91. If the excitation of a 3-phase alternator operating on infinite bus bars is changed, which one of the following shall alter?
- active power of the machine
- reactive power of the machine
- terminal voltage of the machine
- frequency of machine
92. A 3-phase shnchronous genetaor with constant steam input supplies power to an infinite bus at a lagging power factor. If the excitation is increased
- both power angle and power factor decrease
- both power angle and power factor increase
- the power angle decreases while the power factor increases
- the power angle increases while the power factor decreases
93. A turbo alternator set feeds power to a 3-phase constant voltage, constant frequency bus. If the steam supply to the set is cut off, then the set will
- continue to run at rated speed in the same direction
- continue to run at reduced speed in the same direction
- run at rated speed in the reverse direction
- come to stop
94. The mmf produced by single phase winding is
- pulsating and rotating with constant speed
- pulsating and stationary
- constant in amplitude and stationary
- constant in amplitude and rotating
95. A polyphase field is
- pulsating and stationary
- pulsating and rotating
- constant in amplitude and rotating at synchronous speed
- constant in amplitude and stationary in space
96. 4 pole star-connected alternator running at 1500 rpm have 72 slots with 10 conductors per slot. The number of turns per phase and slot angle is
- 120 turns and 20 degrees
- 100 turns and 10 degrees
- 120 turns and 10 degrees
- 100 turns and 20 degrees
97. In a synchronous machine, the stator frame serves the purpose of
- holding the armature stampings in position and also in some cases for circulating cold water for cooling.
- insulating the armature windings
- providing path to the magnetic flux
- protecting the whole machine
98. The angle between induced emf and terminal voltage on no-load for a single phase alternator is
- 180 degree
- 90 degree
- 0 degree
- none of these
99. In a synchronous generator
- the armature mmf leads the air-gap flux and air-gap flux leads the field mmf
- the armature mmf leads the air-gap flux and air-gap flux lags behind the field mmf
- the armature mmf lags behind the air-gap flux and air-gap flux lags behind the field mmf
- the armature mmf lags behind the air-gap flux and air-gap flux leads the field mmf
100. Which one of the following statements is not correct in respect of synchronous machines?
- In salient pole machines, the direct-axis synchronous reactance is greater than the quadrature axis synchronous reactance.
- The damper bars help the motor to self start.
- Short-circuit ratio is the ratio of field current required to produce the rated voltage on open circuit to the rated armature current.
- The V – curve of a synchronous motor represents the variation in armature current with field excitation at a given output power.