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 located 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. 1 shows the experimental set up for conducting load test on three phase induction motor using a d.c. generator.

Fig. 1 Load test on three phase induction motor

       On induction motor side, ammeter reads line current and voltmeter reads line voltage V_L. 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 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 increased 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 x I_L    W
       Assume        η_gen = 80 %
.^..     P_out of induction motor = P_in of d.c. generator
                                           = P_out of d.c. generator / η_gen = (V_t I_L )/η_gen    W
       P_in of induction motor = W₁ + W₂    W
       cos Φ = P_in/(√3V_L I_L )= (W₁ + W₂)/(√3V_L I_L) = power factor

       where         N_s = 120f / P   for a given motor
      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 increase 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. 2.

Fig. 2

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


Related articles :

• Circle Diagram : Introduction
• Circle Diagram for a Series R-L Circuit
• Circle Diagram of a 3 Phase Induction Motor
• No Load Test
• Blocked Rotor Test
• Construction of Circle Diagram

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