Load Test on Three Phase Induction Motor

Load Test on Three Phase Induction Motor

By conducting the load test on three phase induction motor, the performance of the motor viz. slip, power factor, input, efficiency etc. at various loads can be studied.

The induction motor is loaded by any of the following methods :

1. Brake test

2. By connecting a d.c. generator

In case of loading by connecting a d.c. generator, the induction motor is connected to a d.c. generator. The generator is loaded by a lamp bank. Thus inturn an induction motor is loaded. The Fig. 6.7.1

shows the experimental set up for conducting load test on three phase induction motor using a d.c. generator.

On induction motor side, ammeter reads line current IL and voltmeter reads line voltage VL. The two wattmeters are connected as per the two wattmeter method hence,

P_in = Power input = W₁+ W₂ 

On generator side, the ammeter reads load current I_L and voltmeter reads terminal voltage V_t

By varying the lamp bank, load on generator i.e. load on induction motor can be varied. The induction motor can be star or delta connected and can be squirrel cage or slip ring type. The speed readings are taken using tachometer. The load is increases till induction motor carries rated line current. The following observation table is prepared,

Calculations : The output of induction motor is input to a d.c. generator.

Output of d.c. generator = V_t × I_L W

Assume  ƞgen = 80 %

P_out of induction motor = of d.c. generator

= P_out of d.c. generator / ƞgen = V_tI_L / ƞgen W

P_in of induction motor = W₁ + W₂ W

For various loads above parameters are obtained.

As the load on the induction motor increases,

1. The output of motor increases.

2. The power factor increases.

3. The efficiency increases upto certain load and then decreases.

4. The speed decreases marginally.

5. The slip increases.

6. The input current increases.

The various performance characteristics can be obtained as shown in the Fig. 6.7.2.

The graphs indicate the behaviour of various performance parameters against output of the induction motor and not shown to the scale.

Review Question

1. How do conduct load test on an induction motor ? How the various parameters of induction motor change as the load changes ?