A three phase four pole 50 Hz induction motor has a rotor resistance of 0.02 Ω/phase and stand-still reactance of 0.5 Ω/phase. Calculate the speed at which the maximum torque is developed.

• A. 1500 rpm
• B. 1525 rpm
• C. 1475 rpm
• D. 1440 rpm

Answer: (D)

Condition for maximum torque,
Sm = R2/X2
= 0.04
Ns = (120*f)/P
= (120*50)/4
= 1500 rpm
S = (Ns - Nr)/Ns
Nr = Ns(1 - Sm)
= 1500*(1 - 0.04)
= 1440 rpm