In practice many coils of wire wound around an iron core called the armature, the purpose of which is to concentrate the magnetic lines of force, are made to revolve in a powerful field between the poles of adjacent electromagnets. Electromagnets are used because they are capable of producing a stronger magnetic field than magnetized bars of steel. The electromagnets used for this purpose are called field magnets. The central iron portion upon which the revolving coils are wound, called the armature, is usually built up of a number of thin sheets of soft steel called armature disks or laminations.
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.
FIG. 19. Diagram of Dynamo Field Windings.
The compound dynamo is partly excited by shunt coils and partly by series coils.