Electrical Machines Questions and Answers_pag25

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Answer: c 
Explanation: The speed at which rotating magnetic field revolves is called the synchronous 
speed and this effect is equivalent to mechanical rotation of permanent magnets or DC excited 
field poles. 
8. The magnitude of rotating flux _____________ at all instants of time. 
a) Changes 
b) Remains constant 
c) Pulsates 
d) Any of the mentioned 
View Answer 
Answer: b 
Explanation: F​R​ = 3/(2F​m​) (for a 3 phase machine). A constant amplitude rotating MMF or 
rotating field is produced in the air gap of a three phase machine. 
9. The amplitude of rotating MMF wave is proportional to 
a) N​ph​ and P 
b) I and P 
c) N​ph​ and I 
d) N​ph​, I and P 
View Answer 
Answer: c 
Explanation: F​R​ = 3/(2F​m​) = 3/(2 4√2/(ΠK​w​ N​ph​I/P) 
where N​ph​ = series turns per phase and I = RMS current. 
10. If F​1​ is the constant amplitude of fundamental rotating MMF wave, the space harmonics of 
order 6K+1 (where K=1,2,3….), the harmonic MMF wave is of _____________ 
a) Constant amplitude and stationary in space 
b) Constant amplitude and rotates along F​1 
c) Varying amplitude and rotates along F​1 
d) Constant amplitude and rotates against F​1 
View Answer 
Answer: b 
Explanation: Space harmonics of the order of 6K+1(7,13,19,…) have their constant amplitude 
equal to 1/(6K+1)F​1​ and rotate in the direction of fundamental component F​1​ at a speed of 
1/(6K+1) of the fundamental synchronous speed. 
 
 
This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on 
“Rotating Magnetic Field – 2”. 
1. If F​1​ is the constant amplitude of fundamental rotating MMF wave, the for the space 
harmonics of order 6K-1 (where K=1,2,3….), the harmonic MMF wave is of _____________ 
a) Constant amplitude and stationary in space 
b) Constant amplitude and rotates along F​1 
c) Varying amplitude and rotates along F​1 
d) Constant amplitude and rotates against F​1