100 MCQ OF Synchronous Generator or Alternator with Explanation
11.A 10 pole AC generator rotates at 1200 rpm. The frequency of AC voltage in cycles per second will be
Ns = 120f/P 1200= 120 × f/10 f = 100 Hz
12.The output frequency of an alternator depends on
The relation between speed and frequency is Ns = 120f/P
- From the above equation, it is clear that the output frequency of an alternator depends on the number of poles and the rotational speed.
- The speed corresponding to a particular frequency is called the synchronous speed for that frequency.
13.The number of electrical degrees passed through in one revolution of a six pole synchronous alternator is
Electrical degree = p/2 mechanical degree Eectrical deg. = 6/2 × 360 =3 × 360 = 1800
- The prime-mover of an alternator is rotating at a constant power basis, i.e. its power and speed remain constant, now if you make variations in the exciting coils, then the output power from the alternator will vary in accordance.
- The output power of an alternator is proportional to the speed, and excitation voltage.
- Now if the excitation varies then the output currents vary along with voltage, but the speed is constant and power demand or load on the alternator is also constant.
- Since Input to prime mover is kept constant, real power generated remains unchanged (as per the load demand).
- When the excitation current is changed, magnetizing current in armature changes, in order to maintain the air gap flux constant.
- Thus the reactive power consumption/ generation changes for a machine(ie KVAr component).
15.When an alternator is running on no-load the power supplied by the prime mover of an alternator goes to
- If the phase angle θ is exactly 90° the synchronous machine must be driven by the prime mover whose function is to supply power losses.
- In this case, a synchronous machine should be considered as a generator with zero power output.
- Since its power losses are covered by the electric power system its power factor can only be close to zero.
- That is why the synchronous machine at this mode is usually referred to as a synchronous reactor.
16.The power factor of an alternator depends on
- The power factor depends on types of load, according to load the alternator provides both reactive(K VAR) and real power(KW).
- Consider three types of loads
- Pure Resistive Load
- Resistive Inductive Load (RL load)
- Resistive Capacitive Load (RC load)
- Resistive Inductive load:
- When the load is RL type it requires lagging VAR now that power has to be provided by the alternator
- Whenever alternator provides lagging VAR it produces demagnetization armature reaction hence excitation has to be increased to maintain flux inside the machine(constant flux machine).
- At this condition, the alternator power factor is lagging. The more VAR alternator produces, the power factor becomes more lagging.
- Resistive Capacitive Load:
- When the load is of RC type, it requires leading VAR.
- Whenever Alternator provides leading VAR it's power factor is leading. At this condition, the leading VAR current causes magnetization effect
- So excitation can be decreased to main the flux.
17.During the failure of prime mover in an alternator which relay is used to restore the working condition
- If two synchronous generator is running parallel and the total load is divided among two then one generator takes half of the total load of it.
- Due to unbalancing of load one generator shifts his total load on the second generator and run as a motor instead of a generator.
- This situation can cause damage in the winding of the generator.
- In such condition reverse current relay is usually employed to trip the generator on reverse power flow.
18.Pitch factor is the ratio of emf of
Pitch factor Kp is given as
19.Synchronizing torque comes into operation under all of the following cases except
- By changing excitation current power factor changes i.e either leading or lagging.
- Suppose the excitation of the alternator is decreased below normal excitation then reactive power will change and active power output (W or KW) of the alternator will remain unchanged.
- The under-excited alternator delivers leading current to the infinite bus bar.
- It is because the leading current produces an adding m.m.f to increase the under excitation.
- Similarly, an overexcited alternator operates at lagging power factor and supplies lagging reactive power to an infinite bus bar.
20.When a generator is operating by itself supplying the system load then real and reactive power supplied by the generator depends upon
When a generator is operating by itself supplying the system load than
- The real and reactive power supplied by the generator will be the amount demanded by the attachment of the load.
- The governor point on the generator will control the operating frequency of the power system.
- The field current ( field regulator set point) controls the terminal voltage of the power system.
- Note: When there is inductive load on the grid then synchronous generator will supply reactive power as it will act as capacitance which absorbs leading power and gives lagging power which is required by inductive load.but when there is capacitive load then it will act as reactor and it will absorb reactive power from the grid and it will give leading power and absorb lagging power from the grid.
- But when there is a capacitive load then it will act as a reactor and it will absorb reactive power from the grid and it will give leading power and absorb lagging power from the grid.
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