100 MCQ OF Synchronous Generator or Alternator with Explanation

Answer.2. Its speed decreases

Explanation

Synchronous speed is given as

N_s =120f/p

So the speed of an alternator is directly proportional to its frequency therefore by increasing the frequency of an alternator its speed will also increase.

Answer.3. Behave as a synchronous motor with the same rotation direction

Explanation

Here are the various possibilities for a synchronous generator connected to the grid when the Prime mover is accidentally decoupled. The mode depends on whether the field winding is excited.

If Field Excitation is failed ( Field OFF)

• Cylindrical rotor machine: It will work as Induction Motor running in the same direction with speed less than synchronous speed.
• Salient pole machine: It will work as Reluctance Motor running in the same direction at synchronous speed.

If field Excitation is present

• When the alternator is connected to the bus bar and that point of time when the prime mover fails then instead of supplying power to the bus bar it will receive power from the grid. Therefore synchronous generator will behave as the synchronous motor.
• Loss of driving force in prime mover will result in an increase in system voltage far above the safe limits.

Answer.1. Alternator A will deliver more current

Explanation

• Suppose two alternators are operating in parallel and they are supplying half of the active and reactive load.
• If the excitation of Alternator 1 is increased such that E_f1 > E_f2.
• It will cause a circulating current “Ic” which will flow through the armature and around the bus bar.
• Therefore alternator 1 will deliver load current at power factor cosΦ1 and alternator 2 will deliver load current at power factor cosΦ2 where cosΦ1 > cosΦ2 since circulating current “Ic” is added to the load current of alternator 1 and subtracted from the load current of alternator 2.
• It will result in an increase in the KVAR supply of alternator 1 whereas the KVAR supply of alternator 2 will decrease.

Note:

• If the excitation of one of the alternators operating in parallel is increased above its normal value of excitation i.e. overexcited its power factor will change in the lagging direction and its current output will increase without a change in its kW load.
• When the alternator is under excited its power factor will become more leading and its current o/p will increase without changing in kW output.

Thus whether the alternator is overexcited or under excited its circulating current tends to increase thereby increasing the losses and reducing their useful capacity. Therefore it is desirable to keep the circulating current to the minimum value.

Answer.4. 400 Hz

Explanation

• 400 Hz is used in aircraft because this is the optimum frequency.
• Since speed is directly proportional to the frequency and inversely proportional to the flux, therefore, flux will be inversely proportional to the frequency.
• Higher the frequency lesser the flux which means the lesser the core size and weight of the machine
• If a frequency of more than 400 Hz is used it will increase the losses especially skin effect will increase and this will affect the radio system performance of the aircraft.
• Therefore 400 Hz is the optimum frequency which provides the balance i.e. for the given size of component or equipment losses are not that high.

Answer.3. A higher terminal voltage is obtained

Explanation

For the development of a rotating magnetic field in a three-phase machine, it is essential that the current in these three-phase windings must be 120 degrees placed wrt each other both in space and time.

• In order to achieve it, both star or delta configurations can be used, but the star connection has certain advantages over the delta connection which are-
• The phase voltages in star connection are 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn reduces the cost of insulation and conductor material.
• In a star connected system, V phase = √3 V line. and  Emf induced ∝ Turns,
• Therefore a star-connected alternator will require a lesser number of turns than a delta-connected alternator for the same line voltage.
• In star connection, if the neutral is grounded then it also provides a path for the Zero-Sequence currents during faults,
• But in the delta connection, the zero sequence currents flow within the delta circuit and hence increasing the load on the winding.
• Star connection offers high voltage and low current and hence it is more suitable for transmitting high voltages over the transmission lines than that high current.
• Star Connected system is used where the low starting current is required.
• A Delta connection is used where high starting torque is required.

Answer.3. 7α

Explanation

• The chording factor or pitch factor is defined as the ratio of generated emf in the short-pitched coil to the emf that would be generated in the full pitched coil.
  K_p = cosα/2
• The chording angle for rth harmonic is always r times the chording angle.
• Therefore the value of the 7th harmonic will be 7α.

Answer.4. 6/7

Explanation

• One harmonic can be completely eliminated by selecting a coil span (fractional pitch) that results in the pitch factor of zero for that particular harmonic.
• For example, 5th harmonic is completely eliminated by selecting the coil span of 4/5.
• Similarly 3rd harmonic can be eliminated by selecting a coil span of 2/3.
• Therefore eliminating the 7th harmonic coil span of 6/7 is to be selected.

Answer.1. One-half

Explanation

The average E.m.f generated per phase is given as

E = 4 f Φ T volts
Where f = PN/120 Hz
Φ = flux per pole
T = Total number of turns per phase

Since the speed of the alternator is directly proportional to the emf of an alternator. So if the speed is reduced to half then the emf will also become one-half.

Answer.4. Same rating

Explanation

There are five conditions that must be met before when two alternators running in parallel.

1. Equal line voltage
2. Same frequency
3. Same phase sequence
4. Same phase angle
5. Same waveform

We can use 2 alternators of 6 MVA and 4 MVA each instead of using a single 10 MVA alternator because it is economical than using a single alternator of the same rating.

Answer.3. Will Get burn

Explanation

• In a Synchronous machine, the stator carries the armature winding which is having a small resistance.
• Under stationary conditions, the emf induced in stator winding is zero.
• So if such an alternator is connected to a live bus bar, there is always a danger of short circuit (Ra is negligible and frequency is zero so there is no reactance).