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Important Conceptual Question from DC Generators

Read Out “Electrical DC Generator” topic to better understand all those questions

1) Define the principle of operation of a DC generator ?

ANSWER
An electrical generator is a machine that converts mechanical energy into electrical energy. The principle operation of a generator is that whenever flux is cut by a conductor, an e.m.f will induced which in turn will cause current to flow in a conductor circuit if closed. The direction of induced e.m.f can be given by Fleming’s right hand rule.

2) What purpose is served by the pole shoe in a DC machine ?

ANSWER
The pole shoes serve the following purposes:

  1. They spread out the flux in the air gaps
  2. They reduced the magnetic path as they are of large cross sectional area
  3. They supports the field coils

3) What are the advantages and disadvantages of carbon brushes ?

ANSWER
ADVANTAGES:

  1. They lubricate and polish the commutator
  2. If sparking occurs, they damage the commutator less than with the copper brushes
  3. They provide good commutation

DISADVANTAGES:

  1. Due to contact with commutator, it causes a loss of about 2 V. Therefore, they are unsuitable for low voltage machines in which this forms a large percentage loss
  2. Due to high voltage loss, the commutator size should be made larger than for copper brushes
  3. Their low current density necessitates large brush holders

4) What is the role of commutator in an electrical DC generator ?

ANSWER
The function of commutator is to facilitate the collection of current from armature conductors. It converts the alternating current induce within the armature conductors into unidirectional current in the external load circuit.

5) How would you define separately excited DC generator ?

ANSWER
An electrical DC generator in which the magnetic field is created by supplying an energy from external DC source to field magnets is called separately excited DC generator.

6) How would you define self excited DC generator ?

ANSWER
An electrical DC generator in which the magnetic field is created by an output of the generator itself is called self excited DC generator. Depending upon the manner in which the field winding is connected to the armature can be further classified in to three types.

  1. Series Generator: The field winding is connected in series with armature winding so that the whole armature current flows through the field winding as well as the load.
  2. Shunt Generator: The field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it.
  3. Compound Generator: There are two sets of field winding on each pole, one is in series and the other in parallel with the armature. A compound wound generator may be (i) Short Shunt: In which only shunt field winding is in parallel with the armature winding (ii) Long Shunt: In which shunt field winding is in parallel with both series field and armature winding

7) What are the different types of losses in DC Machines ?

ANSWER
The losses in DC machines either (generator or motor) are of three types.

  • Copper losses ( (i) Armature Cu loss (ii) Shunt field Cu loss (iii) Series field Cu loss )
  • Iron losses ( (i) Hysteresis loss (ii) Eddy current loss )
  • Mechanical losses ( (i) Friction loss (ii) Windage loss )

8) Why is the armature of a DC machine made of silicon steel ?

ANSWER
The most important function of an armature is to provide low reluctance path for the flux through the armature from N pole shoe to the S pole shoe. It is, therefore made of a material having high permeability. An additional advantage of silicon steel is that it has low hysteresis loss.

9) Why do we use slotted armature in DC machine ?

ANSWER
The slotted armature has following three advantages

  1. The air gap can be reduced to a value required for mechanical clearance. This reduces the reluctance of the magnetic circuit.
  2. The conductors can be properly placed so that there is a little fear of them being displaced.
  3. The drag on the conductors is greatly reduced so that their insulation is in no danger of damage by pressure.

10) Why is armature placed on the rotor of a DC machine ?

ANSWER
The armature winding is placed on the rotor of a DC machine to facilitate to commutation i.e. to convert the produced alternating voltage in to direct voltage at the brushes

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