Correct Answer - Option 4 : 4
Concept:
The synchronous speed of a three-phase induction motor is given by:
\({N_s} = \dfrac{{120f}}{P}\)
Where Ns = Synchronous speed in rpm
f = Supply frequency
P = Number of poles
Rotor speed (Nr) must be less than synchronous speed (Ns)
Calculation:
Given Nr = 1440 and f = 50 Hz
Ns > Nr
\({N_r}<{N_s} = \dfrac{{120f}}{P}\)
\(⇒ P < \dfrac{120f}{{N_r}}<\dfrac{120\times 50}{1440}<4.166\)
⇒ P = 4
