Fig. 1,464.—Wagner cast steel hub with dovetail grooves for attaching the revolving field magnets. Such construction is generally used on machines of small and medium size.
Fig. 1,465.—Wagner laminated pole piece with horns stamped in one piece. The laminations are held together between two end pieces by through rivets, as shown.
Ques. Describe the construction of a revolving field.
Ans. The entire structure or rotor consists of a shaft, hub or spider, field magnets and slip rings. The magnet poles consist of laminated iron stampings clamped in place by means of through bolts which, acting through the agency of steel end plates, force the laminated stampings into a uniform, rigid mass. This mass is magnetically subdivided into so many small parts that the heating effect of eddy currents is reduced to a minimum. The cores are mounted upon a hub or spider either by dovetail construction or by means of through bolts, according to the centrifugal force which they must withstand in operation, either method permitting the easy removal of any particular field pole if necessary. The field coils are secured upon the pole pieces either by horns in one piece with the laminations, or separate and bolted. All the coils are connected in series, cable leads connecting them to slip rings placed on the shaft.
Fig. 1,466.—Wagner revolving field of 300 kilowatt alternator during construction, illustrating the method of attaching the field magnets to the hub by dovetail joints. After the notched ends of the pole pieces are slid into the grooves in the hub, tapered keys, which are plainly seen, are driven in, thus making a tight joint which will not shake loose.
Ques. What are slip rings?
Ans. Insulated rings mounted upon the alternator shaft to receive direct current for the revolving field, as distinguished from collector rings which collect the alternating currents generated in an alternator of the revolving armature type.