100 MCQ OF Synchronous Generator or Alternator with Explanation

Answer.4. 100 Hz

Explanation

Ns = 120 × f/P

1200= 120 × f/10

f = 100 Hz

Answer.2. Number of poles and Rotational speed

Explanation

The relation between speed and frequency is

N = 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 output frequency of the alternator depends on the rotational speed and the number of Magnetic poles of the rotor.

Answer.3. 1800

Explanation

Electrical degree = p/2 mechanical degree

Electrical deg. = 6/2 × 360 =3 × 360

= 1800°

Answer.3. Reactive components of the output are changed

Explanation

• 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, the real power generated remains unchanged (as per the load demand).
• When the excitation current is changed, the 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).

Answer.2. Meet no-load losses

Explanation

• 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.

Conclusion: Under no-load condition synchronous machine will only draw a small current (active power) to mainly compensate friction and windage losses and it can be used to supply reactive power and control the power factor of an external system, by controlling the field current.

Answer.4. Type of load

Explanation

The power factor depends on the types of load, according to load the alternator provides both reactive (K VAR) and real power(KW). Consider three types of loads

1. 1. Pure Resistive Load
   2. Resistive Inductive Load (RL load)
   3. 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 the alternator provides lagging VAR it produces a 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 a magnetization effect
  • So excitation can be decreased to main the flux.

Answer.1. Reverse Power Relay

Explanation

• 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 the unbalancing of load one generator shifts his total load on the second generator and runs as a motor instead of a generator.
• This situation can cause damage to the winding of the generator.
• In such a condition reverse current relay is usually employed to trip the generator on reverse power flow.

Answer.3. Short pitch coil to the full pitched coil

Explanation

Pitch factor K_p is given as

Answer.3. Changing the excitation current

Explanation

• 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 the 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.

Answer.4. Amount demanded by the load

Explanation

When a generator is operating by itself supplying the system load than

1. The real and reactive power supplied by the generator will be the amount demanded by the attachment of the load.
2. The governor point on the generator will control the operating frequency of the power system.
3. The field current ( field regulator set point) controls the terminal voltage of the power system.

These 3 situations found in an isolated generator in the remote field environment.

Note: When there is an inductive load on the grid then the synchronous generator will supply reactive power as it will act as capacitance which absorbs leading power and gives lagging power which is required by the inductive load, 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.

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.