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
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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
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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
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4. What does the space angle α in the equation F_r(α,t)=3/2F_mcos(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
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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
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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
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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
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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
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9. If φ_m 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) φ_m and 1.5φ_m, both rotating
b) φ_m and 1.5φ_m, both stand still
c) φ_m stand still and 1.5φ_m rotating
d) 1.5φ_m and 2φ_m, both rotating
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