Synchronous Generator Objectives Part 07

121 . The windings for an alternator are I. 36 slots, four poles, span 1 to 8 II. 72 slots, six poles, span 1 to 10 III. 96 slots, six poles, span 1 to 12. The windings having pitch factors of more than 0.9 are (A) I and II only

Questions 122 to 124 refer to data given below:

A 500 kVA ,2300-volt three phase star connected alternator has a full load armature-resistance drop per phase of 50 volts and a combined armature reactance plus armature-reaction drop of 500 volts per phase

122 . The percent regulation of the alternator at unity power factor is) 10.5

123 . The percent regulation of the alternator at 0.866 power factor lagging is) 26.3

124 . The percent regulation of the alternator at 0.8 power factor leading is – 13.2.

125 . The imaginary or fictitious part of synchronous reactance takes care of armature reaction

126 . In an alternator, the use of short pitch coils of 160° will indicate the absence of) ninth harmonic

127 . When a generator designed for operation at 60 Hz is operated at 50 Hz operating voltage must be derated to (50/60) of its original value

128 . Overheating of generator’s winding reduces life of the machine

129 . Rotor shaft of 500 MW alternator is supported in journal bearings.

130 . The voltage of field system for an alternator is usually less than 200 V

131 . Maximum electric power output of a synchronous generator is VtEf / Xs

132 . The electrical angle between the field axis and axis of armature reaction of a loaded synchronous generator with armature current lagging behind the excitation emf by ψ is ψ + 90

133 . Two synchronous generators G1and G2 are equally sharing the KVAR of the load while operating in parallel. Keeping the terminal voltage fixed in order to shift part of the KVAR load from G2 to G1 The field current of G1 is raised and of G2 lowered

134 . A synchronous generator is operating with excitation adjusted for unity power factor current at constant load. When on increasing the excitation the power factor will lag

135 . On changing the speed of an alternator from 4000 rpm to 2000 rpm, the generated emf phase will become ½

136 . Zero power factor method of an alternator is used to find its voltage regulation.

137 . The power factor of an alternator is obtained from its load

138 . For parallel operation, alternators must have same voltage rating

139 . For alternation having fractional pitch of 5/6 the coil span is 150°

140 . Fractional pitch to eliminate 7th harmonic from alternator emf is 6/7