No Load Test of Induction Motor

     This test is similar to the No-Load test on a transformer. The purpose of this test is to determine the no-load losses, no-load power factor and constants Rand Xo of  the equivalent circuit.

     The no-load losses include the frictional windage loss, core loss and a small copper loss in stator winding. As the slip at no-load is very small and negligible, the rotor copper loss is negligible as rotor current at no-load is very small and neglected.

No Load Test of Induction Motor

     Figure above shows the connection diagram and equivalent circuit of the motor at no-load. The normal voltage is applied to the stator winding and the rotor is allowed to rotate freely without any mechanical load on its shaft. Two wattmeters W1 and  W2 measure the power input to the motor.

No Load Test of Induction Motor

     Since the motor is not supplying any load, the no-load current is small and stator copper losses are negligible. The input power, Wo equals mechanical and core losses. To be more accurate the stator copper loss at no-load current should be deducted from the input when calculating the above losses.

Calculation of No-Load Test of Induction Motor :

Let ,

     W =  Power input 

     V =   Applied voltage per phase  

      Io  =   Applied current per phase under no-load conditions. 

Then the no-load power factor is ,

Where ,

        IW    =   Working component of no-load current Io
        Iµ   =   Magnetising component of no-load current Io

The no-load losses for different voltages my be plotted as, 

No Load Test of Induction Motor

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