3-phase induction motor is wound for 4 Poles and is supplied from a 50Hz system. Calculate;

1. Synchronous speed
2. Rotor speed when slip is 4%
3. Rotor frequency (5 marks)

SOLUTION
I) Synchronous speed = f/p = 50/2 = 25rev/sec
II) Rotor slip 4% = Ns – Nr/ Ns × 100.
4/100 = 25- Nr/25
100 = 2500 – 100 Nr
100Nr = 2400Nr
Nr = 24rev/sec [RPM]
III) Rotor frequency = [Ns-Nr]P
= [25-24]2 = 2Hz