# Electrical Machines Questions and Answers – Rotating Magnetic Field – 2

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Rotating Magnetic Field – 2”.

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

Explanation: Space harmonics of the order of 6K-1(5,11,17,…) 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.

2. Which of the following harmonics are present in the resultant MMF produced by 3 phase currents flowing in the 3 phase windings?
a) Triplen harmonics (3K), where K is an integer
b) 6K+1 harmonics only, where K is an integer
c) 6K-1 harmonics only, where K is an integer
d) 6k+1 and 6K-1 harmonics, where K is an integer

Explanation: Space harmonics of the order 3,9,15,… i.e, triplen harmonics are absent.

3. When a single-phase winding is excited by an alternating current, then which of the following statements are correct?
a) It produces one stationary MMF wave
b) Produced MMF wave pulsates along its magnetic axis
c) It doesn’t produce two counter rotating MMF wave
d) All of the mentioned

Explanation: It is the decomposition of the MMF of a single phase winding into two counter rotating MMF waves, but are not actually produced and only one stationary MMF wave pulsating along its magnetic axis is produced.

4. What does the space angle α in the equation Fr(α,t)=3/2Fmcos(α-ωt) for MMF produced when 3-phase balanced windings excited by three phase balanced currents represent?
a) MMF wave at any moment is sinusoidally distributed in space
b) Entire MMF waveform is travelling at (constant angular speed) synchronous speed
c) MMF wave is of constant amplitude
d) None of the mentioned

Explanation: The space angle α means that MMF wave at any moment is sinusoidally distributed in space along the air-gap periphery.

5. Three phase voltages are applied to the three windings of an electrical machine. If any two supply terminals are interchanged, then the rotating MMF wave _____________
a) Direction reverses, amplitude alters
b) Direction reverses, amplitude unaltered
c) Direction remains same, amplitude alters
d) Direction remains same, amplitude unaltered

Explanation: The direction of rotating MMF wave is reversed, though its amplitude remains unaltered.
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6. The rotating MMF wave of constant amplitude can be produced, only if the time angle displacement between currents is _____________
a) Less than the space angle displacement between winding axis
b) More than the space angle displacement between winding axis
c) Equal to the space angle displacement between winding axis
d) All of the mentioned

Explanation: If the time angle displacement between currents and space angle displacement between winding axis are not equal, the magnitude of resultant MMF wave is always zero.

7. A 3 phase star connected winding is fed from symmetrical 3 phase supply with their neutrals connected together. If one of the 3 supply lines gets disconnected, then the revolving MMF wave will have a ___________
a) Constant amplitude but sub synchronous speed
b) Varying amplitude but synchronous speed
c) Constant amplitude but super synchronous speed
d) Varying speed and a speed fluctuating around the synchronous speed

Explanation: With one supply phase disconnected, a rotating MMF wave of varying amplitude is produced, but at a speed fluctuating around the synchronous speed.

8. In case of the 3 phase winding with delta or star connection without neutral, if one of the supply lines gets disconnected, then the MMF becomes __________
a) Stationary and pulsating
b) Rotating with synchronous speed
c) Rotating with sub synchronous speed
d) Rotating with super synchronous speed

Explanation: Disconnection of one of the supply leads, results in single-phase operation of the machine.

9. If φm is the maximum value of flux due to any one phase, then resultant flux in 2 phase and 3 phase AC machines would respectively be given by _________
a) φm and 1.5φm, both rotating
b) φm and 1.5φm, both stand still
c) φm stand still and 1.5φm rotating
d) 1.5φm and 2φm, both rotating

Explanation: In a 2 phase and 3 phase AC machine, rotating MMF is produced with magnitudes of φm and 3/2φm.

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