Ques. How should the rotor be designed?
Ans. It should be of very substantial construction.
Ques. Why?
Ans. Because water wheel alternators are frequently required to operate safely at speeds considerably in excess of normal.
Fig. 1,441.—Allis-Chalmers revolving field for water wheel alternator. In this type of alternator it is essential that the rotating part be designed to have a liberal factor of safety not only at the ordinary operating speed, but also at speeds much in excess of normal. Frequently machines are required to operate safely at a speed 50 to 75 per cent. in excess of normal, so that there may be no danger in case the water wheel races. In most machines the field spider is of steel cast in a single piece for the smaller alternators and in two or more parts for the larger sizes. For alternators running at high peripheral speed, the rim is built up of steel laminations supported by a cast steel spider; the latter serves merely to rotate the rim, which is in itself able to withstand all stresses due to the high speed. The field poles are laminated, being built up of steel punchings held between malleable iron or bronze end plates, the latter being used on high speed machines. With but very few exceptions the poles are attached by dovetail projections that fit into corresponding slots. Steel tapered keys are driven in alongside the dovetails, and the pole pieces cannot become loose. All field coils, except on a few of the smallest machines, are of edgewise wound copper strip. This style of coil is essential for revolving field alternators where the pressure on the insulation, due to centrifugal force, is so great that cotton insulation on round wire will not stand. Current is led into the rings by means of carbon brushes, the number of brushes being such that the current density at the rubbing contact is kept within conservative limits. At least two brushes per ring are always provided so that one can be removed for inspection without interrupting the exciting current. In large machines the brush holder studs are mounted on a stand supported from the base; in small alternators they are usually fastened to the cap of one of the bearing pedestals.
Figs. 1,442 to 1,444.—Diagram of turbine alternator windings for revolving armature. Fig. 1,442 illustrates a two pole design in which all overlapping is avoided. It has 72 slots of which only 48 are filled, giving 8 slots per phase. The projecting claws from the brass end shield which hold the coils in position are shown in section. Fig. 1,443 shows a four pole design having 48 slots or 4 slots per phase per pole, the coils being made up of 8 inductors per slot taped together, the end bends forming two ranges. Fig. 1,444 shows a two pole design for a two phase armature with 18 slots per pole per phase. The core discs are spaced out as for 108 slots, but of these, 4 lots of 7 each are not stamped out, and 8 of those stamped are left empty, so that there are 72 slots filled.