lab-

Volt test-bench circuit and compare the response of the actual motor to a step load of

1.0 N-m with the output of the PSIM circuit. Discuss the response times, overshoot,

oscillation and maximum current levels. Note that the inertia constant, J, has been

doubled ( Fig. 2-6, TFCN3 , J = .0025 not .0025) to account for the addition of

the prime mover – dynamometer.

2. In the PSIM circuit of Fig. 2-4 replace the PI block with a proportional gain, KP. Set

the load torque to 0.0 N-m. Raise the value of the gain until the output variable

begins to oscillate. Use the Zeigler-Nichols method to recalculate the values of the

proportional and integral gains, KP and KI. Put the new PI controller in the circuit

and replace the step load of 1.0 N-m at 2 seconds. Run the simulation with the new

values. Discuss the difference in the responses of the two circuits – the new

simulation versus the simulation using the test-bench values. What is the nature of

the speed and current responses?

3. Discuss the need for an inner current-control loop. Would it include a PI element?

How would the time constant compare to the outer speed loop? How has this been

addressed in the test-bench circuit?

61

EXPERIMENT 4

INDUCTION MOTOR DRIVES

I. OBJECTIVES There are two objectives to this experiment:

- To observe the performance of the squirrel-cage induction motor under voltage

control;

- To observe the performance of the squirrel-cage induction motor as the frequency of

the applied power changes.

II. INTRODUCTION Squirrel –cage induction machines have been used extensively for many years in variable

speed drive applications, such as in automobiles, trains, aircraft, air conditioners, heaters

and defrosters. Control of the performance of this type of induction machine falls into

two classes: scalar and vector.

The scalar control methods include voltage control, frequency control, volt/frequency

control and slip energy. The easiest methods to implement are the voltage and frequency

techniques. An appreciation of the characteristics and limitations of these approaches

provides the basis for the study of more advanced and more efficient techniques.

III. AIR-GAP FLUX AND SUPPLY FREQUENCY The air-gap induced emf, E1, of the 3-phase squirrel cage induction motor is given by

4.44 · · · ·

The coefficients are: kω1 is the stator winding factor (constant), φm is the air-gap flux, fs is

the supply frequency, and T1 is the number of turns per-phase in the stator (constant).

62

Combing the constants yields

4.44 · ·

By omitting the stator impedances Rs and jXls , the phase voltage of the motor, Vph,m, is

equivalent to the air-gap emf E1. This produces the essential relationship

, ·

Thus the phase voltage of the induction machine is linked to the frequency of the supply

and the flux in the air-gap. The final shaft-speed of the motor if is related to the supply

frequency, fs, by the slip. (Krishnan, Chap. 7).

In this experiment the motor will first be operated with a variable supply-voltage

amplitude and a set of fixed parameters (supply frequency). Then, the motor will

performance will be tested with a set of fixed parameters (supply-voltage) and a variable

supply frequency.

IV. PROCEDURE

Warning:

High voltages are present in this experiment! DO NOT make any connection while the power is ON.

1. Connect the circuit shown in Fig. 4-1. Do not connect the current and voltage

isolators assigned to the switch Q1 (e1, i1). The interconnection switch on the

MOSFET module should be in the closed or I position. Connect the Lab-Volt

prime mover – dynamometer to the shaft of the motor with the timing belt.

Choose the following settings:

On the Power Supply:

Voltage Selector ..................................................................................................... 4-N

On the Chopper/Inverter Control Unit:

MODE ................................................................................................... 3 ∼ Phase 180o

63

DC SOURCE 1 ............................................. 3/4 position, clock-wise, CW, 3 o’clock

DC SOURCE 2 ................................ mid position On the Power MOSFETS module:

Interconnection Switch S1 ...........................................................................................1

On the Oscilloscope:

Channel-1 Sensitivity ................................................................ 5 V/DIV (dc coupled)

Channel-2 Sensitivity ................................................................ 5 V/DIV (dc coupled)

Mode ................................................................................................................. Digital

Time Base ................................................................................................... 10 ms/DIV

Trigger Source .......................................... Ext, from Synch Out of MOSFET module

Trigger Coupling ..................................................................................................... DC

A) Connect channel 1 of the oscilloscope to the voltage isolator, e2. Connect

channel 2 to the current isolator, i2. Connect the synchronizing output of the

MOSFET module (Synch Out) to the external trigger of the oscilloscope. (Fluke

model, Refer to the Appendix for setting the oscilloscope).

B) The prime mover – dynamometer should be in the dynamometer mode under

manual control with the manual potentiometer in the minimum position (full

counterclockwise).

C) The tacho-generator should be connected to the motor. The output from the

meter box should be connected to the Lab-Volt data acquisition module (8, N).

dc. An Excel spreadsheet should be opened to store the observed data.

D) The resistor in the dc link consists of three, 172 Ω resistances connected in

series. Each series element is composed of three parallel resistors: 300 600

1200 Ohms.

E) Set up the Lab-Volt data table to record the following parameters:

E1, I1, P1, E2, I2, P2, E3, I3, P3, I4, E4, P4 and PF4. Also, record motor speed,

n, and torque. Assign a meter to report the power factor for E1 and I1.

64

Figure 4-1. Test Circuit for Voltage and Frequency Speed Control

65

2. Speed Control by Variable Voltage

a) Turn on the power supply and slowly increase the voltage until the motor starts

to rotate. The direction should be in a clockwise direction.

b) Adjust the oscilloscope so that the waveform of the line voltage of the motor,

Vab, (E3) is visible on the oscilloscope as well as the frequency.

c) Set VOLTAGE SOURCE 1 on the Chopper-Inverter controller so that the

switching frequency is 20 Hz. Turn the variable voltage supply (VARIAC) to

20%.

Verify that the reported tachometer speed is not greater than 600 rpm.

Adjust the torque until the motor phase-current is 1.5 A or the motor stalls.

Record the data in the Lab-Volt date table. Repeat with the Variac at 30% to

90% in 10% increments. The maximum phase current, I3, should be 1.5 Amps.

d) Complete the data set shown in the Table 4-1 below.

Table 4-1 Data for the Variable Speed Tests (20 Hz, Iph) Voltage (% Varaiac)

fs , Hz Motor Current, I3 1.5 A max.

All Other parameters

20 20 - 30 20 - 40 20 - 50 20 - 60 20 - 70 20 * Caution 80 20 * Caution 90 20 * Caution

e) Repeat the test sequence for a fixed frequency of 60 Hz as shown in Table 4-2

below. After the tests are finished turn off the motor and have the laboratory

demonstrator check your data.Verify that the reported tachometer speed is

not greater than 600 rpm.

Table 4-2. Data for the Variable Speed Tests (60 Hz, Iph) Voltage (% Varaiac)

fs , Hz Motor Current, I3 1.5 A max.

All Other parameters

66

20 60 *Caution 30 60 *Caution 40 60 - 50 60 - 60 60 - 70 60 - 80 60 - 90 60 -

3. Variable Frequency Speed Response

a) Turn on the power supply and slowly increase the voltage until the motor starts

to rotate. The direction should be in a clockwise direction.

b) Measure the oscilloscope so that the waveform of the line voltage of the motor,

Vab, (E3) is visible on the oscilloscope as well as the frequency

c) Set VOLTAGE SOURCE 1 on the Chopper-Inverter controller so that the

switching frequency is 20 Hz. Turn the variable voltage supply (VARIAC) to

50%. Adjust the torque until the motor phase-current is 1.5 A or the motor

stalls. Record the data in the Lab-Volt date table. Repeat with the frequency 20

Hz and at 40 Hz to 60 Hz in 10 Hz increments. The maximum phase current, I3,

should be 1.5 Amps. The test sequence is shown in Table 4-3. When the test is

finished have the laboratory demonstrator check your data.

d) The test is repeated with the variable voltage source at 90%. The data sequence

is shown in Table 4-4. When the test is finished have the laboratory

demonstrator check your data.

Table 4-3. Data for the Variable Frequency Tests (Vs = 50%, Iph) fs , Hz

Voltage (% Varaiac)

Motor Current, I3 1.5 A max.

All Other parameters

20 50 -

67

30 50 - 40 50 - 50 50 - 60 50 -

Table 4-4. Data for the Variable Frequency Tests (Vs = 90%, Iph) fs , Hz

Voltage (% Varaiac)

Motor Current, I3 1.5 A max.

All Other parameters

20 90 - 30 90 - 40 90 - 50 90 - 60 90 -

4. Test of the DC Link Filter

A) Set the voltage supply and frequency to the test point done previously: variable-

voltage supply set at 50% or 60% and frequency at 20 Hz. Repeat the

measurements with the addition of an oscilloscope trace taken using E1 and I1 on

the Lab-Volt oscilloscope or by using a second set of isolators connected to the

oscilloscope. Also observe the phasor angle and the first 10 input harmonics of

the current (THD and HD1)

B) Disconnect the smoothing inductor and the dc-link resistors. Repeat the previous

exercise.

V. QUESTIONS

68

1. Plot the effect of varying the supply frequency on the speed and the torque at rated

motor current. (Use frequency in Hz as the x-axis. If possible, use the primary and

secondary vertical axis functions in Excel to place both curves on the same plot.

Copy the data into 3 columns. Insert scatter plot. The plot will have a flat curve (data

series). Highlight the finished plot. Use FORMAT to select the desired series. In the

Format-Data-Series pane choose secondary axis.)

2. Plot the effect of varying the supply voltage on the speed and the torque at rated

motor current. (Use the amplitude of the supply voltage as the x-axis.)

3. What is the relative advantage of each method of speed control – voltage control

versus frequency variation? Cite the percent differences in output speed and torque

and indicate to the typical performance range of each method.

4. Using the data from the operating point where fs = 20 Hz and the voltage supply was

at 50% or 60% of maximum amplitude, calculate the efficiency and the input power

factor of the motor-drive combination with the dc link filter and without the

smoothing inductor-resistor combination. What is the improvement in each

parameter, n and PF? What is the percent change in the harmonic components. Are

the harmonics odd or even?

69

EXPERIMENT 5

VOLTS/FREQUENCY CONTROL OF INDUCTION MOTORS

I. OBJECTIVES

There are three objectives to this experiment:

- To become familiar with the theory of V/f control of induction motors;

- To build an open-loop control circuit that provides V/f operation of an induction

machine;

- To measure the operational characteristics of a V/f based drive circuit.

II. INTRODUCTION

Control of the speed of an induction motor (IM) can be accomplished by changing the

voltage applied to the stator of the motor, Vs, or by changing the frequency of the supply,

fs. Changing either parameter independently of the other results in the pattern of curves

shown in Fig. 5-1(a). Neither approach yields access to the full range of speed and torque

that the IM is designed for. Voltage-control produces large currents in the stator as the

speed is reduced and the effective speed is 66% of the synchronous speed. Attempts to

increase the frequency beyond the rated value will saturate the air-gap flux and cause

overheating. Also, as the frequency is increased, the maximum torque falls quickly. The

preferred technique of control is one that allows simultaneous manipulation of both

parameters: the constant Volts/frequency method. The diagram of Fig. 5-1(b) shows how

the amplitude of the voltage supplied to the stator is varied as the frequency is raised.

Figure 5-1(c) shows the effect on the developed torque, Te, the stator current, Ia, and the

slip. The net performance can be summarized as follows:

1. At frequencies below the rated value, the voltage is decreased and the flux remains

constant;

70

2. The air-gap flux does not saturate and overheating of the rotor is avoided;

3. The torque, slip and stator current are constant while the frequency is below frated (1.0

pu);

4. At frequencies immediately above the rated value the machine enters a constant-

power region.

Figure 5-1. Vols/f performance characteristics. (a) Voltage and frequency controlled

independently. (b) Volts/f control. (c) Circuit performance.

71

III. BASIC V/F DRIVE CIRCUITS 1. Volts/f Circuits There are a variety of methods available for controlling the amplitude and frequency of

the voltage supplied to an induction motor. Figure 5-2 shows three basic open-loop

variations. All three circuits use a voltage-source inverter (VSI) in combination with a

rectifier and a dc-link filter.

a) Manual control of the voltage with a variable-ac source and frequency control

with a VSI.

b) Voltage control with a phase-controlled rectifier and frequency control with a

VSI.

c) Voltage and frequency control using a VSI and a PWM pattern the varies the

amplitude (modulation index) and the frequency of the reference sine-wave.

Option a) was done in the previous experiment. All three methods serve as the basis for

more sophisticated schemes that will include closed-loop controllers.

2. Phase-Controlled Rectifier and VSI The circuit shown in Fig. 5-2(a) was used the generate the curves shown in Fig. 5-1(a)

(Experiment 4). The shortcomings of this approach will be addressed in the circuit

shown in Fig. 5-2(b). The circuit consists of four parts:

a) The amplitude of the source voltage is controlled by a phase controlled-rectifier

thru the dc link.

b) The dc link consists of an inductor and a resistor.

c) The frequency of the voltage that is supplied to the IM is determined by the

switching frequency of a VSI.

d) The controller uses the input command-frequency to set the switching

frequency, fsw, and the point on the V/f slope that determines the stator voltage,

Vs.

72

(a)

Variable AC

Source

Diode Rectifier

DC Link Filter

Voltage Source Inverter

Switching Control Circuit

Frequency Command

Induction Motor

AC Source

Thyristor Phase-Controlled Rectifier

DC Link Filter

Voltage Source Inverter

Switching Control Circuit

Frequency Command

Induction Motor

Firing Control Circuit

V/f Ratio

Control

AC Source

Diode Rectifier

DC Link Filter

Voltage Source Inverter

Switching Control Circuit

Programmed V/f PWM

Induction Motor

(b)

(c)

Figure 5-2. V/f circuits. (a) Manual open-loop. (b) Controlled rectifier and VSI bridge. (c) VSI with PWM.

73

IV. PROCEDURE

Warning:

High voltages are present in this experiment! DO NOT make any connection while the power is ON.

1. Circuit Connection Connect the circuit shown in Fig. 5-3. Connect the Lab-Volt prime mover –

dynamometer to the shaft of the motor with the timing belt. This circuit will use both a

Thyristor bridge to control the voltage of the dc-link and a voltage-source MOSFET

inverter to control the frequency.

2 Circuit Settings The sections of the circuit should be set to the initial settings listed below. On the Power Thyristors Module:

Upper Interconnection Switch .................................................................................... I

Upper Interconnection Switch .................................................................................... I

24 Vac source .................................................................. Connected, green LED is on

9 Conductor Cable ....................................................... Connected to the Control Unit

On the Thyristor Firing Unit:

MODE .......................................................................................................... 3 ∼ Phase

ARC COSINE ......................................................................................... I (Pushed IN)

COMPLEMENT ............................................................................... O (Pushed OUT)

Firing Angle ............................................................................................................ 90˚

74

Figure 5-3. The V/f circuit.

The DC-Link :

Smoothing Inductors ..................... 1 section, 2 windings connected in parallel, 0.8 H

Resistance ............... All switches up (171 Ω per section), 3 section in series (513 Ω)

75

On the POWER MOSFETS Unit:

Interconnection Switch ............................................................................................... I

24 Vac Source ................................................................. Connected, green LED is on

9 Conductor Cable ....................................................... Connected to the Control Unit

On the Chopper/Inverter Control Unit:

MODE ................................................................................................... 3 ∼ Phase 180o

DC SOURCE 1 ........................................................................................ mid position

DC SOURCE 2 ........................................................................................ mid position

Prime mover – Dynamometer:

MODE .....................................................................................Dynamometer, No-load

Display ................................................................................................................ Speed

Manual Adjust ............................................................................ Minimum, Full CCW

P. I. D. Controller:

Power .......................................................................................................... + - 15 Vdc

Proportional Gain Range............................................................ Low, Pot is pushed in

Proportional Gain ................................................................................ Min, Full CCW

Oscilloscope:

Channel-1 Sensitivity ................................................................... 5 V/DIV (dc coupled)

Channel-2 Sensitivity ................................................................... 5 V/DIV (dc coupled)

Mode .................................................................................................................... Digital

Time Base ...................................................................................................... 10 ms/DIV

Trigger Source ............................................. Ext, from Synch Out of MOSFET module

Trigger Coupling ........................................................................................................ DC

Measure ............................................................................................................ Frequency

Cursors .......................................................... Vertical – Frequency, Horizontal - Voltage

76

3. Procedure 1. Turn on the PC. Make sure that the Lab-Volt Data Acquisition Module is powered-

on (the green LED is on). Start the Lab-Volt software LVDAM. The data to be

recorded is given in Table 1

Table 1. Lab-Volt Data Set Meters Meters Meters Meters Meters Meters

E1, Vac E2, Vdc E3, Vac T, N-m E4, Vac PF (E1, I1)

I1, Aac I2, Adc I3, Aac N, rpm I4, Aac

P1, W P2, W P3, W PF (E4, I4), P(E4, I4)

Thyristor Controller Oscilloscope, e1

Firing Angle, deg Switching Frequency, Hz

During the experiment verify that the motor speed, n (rpm) is less than the

frequency of the output voltage of the VSI (e1). There should always be some slip.

2. Turn on the oscilloscope. Start the Fluke software for the Combiscope. Set the

Cursors and Meas. functions to monitor the output voltage of the VSI (e1).

3. Turn on the Lab-Volt power supply. Set the voltage dial on the autotransformer

(VARIAC) to 90 or 90%.

4. On the Chopper/Inverter Control Unit set the DC SOURCE 2 control knob so the

firing angle on the Thyristor Firing Unit display is 90˚. Measure DC Source 1 and

DC Source 2 with the DMM. Each should be approximately 0.0 Vdc and the motor

should not be moving.

5. Make measurements at 3 points as indicated in Table 2: a) Use the DMM to record

the voltage at the ANGLE CONTROL INPUT of the THYRISTOR controller, b)

visually note the firing angle of the on the THYRISTOR FIRING UNIT, and c)

measure the dc voltage at the input of the VSI.

77

Table 2. INITIAL SETTINGS Voltage, Vdc Firing Angle,

degs Angle Control Input

na

Angle Control Input

na

VSI Input na

6. Adjust DC SOURCE 1 so that the firing angle of the Thyristor bridge is at a

maximum (approximately 90˚ or slightly greater). Adjust DC SOURCE 2 so that the

firing angle now reduces to approximately 75˚. The IM should begin to rotate.

Record this data set on the Lab-Volt Data Table. Remember to transfer the

data to an Excel file. (E2, Speed, Firing Angle, switching frequency, fsw)

7. Now adjust DC SOURCE 1 (CW) until the VSI output waveform, e1, is operating at

the frequency of the ac source voltage. On the PID Controller set the proportional

gain CW so that the firing angle of the thyristor bridge decreases to 0˚,(0˚ - 7˚). The

line voltage at the VSI output should be approximately equal to the rated voltage of

the IM. Verify that n ≈1800 rpm, fsw = 60 Hz, Vphase (E4) ≈ 120 Vac. Record

this data set on the Lab-Volt Data Table.

8. Slowly turn the DC SOURCE 1 until the motor is turning clockwise at 150 rpm.

Using the dynamometer apply a load of 0.1 N-m (0.5 N-m for less stable results).

Record the data-set with the Lab-Volt software.

9. Increase the machine speed to 250 rpm and raise the speed in steps of 250 rpm, from

250 rpm, to 2000 rpm and the maximum speed (10 steps: 150 rpm, 250 rpm … 2000

rpm, > 2000 rpm). Take all measurements including the power factor at E1, I1, the

switching frequency at the VSI output, e1, and the power at E4, I4 (phase a of the

motor). For best results take the readings in continuous sequence and allow the

circuit to stabilize 30 secs. before each sample is taken. Record these data sets

on the Lab-Volt Data Table. Remember to transfer the data to an Excel file.

10. Turn off the motor and the power to the bench. Have the TA check your data before

you disassemble your circuit.

78

V. QUESTIONS

1. What part of the V/f curve was set in step 6 of the procedure? What does DC

SOURCE 2 determine? What were the parameters with respect to Fig. 5-1 (b)?

Starting voltage, maximum voltage

2. What part of the V/f curve was set in step 7 of the procedure? What does DC the

proportional gain of the PID controller determine? What were the parameters with

respect to Fig. 5-1 (b)?

3. What parameter is set by DC SOURCE 1 in Fig. 5-2 (b)?

4. Reproduce the plot of Fig. 5-1 (b). Plot induced torque (Te), motor power, if

applicable , stator current and slip versus the switching frequency of the VSI. If it is

not possible to plot all the data on a single set of axes then combine two parameters

on a single graph and make several plots. What is the ratio of V/f (V/Hz)? Over

what range of switching frequencies (VSI) and speed does this circuit behave as

predicted?

5. From the data collected in step 9, plot input power and power factor versus the

switching frequency.

79

APPENDIX

80