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?
Answer & step-by-step solution
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.
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.
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.
एक सिंक्रोनस मशीन में फील्ड वाइंडिंग स्टेटर पर और पॉलीफेज़ आर्मेचर वाइंडिंग रोटर पर है। स्थायी अवस्था में इसके एयर गैप क्षेत्र के लिए निम्नलिखित में से कौन-सा सही नहीं है?
उत्तर और चरणबद्ध हल
GPSC का अंतिम उत्तर B है। स्टेटर पर फील्ड वाइंडिंग स्टेटर के सापेक्ष स्थिर एयर-गैप क्षेत्र बनाती है। सिंक्रोनस गति से घूमता रोटर इस स्थिर क्षेत्र को Ns गति से पीछे जाता देखता है, 2Ns से नहीं।
विस्तृत हल (मूल भाषा में)
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.
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.
એક સિંક્રોનસ મશીનમાં ફીલ્ડ વાઇન્ડિંગ સ્ટેટર પર અને પોલિફેઝ આર્મેચર વાઇન્ડિંગ રોટર પર છે. સ્થાયી સ્થિતિમાં તેના એર ગેપ ક્ષેત્ર માટે નીચેનામાંથી કયું સાચું નથી?
જવાબ અને પગલાવાર ઉકેલ
GPSC નો અંતિમ જવાબ B છે. સ્ટેટર પરનું ફીલ્ડ વાઇન્ડિંગ સ્ટેટરની સાપેક્ષે સ્થિર એર-ગેપ ક્ષેત્ર બનાવે છે. સિંક્રોનસ ઝડપે ફરતો રોટર આ સ્થિર ક્ષેત્રને Ns ઝડપે પાછળ જતું જુએ છે, 2Ns થી નહીં.
વિગતવાર ઉકેલ (મૂળ ભાષામાં)
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.
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.
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