Causes of Failure to Build Up Voltage

Causes of Failure to Build Up Voltage Points : Causes of Failure to Build Up Voltage There may be one or more of the following reasons due which a self excited generator may fail to build up voltage. 1. No residual magnetism The start of the build up process needs some residual magnetism in the magnetic circuit of the generator. If there is little or no residual magnetism, because of inactivity or jarring in shipment, no voltage will be induced that can produce field current. 2. Reversal of Field Connections The voltage induced owing to residual magnetism acts across the field and results in flow or current in the field coils in such a direction as to produce magnetic flux in the same direction as the residual flux. Reversal of connections of the field winding destroys the residual magnetism which causes the generator failure to build up voltage. 3. In case of dc series wound generators The resistance in the load circuit may be more than its critical resistance, which may be due to
(i) open-circuit
(ii) high resistance of load circuit
(iii) faulty contact between brushes and commutator and
(iv) commutator surface dirty or greasy. 4. In case of shunt wound generator (a) the resistance of the shunt field circuit may be greater than the critical resistance;
(b) the resistance in the load circuit may be lower than the critical resistance;
(c) the speed of rotation may not be equal to rated one. Remedy In case the generator is started up for the first time, it may be that no voltage will be built up either because the poles have no residual magnetism or the poles have retained some residual magnetism but the field winding connections are reversed so that the magnetism developed by the field winding on start has destroyed the residual magnetism and the machine can not “build up”. In both the cases, the field coils must be connected to a dc source (a storage battery) for a short while to magnetise the poles. The application of external source of direct current to the field is called flashing of the field.