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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