100 MCQ OF Synchronous Generator or Alternator with Explanation

Answer.2. It will supply a greater portion of the load

Explanation

• If the steam supply of one of the alternators is increased then its power input to the prime mover will also increase.
• As we know that for parallel operation the speed must be the same for both the alternators, therefore the alternator with greater steam cannot overrun the alternator having the lesser steam supply.
• The alternator with more steam supply utilizes its increased power to carry more load by advancing its angular position.

Answer.1. Voltage Regulation

Explanation

• Zero power factor means that the load connected is either purely capacitive (zero leading) or purely reactive (zero lagging), therefore, there’s no KWatt (real power) consumed by the load.
• As we know, the armature MMF depends on the armature current (Ia). when the load is purely resistive, the armature MMF is 90° Electrical behind the field MMF.
• When the load is purely capacitive (zero leading pf), (Ia) advances by 90°, thus armature MMF is in phase with the field MMF and in this case, it helps in increasing the resultant MMF.
• When the load is purely reactive ( zero lagging pf), Ia lags behind by 90° thus the voltage leads the current by 90°.

NOTE: The power factor being zero means the angle between the voltage and current phasors in 90 degrees. The term lagging implies that voltage leads to current. Cosine 90 is Zero and hence called Zero Power Factor load.

Answer.3. Steam supply to the prime mover

Explanation

• Zero power factor means that the load connected is either purely capacitive (zero leading) or purely reactive (zero lagging), therefore, there’s no KWatt (real power) consumed by the load.
• As we know, the armature MMF depends on the armature current (Ia). when the load is purely resistive, the armature MMF is 90° Electrical behind the field MMF.
• When the load is purely capacitive (zero leading pf), (Ia) advances by 90°, thus armature MMF is in phase with the field MMF and in this case, it helps in increasing the resultant MMF.
• When the load is purely reactive ( zero lagging pf), Ia lags behind by 90° thus the voltage leads the current by 90°.

NOTE: The power factor being zero means the angle between the voltage and current phasors in 90 degrees. The term lagging implies that voltage leads to current. Cosine 90 is Zero and hence called Zero Power Factor load.

Answer.2. Changing prime mover speed

Explanation

• The alternator rotor has to maintain a constant speed as it is designed to produce a certain voltage at the desired frequency.
• However, the real power demand is variable and at any instant, it is the duty of the generator to match (supply) this real power demand.
• When an alternator is connected to the grid if the electrical load increases that mean real power increases. This increase in real power takes place by rotor deceleration for a short duration of time.
• The loading on the generator should not exceed the generator rating as it may lead to the heating of the stator.

Answer.3. Leading power factor load

Explanation

The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, to the rated terminal voltage.

E_ph =No load-induced e.m.f

• The value of regulation depends upon the load current and power factor of the load.
• For lagging and unity power factors there is always a drop in the terminal voltage hence regulation values are always positive.
• For leading or capacitive load the terminal voltage increase as the load current increase because armature flux is added up with the main field flux hence regulation is negative.

Answer.4. The exciting field winding of the main exciter

Explanation

An exciter is a device or combination of devices that supply the magnetizing current to generate the working flux.

• In industrial applications basically, 3 types of exciters are used for an alternator.
  1. Static exciter
  2. Permanent magnet/Brushless Exciter
  3. DC/Pilot exciter
• To maintain the stability of the overall system the synchronous generator should respond quickly for sudden changes in the load and in order to attend these two dc generators are used, one as the main exciter and the other as a pilot exciter.
• A pilot exciter is used to make the excitation of the generator independent of an external power source.
• A pilot exciter is mounted on the rotor shaft and it excites the field coil of a separately excited dc generator called “ the main exciter.

Answer.4. All option are correct

Explanation

The advantages of the parallel operation of an alternator are

• Proper load sharing
• Increase in reliability
• Increase in efficiency
• Reduce losses
• Easy to operate
• Cheaper (reduce capital cost)
• Continuity of supply

Continuity of the supply

• If the single large unit is disabled for maintenance purposes then the station will be no longer functional; Whereas if one of the several smaller units is in need of repair, the other smaller units are still available to maintain continuity of the supply.

Efficiency

• During light load, one or more alternators may be shut off while a few operate at full load. This provides more efficiency.

Future Expansion & reduction in cost

• An additional set of an alternator can be connected in parallel to meet the increasing demand thereby reducing the initial cost of setting up another unit.

Answer.1. Field current on X-axis and armature current on Y-axis

Explanation

• V curve is the graph showing the relation of armature current as a function of field current in synchronous machines.
• In the case of a synchronous generator, the V curve is drawn between field current on the X-axis and armature current on the Y-axis.
• The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied.

Answer.4. Field current on X-axis and Power factor on Y-axis

Explanation

• The Inverted V Curve is a graph showing the relation of power factor as a function of field current.
• In the case of a synchronous generator, the inverted V curve is drawn between field current on X-axis and Power factor on Y-axis.
• Similar to V Curve, its purpose is to show variation in power factor as the field current of the machine is varied.

Answer.1. 24 poles

Explanation

Number of poles of alternator Pa = 10

F = 25 Hz (alternator)
F = 60 Hz (motor)

Then the number of poles of motor Pm =?

Since the synchronous motor is directly coupled hence
The synchronous speed of an alternator = Synchronous speed of the motor

(120 × 25)/10 = (120 × 60)/ Pm

P_m = 24 poles