In a 3-phase induction motor, the maximum torque under standstill occurs when the rotor resistance (R2) is:

Torque in the 3-phase induction motor

The torque of a 3-phase induction motor is given by:

\(T={3\over ω_s}{V^2\over ({R_2\over s}^2)+X_2^2}{R_2\over s}\)

where, V = Supply

ωs = Synchronous speed in radian/sec

R2 = Rotor resistance

X2 = Rotor reactance

s = Slip

Condition for maximum torque

The maximum torque of a 3-phase induction motor occurs at:

\(s={R_2\over X_1+X_2}\)

If the maximum torque has to occur in starting, the value of the slip (s)=1

\(1={R_2\over X_1+X_2}\)

\(R_2=X_1+X_2\)

The maximum torque of a 3ϕ induction motor occurs when rotor resistance is equal to leakage reactance (X1 + X2).