Why a dc generator fails to build up voltage?

How can the problem be remedied :

No residual magnetism, reversed field connection and high resistance of field circuit are responsible for the failure of build up process of a dc generator.

The factors are explained below:

No residual magnetism:   For the starting build up process requires some residual magnetism in the magnetic circuit then no voltage will be generated that can produce current flow through field circuit.

Reversed field connection:   The voltage induced inside the coil of field circuit due to residual magnetism that can permit the flow of current. For the following direction of this flux will be created and the direction of this flux will be same direction as the residual flux. If the field connections are reversed the lines of flux produced by current flow will oppose the residual flux, as a result generate voltage will decrease rather than increase. So voltage build up process failed.

Field circuit resistance too high:  A field circuit resistance greater critical resistance value will permit an appreciable build up. At no load resistance greater than the critical may be caused by following:

Open field circuit connection: The effects of an open circuit are apparent. The field circuit resistance is much greater than critical value, hence the generator will not build up.

Dirty commutation: A dirty commutator doesn’t permit good contact between the brush and commutator. This poor contact shows up a high resistance to flow of current in the field circuit and produces the same effect as a higher field circuit resistance.

This problem can be solved by following ways:

·                     In case of little or no residual magnetism and external source of direct current is applied to the field for a short time and then removed as a result the magnetic field is able to allow voltage to build up. This process is known as flashing the field.

·                     For reversed field circuit connection it is necessary to preset or appropriate connection with respect to armature.

The resistance of field circuit may less than the critical resistance and the commutator will be neat and clean