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Electrical Engineering Objective Questions { Alternators }

162. For proper synchronisation of a large synchronous machine to a bus, the frequency of the incoming machine
A. should be exactly the same as that of the bus
B. should be slightly higher than that of the bus
C. should be slightly lower than that of the bus
D. can be of any value
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163. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by 15?. Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be
A. 30? electrical
B. 60? electrical
C. 90? electrical
D. 1500 electrical
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164. A 3-phase, 400 V, delta connected alternator has a synchronous impedance 01 (0 +j20) ohms per phase. If it delivers a balanced load of 12 kVA at zero power factor leading, then the percentage voltage regulation of the alternator is equal to
C. ?150
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165. For a uniformly distributed winding of an alternator with a phase spread of 13 degrees, the distribution factor is
B. 2 sinf3
C. 2sin3
D.sinf3 / 2
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166. A single-phase alternator has a synchronous reactance of 2 ohms and negligible resistance. If it supplies 10 A to a purely capacitive load at 200V, then the generated emf will be
A. 240 volts
B. 220 volts
C. 200 volts
D. 180 volts
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167. Suppose a synchronous generator connected to an infinite bus is supplying electrical power at unity PF to the bus. If its field current is now increased.sinp
A. both the active and reactive power output of the generator will remain unchanged
B. the active power supplied will remain unchanged but the machine will also supply lagging reactive power
C. the active power supplied will increase and the machine will draw leading reactive power
D. the active power supplied will decrease and the machine will supply leading reactive power
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168. In the phasor-diagram of a round-rotor synchronous generator, the voltage equations is Et. =V + la (ra + jXs), where Et- = excitation voltage, Vt = terminal voltage, Ta = armature current at lagging power factor, ra = armature resistance; Xs = synchronous reactance. While remaining synchronized to the bus-bars (infinite bus), if th power input from the prime mover is gradually decreased and finally stopped, it will result in
A. reversal of Ia, and Ef lagging Vt
B. reversal of la, but Ef leading Vt
C. the sign of Ia remaining unchanged, but Et. leading Vt
D. the sign of la remaining unchanged, but Er lagging Vt
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