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Monday, 16 December 2019

Voltage build up in a Self Excited Generator


The following are the essential conditions for building up of voltage in self excited generator.

a. There must be residual magnetism in the poles.

b. The field must be connected in such a way as to strength the residual magnetism, when the current due to induced e.m.f. flows through them.

c. If excited on open circuit its shunt field resistance should be less than the critical resistance value.

d. If excited on load, then the shunt field resistance should be more than a certain minimum value of resistance as given by internal characteristics.

e. In the case of a series generator there must be some load on the generator for building up of e.m.f.

Assuming that some residual magnetism is present in the poles, the shaft is rotated to rated speed. Due to this residual magnetism initially some amount of e.m.f. is produced which circulates small current in the field coil. It results in increase in flux which in turn increases the induced e.m.f. and this process continues till full rated e.m.f. is generated.
The generated e.m.f. has to overcome ohmic drop in the field winding (If Rsh). So long as the generated e.m.f. is in excess, this energy would continue to be stored in the pole fields. In Figure, the emf generated corresponding to field current OA is OB out of which AC corresponds to ohmic drop in the field. While whole of the e.m.f. generated to corresponding field current OF is utilised towards ohmic drop leaving nothing for L.di/dt. Hence no energy is stored in the poles. Consequently no further increase in the flux and generated e.m.f. is there. If OP line represents the shunt field resistance, the maximum e.m.f. generated is OE. If this resistance is decreased building up of voltage will be higher. If a tangential line is drawn to the linear portion of the OCC curve, it gives the value of resistance of the field winding for generation of e.m.f. This resistance is called critical resistance. The generator would fail to generate e.m.f. if the shunt field resistance is greater than critical value.
In case the generator fails to build up e.m.f. the following may be the causes

a. Improper direction of rotation
b. Shaft speed is too low
c. Improper brush contacts
d. Short circuit or open circuit armature or field
e. Improper connections of field
f. Lack of residual magnetism


Some of the causes and remedies for non building up of e.m.f. in the D.C. generator is given in Table.

Causes and Remedies for Failure of E.M.F. in Generator

The direction of rotation must have been reversed in which case residual magnetism will he opposed.
Change the direction of rotation

Speed too low
Generator should be rotated at the constant speed
Brushes not resting on the commutator or in the wrong position
Brushes to be set or shifted to the correction position of MNA
Residual magnetism is completely lost
A generator should give a reading upto 5% of its full voltage when it run without excitation. If it does not fulfil the above condition, then run the generator as d.c. motor for few seconds. If d.c. supply is not available, a battery of e.m.f. 10% of the generator voltage must be used for sending current to re-establish the residual magnetism. This is called flushing of the field
Short circuit in the armature
Remove the short circuit which may lie in the generator itself, the switch hoard or external circuit

lf the short circuit is in the external circuit, the generator will excite when it is disconnected from the load
Short circuit in the field circuit
Test and remove short circuit which may be on the terminals or within the coil. Faulty coil will show much less resistance than the perfect coil
Open circuit in the armature
Test and repair for open circuit if possible
Open circuit in the field winding
Test for open circuit. If the break is in the field coil, remove it or rewind it if required
High resistance in the field winding
High resistance in the field circuit will not allow the generator to build up its normal voltage, so cut it out from the field circuit
Wrong connection of the field winding
Reverse its connection
Load on the machine
In case of series generator there must be some load on it, while in other case no load or small load is required on the generator
Series field opposes shunt field flux in compound machine
Reverse the connection of the series field winding

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