This test is carried out on an induction motor running without any external load and at rated voltage and rated frequency. No load test on an induction motor gives information about excitation current and no-load rotational losses (i.e. windage, friction and core losses).
Rotor current is negligible under no-load condition and, hence, the rotor copper losses are also negligible. Thus, the total input power measured under no-load is equal to the sum of stator copper losses and the rotational losses.
In blocked rotor test, the rotor of induction motor under test is locked by means of a belt or other mechanical arrangement. A low voltage is applied at the stator terminals. Because, if full voltage is applied then the current through the stator winding would be high enough to overheat and damage the stator winding. Also, this test is conducted at 1/4th of the rated frequency. This test is carried out to find short circuit current at normal voltage, leakage reactance and starting torque of the motor.