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d) Fm where Fm is maximum MMF in any one phase, when current is maximum in that phase. View Answer Answer: c 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) Any of the mentioned View Answer Answer: a 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 View Answer Answer: a 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 View Answer Answer: c 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 View Answer
