Q4. A three phase induction motor is wound for four poles and is supplied by 50 Hz supply system. Calculate: (i) Synchronous speed (ii) Rotor speed when slip is 4% (iii) Rotor frequency when rotor runs at 600RPM (iv) Rotor frequency at standstill.

Given data: P=4, Freq=50Hz

To find: (i) Ns ,    (ii) Nr at S=4%,    (iii) F’ at Nr =600rpm,       (iv) F’ at standstill i.e S=1

Solution: (i)   Ns = 120F/ P = (120*50)/4

Ns = 1500 rpm

                  (ii) Nr = Ns(1-S) = 1500(1-0.04)

Nr =1440 rpm

               (iii) We know, (Ns-Nr)/Ns = F’/F

F’=[(1500-600)/1500]*50

F’ = 30 Hz

               (iv) F’ = S*F

F’ = 1*50

F’ =50 Hz

 

Hence, Ns=1500rpm

               Nr=1440rpm

               F’=30Hz

               F’=50Hz at S=1