How and why do you do a load test on an induction motor?

Answer 1

This was a good experiment in terms of it was to show us the direct result of the effect of increasing the load current on the motor power factor. It showed that the higher the current the better the power factor. The Electrical Supply Board desire a high power factor for installations within Ireland and penalise those which do not meet required standards as they see poor power factor as a wasteful loss of energy. It also showed that there are two ways in which the rotating magnetic field could be used to provide the rotating motion.

Answer 2

AIM :

To determine the performance characteristics of 3-phase squirrel cage induction motor by direct loading.

APPARATUS REQUIRED:

SI NO

APPARATUS REQUIRED

TYPE

RANGE

QUANTITY

NAME PLATE DETAILS :

FUSE RATING CALCULATION :

125% of rated current.

No-load test - 25% of rated current.

THEORY :

The load test on 3-phase induction motor is performed to obtain its various characteristics including efficiency. A belt and brake drum arrangement as shown in the circuit diagram can load the motor. If S1 and S2 are the tensions provided at the two sides of the belt, then the load torque is given by

T = (S1 - S2) * 9.81 * R N-m.

Where R is the radius of the brake drum in metre. The mechanical output of the motor is given by

Pm = 2 * 3.14 * N * T Watt

60

Where N is the speed of the motor in RPM. The power input to the motor

Pi = VLIL watt

The efficiency of the motor is given by

Efficiency = Pm / Pi

FORMULA :

Torque,

T = (S1 - S2) * 9.81 * r (Nm)

Input power

(Pi) = (W1 + W2) (Watt)

Output power

(Po) = 2 P NT / 60 (Watt)

Efficiency

h = Po X 100

Pi

Cos f = W/(31/2 VLIL)

Slip = (Ns - N) / Ns *100

PRECAUTION:

1. TPST switch should be at open position.

2. 3-phase autotransformer should be at minimum voltage position.

3. There should be no-load at the time of starting(Loosen the belt on the brake drum)

4. Brake drum should be filled with water.

PROCEDURE:

1. The connections are made as per the circuit diagram.

2. Power supply is obtained from the control panel.

3. The TPST switch is closed.

4. Rated voltage of 3-phase induction motor, is applied by adjusting autotransformer

5. The initial readings of ammeter, voltmeter and wattmeter are noted.

6. By increasing the load step by step, the reading of ammeter, voltmeter and wattmeter

7. Step1 to 6 is repeated till the ammeter shows the rated current of 3-phase induction motor.

8. Decrease the load, bring auto-transformer to its minimum voltage position.

9. Switch off the supply.

CIRCUIT DIAGRAM

LOAD TEST ON THREE PHASE SQUIRREL CAGE INDUCTION MOTOR:

(0 - 10) A, MI

M L R

R C V

TPST

( 0 -600)V,MI

Y B

415V,50Hz, 3f SUPPLY

STATOR

Y

M L

C V

N

OBSERVATION TABLE :

S.NO

V (volt)

I (A)

Speed (rpm)

Spring Balance

S1(Kg) S2(Kg)

Torque

=((S1 - S2) *

9.81 * R)

N-m

I/P

(V*IL)

watt

O/P

2 P NT

60 (watt)

Efficiency = Output Power

Input Power

100

%h

%

slip

RESULT :

Answer 3

By coupling a dynamo meter to output shaft of the motor. By means of dynamo meter, you can create mechanical load which can be easily adjustable. Just Google search "dynamometer" for getting information.

Eddy current (EC) dynamometers, regenerative induction motor dynamometers, water dynamometers are some of the types of dynamometers, which can be used for testing the motors.