What is No Load Test of an Induction Motor?

The No - Load Test serves as an indirect approach for ascertaining the efficiency of three - phase induction motors. It also enables the determination of the circuit parameters of their equivalent circuits. Similarly, the open - circuit test is carried out on transformers. In fact, the no - load test on an induction motor is conceptually equivalent to the open - circuit test on a transformer.

During this test, the motor is decoupled from its load. Then, the rated voltage at the rated frequency is supplied to the stator, allowing the motor to operate without any load attached. Two wattmeters are utilized to measure the input power of the motor. The circuit diagram for the No - Load Test is presented as follows:

An ammeter is employed to measure the no - load current, while a voltmeter indicates the normal rated supply voltage. The I²R losses on the primary side are disregarded because these losses vary in proportion to the square of the current. It is known that the no - load current typically ranges from 20 - 30% of the full - load current.

Since the motor is operating under no - load conditions, the total input power is equivalent to the sum of the constant iron losses, as well as the friction and windage losses within the motor.

Since the power factor of an induction motor under no - load conditions is generally less than 0.5, the reading of one of the wattmeters will be negative. As a result, it is essential to reverse the connections of the current coil terminals of that wattmeter to obtain proper readings.

In the no - load test of a transformer, the constant values of the equivalent resistance (R0) and reactance (X0) can be computed from the test measurements.

Let:

• (Vinl) represent the input line voltage.

• (Pinl) denote the total three - phase input power at no - load.

• (I0) be the input line current.

• (Vip) stand for the input phase voltage.

Therefore,