ABSTRACT
The polyphase induction machine has one obvious characteristic: there must be relative motion between the rotating stator field and the rotor if induction is to occur. If the rotor rotates at synchronous speed, the induction process breaks down, and rotor voltages and currents drop to zero. Also, if rotor currents were to exist, their frequency would have to be zero to produce nonzero average torque: But zero frequency AC is DC. It is possible to modify the design of the machine to supply the rotor with DC current, not by induction processes but rather by conduction means. The resultant machine is characterized as a synchronous machine since the EM energy conversion process can only occur when the rotating stator field and the rotor are synchronized; that is, they rotate at the same speed.1 Hence, the speed of the machine is
(7.1)
Observe that for motor operation, the speed control problem is greatly simplified in that the rotor speed will no longer depend on load. We start our study by examining the details of synchronous machine construction.
