Generator Interview Questions & Answers

Question 1. What Is A Generator?

Answer :

A Generator is a machine which converts Mechanical Energy into Electrical Energy.

Question 2. On Which Principle A Generator Works?

Answer :

Generator works on Faradays laws of “Electro Magnetic Induction ”

Question 3. What Are Faraday's Laws Of Electro Magnetic Induction?

Answer :

First Law: Whenever a conductor cuts magnetic flux and E.M.F is induced in that conductor.

Second Law: The magnitude of the induced e.m.f. is equal to the rate of change of flux linkage.

Question 4. What Are The Two Main Parts Of An Ac Generator?

Answer :

1. Stator
2. Rotor

Question 5. What Is The Main Difference Between An Ac Generator And Dc Generator?

Answer :

In an AC Generator the field is rotating and the armature is stationary whereas in DC Generator the field is stationary and the armature is rotating.

Question 6. What Are The Advantages Of Stationary Armature And Rotating Field In An Ac Generator?

Answer :

1. It is easy to take output from the stationary armature.
2. It is easier to insulate stationary armature for higher voltage.
3. Low voltage excitation supply can be easily supplied to the rotor through slip rings.

Question 7. What Is The Relation Between The Frequency, Pole And Speed Of A Generator?

Answer :

F = PN/120 Hz

   Where P is the No.of Poles

              N is the Speed in RPM.

Question 8. In A Generator With 2 Pole, And Speed 3000 Rpm, What Is The Frequency Of The Induced Emf ?

Answer :

F = PN/120

= 2 x 3000/120

  = 50 Hz.

Question 9. What Is The Connection Of Winding In A Stator?

Answer :

Double star connection.

Question 10. What Is The Efficiency Of A Generator?

Answer :

98.55%

Question 11. What Is The Coolant Used To Cool Stator Winding?

Answer :

Demineralised water

Question 12. What Is The Coolant Used To Cool Rotor Winding?

Answer :

Hydrogen.

Question 13. What Are The Advantages Of Hydrogen Gas As Coolant Over Others?

Answer :

1. Density is 1/14 of air.
2. Windage loss is low.
3. Low noise.
4. Heat transfer co efficient is 1.5 times higher than air.
5. Thermal conductivity is 7 times higher than air.

Question 14. What Are The Losses In A Generator?

Answer :

1. Stator copper
2. Stator iron
3. Rotor copper
4. Windage loss 
5. Stray loss
6. Friction loss

Question 15. What Is Short Circuit Ratio (scr) Of A Generator?

Answer :

It is the ratio of the field current required to produce rated voltage on open circuit to the field current required to circulate rated current on short circuit.

Question 16. What Is The Value Of Scr In A 210 Mw Turbo Alternator?

Answer :

SCR = 0.49

Question 17. Why Over Current Protection Is Not Necessary For Modern Generators?

Answer :

Over Current protection is not considered necessary for modern alternators because these are capable of withstanding a complete short circuit at their terminals for sufficient time without much over heating and damage.

Question 18. What Type Of Protection Is Provided For The Generators Against Over Heating Of The Generator Stator?

Answer :

Resistance temperature detector.

Question 19. Which Type Of Relays Are Used For The Merz-price Protection System For Alternator?

Answer :

Merz-Price protection is differential protection provided for the alternator. The relays used in the Merz-Price protection system of alternator are instantaneous electro-magnetic type protection.

Question 20. Why Large Alternator Is Grounded With Large Resistance?

Answer :

Large capacity of alternators are typically provided with resistance grounding. High value of resistor is connected to the neural path to the ground. If the generator is delta connected, then it is grounded with the help of zig-zag transformer or (Delta-Star) transformer such that a high resistor is connected between the neutral point and the ground. Resistance grounding is provided so as to limit the short circuit current flowing during earth fault to stator winding in order to provide protection against mechanical stresses and melting of winding during Line to Ground short circuit. In large generators fault current is limited as low as 10 to 15 amperes during short circuit.

Question 21. Why It Is Not Necessary To Provide Protection For Turn To Turn Fault In Alternator?

Answer :

The coils of the modern alternators are single turn and therefore it is not necessary to provide protection for turn to turn faults.

Question 22. Why It Is Necessary To Suppress Field Immediately After Disconnection Of Faulty Alternator From The System?

Answer :

In the event of fault on the generator windings even though the generator circuit breaker is tripped, the fault is continuous to fed as long as the excitation will exist because the emf is induced in the generator. Hence it is necessary to suppress the field immediately after disconnecting the faulty generator from the system.

Question 23. Why It Not Necessary To Provide Over-voltage Protection In Turbo-alternator?

Answer :

The over voltage occurs when the prime mover speed increases due to sudden loss of the load on the generator and the speed control governors in case of turbo-generators are very sensitive to the speed variations and therefore generator over voltage of significant duration or magnitude does not generally occur. This is the reason why generators are not provided with over voltage protection.