# Electrical Machines Questions and Answers – Rotating Magnetic Field – 1

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Rotating Magnetic Field – 1”.

1. For a p-pole machine, which of the following statements are correct regarding the rotating field speed?
a) The rotating field speed is 1/(p/2) revolutions in one cycle
b) f/(p/2) revolutions in f cycles
c) f/(p/2) revolutions in one second
d) Any of the mentioned

Explanation: f/(p/2) revolutions in one second is also correct because f cycles are completed in one second. ns(rotating field speed in revolutions per second) = 2f/p or Ns = 120f/p r.p.m.

2. The speed at which rotating magnetic field revolves is called?
a) Induction speed
b) Synchronous speed
c) Relative speed
d) Rotating speed

Explanation: Rotating field speed (Ns) = 120f/(p r.p.m) = synchronous speed.

3. What is the amplitude of rotating MMF produced as a result of m-phase currents flowing in m-phase windings? (where Fm is maximum MMF in any one phase, when current is maximum in that phase.)
a) (2/m)Fm
b) mFm
c) (m/2)Fm
d) Fm

Explanation: In general m phase current flowing in m phase winding results in rotating MMF wave of constant amplitude, equal to m/(2Fm).

4. The peak of rotating MMF wave (FR) is directed along which of the following axis?
a) The axis of that phase which carries the maximum current at that instant
b) The axis of that phase which carries half of the maximum current at that instant
c) The axis of that phase which carries minimum current at that instant
d) All of the mentioned

Explanation: The resultant MMF FR = 3/(2Fm) is directed along the axis of that phase, which carries the maximum current at that instant.

5. If the phase sequence of supply currents are reversed, then the direction of rotation of the resultant MMF wave will ___________
a) Be reversed
b) Remains unchanged
c) Cannot be determined
d) None of the mentioned

Explanation: A reversal of the phase sequence of supply currents will reverse the direction of rotation of the resultant MMF wave.

6. Which of the following statements are correct regarding individual phase MMF in rotating machines?
a) It is a rotating MMF wave
b) It is not a rotating MMF wave and its amplitude doesn’t alternate along its own phase axis
c) It is not a rotating MMF wave but its amplitude merely pulsates
d) None of the mentioned

Explanation: It should be remembered that individual phase MMF is not a rotating MMF wave, its amplitude merely pulsates or alternates along its own phase axis.

7. The effect of poly phase currents in poly phase winding can be compared to _______________
a) Mechanical rotation of permanent magnets at synchronous speed
b) Mechanical rotation of DC excited field poles at synchronous speed
c) Mechanical rotation of either permanent magnets or DC excited field poles, at synchronous speed
d) None of the mentioned

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

Explanation: FR = 3/(2Fm) (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) Nph and P
b) I and P
c) Nph and I
d) Nph, I and P

Explanation: FR = 3/(2Fm) = 3/(2 4√2/(ΠKw NphI/P)
where Nph = series turns per phase and I = RMS current.

10. If F1 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 F1
c) Varying amplitude and rotates along F1
d) Constant amplitude and rotates against F1

Explanation: Space harmonics of the order of 6K+1(7,13,19,…) have their constant amplitude equal to 1/(6K+1)F1 and rotate in the direction of fundamental component F1 at a speed of 1/(6K+1) of the fundamental synchronous speed.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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