SELF-STARTING SYNCHRONOUS-RELUCTANCE MOTORS BASIC INFORMATION



Any one of the induction-motor types described above can be made into a self-starting synchronous reluctance motor.

Anything which makes the reluctance of the air gap a function of the angular position of the rotor with respect to the stator coil axis will produce reluctance torque when the rotor is revolving at synchronous speed.

For example, suppose some of the teeth are removed from a squirrel-cage rotor, leaving the bars and end tings intact, as in an ordinary squirrel-cage induction motor. Figure 9.9a shows a lamination for such a rotor designed for use with a four-pole stator.

The stator may be polyphase or any one of the single-phase types described above. The motor will start as an induction motor and at light loads will speed up to a small value of slip.

The reluctance torque arises from the tendency of the rotor to try to align itself in the minimum reluctance position with respect to the synchronously revolving forward air-gap flux wave.

At a small slip, this torque alternates slowly in direction; the rotor is accelerated during a positive half cycle of the torque variation and decelerated during the succeeding negative half cycle.

If the moment of inertia of the rotor and its mechanical load are sufficiently small, the rotor will be accelerated from slip speed up to synchronous speed during an accelerating half cycle of the reluctance torque. The rotor will then pull into synchronism and continue to run at synchronous speed.

The presence of any backward-revolving stator flux wave will produce torque ripple and additional losses, but synchronous operation will be maintained provided the load torque is not excessive.

A typical torque-speed characteristic for a split-phase-start synchronousreluctance motor is shown in Fig. 9.9b. Notice the high values of induction-motor torque.

The reason for this is that in order to obtain satisfactory synchronous-motor characteristics, it has been found necessary to build synchronous reluctance motors in frames which would be suitable for induction motors of two or three times their synchronous-motor rating.

Also notice that the principal effect of the salient-pole rotor on the induction-motor characteristic is at standstill, where considerable "cogging" is evident; i.e., the torque varies considerably with rotor position.

Figure 9.9 Rotor punching for four-pole synchronous-reluctance motor and typical torque-speed characteristic.

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