Fig. 1,467.—General Electric field coil, showing one method of winding. In the smaller machines the wire is wound on spools which are slipped over the pole pieces, which are built of sheet iron, spreading at the pole face so as to secure not only a wide polar arc for the proper distribution of the magnetic flux, but also to hold the field windings in place.
In construction provision is made for attaching the field winding leads. The rings are usually made of cast iron and are supported mechanically upon the shaft, but are insulated from it and from one another.
The current is introduced by means of brushes as with a commutator. Carbon brushes are generally used.
A good design of slip ring should provide for air circulation underneath and between the rings.
Fig. 1,468.—General Electric field coil showing another method of winding. The field coils on the larger machines consist of a single strip of flat copper, wound on edge as shown, so that the surface of every turn is exposed to the air for cooling. The flat sides of the copper strip rest against each other and the entire coil forms a structure of great solidity which can be easily removed for inspection and repair.
Ques. What form of spider is used on large alternators?
Ans. It is practically the same form as a fly wheel, consisting of hub, spokes, and rim to which the magnets are bolted.