Page 402 - Power Electronics Handbook

Electrical machine control 391
In general, the electronic commutator motor is best suited to
applications requiring high starting torque, good efficiency at low speeds
and continuous speed variation from standstill to full speed. Due to their
poor efficiency at low speeds, induction motors driven from inverters are
more suited for use in applications where the output is a fixed high speed,
and where, by using crystal-controlled oscillators in the inverter and a
synchronous motor, the speed regulation can be made exceptionally good.
14.3.4 A.C. motors

As outlined in Figure 14.12, there are three basic types of a.c. motors
popularly used with power semiconductor drives, induction, synchronous
and commutator. There is yet another type of motor, the reluctance
machine, which has not been mentioned so far since it can be considered as
a special form of synchronous motor.
Reluctance motors were not popular, since they suffered from several
disadvantages, chiefly connected with the low power factor, but develop-
ments in new materials has resulted in machines with greatly improved
performance. All reluctance motors have a certain salience in their rotors.
The stator produces a rotating field similar to an induction motor, which
causes the motor to run up close to synchronism by induction motor
principles. Thereafter the salient poles lock in onto the stator field and it
runs at synchronous speed, as in a synchronous motor, even though the
rotor poles are not externally excited. It is this synchronous-induction
action of the reluctance motor that has resulted in its use in applications
such as multi-motor converter drives in the textile, steel and plastics
industries. When used to drive an alternator it can also provide highly
stable computer supplies.

Quadrature
I axis
0
fi

Direct
axis

Quadrati Jre
field
Figure 14.42 Conventional two-pole reluctance motor