The frequency of the emf in the stator of a 4-pole induction motor is 50 Hz, and that in the rotor is 2 Hz. What is the slip and at what speed is the motor running?

• A. 3%, 1440 rpm
• B. 3%, 1455 rpm
• C. 4%, 1455 rpm
• D. 4%, 1440 rpm

Answer: (D)

Rotor frequency, f_r = s*f_s
s = 2/50
s = 0.04

Ns = 120f/P
Ns = 1500 rpm
s = (Ns - Nr)/Ns
Nr = Ns(1 - s)
Nr = 1500*(1 - 0.04)
Nr = 1440 rpm