When the field winding of a d.c machine is connected in parallel with the armature, as shown in Fig (a), the machine is said to be shunt wound. If the field winding is connected in series with the armature, as shown in Fig (b), then the machine is said to be series wound. A compound wound machine has a combination of series and shunt windings.
Depending on whether the electrical machine is series wound, shunt wound or compound wound, it behaves differently when a load is applied. The behaviour of a d.c machine under various conditions is shown by means of graphs, called characteristic curves or just characteristics. The characteristics shown in the following sections are theoretical, since they neglect the effects of armature reaction.
Armature reaction is the effect that the magnetic field produced by the armature current has on the magnetic field produced by the field system. In a generator, armature reaction results in a reduced output voltage, and in a motor, armature reaction results in increased speed.
A way of overcoming the effect of armature reaction is to fit compensating windings, located in slots in the pole face.
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