Multi-Polar Field Magnets.—In the multi-polar machine, the subdivision of the magnetic flux reduces the amount of material of both magnet and armature. Moreover, there is less heating on account of the greater capability of dissipating the heat, offered by the increased area of surface per unit of volume in each magnet pole and winding.
There may be four, six, eight, or more poles, arranged in alternate order around the armature. Fig. 204 shows a four pole field magnet having a common yoke or iron ring, with four pole pieces projecting inwardly, and over which the exciting coils are slipped.
In the larger machines the yoke is made in two parts bolted together as shown in fig. 206, so that the upper portion may be lifted off for examination of the armature.
Ques. Can the number of poles in a multi-polar machine be advantageously increased to 16, 32, or more?
Ans. A large number of poles is not advisable except in very large machines, since it involves an increase in the expense of machine work, fittings, etc., somewhat out of proportion to the reduction in cost of material and increase in efficiency.
Ques. What materials are generally used for field magnets?
Ans. Wrought iron, steel and copper.