Question

A 4 pole, 3 phase, 50 Hz star connected induction motor has a full load slip of 4%. Calculate full load speed of motor.
1. 1500 rpm
2. 1440 rpm
3. 1425 rpm
4. None of these

Answer 1

Correct Answer - Option 2 : 1440 rpm

Concept:

In a 3-phase induction motor, the synchronous speed can be calculated as

\({N_s} = \;\frac{{120f}}{P}\)

\(s = \;\frac{{{N_s} - {N_r}}}{{{N_s}}}\)

Where, Ns = Synchronous speed in rpm,

Nr = Rotor speed in rpm,

s = Slip of the machine,

f = Supply frequency in Hz,

P = Number of poles.

Slip speed: The difference between the synchronous speed and the actual speed of the rotor is known as the slip speed.

Slip speed sNs = Ns - Nr

Rotor speed ⇒ N_r = (1-s)N_s

Calculation: 

Given,

P = 4

f = 50 Hz

Full load slip = s = 4% = 0.04

\(N_s = \dfrac{120f}{P}=\dfrac{120× 50}{4} = 1500\;rpm\)

Full load rotor speed ⇒ N_r = (1-s)Ns = (1-0.04) × 1500 = 1440 rpm