A synchronous machine has field winding on stator and polyphase armature winding on rotor. At steady state, which of the following is not true for its air gap field?
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Correct AnswerB. Rotating at double the speed Ns w.r.t. rotor
Quick Explanation
The final GPSC answer is B. A field winding placed on the stator produces an air-gap field stationary with respect to the stator. A rotor moving at synchronous speed sees that stationary field moving backward at Ns, not at 2Ns.
Formula / Key Relation
Relative speed:
Nfield/stator = 0
Nrotor/stator = Ns
Nfield/rotor = 0 - Ns = -Ns
Detailed Explanation
Concept & ReasoningTake the stator as the stationary reference. The DC field winding on the stator establishes a stationary spatial field. The rotor carrying the polyphase armature rotates mechanically at synchronous speed Ns. Relative to the rotor, a field that is stationary in the stator frame appears to move at Ns in the opposite direction. Therefore 'rotating at double the speed Ns with respect to the rotor' is the statement that is not true. The other statements describe the same relative-motion picture in different frames.How to Solve It in the ExamAlways subtract angular speeds when changing reference frames.Important Exam PointWrite…
Concept & Reasoning
Take the stator as the stationary reference. The DC field winding on the stator establishes a stationary spatial field. The rotor carrying the polyphase armature rotates mechanically at synchronous speed Ns. Relative to the rotor, a field that is stationary in the stator frame appears to move at Ns in the opposite direction. Therefore 'rotating at double the speed Ns with respect to the rotor' is the statement that is not true. The other statements describe the same relative-motion picture in different frames.
How to Solve It in the Exam
Always subtract angular speeds when changing reference frames.
Important Exam Point
Write both field and rotor speeds in the same reference frame before judging relative direction.
Editorial / Answer-Key Note
The official GPSC final answer key revised the provisional outcome for this question from D to B.
Related Revision Path
This question sits in the revision path Electrical Machines → Synchronous Machines → Rotating Magnetic Fields .
Quick Trick
Always subtract angular speeds when changing reference frames.
Common Mistake
Adding speeds instead of subtracting them is what produces the incorrect 2Ns conclusion.
Exam Tip
Write both field and rotor speeds in the same reference frame before judging relative direction.
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