ecce4356 lab manual 2

7/28/2019 ECCE4356 Lab Manual 2

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Sultan Qaboos University College of Engineering Electrical & Computer Engineering Department

ELECTRICAL MACHINES

LAB.MANUAL

ELEC4356

Semester 7

2005

Sultan Qaboos UniversityCollege of Engineering

Electrical & Computer Engineering Department


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ECCE4356 LAB. MANUAL

II

AUTHORS

This manual is the result of the contribution of the following SQU-EE faculty members:

Dr. M. M. El-Attar (1987-1992)

Dr. M. M. Ahmed (1988-1994)

Dr. M. A. El-Khazendar (1989-1995)

Dr. A. Gastli (1995-1998)

Dr. A. Adly (Fall 2005)

This manual has been continuously revised to match the modifications made on the contents of the ElectricalMachines Courses.

Dr. A. Gastli is responsible for changes made on the 1998 edition of this manual. During the Fall of 2005,Dr. A. Adly has applied some re-arrangements, as well as omissions, to make the manual in agreement withthe ECCE4356 course material.


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III

PREFACE

The electrical machine laboratory is intended specifically to meet the needs of modern courses in electricalmachines. It is used at undergraduate level for studying the characteristics and operation of dc, induction,synchronous and special machines using state of the art teaching modules. The subjects that are covered inthis manual include: characteristics of dc motors and dc generators (separate, series, shunt and compound),determining the parameters and performance characteristics of transformers, and finally the starting methods,speed control and performance of 3-phase induction motors.


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IV

CONTENTS

EM-I. INTRODUCTION TO TERCO ELECTRICAL MACHINE TEACHING UNIT I-1

EM-I.1 AIM OF THE EXPERIMENT: I-1EM-I.2 EQUIPMENT AND COMPONENTS: I-1EM-I.3 CONNECTION DIAGRAM I-1EM-I.4 INTRODUCTION I-2EM-I.5 CONDUCTING THE EXPERIMENT I-2EM-I.5.1 POWERPACK I-2EM-I.5.2 SPEED CONTROL OF DCMOTORS I-3EM-I.5.3 TORQUE METER I-4

EM-II. DC GENERATORS II-5

EM-II.1 AIM OF THE EXPERIMENT: II-5EM-II.2 EQUIPMENT AND COMPONENTS: II-5EM-II.3 CONNECTION DIAGRAM II-5EM-II.4 CONDUCTING THE EXPERIMENT II-6EM-II.4.1 NO-LOADCHARACTERISTICS II-6EM-II.4.2 LOAD CHARACTERISTICS (VT-IL) II-7

EM-III. DC MOTORS III-10

EM-III.1 AIM OF THE EXPERIMENT: III-10

EM-III.2 EQUIPMENT AND COMPONENTS: III-10EM-III.3 CONDUCTING THE EXPERIMENT III-10EM-III.3.1 MAGNETIZATION CHARACTERISTIC (SERIES MOTOR) III-10EM-III.3.2 LOADTESTS III-11

EM-IV. SINGLE PHASE TRANSFORMERS IV-13

EM-IV.1 AIM OF THE EXPERIMENT: IV-13EM-IV.2 EQUIPMENT AND COMPONENTS: IV-13EM-IV.3 OVERVIEW IV-13EM-IV.4 CONDUCTING THE EXPERIMENT IV-14

EM-V. THE 3-PHASE INDUCTION MOTOR V-19

EM-V.1 AIM OF THE EXPERIMENT: V-19EM-V.2 EQUIPMENT AND COMPONENTS V-19EM-V.3 CONNECTIONDIAGRAM V-19EM-V.4 CONDUCTING THE EXPERIMENT V-20EM-V.4.1 RESISTANCE MEASUREMENT V-20EM-V.4.2 NO-LOAD TEST V-21EM-V.4.3 BLOCKED-ROTORTEST AND STANDSTILL TEST V-21


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V

EM-V.4.4 LOADING WITHOUT 3-PHASE ROTORRESISTANCE V-22EM-V.4.5 LOADING TESTWITH A 3-PHASE ROTOR RESISTANCE IN CIRCUIT V-23EM-V.4.6 DETERMINATION OF STARTING TORQUE AND CURRENT V-24


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I-1

EM-I. Introduction to Terco Electrical Machine Teaching Unit

EM-I.1 Aim of the experiment:The object of this experiment is to:

1. Study the different output power available and the constraints imposed upon the different switches of thepower pack unit.

2. Understand and use the torque meter unit.

EM-I.2 Equipment and components: M Electric torque meter MV 1036

G DC machine MV 1006

TG Tachometer MV 1025

S Switch MV 1500

F Power pack MV 1300

V Voltmeter 300V MV 1926

EM-I.3 Connection DiagramThe circuit diagram is shown in Fig. 1.

++-+-

S

rpm

MV 1036

A A

M

F1

F2

A1

A2

TG G

V

F2

F1

-

220V = 0-220V =F

MV 1300

Figure 1. Circuit Diagram


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I-2

EM-I.4 Introduction The power pack contains a three-phase transformer, rectifier, and variable voltage transformer connected

so that fixed and variable DC and AC voltages can be supplied.

The torque meter can be used to drive or brake an electric machine in order to measure its characteristics.It consists of a DC machine, the stator of which is mounted in bearings so that its torque reaction is

transmitted to a pressure sensor. The range of the torque meter is 25 Nm. The reaction between theshaft torque, electromagnetic torque and losses torque, for both motoring and generating, is given in Fig.2. The dynamometer reading is the net coupling torque (shaft torque).

Forceonbalance

Torque-armlength

STATOR

ROTOR Air gap

Te

Reactionon Stator

Reaction on stator due to oil lossforces; for anti-clockwise rotation.

Bearing frictionforce opposingrotation

m

(a) Air-gap forces (b) Loss forces

m

Te

TlossTr

Tsh

(d) Motoring

m

Te

Tloss

Tr

Tsh

(c) Generating

Tsh=Te+Tloss

Tr=-Tsh

Tsh=Te-Tloss

Tr=-Tsh

Figure 2. Dynamometer for loading and torque measurement: (a) air gap forces; (b) loss forces.

EM-I.5 Conducting the experimentEM-I.5.1 Power Pack1. Find the main switch Sm and study the different voltage output terminals. From left to right these are:

a) Fixed DC voltage, 220V, with switch S1.

b) Variable DC voltage, can be varied using the central knob VVC. Its range is 0-220V, with switch S 2.

c) 3-phase AC voltage which can also be varied using VVC. Its range is 3x0-220V, and its switch is S 3.

d) Fixed 3-phase AC voltage, 220/127V, with switch S4.

2. Make sure that all switches are off and the variable voltage controller (VVC) is set to minimum.


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I-3

3. Switch on the different switches in the sequence mentioned in the following tables and measure the specifiedDC and AC voltages.

Table 1. DC Supply Voltages

Initial Position ofVVC

Sequence ofClosing Switches

VVC rotated toPosition

Fixed DC Voltage(V)

Variable DCVoltage (V)

0 SmS1S2 00 SmS1S2 500 SmS1S2 100

100 SmS1S2 1000 Sm S2 100

Table 2. AC Supply Voltages

InitialPosition

of VVC

Sequenceof Closing

Switches

VVCrotated to

Position

Fixed AC Voltage (V) Variable AC Voltage (V)

RS ST TR R0 S0 T0 RS ST TR R0 S0 T0

0 SmS3S4 0

0 SmS3S4 100

0 SmS3 100

100 SmS3S4 100

Question 1

Based on the previous results, guess what are the limitations imposed upon the different switches ? Why thismachine was designed with such limitations ?

EM-I.5.2 Speed Control of DC Motors1. Connect up the torque meter in accordance with the connection diagram, without exciting the coupled

DC generator (switch S should be off).

2. Set the VVC to zero. Turn on Sm, S1, and S2 in sequence.

3. Adjust the field rheostat such that the motor excitation current is 0.7 A.

4. Carefully increase the variable DC voltage up to 120V. The motor should rotate. Note the speed n and

the field currentI

f.

5. Reduce the field current to 0.3A in steps of 0.1A, and record the corresponding speedn.

Table 3. Variation of speed as a function of field current at constant armature voltage Vt=120V.

Field CurrentIf(A) 0.7 0.6 0.5 0.4 0.3

Speedn (rpm)


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I-4

6. Turn back the field rheostat to getIf=0.7A. Increase the terminal voltage from 60V to 210V in steps of30V and record the corresponding speedn.

Table 3. Variation of speed as a function of armature voltage at constant field current If=0.7A.

Question 2

Draw the relation between speedn and field current If at constant armature voltage. Comment on thismethod of speed control.

Question 3

Draw the relation between speedn and armature voltage Vt at constant field current. Comment on thismethod of speed control.

EM-I.5.3 Torque Meter1. Connect the circuit as shown in Fig. 1 with switch S is opened.

2. Adjust the field current to 0.7A. Increase the armature voltage gradually to increase motor speed from500rpm to 1500rpm in steps of 250rpm, and record the corresponding torque (Tsh).

3. Close the switch S and notice the change in torque reading. Give the reason.

4. Reduce the speed from 1500rpm to 500rpm in steps of 250rpm and record the corresponding torque(Tsh).

Table 5. Variation of Torque as a Function of Speed at constant field current If=0.7A.

Question 4

Draw the relation between torque and speed for the two cases. Explain the action of the dynamometer andcomment on the difference between the torque speed curves.

Armature Voltage Vt (V) 60 90 120 150 180 210

Speedn (rpm)

Speed n (rpm) 500 750 1000 1250 1500

Torque Tsh S: opened

(Nm) S: closed


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II-5

EM-II. DC Generators

EM-II.1 Aim of the experiment:The object of this experiment is to: measure the following characteristics of DC generators.

1. No-load characteristics (Ea-Im).

2. The load characteristics (Vt-IL) of :

3. Separately excited DC generators.

4. Shunt generators.

5. Compound generators.

EM-II.2

Equipment and components: Electric torque meter MV 1036

DC machine MV 1006

Tachometer MV 1025

Power pack MV 1300

Switch MV 1500

Shunt rheostat MV 1962 or MV1961

Load resistor MV 1100

DC Voltmeter 300V

DC Ammeter 12A

DC Ammeter 1A

EM-II.3 Connection DiagramThe circuit diagram is shown in Fig. 1.


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II-6

++-+-

S

rpm

MV 1036

A A

M

F1

F2

A1

A2

TG G

V

F2

F1

-

220V = 0-220V =FMV 1300

ImIL

A1

A2

Rmy

RL

Figure 1. Circuit Diagram

EM-II.4 Conducting the experimentEM-II.4.1No-Load Characteristics1. Connect the circuit as shown in Fig. 1 with switch S open.

2. Take note of the ratings of the DC generator and torque meter. The ratings are shown on their rating plate.Theses ratings must not be exceeded at any time.

Generator Rating Torque Meter Rating

3. Turn on the fixed DC voltage switch and adjust the field current of the torque meter to maximum excitationvalue.

4. Adjust the DC generator field current to obtain minimum excitation current.

5. Set the variable DC voltage to zero and turn on its switch. Then, increase the armature voltage up to 220V.

The machine then rotates at about 1500 rev/min in the direction of the arrow (see above the shaft of the DCgenerator).

6. Adjust the speed to 1400rev/min. It should be maintained constant throughout the experiment.

7. VaryIm in steps of 0.1A from zero to maximum and take the corresponding reading (Ea) of the voltmeter V.Fill results in Table 1.


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II-7

Table 1. DC Supply Voltages

1400 r. p. m. 1200 r. p. m.

IncreasingIm DecreasingIm IncreasingIm DecreasingIm

Ea(V) Im(A) Ea(V) Im(A) Ea(V) Im(A) Ea(V) Im(A)

8. VaryIm in steps from maximum value to zero at constant speed 1400 rev/min. Fill the results in Table 1.

9. Repeat steps 7 and 8 at speed of 1200 rev/min and fill results in Table 1.

10. Draw on the same graph with a commonIm axis the no-load characteristics for increasing and decreasingImat both 1400 rev/min and 1200 rev/min.

Question 1

Why does the no-load characteristics differ fot the increasing and decreasingIm ?

Question 2

How the reduction in the speed from 1400 rev/min to 1200 rev/min has affectedEa at the same field current.

EM-II.4.2Load Characteristics (Vt-IL)EM-II.4.2.1 A. Separately Excited Generator

1. Adjust the speed of the torque meter to 1400 rev/min and maintain it constant during the entireexperiment.

2. AdjustIm to bring Vtto 220V with S still open. Record the value ofImin Table 2. KeepIm at this valueduring the rest of the experiment.

3. AdjustRL to maximum resistance (minimum current).


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II-8

4. Turn S on and vary RL such that IL varies in steps of 1A up to the rated current. Record thecorresponding reading Vtand fill the results in Table 2.

5. Stop the machines and turn off all switches and supplies.

EM-II.4.2.2 Shunt Generator

1. Connect the DC generator with shunt excitation as shown in Fig. 2.

2. Start the torque meter as mentioned before in Part 1 (3-5) with S open. Adjust the speed to 1400 rev/min.

3. AdjustRmy of the generator so that the voltage becomes 220V. The setting ofRmy must not be changedduring this experiment.

4. Turn on S withRL set to maximum resistance (minimum current). UsingRL varyIL in steps of 1A up tothe rated current.

5. Record for each stepIL and voltage Vt and fill the results in Table 2. Make sure that the speed is keptconstant at 1400 rev/min before each reading.

6. Stop the machine.

S

G

Vt

F2

F1

Im

IL

A1

A2

Rmy

RL

Figure 2. Shunt Generator Connection Diagram.

EM-II.4.2.3 Shunt Generator

1. Connect the DC generator to compound excitation by passing the armature current through the series

winding D2-D3 as shown in Fig. 3.

2. Repeat steps 2 to 6 of Part B.

3. Connect the entire winding (D1-D2) and repeat steps 2 to 6 of Part B.

4. Draw the external voltage characteristics (Vt-IL) for separate, shunt and compound excitation in the samediagram with commonIL axis (x-axis).


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II-9

S

G

Vt

F 2

F 1

Im

IL

A 1

A 2

R m y

R L

D 2

D 3

D 1

Figure 3. Compound Generator Connection Diagram.

Table 2. External Characteristic for Separate, Shunt and Compound Generators.

Separate:Im=const. Shunt Compound D2-D3 Compound D1-D2

Vt (V) IL (A) Vt (V) IL (A) Vt (V) IL (A) Vt (V) IL (A)

Question 3

Explain why does Vt decrease with increasingIL with separate excitation.

Question 4

Explain why does Vt decrease more with shunt excitation than with separate excitation.

Question 5

What would happen to Vt if the connection of the series winding were reversed in Part C section 2 ? What isthis connection called ? Explain one application for that kind of excitation.

Question 6

Why doesn't the generator take up voltage in Part B section 2 (shunt excitation) if the field is wronglyconnected. Explain the necessary conditions for successful build up of the terminal voltage.


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III-10

EM-III. DC Motors

EM-III.1 Aim of the experiment:The object of this experiment is to examine the load characteristics of DC motors when the field windings are

connected:

Shunt

Series

Compound

EM-III.2 Equipment and components: DC shunt/series machine (Nickerson)

DC dynamometer machine

Voltmeters,

Ammeters

Resistors

EM-III.3 Conducting the experimentEM-III.3.1Magnetization Characteristic (Series Motor)

Connect the circuit as shown in Fig. 1.

250V

M

10A

0-50V

DC

D1A1

A2

1A300V

Dynam.

-15A/0/15A

0-240VDC

A1

A2

F2

F1

500

25 500W

D2

IaVt

Figure 1. Circuit Diagram I

Take note of the ratings of the Dynamometer and the DC motor. The ratings are shown on their rating plate.Theses ratings must not be exceeded at any time unless when it is specified by the manual.


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III-11

Dynamometer Rating DC Motor Rating

1. Make sure that field current of the dynamometer is set to maximum value (500 variable resistance set tominimum). Switch ON the 0-240V DC supply and gradually increase it until the speed reaches 1300rpm.

2. Adjust the field current and set the speed to 1400rpm. Make sure that the speed is kept constant duringthis experiment.

3. Take readings ofVtandIa when the conditions have settled.

4. Switch ON the 0-50V DC supply and increase it gradually up to a maximum field current value of 10A.Record the field currentIsrand the voltage Vtfor each step and fill in Table 1. Use both the variable DC supply

and the variable resistance 25W in order to adjust the field current to the values given in Table 2.

5. Switch off the 0-50V DC supply..

6. Reduce the 0-240V DC supply and switch OFF.

7. Plot the magnetizing characteristicEf/Isrof the series DC motor.

Table 1. Magnetization Characteristic of the Series Motor.

EM-III.3.2Load TestsConnect the circuit as shown in Fig. 2.

1 2 1 2 1 2

S1

30

0.5A250V

M

10A

0-240VDC

S2

D1A1

A2

F2

F1

1A

D nam.

-15A/0/15A

200VDC

3x60

A1

A2

F2

F1

500 500

D2

1 2 1 2

IaIfVt

Figure 2. Circuit Diagram II.

Vt(=Ef) (V)

Isr (A) 0 1 3 5 7 8 9 10


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III-12

EM-III.3.2.1 Series Motor

1. S1 should be in position (1) and S2 should be in position (OFF).

2. Switch ON the 0-240V DC supply. Increase the voltage up to 60V and keep it constant throughout thistest.

3. Switch ON the 200V DC supply. Vary the dynamometer field resistance to apply more torque to lowerspeed successively in steps of about 100 rev/min. Each time adjust the voltage to be 60V.

4. Take the readings of armature current Ia, torque Te, and speed n at each setting until the series motorarmature current reaches 10A. Fill in Table 3.

EM-III.3.2.2 Shunt Motor

1. S1 should be in position (2) and S2 should be in position (1).

2. Switch ON the 0-240V DC supply. Increase the voltage gradually until the motor starts rotating. Checkthat the rotation is in the forward direction (in this case shunt field is said to be cumulative). If rotation is

backward, switch OFF and reverse the shunt field connection.

3. Switch ON the 0-240V DC supply. Increase the voltage up to 180V and keep it constant throughout thistest. Set the field current to 400mA. Make sure that the voltage and the filed current are kept constant

before each reading.

4. Repeat steps 3 and 4 in EM-IV3.2.1 (speed step of 10rev/min), to get the shunt motor characteristics.

EM-III.3.2.3 Compound Motor

1. S1 should be in position (1) and S2 should be in position (1).

2. Repeat steps 2 to 4 above (in EM-IV3.2.2) to get the compound motor characteristics.

3. Plot for each motor type the following curves:

Torque/Speed (Te/n) Torque/Armature current (Te/Ia) Speed/Armature current (n/Ia)

Table 2. Load Test Results

Series Motor Shunt Motor Compound Motor

Te (Nm) Ia (A) n (rpm) Te (Nm) Ia (A) n (rpm) Te (Nm) Ia (A) n (rpm)


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IV-13

EM-IV. Single Phase Transformers

EM-IV.1 Aim of the experiment:The objectives of this experiment are to:

Carry out open and short circuit tests. Determine the equivalent circuit parameters. Carry out load test. Calculate voltage regulation and efficiency.

EM-IV.2 Equipment and components: 1 U - Core I Yoke 1 Coil 1000 turns 1 Coil 500 turns

1 Variable transformer 2 Voltameters 0 - 50 V 2 Multimeters 1 Rheostat 0 - 30 ohm 1 Variable Capacitor I Variable inductor 1 Wattmeter

EM-IV.3 OverviewA single - phase transformer can be represented by an equivalent circuit. An equivalent circuit can

be either referred to primary side or secondary side. Fig. a shows a single - phase two winding

transformer. The exact equivalent circuit of the transformer referred to primary side is shown in

Fig. b while the approximate equivalent circuit referred to primary side is shown in Fig. c.


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IV-14

Open - circuit and short circuit tests are carried out to determine the parameters of the equivalent circuit.

The power losses in the transformer can also be determined from these tests.

EM-IV.4 Conducting the experimentPart 1: Open Circuit Test (O.C.T.)1. Connect the circuit as shown in Fig. 1.2. Adjust the primary voltage VI On the variable transformer to 22 V.

3. Measure the no-load current Io , no-load power Poc (core loss), primary voltage V1 andsecondaryvoltage V2.

4. Enter the measurements in Table 1.

/2112

2

1/22

2

2

1/2

/2112

1

2/22

2

2

1/2

XXXVN

NVX

N

NX

RRRI

N

NIR

N

NR

e

e

+=

=

=

+=

=

=


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IV-15

5. Calculate the no-load power factor cos o using the relationo1

oco

IV

P=cos and enter the result in

Table I.6. Calculate the magnetizing (reactive) current component Iomand core-loss (active) current component Iol

using the following relations;

ooom

oo1o

II

II

sin

cos

=

=and enter the results in Table I.

7. Calculate the parameters X (magnetizing reactance) and R (Core loss equivalent resistor) of the

equivalent circuit using the following relations

=

=

ol

om

IVR

IV

X

1

1

and enter the result in Table 1.

8. Repeat; the above steps with an air gap between U-Core and Yoke.

Questions:-1. Why is the no-load current Iowith air gap greater than that without air-gap?2. The value of R and X in Table 1 are referred to primary side. Calculate R and X referred to

secondary side.R (referred to secondary) = ___________________X (referred to secondary) = ___________________


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IV-16

3. Calculate the no-load power Poc (core loss) using the following relation

== RIP oloc2

____________________________ W

and compare it with the value of Poc in Table I.4. Identify the types of the core-loss Poc .5. If the primary side is opened and the no load current Io is measured from the secondary side1

what would be the value of IoIo = ________________ (without air gap)Io = ________________ (with air gap)

Part 2: Short Circuit Test (S.C.T.)1. Connect the circuit as shown in Fig. 2.

2. Adjust the primary voltage V1 on the Variable transformer so that the short circuit current onprimary side Isc1 is 170 mA.

3. Measure the short circuit power Psc (copper losses), short circuit currents Isc1 ,Isc2 and shortcircuit voltage Vsc.

4. Enter the measurements in Table 2.

5. Calculate the short circuit power factor cos sc using the relation1

cosscsc

scsc

IV

P= and

enter the result in Table 2.

6. Calculate the short circuit equivalent impedance Zsc using the relation1

1

sc

scesc

I

VZZ ==

and enter the result in Table 2.7. Calculate the equivalent resistance and reactance using the following relations

scee ZR cos11 = , scee ZX sin11 = and enter the results in Table 2.

Questions:-1. The value of Zel in Table 2 is referred to primary side. Calculate the

equivalent impedance referred to secondary side Ze2.Ze2 = ___________________ ohm


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IV-17

2. Calculate the efficiency of the transformer at 125%, 100%,50% and 25% of the full load using

the relation:( )

( )scocLL

PnPnS

nSEfficiency

2cos

cos

++=

, take 8.0cos =L and S=3.3 VA.

Part 3: Load Test

1. Connect the circuit as shown in Fig. 3

.

2. Adjust the voltage V1 on the Variable transformer to 22 V.3. Vary the current I2 in steps of 100 mA by varying the rheostat (resistive load).4. Measure (secondary voltage) Voltage V2.5. Enter the measurements in Table 3.6. Calculate the voltage regulation of full load using the following relation

% Voltage Regulation)

100,2

,2,2

=NL

FLNL

V

VV%

where, =NLV ,2 no load (I2 = 0) secondary voltage

==

1

21

/1

N

NVV

=FLV ,2 full load (I2 = 300 mA) secondary voltage.

7. Draw the relation between I2 and V2 and from which calculate the voltage regulation at unitypower factor (resistive load).

8. Repeat steps 1-7 but with capacitive and inductive loads.


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IV-18

Questions:-1. Is it possible to get a -ve value for the voltage regulation? Explain.2. What is the best value of voltage regulation? Is it possible to get it practically?3. Calculate the power factor which yields the voltage regulation best value.


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V-19

EM-V. The 3-Phase Induction Motor

EM-V.1 Aim of the experiment:The object of this experiment is to :

Determine the parameters of the equivalent circuit per phase.

Plot the torque/speed and current/speed relation at different voltages.

Study the effect of rotor resistance on the motor performance.

Determine the starting torque and current at different voltages.

EM-V.2 Equipment and Components Scan Drive Unit

3-phase rotor resistance MV 1013

Moving iron ammeter 0 6A

Digital Ohmmeter

EM-V.3 Connection Diagram(see next page)


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V-20

OVER TEMP

OVER TEMP

OVER TEMP

MT1SW1BR1

ccw

cw

DR1 DR2

DigitalMechanicalTransducer

L3L1 L

A2A1F2F1VV2VV1

MS3MS2MS1MAMF0-30V/5A 0-30V/5A0-230V/1A 0-230V/1A

R1

L3L2L1 N

OVER TEMP

L1S1 L2S2

N

L3S3

L3L2L1

Rst

IR

EM-V.4 Conducting the ExperimentEM-V.4.1Resistance Measurement

1. Using a digital ohmmeter measure the resistance between R1 and R2.

2. Measure the resistance between S1 and S2 and fill the results in Table 1.

3. The stator resistance per phase =1.11xRS1S2/2=_____________()

4. The rotor resistance per phase =1.11xRR1R2/2=_____________()

Table 1.

Resistance R1+R2R

R R

ph=

+ ( ) .1 2 111

2

Rotor

Stator


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V-21

EM-V.4.2No-Load Test1. Connect the circuit as shown in Fig. 1.

2. Short circuit the rotor terminals R1, R2, and R3.

3. Turn off the switch of VV1 and DR1 to make sure there is no load on the induction motor.

4. With the 3-phase voltage supply of 400V and the motor winding connected as star, use theautotransformer to start the motor from zero voltage and up to full voltage.

5. Fill in Table 2.

6. Reduce the voltage back to zero and switch off the 3-phase supply.

Table 2.

Question 1

Why is the slip of the motor relatively small at no-load ?

Question 2

What does Pact|NL represents on no-load ? Calculate the resistive and inductive component of the magnetizingbranch ?

EM-V.4.3Blocked-Rotor Test and Standstill Test

1. Record the full load value of the current from the nameplate of the motorIFL=______(A).

2. Switch on the 3-phase supply and block the rotor manually to prevent the rotation.

3. Increase the voltage across the motor using the autotransformer until the full load current flows.

4. Fill in the results of Table 3.

1. Reduce the voltage across the motor to zero and switch off the supply.

Table 3.

VNL

(V)

INL

(A)

Pact|NL

(kW)

Qreact|NL

(kVAR)

cos nNL

(rpm)

SlipNL

(p.u.)

VBL

(V)

IBL

(A)

Pact|BL

(kW)

Qreact|BL

(kVAR)

cos


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V-22

Question 3

What is the relation betweenIBL andIstarting ?

Question 4

What does Pact|BL represent ? Calculate leakage impedance of the stator and rotor windings referred to the

stator.

EM-V.4.4Loading Without 3-phase Rotor Resistance1. With the switch of VV1 in the off position start the induction motor by increasing the voltage from zero to

80V and fill the results in Table 4.

2. Now, turn the switch VV1 on to provide loading on the motor and take the readings of the digital panel.

3. Fill the results in Table 4 at different values of loadings.

4. You can get more loading by reducing the load resistanceR1.

5. Plot the relation Tversus slip and current versus slip.

6. Turn the switch of VV1 off and reduce the motor voltage to zero and switch off the supply.

7. Efficiency may be calculated using the relation

=

T n

Pact

2

1000 60

where Tis in Nm, n in rpm andPactin kW.

Question 5

Plot the PF and current versus slip on one graph paper all at Vph=80V then plot the current at Vph=230V on thesame graph.

Question 6

What is the relation between torque and voltage and the relation between current and voltage ?

Plot Tversus slip at Vph=230V on the same graph paper and comment.


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V-23

Table 4.

EM-V.4.5Loading Test With a 3-phase Rotor Resistance in Circuit1. Include the 3-phase rotor resistanceRstin the rotor circuit and repeat all the steps 1 to 1 in part iv at the

same voltage.

2. Fill the results in Table 5.

3. Switch off the main supply and DC supply.

Table 5.

Question 7

Repeat question 5 with the new results on the same graph paper.

Question 8

Explain with figures the effect of increasing the rotor resistance.

Vph

(V)

IL

(A)

Ppact

(kW)

Qreact

(kVAR)

cos T

(Nm)

n

(rpm)

s

(pu)

Pout

(kW)

Effi.

(%)

Vph

(V)

IL

(A)

Ppact

(kW)

Qreact

(kVAR)

cos T

(Nm)

n

(rpm)

s

(pu)

Pout

(kW)

Effi.

(%)


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EM-V.4.6Determination of starting torque and current1. Short circuit the rotor terminals R1, R2 and R3.

2. Start the motor from zero voltage and set the voltage constant at Vph=50V.

3. Load the motor using the DC generator until the speed reaches zero.

4. Take readings of digital panel and fill the results in Table 6.

5. Switch off the supplies.

6. Repeat the same procedure 2 to 5 but with the 3-phase external resistanceRstin the rotor circuit.

7. Fill the results from the digital panel in Table 6.

Table 6.

Question 9

Calculate the starting torque and current at Vph=230V with and without external rotor resistance and comparethe values.

Question 10

Draw the equivalent circuit per phase with values on at a slip of 0.05. What happen to the total impedance ofthe motor when s is increasing from 0.05 to 0.5 ?

IL

(A)

Pact

(kW)

Qreact

(kVAR)

cos Tst

(Nm)

WithoutRst

WithRst

ecce4356 lab manual 2

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