Field Excitation

The electromagnets used to produce the magnetic field of a generator are called field coils. Both the field together are called winding. If the current required to excite the magnet is obtained from a separate source, the dynamo is said to be separately excited. If the current is taken from the dynamo itself, the dynamo is said to be self excited. In the case of self-excited dynamos, the field winding is connected directly to the generator output.

There are mainly three different types of field coil winding in a DC dynamo.

(i)    Series wound dynamo. In a series wound dynamo, the field coil consists of a few numbers of turns of thick copper wire connected in series with the armature coil (A). The load is connected across the output terminals.





In this type, the emf will be built up only when a load is connected at the output. Otherwise the circuit is not closed. As the load increases (resistance R1 decreases), the current through the field coil increases. This in turn increases the induced emf in the armature coil. Thus the generated voltage depends on the load resistance and no appreciable voltage is generated with no load. The characteristics curve of generated emf vs load current. The maximum value of the emf is limited by the electromagnetic, considerations of insulation, heat losses, etc. It is used for working arc lamps and search light, which do not involve variation of load. It is unsuitable for electrical installations requiring a constant supply at varying loads.

(ii)    Shunt wound dynamo. In a shunt wound dynamo, the field coil is connected parallel to the armature coil. The field coil is of large number of turns and a large resistance. With no load on the dynamo, the field coil acts as load. Hence without any load connected to the dynamo, there is current through the field coil and emf is generated. When the load current increases, the current in the armature coil also increases so that there is a greater potential drop (VI) across the coil which decreases the output voltage. As a result, the current through the field winding decreases which, in turn, produces a weak magnetic field and decreases the output voltage still further. Therefore the output voltage of the generator decreases with the increase of load currents.
Shunt wound dynamo is used for domestic lighting where there is not much variation in load. It is also used for charging batteries.

(iii)    Compound wound dynamo. As the emf rises with increasing current in series and the emf decreases in shunt wound dynamo, the combination of these two may give a constant emf for all external currents. In the compound wound dynamo, there are two sets of field coils one set being in series with the armature coil. When the load current increases, the current through the shunt winding decreases, thus decreasing the strength of the magnetic field whereas the current in the series winding increases, thus increasing the strength of the magnetic field. With proper number of turns in the series and the shunt winding, the two fields are made to compensate each other keeping the strength of the magnetic field almost stable.