Ques. Describe briefly the operation of a polyphase induction motor.
Ans. Its operation is due to the production of a rotating magnetic field by the polyphase current furnished. This field "rotating" in space about the axis of the armature induces currents in the latter. The reaction between these currents and the rotating field creates a torque which tends to turn the armature, whether the latter be at rest or in motion.
Figs. 1642 and 1643.—Western Electric end flange rivets and punchings of riveted frame induction motor. The riveted frame is constructed of two cast iron flanges between which the stator laminations of sheet steel are securely clamped and riveted under hydraulic pressure. This construction exposes the laminations directly to the air and improves the radiation, thus insuring high overload capacity and low operating temperatures. The field slots are overhung or partially closed, affording mechanical protection to the coils.
Ques. Why are induction motors called "asynchronous"?
Ans. Because the armature does not turn in synchronism with the rotating field, or, in the case of a single phase induction motor, with the oscillating field (considering the latter in the light of a rotating field).
Ques. How does the speed vary?
Ans. It is slower (more or less according to load) than the "field speed," that is, than "synchronism" or the "synchronous speed."
