Previous Year Question
The frequency of rotor EMF of a 3-phase, 60 Hz, 8 pole induction motor is 2 Hz. Its speed of rotation will be
Correct answer: B
Explanation
Rotor frequency is slip times supply frequency. Thus s=2/60=1/30. For 8 poles at 60 Hz, Ns=900 rpm and N=(1−1/30)×900=870 rpm.
Formula
s=fr/f=2/60=1/30; Ns=120f/P=900 rpm; N=(1−s)Ns=870 rpm
Step-by-step solution
Correct Answer
B — 870 rpm
Concept and Reasoning
Rotor frequency is slip times supply frequency. Thus s=2/60=1/30. For 8 poles at 60 Hz, Ns=900 rpm and N=(1−1/30)×900=870 rpm.
Calculation / Key Relation
s=fr/f=2/60=1/30
Ns=120f/P=120×60/8=900 rpm
N=(1−s)Ns=(29/30)×900=870 rpm
Exam Takeaway
Rotor frequency directly gives slip: fr=sf.
Common Mistake
Do not use rotor frequency itself in the synchronous-speed formula.
Common mistake
Do not use rotor frequency itself in the synchronous-speed formula.
Exam tip
Rotor frequency directly gives slip: fr=sf.
Quick method
Rotor frequency directly gives slip: fr=sf.