ALTERNATING-CURRENT MOTORS

146. General construction of polyphase induction motors. Polyphase induction motors have two windings, one on the stationary part of the machine, or stator, and one mounted on the revolving part, or rotor. The stator winding is embedded in slots in the inner surface of the frame of the machine and is similar to the armature winding of a revolving-field type of ac generator. The rotor winding may be one of two types, squirrel-cage or wound-rotor. In a squirrel-cage machine the rotor winding consists simply of copper or aluminum bars embedded in slots in the iron core of the rotor and connected together at each end by a copper or aluminum ring. The rotor winding thus forms a complete closed circuit in itself. The rotor winding of the wound-rotor machine is similar to the armature winding of a revolving-armature type of ac generator. The free ends of the winding are brought out to slip rings. The rotor circuit is not closed until the slip rings are connected either directly together or through some resistances external to the machine.

147. Principle of operation of the induction motor. The induction motor differs from other types of motors in that there is no electrical connection from the rotor winding to any source of electrical energy. The necessary voltage and current in the rotor circuits are produced by induction from the stator winding. The operation of the induction motor depends upon the production of a revolving magnetic field. The...