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An 3 phase alternator star connected 400 V synchronous motor takes a power input of 5472 watts at rated voltage. Its synchronous reactance is 10** per phase and negligible resistance. If its excitation voltage is adjusted equal to the rated voltage of 400 V, its power factor is _______

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An 3 phase alternator star connected 400 V synchronous motor takes a power input of 5472 watts at rated voltage. Its synchronous reactance is 10** per phase and negligible resistance. If its excitation voltage is adjusted equal to the rated voltage of 400 V, its power factor is _______

(a) cos(10°)

(b) cos(20°)

(c) cos(40°)

(d) cos(30°)

The question was asked in an interview for internship.

Question is taken from Power Flow Through an Inductive Impedance topic in chapter Efficiency and Regulation of Electrical Machines

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The correct choice is (a) cos(10°)

Best explanation: Vt = Ef = 400/1.73 = 231 V

                  V*Esinδ/X = P

                  δ = 20°

                  θ = δ/2.

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