DC Generators - Question and Answer

1. What is an electrical generator?

An electrical generator is a machine which converts mechanical energy to electrical energy.

2. What is the basic principle of a dc generator?

Basic principle of a dc generator is Faraday’s law of electromagnetic induction. whenever a conductor is moved in a magnetic field, dynamically induced emf is produced in that conductor.

3. What are the essential parts of a d.c. generator?

Magnetic frame or yoke
Poles
Armature
Commutator, pole shoes, armature windings, interpoles
Brushes, bearings and shaft.

4. What is the purpose of yoke in a d.c. machine?

It acts as a protecting cover for the whole machine and provides mechanical support for the poles.

It carries the magnetic flux produced by the poles. The flux per pole divides at the yoke so that, the yoke carries only half the flux produced by each pole.

5. What is the purpose of interpoles in modern d.c. machine?

In modern d.c machine commutating poles or inter poles are provided to improve commutation.

6. The core of the armature is laminated in DC machines. Justify.

The armature core is made of laminations, in order to reduce the eddy current losses.

7. How do you reduce the hysteresis loss in armature?

The hysteresis losses can be reduced by using low hysteresis steel containing a few percentage of silicon.

8. What is the purpose of commutator in a DC generator?

The brushes are mainly used to collect current from the commutator.

9. What are the different types of d.c. generator?

Separately excited d.c. generator.
Self excited d.c. generator
1. Series generator
2. Shunt generator.
3. Compound generator
  1. Long shunt compound
  2. Short shunt compound

10. What is a separately excited d.c. generator?

If the field winding of a generator is excited by a separate d.c. supply, then the generator is called separately excited d.c. generator.

11. What is a self excited d.c. machines?

If the field winding of a dc machine is supplied from the armature of the dc machine itself then it is called a self-excited d.c. machine.

12. What are the characteristics of d.c. generator?

No load magnetization curve or open circuit characteristic.
Load characteristics.

13. What is meant by open circuit characteristics?

When the speed of a d.c. generator is maintained constant and the field current varied, then the emf induced also varies. The variation of flux with induced emf is called the open circuit characteristics (O.C.C.) of the generator.

14. What are the conditions for build up of voltage in a shunt generator?

The residual magnetism should be present in the poles.
The field winding should be properly connected with armature.
Under no load condition, the shunt field resistance should be less than the critical resistance.
Under loaded condition, the shunt field resistance should be more than the critical resistance.

15. Why is the airgap made as small as possible between the armature and the poles in a de machine?

The airgap is made as small as possible between the armature and the poles in a dc machine in order to reduce the reluctance of the magnetic path.

16. Define critical resistance.

A tangent line is drawn to the linear portion of open circuit characteristics from origin. The slope of this tangent is called critical resistance.

17. What is the function of armature in a dc generator?

Its function is to rotate the conductors in a uniform magnetic field and provide a path of very low reluctance to the magnetic flux.

18. List out the main parts of stator of DC machine.

Yoke
Main Poles
Field winding
Interpoles

19. What are the causes of failure of dc shunt generator to excite?

The residual magnetism may not be present in the poles.
The field winding may not be properly connected with armature.
Under no load condition, the shunt field resistance should be greater than the critical field resistance.
Under loaded condition, the shunt field resistance should be less than the critical field resistance.