Field Magnets.—The early forms of alternator were built with permanently magnetized steel magnets, but these were later discarded for electromagnets.
Alternators are built with three kinds of electromagnets, classed according to the manner in which they are excited, the machines being known as,
- 1. Self-excited;
- 2. Separately excited;
- 3. Compositely excited.
Figs. 1,448 and 1,449.—Westinghouse laminated hub and laminated pole piece for revolving field having squirrel cage winding. Thin steel is used for the laminations of both hub and pole piece; these are assembled and firmly riveted together under hydraulic pressure. The laminations are of the same thickness in both hub and spider.
Figs. 1,450 to 1,452.—Views of Triumph pole pieces. These consist of laminated punchings securely clamped between two cast steel end plates. The laminations are shaped with polar horns or shoes as shown, and which serve to keep the field coils securely wedged in position. In some designs the horns are separate. The two holes in each pole piece are for through bolts which secure the pole piece and coil to the spider run. Dovetail joints are sometimes used instead of through bolts, as in figs. 1,448 and 1,449.
Ques. What is a self-excited alternator?