Correct Answer - Option 3 : 970 rpm
Slip of Induction Motor (S):
Slip in Induction Motor is the relative speed between the rotating magnetic flux and rotor expressed in terms of per unit synchronous speed.
If Ns and Nr being the synchronous speed of rotating magnetic flux and rotor speed respectively, then the relative speed between them is equal to (Ns – Nr).
Therefore, slip is defined as
\(s=\frac{N_s-N_r}{N_s}\)
Where,
\(N_s=\frac{120f}{P}\)
f is the frequency in Hz and P is the number of poles
Application:
Given,
f = 50 Hz
P = 6
s = 3% = 0.03
\(\therefore N_s=\frac{120f}{P}=\frac{120×50}{6}=1000\ rpm\)
from above concept,
\(0.03=\frac{1000-N_r}{1000}\)
or, 1000 × 0.03 = 1000 - Nr
Nr = 970 rpm