Question

Mention and explain the various causes for the failure of the generator to build up.

Answer.

Principal causes due to which a generator may fail to excite are:

(1) Brushes not properly adjusted—if brushes are not in their proper positions, then whole of the armature voltage will not be utilized and so would be insufficient to excite the machine.

(2) Defective contacts—unclean contacts may interpose large resistance in the path of the exciting current and reduce it to such a small value that it fails to excite the machine.

(3) Incorrect adjustment of regulators—in the case of shunt and compound generators, it is possible that the resistance of field regulator may be too great to permit the passage of sufficient current through the field windings.

(4) Speed too low—in the case of shunt-and compound-wound generators, there is certain critical armature speed below which they will not excite,

(5) open-circuit—in the case of series machines.

(6) Short circuit—in the generator or external circuit.

7) Reversed field polarity—usually caused by the reversed connections of the field coils.

(8) Insufficient residual magnetism—this trouble normally occurs when' the generator is new.

It can be remedied by passing a strong direct current through the field coils.