91.MET March 2020 Q.6(b)

A 4 pole 3 phase induction motor operates from a 50 Hz system. Calculate. i. The speed at which magnetic field of stator is rotating; ii. The speed of the rotor when the slip is 0.04; iii. The motor frequency when slip is 0.03

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Given 

No of poles P = 4
Frequency f = 50 Hz
Slip s = 0.04

To Find

 i. The speed at which magnetic field of stator is rotating
ii. The speed of the rotor when the slip is 0.04
iii. The motor frequency when slip is 0.03

Solution

i) Speed at which stator is rotating is called Synchronous speed 

   Synchronous speed Ns = 120f / P
                                         = (120 x 50) / 4
                                         = 1500 rpm

                                slip s = (Ns - N) / Ns

ii) Rotor speed N = Ns (1 - s)
                             = 1500 (1 - 0.04)
                             = 1440 rpm

iii) Motor frequency when s = 0.03
                 Motor frequency = slip x f
                                            = 0.03 x 50
                                            = 1.5 Hz