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Equivalent Circuit of an Induction Motor

The equivalent circuit for a polyphase induction motor is shown in Fig. 24, where

V1 = applied voltage per phase

R1 = stator resistance/phase

R2 = rotor resistance/phase

X1 = stator leakage reactance/phase

X2 = rotor standstill leakage reactance/phase

K = turn-ratio of secondary to primary

R0 = no-load resistance/phase

X0 = no-load reactance/phase

Equivalent Circuit of an Induction Motor

Fig. 24. Equivalent circuit of an induction motor.

As shown in Fig. 25 the exciting circuit may be transferred to the left, because inaccuracy involved is negligible but the circuit and hence the calculations are very much simplified. This is known as the approximate equivalent circuit of the induction motor.

Maximum Power Output. Refer Fig. 25.

Equivalent Circuit of an Induction Motor

EQUIVALENT CIRCUIT OF AN INDUCTION MOTOR 

Example 16. In a 3-phase induction motor maximum torque occurs at a slip of 10 per cent. The equivalent secondary resistance of the motor is 0.09 Ω/phase. If the gross power output is 8.5 II W. Calculate.-

(i)     Equivalent load resistance,

(ii)   Equivalent load voltage, and

(iii)Current at this slip.

Solution. Slip at maximum torque, s = 10 per cent

The equivalent secondary resistance of the motor,

R2 = 0.09 Ω/phase

Gross power output, Pg = 8.5 kW or 8500 W

Equivalent Circuit of an Induction Motor