Electrical Engineering ⇒ Topic : Blocked Rotor or Short-Circuit Test

Blocked Rotor or Short-circuit Test. This test is performed to determine :

• Short-circuit current Ç with normal voltage applied to stator.
• Power factor on short-circuit.
• Total equivalent resistance and reactance of the motor as referred to stator.
• Full-load copper loss.

The test is conducted as follows (Figure (a)).

   The rotor is preliented from turning by blocking it, and a low voltage of less than 10 per cent of rated is applied to the stator. This voltage is best applied through a variable,auto-transformer, or three-phase Variac. Suitable resistors in series with the motor- stator terminals may also be used, but care must be taken that the resistors are identical so that balanced voltage may be applied to the stator

                                                                 Fig. (a) Efficiency test of induction motor.

   The voltage is then gradually decreased until rated current is obtained in Since the voltage required the stator. is quite low, the power supplied to the stator will be dissipated as copper loss in both stator and rotor. It is thus assumed, as in the static transformer short-circuit test, that the core loss is negligible. This will enable us to calculate an equivalent resistance of the motor referred to the stator terminals and to use this value of resistance to correct our readings of the no-load test.

If at short-circuit,

                       v_S= applied voltage/phase,

                       I_S = short current/phase in the stator winding,

                       P_s= total input power, then

Since input on short-circuit meets with stator and rotor copper losses and core losses, core losses being very small can be neglected, therefore,

       Input on short-circuit = 

or Equivalent resistance per phase referred to stator,

Motor equivalent impedance per phase as referred to stator,

Motor equivalent reactance per phase as referred to stator

The wiring diagram to be used for both tests is phown in Fig.(a)

BLOCKED ROTOR OR SHORT-CIRCUIT TEST

Figure (a) shows the schematic diagram of blocked rotor test of an induction motor. The induction motor is locked so that it cannot move. The rotor winding is short circuited. In squirrel cage motors, the rotor bars are permanently short circuited. In a slip-ring motor, the rotor winding sare short circuited by the slip rings.

                                    Figure (a) Blocked rotor test of three-phase induction motor

Let P_sc is the total power input during this test, I_Sc is line current on short circuit, and V_sc is the line voltage on short circuit.