Why must we supply rated current at block rotor test condition instead of rated voltage?

In the blocked rotor test, it should be kept in mind that the applied voltage on the stator terminals should be low otherwise normal voltage could damage the winding of the stator. The slip is unity related to zero speed of rotor hence the load resistance becomes zero.

Why blocked rotor test is performed at rated current?

This test is performed to determine the short circuit current with normal voltage applied to stator; power factor on short circuit; total equivalent of resistance and Reactance referred to stator.

Why the no load test is conducted at rated voltage while the blocked rotor test is conducted at rated current?

The efficiency of large motors can be determined by directly loading them and by measuring their input and output powers. For larger motors it may be difficult to arrange loads for them. Moreover power loss will be large with direct loading tests. Thus no load and blocked rotor tests are performed on the motors.

Why no load test is performed at rated frequency and blocked rotor test is performed at reduced frequency?

This test should be performed at a reduced frequency. In order to obtain accurate results, the Blocked Rotor Test is performed at a frequency 25 percent or less than the rated frequency. The leakage reactances at the rated frequency are obtained by considering that the reactance is proportional to the frequency.

Why blocked rotor test is known as short circuit test?

It is also known as short-circuit test (because it is the mechanical analogy of a transformer short-circuit test), locked rotor test or stalled torque test. From this test, short-circuit current at normal voltage, power factor on short circuit, total leakage reactance, and starting torque of the motor can be found.

Why we do no load and blocked rotor test?

The efficiency of small motors can be determined by directly loading them and by measuring the input and output powers. But in the case of large motors, it is difficult to arrange that much load for them. The power loss will be large if we directly test the load.

What is the purpose of no load and blocked rotor test?

1. To obtain the variation of no load power and current and blocked rotor power and current with changes in the applied voltage to the stator. 2. To determine the equivalent circuit parameters of an induction motor.

Why do we need short-circuit test in transformer?

The purpose of a short-circuit test is to determine the series branch parameters of the equivalent circuit of a transformer.

What is locked rotor condition?

Locked rotor is a case when rotor isn’t allowed to rotate by supplying rated voltage. Under this condition, the rotor winding cut the rotating flux without rotating which is equivalent to short circuit of a transformer secondary. So, locked rotor current is actually short circuit current in a motor.

How can the blocked rotor test be performed to obtain induction motor characteristics in synchronous speed?

As in Transformer short circuit test, we apply a voltage which can circulate rated current in the winding, likewise in Blocked Rotor Test we apply a Voltage to the Stator winding of an Induction Motor by using VARIAC so that rated current flows through the stator winding.

Why blocked rotor test is called as short-circuit test?