Exp. 3_ Load Test and Equivalent Circuit Determination on Three Phase Squirrel Cage Induction Motor...

7/30/2019 Exp. 3_ Load Test and Equivalent Circuit Determination on Three Phase Squirrel Cage Induction Motor and (1)

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Indian Institute of Technology Gandhinagar

Department of Electrical Engineering

EE 311 Electrical Machines and Power Electronics Lab. B. Tech.: Electrical, Sem. : V

Date of Exp. : 21st Aug. 2012 Submitted by:

Date of Submission: 28th Aug.2012 Nitya Pawar(10002022)

EXPERIMENT 3: LOAD TEST ON THREE PHASE SQUIRREL CAGE INDUCTION MOTOR AND

EQUIVALENT CIRCUIT DETERMINATION USING NO LOAD AND BLOCKED ROTOR TESTS.

AIM:

1. To conduct load test on 3 squirrel cage induction motor and obtain its performancecharacteristics.

2. Equivalent circuit model determination using OC and SC tests and performance analysis.

APPARATUS REQUIRED:

Sl. No. Apparatus Specification Quantity

1. Rotating Machines

from Test bench

The machines from the Test Bench are to be

configured for running the DC machine as

Generator and the Induction machine as Motor

the prime mover for the DC generator.

1

2 Extension Panel Panel that facilitates having terminals extended

from rotating machinery, power supplies, Load

banks and panel meters.

1

THEORY

A three phase induction motor consists of a stationary stator and rotating rotor separated by

uniform air gap. Two types of rotor used are slip ring rotor and squirrel cage rotor.

When a three phase supply is given to a stator, a revolving magnetic field rotating at

synchronous speed is produced. The lines of force of stator field cut the rotor conductors and

alternating emf is induced in the rotor conductors due to the relative motion between the

stator field and the rotor conductors. The Interaction between the stator magnetic field and the

rotor current carrying conductors causes a force upon the rotor conductors ending them to

turn in the direction of flux.


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Torque is produced only when the speed of the rotor is less than N s. When the rotor is loaded

the speeds falls causing an increase in relative motion between stator and rotor conductors,

rotor current and hence the torque in order to cope with the increased load.

With the circuit model developed for the induction motor, the parameters of the circuit can be

obtained from no load and blocked rotor tests. When the machine is run on no load, very little

torque is developed by it. The rotor Branch acts like an open circuit. The slip is near to zero

resulting in infinite impedance on the rotor branch.

When the Machine is prevented from rotating the slip is unity. The elements representing the

magnetising branch Rm and Xm are high impedances much larger than RT and XTR in series. Thus

the equivalent circuit under this condition, the magnetising branch is neglected.

The equivalent circuit under these conditions are shown below.

CIRCUIT DIAGRAM (LOAD TEST)

G

Variable

Power

Supply

A

A

V

V_

Control

1 Supply

Sensors

T & S

Display

T/I

MCB 16 A

L

O

A

DInduction

Motor

R

YB

V

A M L

C V

M L

C V

W1

W23,4

15VAC,

50Hz

R

Y

B

NO LOAD AND BLOCKED ROTOR TEST

Induction

Motor

R

Y B

3PhasePowerSupply

A

V

R

Y

B

B

Y

R3

Auto

Xmer

W1

W2

No

Load or

Blocked


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PROCEDURE (LOAD TEST)

1. The Induction Machine is selected and configured as Motor running directly on theIncoming Supply.

2. The DC machine is configured as Generator and the field is kept at minimum initially. TheLoad is not connected to the DC output of the Generator. The DC machine is the load for the

Induction Motor.

3. The Induction Motor is started and brought to rated rpm.4. The DC Machine field is gradually increased till the output voltage reaches the rated voltage

of the Generator.

5. The input supply current, voltage, power and Power factor of the Induction Machine can bemeasured using the required meters.. Also the output of the IM parameters namely the

speed and the Torque are also noted from the Monitoring Panel of the DC machine. The first

set of readings when the Generator is on no load is recorded.

6. The Load on the DC generator is added gradually and the corresponding readings arerecorded for each load step.

7. The above step is repeated till the IM current reaches the rated value.8. Remove the load completely and bring down the DC machine field to minimum.9. Switch off the Induction motor and the main supply.

TABULATION:

Sr.

No.

Supply

Voltage

VL

(Volt)

Line

Current

IL

(Amp.)

Speed

N

(RPM)

I/P

Power

(Watt)

Torque

T

(NM)

O/P

(Watt)

Effici-

ency

(%)

Slip

S

(%)

Power

Factor

(Cos)1 377.22 1.25 1493 356.43 1.1 without excitan .28

2 377.46 1.22 1490 445.47 1.7 Excitation without load .31

3 379.55 1.42 1483 812.25 3.5 539.81 66.45 1.1 .50

4 382.81 1.61 1475 1169.4 5.2 797.68 68.21 1.7 .62

5 381.48 1.83 1469 1497.3 6.9 1054.15 70.40 2.1 .71

6 380.90 2.05 1462 1776.3 8.6 1307.61 73.61 2.5 .77

7 381.72 2.28 1454 2070.5 10.2 1542.40 74.49 3.06 .80

8 379.72 2.46 1447 2308.2 11.7 1760.70 76.28 3.53 .83


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CALCULATION

Input to motor : W Watt

Motor output :

% Efficiency :

100% Slip :

Where, Ns = synchronous speed,

N= actual speed

Power factor, cos : MODEL GRAPHS:

T

N

0 Output

pf

0 % Slip

TNm

Observed graph:

1.

0

2

4

6

8

10

12

14

0 250 500 750 1000 1250 1500 1750 2000

Torque

Output Power

Torque v/s Output Power


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2.

3.

4.

1445

1450

1455

1460

1465

1470

1475

1480

1485

0 250 500 750 1000 1250 1500 1750 2000

Speend

Output Power

Speed v/s output power

64

66

68

70

72

74

76

78

0 250 500 750 1000 1250 1500 1750 2000

Efficiency

Output Power

Efficiency v/s Output Power

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 250 500 750 1000 1250 1500 1750 2000

PowerFactor

Output Power

Power Factor v/s Output Power


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PROCEDURE( Equivalent Circuit Determination)

NO LOAD TEST

1. The Rotary Bench is configured for the Induction Motor operation. The Motor shaft isdelinked from the DC machine.

2. Using the DOL starter mode the machine is started and made to run under no load.3. The applied voltage, the no load current and the No load power are noted down.BLOCKED ROTOR TEST:

1. Rotary Bench is configured for the Induction Motor operation thro external supply source.2. The external supply source with fed with a Variable 3 phase AC supply ( using an

Autotransformer)

3. The rotor is held/Blocked. The 3 ph voltages to the motor is gradually applied keeping awatch on the stator current. Once the rated current is reached, (with the rotor in blocked

condition) the voltage, current and the power readings are noted. The Voltage to the

machine is brought to zero and the power is switched off.

TABULATION:

No load test:

Sr. No. Volt IO Amp 3 Phase Power Power factor

1 414.26 1,57 278.1 W .137(82.3)

Blocked rotor test:

Sr.

No.

VS Volts IS Amps. WS = Total system

power in Watts

Power factor

1 71.6 2.4 310.5 .59

CALCULATION OF EQUIVALENT CIRCUIT PARAMETERS

In no load the current drawn is primarily by the magnetising branch and the power consumed

by the core loss is too high compared to the loss in the stator resistance. The stator cu loss is

neglected and the input power is consumed at the Rm in meeting the core loss. Hence f or the

data of Input voltage, current and the Power the following circuit parameters are calculated,

Let the applied voltage = Vs Then the current drawn is given by

Is =

+

-------------- (1)

-------------- (2)

From these equations the Rm and Xm are calculated.


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In blocked rotor test the rotor is prevented from running and hence the slip of the operation is

unity. Since the current drawn is decided by the resistance and leakage reactance alone, the

measured power is entirely consumed in the stator resistance plus the equivalent resistance of

rotor referred to the stator.

Using the equivalent circuit of this test, the resistive and the reactive component of the stator

and the equivalent rotor component referred to stator can be evaluated as shown below,

------------- (1)

------------- (2) ------------- (3)Assume,

The Stator resistance is measured using a voltmeter and ammeter method employing a

variable DC source. The Equivalent circuit thus determined is shown below.

EQUIVALENT CIRCUIT:

Rs Xls R/s Xlr

XmRm

Rr (1-s)/s

PERFORMANCE CHARACTERISTICS EVALUATION

From the equivalent circuit many aspects of the steady state behaviour of the machine can be

deduced. The most important of all is the speed Torque characteristics of the machine.


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In order to estimate the speed torque characteristics let us suppose that a Sinusoidal voltage

is impressed on the machine.

The Current drawn by the circuit is,

Neglecting the magnetising current the air gap power is given by

The mechanical power output P given by (1-s) Pg. The Torque is obtained by the relation,

Overall Torque,

The Torque may be plotted as function of s and t Called Torque Slip characteristics.

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8

Torque

% slip

Torque v/s % Slip


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RESULT AND INFERENCE:

1. The observed curve for efficiency, speed, torque, and power factor v/s output (load)current is found to be similar to the model curve, the curve is not complete as we have

worked out few readings only.

2. The performance characteristic curve of the motor i.e. torque-slip curve is also foundmatching with the model graph given.

3. By calculating equivalent circuit parameters, we now know,Rm=617.08

Xm=462.9

Rs=Rr=26.95

Xs=Sr=22.44

4.

Exp. 3_ Load Test and Equivalent Circuit Determination on Three Phase Squirrel Cage Induction Motor...

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