The starting torque in a three phase induction motor, will be the maximum when, 

The correct answer is option 3): 1

Concept:

Torque equation: The torque equation of a three-phase induction motor is given by,

Where Ns is the synchronous speed

V = supply voltage

R2 = rotor resistance

X2 = rotor reactance s is the slip

By the above expression, we can say that the torque of an induction motor depends on rotor resistance and slip. Condition for maximum torque: The condition to get the maximum torque at starting is,

X_m = R_­m

Where,

Rm = Motor resistance per phase

X_m = Motor reactance per phase At the starting of the three-phase slip ring induction motor

Slip (s) = 1 (At the starting Nr = 0)

X_m = R_m

Hence the starting torque of an induction motor is maximum when rotor resistance equals rotor reactance.