THREE-PHASE INDUCTION MOTOR WITH SINUSOIDAL SUPPLY VOLTAGES

A three-phase induction motor contains a three-phase distributed winding that is housed in slots on the stationary part of the motor, usually called the stator. The rotating part of the machine, or rotor, also contains either a distributed three-phase winding or a cage of interconnected copper bars that serve as rotor winding conductors. When the rotor contains a distributed winding the three phases of this winding are connected to three slip rings on the motor shaft and the motor is known as a wound-rotor machine or slipring machine. When a cage of copper bars is used these bars are electrically connected by end rings inside the rotor, no electrical connection can be made to them and the motor is known as a squirrel-cage motor or, more simply, a cage motor.

One set of three-phase windings is connected to a three-phase voltage supply and this set becomes the primary or excitation (field) windings. With a slip-ring motor either the stator or the rotor windings may act as primary windings, although invariably the stator is used. With a cage motor only the stator windings can be used as primary windings. The other set of motor windings, known as secondary windings, is not connected to the electrical supply but is closed on itself. There is no electrical connection between the primary windings and the secondary windings but these are linked magnetically, as in a transformer.