Blocked Rotor Test

In this test, the rotor is locked and it is not allowed to rotate. Thus the slip s = 1.


When s = 1 ⇒ Electrical equivalent of Mechanical load becomes equal to Zero.

If the motor is slip – ring induction motor then the windings are short – circuited at the slip rings. The situation is exactly similar to the short circuit test on transformer.

If under short circuit condition ⇒ Primary is excited with rated voltage ⇒ A large short circuit current can flow which is dangerous from the windings point of view. So similar to the transformer short circuit test ⇒ The reduced voltage (about 5 to 10% of rated voltage) just enough such that stator carries rated current is applied.

Vsc = Short circuit reduced voltage (Line value)

Isc = Short circuit current (Line value)

Wsc = Short circuit input power
Now Wsc = √3 Vsc Isc Cos Φsc
⇒ Cos Φsc = Wsc / √3 Vsc Isc
This is the short circuit power factor.
⇒ Wsc = 3 (Isc)2 R1e

Where Isc per phase value.
⇒ (Equivalent resistance referred to stator) R1e = Wsc / 3 (Isc)2
Z1e = Vsc (Per phase) / Isc (Per phase)
⇒ (Equivalent reactance referred to stator) X1e = √(Z1e)2 – (R1e)2
During this test, the stator carries rated current hence the stator copper loss is also dominant. Similarly the rotor also carries short circuit current is to produce dominant rotor copper loss. As the voltage is reduced, the iron loss which is proportional to voltage is negligibly small. The motor is at standstill hence mechanical losses i.e. friction and windage loss is absent.
Wsc = Stator Cu loss + Rotor Cu loss
It is necessary to obtain short circuit current when normal or rated voltage is applied to the motor. This is practically impossible. But the reduced voltage test results can be used to find short circuit current at normal or rated voltage.

VL = Normal rated voltage (Line to Line)

Vsc = Reduced short circuit voltage (Line value)
then ISN = (VL / Vsc) × Isc
Where Isc = Short circuit current at reduced voltage.
And ISN = Short circuit current at normal voltage.
Now power input is proportional to square of the current.
So Wsn = Short circuit input power at normal or rated voltage.
⇒ Wsn = (Isn / Isc)2 Wsc
But at normal voltage core loss can not be negligible.
Wsn = Core loss + Stator and rotor copper loss

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