Question 4.13: The frequency of the rotor-induced EMF of 400 V, three-phase...

N_{s} = \frac{120  f}{P} = \frac{120  ×  50}{6} = 1000 rpm

Rotor frequency,           f_{r} = Sf
or,                                     2 = S × 50
or,                                     S = 0.04

Again,                               S = \frac{N_{s}  –  N_{r}}{N_{s}}

Substituting values       0.04 = \frac{ 1000  –  N_{r}}{1000}

or,                                     N_{r} = 960 rpm