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Blocked Rotor Test or Short Circuit Test of Induction Motor
The figure shows the circuit diagram for the blocked rotor test of an induction motor. The blocked rotor test enables us to determine the efficiency and the circuit parameters of the equivalent circuit of a 3-phase induction motor.
In the blocked rotor test, the shaft of the motor is locked so that it cannot rotate and the rotor winding is short circuited. In a slip-ring induction motor, the rotor winding is short-circuited through the slip-rings while in a squirrel cage induction motor, the rotor bars are permanently short-circuited with the help of end rings.
In the blocked rotor test, a reduced voltage at reduced frequency is applied to the stator of the induction motor through a 3-phase autotransformer so that the rated current flows in the stator winding. The readings obtained are given as follows −
- Total input power on short circuit is measured by the two wattmeter method and is given by the algebraic sum of the two watt-meter readings. Here, a reduced voltage is applied to the stator and rotation of the rotor is not allowed, thus, the core and mechanical losses are negligible. Therefore, the total input power in the blocked rotor test is equal to the sum of stator copper losses and rotor copper losses for all the 3-phases.
- The ammeter reads the value of line current (Iscl) with blocked rotor which is corresponds to the short circuit condition.
- The voltmeter reads the value of reduced line voltage with blocked rotor (Vscl).
Therefore, the input power under blocked rotor condition is given by,
$$\mathrm{𝑃_{𝑠𝑐} = \sqrt{3} 𝑉_{𝑠𝑐𝑙}𝐼_{𝑠𝑐𝑙} cos \varphi_{𝑠𝑐}}$$
Where, ϕsc is the power factor under blocked rotor condition.
The equivalent resistance of the motor referred to the stator is
$$\mathrm{𝑅_{𝑒1} =\frac{𝑃_{𝑠𝑐.𝑝ℎ}}{𝐼_{𝑠𝑐.𝑝ℎ}^{2}}}$$
Where,
- Psc.ph = per phase power under blocked rotor condition.
- Isc.ph = per phase current under locked rotor condition.
Equivalent impedance of the motor referred to the stator is
$$\mathrm{Z_{𝑒1} =\frac{𝑃_{𝑠𝑐.𝑝ℎ}}{𝐼_{𝑠𝑐.𝑝ℎ}}}$$
And the equivalent reactance of the motor referred to the stator is
$$\mathrm{𝑋_{𝑒1} = \sqrt{𝑍_{𝑒1}^{2} − 𝑅_{𝑒1}^{2}}}$$
The blocked rotor test of the induction motor should be performed under the same conditions of rotor current and frequency that will exist under the normal operating conditions. Under the normal operating conditions, the slip of induction motors is about 2 to 4 percent and the rotor current frequency is from 1 to 2 Hz for the stator frequency of 50 Hz.
Thus, the blocked rotor test should be performed at a reduced stator frequency because the effective resistance and the leakage reactance of the rotor at the reduced frequency may differ considerably from their values at the rated frequency. Hence, in order to obtain the accurate results, the blocked rotor test is performed at a frequency 25 % less of the rated frequency.
Although, for the induction motors of less than 20kW rating, the effect of the frequency is negligible and hence the blocked rotor test can be performed directly at the rated frequency.
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