The field circuit and the armature circuit are inter-locked in numerous ways in which to produce a good sort of performance characteristics an excellent benefit of DC machines. Also, the field poles will be excited by 2 field windings, a shunt field coil and a series field coil as shown. The shunt winding contains an ample amount of turns and takes solely a tiny low current (less than 5% of the rated coil current). This winding is connected across the coil (i.e., parallel with it), thus the name shunt winding. The series winding has less turns however carries large current. It’s connected nonparallel with the coil, thus the name series winding. If each shunt and series winding's are present, the series winding is wound on topmost of the shunt winding.
The different connections of the field circuit and armature circuit are shown above. Within the severally excited DC machine, the field winding is worked up from a separate supply.
In the self-excited DC machine, the field winding is connected in 3 alternative ways. The field winding is also connected nonparallel with the coil, leading to a series DC machine; it should be connected across the coil, leading to a shunt machine; or each shunt and series winding's is also used, leading to a compound machine. If the shunt winding is connected across the coil, it's referred to as short-shunt machine. In an armature connection, the shunt winding is linked across the series connection of armature and series winding, and therefore the machine is thought as long-shunt machine. There’s no important distinction between these 2 connections. In the interior the compound machine, the series winding mmf could aid or oppose the shunt winding mmf, leading to completely different performance characteristics.
A resistance is generally enclosed within the circuit of the shunt winding to regulate the field current and thereby to vary the field mmf.
Field excitation might also be provided by permanent magnets. This could be thought-about as a sort of severally excited machine, the magnet providing the separate however constant excitation.
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