Blocked Rotor Test of an Induction Motor

05/24/2025

The blocked - rotor test of an induction motor is analogous to the short - circuit test of a transformer. In this test, the motor's shaft is immobilized to prevent any rotation, and the rotor winding is short - circuited. For a slip - ring motor, the rotor winding is short - circuited via the slip rings. In the case of cage motors, the rotor bars are inherently short - circuited. This test is also referred to as the Locked Rotor Test. The circuit diagram for the blocked - rotor test is presented below:

A reduced voltage at a reduced frequency is supplied to the stator via a three - phase autotransformer, ensuring that the full - load rated current circulates in the stator. The blocked - rotor test yields the following three measurements:

Total power input during short - circuit $Psc$ : This is the algebraic sum of the readings from the two wattmeters. The power input during the blocked - rotor test is equivalent to the combined copper losses of the stator and rotor for all three phases. Since a reduced voltage is applied to the stator, thereby preventing rotor rotation, core and mechanical losses can be disregarded.
Ammeter reading

Reading of the voltmeter

where cosϕ represents the power factor of the short - circuit. The equivalent resistance of the motor, referred to the stator side, is expressed by the following equation:

The equivalent impedance of the motor referred to the stator side is given by the equation shown below:

The equivalent reactance of the motor referred to the stator side is given by the equation shown below.

The blocked - rotor test is carried out under normal operating conditions, with the rotor current and frequency being in their typical states. Generally, for an induction motor, the slip typically ranges from 2% to 4%. When the stator frequency is 50 hertz under normal conditions, the resulting rotor frequency falls within the range of 1 to 2 hertz.

This test should be executed at a reduced frequency. To achieve accurate results, the blocked - rotor test is performed at a frequency that is 25% or less of the rated frequency. The leakage reactances at the rated frequency are derived based on the principle that reactance is proportional to frequency.

Nevertheless, for motors with a rating of less than 20 kilowatts, the influence of frequency is negligible, and the blocked - rotor test can be directly conducted at the rated frequency.

Topics

Machines Electric Motors

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