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Explanation: The direction of rotating MMF wave is reversed, though its amplitude remains unaltered. 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) Any of the mentioned View Answer Answer: c 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 View Answer Answer: d 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 View Answer Answer: a 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 View Answer Answer: a Explanation: In a 2 phase and 3 phase AC machine, rotating MMF is produced with magnitudes of φm and 3/2φm.
