No Load Test of Induction Motor

This test is similar to the no-load test performed on a transformer. The purpose of this test is to determine the no-load losses, no-load power factor, and constants R_o and X_o of the equivalent circuit.

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

The 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.

Since the motor is not supplying any load, the no-load current is small and stator copper losses are negligible. The input power, W_o 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_o = Power input.
• V₁ = Applied voltage per phase.
• I_o = Applied current per phase under no-load conditions.

Then the no-load power factor is,

Where,

• I_W = Working component of no-load current I_o.
• I_μ = Magnetising component of no-load current I_o.

The no-load losses for different voltages are plotted as,

The below table helps you to note down the readings while performing the no-load test on the induction motor. For easier calculation of no-load losses and power factor.

S.No.	Stator Voltage (V1)	Stator Current (Io	Rotor Current (Ir)	Wattmeter Readings W1 and W2	Power Input (Wo)

Readings observed from the test>

But there is a thing you have to remember while performing the no-load test on an induction motor, i.e., changes in speed, power factor, stator current, and rotor current due to fluctuations in applied voltage.

When there are changes (fluctuations) in the applied voltage the readings noted from the test will be more accurate. Hence it also increases the accuracy of the calculations made from the test.

• While performing a no-load test, the speed of an induction motor obviously remains constant during the test. The speed of an induction motor changes when there is a heavy load on the motor. Therefore, here the applied voltage should remain constant so that the speed of the motor will remain unchanged.
• When there is an increase in the applied voltage to a larger value, the stator current that supplies the magnetizing component of the stator winding will also increase. Thus, there we can see a fall in the power factor.
• The torque of an induction motor is proportional to the rotor current and flux generated by the stator winding. Here flux in an induction motor will depend upon the amount of applied voltage to the stator. When the applied voltage is kept minimum the torque generated will remain constant, but the rotor current will go on increasing.