Each end of the armature winding is connected to one half of a brass ring called the commutator and marked "C, C," in the illustration. The two halves of the commutator are insulated from each other and are mounted on the armature shaft so that they revolve together with the armature.
The armature revolves between the ends of a horseshoe shaped piece of iron called the field. The field is also wound with a coil of wire called the field winding or sometimes the field coil.
The armature and the field are both electromagnets.
Two strips of copper, "B, B," bear against the commutator. These are the brushes, and their purpose is to lead the current to the armature coil.
One brush is connected to one end of the field coil. The other end of the field coil and the other brush are connected to a source of electric current.
As soon as the current is turned on, the armature and the field both become magnets. The North pole of the field attracts the South pole of the armature and vice-versa. The armature starts to move so that the poles will come opposite but as the commutator moves around and is turned over, the current flows through the armature coil in the opposite direction. This reverses the magnetism of the armature and that which was the South pole become the North pole and vice-versa.
FIG. 7.—Diagrams showing the difference between a Shunt and a Series Motor.
The armature poles will therefore have to move 180 degrees in order that the South pole may come opposite the North pole of the field. Before it gets there, however, the commutator will have turned over again, reversing the current in the armature and making it necessary to continue its journey again. This process keeps up and so the armature revolves always trying to seek a new position which it is prevented from remaining at by the action of the commutator.
Motors are said to be series or shunt wound depending on whether all the current flowing through the armature also passes through the field or whether it divides between the two as shown in Figure 7.