Classic AC motors: The synchronous motor

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.A basic synchronous motor is depicted in FIG. 22. In general, the synchronous motor, whether polyphase or single phase, uses a stator similar to those in corresponding induction motors. As with the induction motor, the primary function of the stator is to set up a rotating field. This rotating field can be derived from polyphase windings or from some method such as phase splitting by resistance or by reactance. Now focus attention on the fundamental difference between the two machines.

As shown in FIG. 22, the rotor of the synchronous motor has fixed magnetic poles. These poles can result from the current delivered from a DC source, or they can be incorporated in a permanent-magnet rotor. There is a third possibility iron rotor without windings can be used, in which case the required poles develop as the motor pulls into synchronism with the rotating field. In all cases, the rotor is “locked” to the rotating field and can deviate its speed only on a transitory basis. Stated another way, the average slip of an operating synchronous motor is zero. Whereas the induction motor cannot attain synchronism because it would then develop zero torque, the synchronous motor behaves in an opposite manner. Its torque is developed only at synchronous speed; if forced to slow down by an excessive load, it will first try to fail back a small amount and then try to resume the speed of the rotating field. If it cannot develop sufficient torque to do this, it will immediately come to a halt.

The synchronous motor is not a self-starter. Even with the symmetrical rotating field provided by a three-phase stator, no torque is developed at standstill. Large synchronous motors are sometimes brought up to synchronous speed by means of another motor coupled to the shaft. This has worked out well in some applications because the other machine could be a DC generator that was operated as a DC motor for starting purposes, then switched over to function as an exciter for the field windings of the synchronous motor. No field current is necessary, or desirable, when the synchronous motor is being accelerated during starting.


FIG. 22 The three-phase synchronous motor.

A more sophisticated starting technique can be developed from this knowledge of induction motors. By embedding segments of a squirrel-cage structure in the pole faces of the rotor, the same machine becomes both an induction motor and a synchronous motor. Such a “hybrid” motor will start as an induction motor and then it will attain synchronism because a synchronous motor has a certain amount of “pull- in” capability.

In FIG. 22, the squirrel-cage windings are designated “damper windings.” This is because of their inhibiting action when the rotor tends to oscillate or “hunt” about its average speed. Even though the average speed is synchronous with the rotating field, the rotor is subject to deviations in its instantaneous speed when there are fluctuations in line voltage or when the mechanical load varies. Another name for these short-circuited conductors in the pole faces is amortisseur windings—literally, the “killer” windings.

If the synchronous motor in FIG. 22 is driven, it becomes a three-phase alternator. The most familiar example of such an alternator is the one used in automobiles for charging the storage battery. Another interesting characteristic of the synchronous motor is that it’s the only AC motor that can control its own power factor. All other AC motors must operate at less than unity power factor, that is, they appear as an inductive reactance to the AC line. This causes a higher current consumption than would prevail at unity power factor, and therefore must be considered as a degradation of the overall operating efficiency of the AC motor. By the simple expedient of adjusting the field cur rent in the synchronous motor, the power factor can be varied. The field current can be “resonated” so that the motor appears to the AC line as a resistive load having unity power factor. Even better, the synchronous motor can be operated at a leading power factor to produce an overall unity power factor, taking into consideration other motors and inductive loads. When so operated, the synchronous motor behaves as a capacitor and cancels the inductive reactance of other loads. Indeed, synchronous motors have been made specifically for this purpose; they are called synchronous condensers and have no protruding shaft, because they don’t drive a mechanical load.

The effects of field current in the synchronous motor are shown in FIG. 23. The motor behaves similarly to a resonant circuit because it can be made to have the characteristics of inductance, resistance, or capacitance. Note, also, that its Q is degraded when loaded. These curves indicate the importance of instrumentation in optimizing the performance of the synchronous motor.

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