[Illustration: FIG. 18. Simple Dynamo showing Commutator.]
The modern armature is very complex. A simple coil such as those shown in Figs. 17 and 18 will not yield a steady current for twice in each revolution the electromotive force dies away to zero. The coils of large dynamos are grouped so that some of them are always active.
There are three general methods of supplying current to the held magnets of a dynamo, known as the _series_, _shunt_ and _compound_ windings.
The series dynamo is arranged so that the coils of the held magnets are in series with those of the armature.
In the shunt dynamo, the coils of the held magnet form a shunt to the main circuit and being made of many turns of thin wire, draw off only a small fraction of the whole current.
[Illustration: FIG. 19. Diagram of Dynamo Field Windings.]
The compound dynamo is partly excited by shunt coils and partly by series coils.
[Illustration: FIG. 20. Motor Generator.]
Each variety of dynamo winding has a certain advantage depending upon the condition of use.
In the case of alternating current dynamos, the field magnets are sometimes supplied from a separate dynamo called an "exciter." In other cases the dynamo is provided with two sets of windings, one connected to a commutator producing a direct current which excites the field coils and the other connected to a set of rings and supplying the alternating current.
In case a supply of either direct or alternating current is available and it is desirable to change the supply from direct to alternating or vice versa, it may be accomplished by employing a Motor-Generator. A motor-generator consists of an electric motor operating from the source of current supply on hand and driving a dynamo which supplies current of the kind desired.
A motor is exactly the reverse of a dynamo. If a current of electricity is passed into a dynamo, the armature will be dragged around by the mutual action of the currents flowing in the copper conductors and the magnetic field in which they lie. Such a device constitutes a motor and may be employed to do useful work.
Motors are classified as alternating and direct current machines accordingly as they are built to operate on either kind of current.
