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View Answer Answer: b 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 View Answer Answer: d 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 View Answer Answer: d 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 View Answer Answer: a 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 View Answer Answer: b
