Fig. 1,745.—Mechanical Appliance Co. solid core discs as used on small and medium size induction motors.
Fig. 1,746.—Allis-Chalmers squirrel cage armature construction. The core laminæ are mounted on a cast iron spider having arms shaped to act as fan blades for forcing air through the motor. The spider is pressed on to the shaft. In the smallest sizes the punchings are mounted directly on the shaft, which is properly machined to hold them firmly. Copper bars are used as inductors in the larger sizes, and copper rods in the smaller sizes. The ends of the inductors are turned down somewhat smaller than the body and fit in holes in the end rings. The shoulder thus formed fits firmly against the end rings. Good electrical contact is obtained by expanding the inductors in the end ring holes. In large armatures both bars and end rings are of rectangular cross section, the bars and rings being fastened by machine steel cap screws.
Obviously, a better result is obtained if, in fig. 1,738, the downward returning currents of the eddies are led into some path where they will return across a field of opposite polarity from that across which they ascended, as in such case, the turning effect will be doubled. Accordingly the design of fig. 1,738 was modified by cutting a number of parallel slits which extended nearly to the ends, leaving at each end an uninterrupted "ring" of metal. This may be called the first squirrel cage armature, and in the later development Dobrowolsky was the first to employ a built-up construction, using a number of bars joined together by a ring at each end, as in fig. 1,740, and embedded in a solid mass of iron, as in fig. 1,741; he regarding the bars merely as veins of copper lying buried in the iron.
Fig. 1,747.—Triumph squirrel cage armature. In construction thin sheet steel laminations, japanned, are built up to form the core, and are rigidly clamped together by heavy malleable iron end plates. Semi-enclosed slots are punched in the outer periphery to receive the windings, so that none of the centrifugal force is carried by the inductors. These inductors are set edge on, and are riveted and soldered into resistance rings. These rings are punched to receive the inductors in such a manner that there is an unbroken strip of metal completely surrounding them. Moreover, the short circuiting rings are set some distance from the end of the core, so that the inductors between the core and ring act as vanes to force air through the coils for ventilation.
Fig. 1,748.—General Electric soldered form of end ring construction on squirrel cage armatures. The armature inductors or copper bars laid in the core slots are short circuited by these end rings, which are also made of copper. For the smaller sizes the rings are thin, but of considerable radial depth and are held apart by spacing washers. They have rectangular holes punched near their outer peripheries through which the bars pass. Lips are formed on the rings, as shown, to which the bars are soldered.
