There are three types of motors that
can start and run as induction motors yet can lock into the supply
frequency and run as synchronous motors as well. They are (1) the
wound-rotor motor with dc exciter (2) the permanent-magnet (PM)
synchronous motor, and (3) the reluctance-synchronous motor.
The latter two types have been used
primarily with adjustable frequency inverter power supplies. In
Europe, wound-rotor induction motors have often been provided with
low-voltage dc exciters that supply direct current to the rotor,
making them operate as synchronous machines.
With secondary rheostats for starting,
such a motor gives the low starting current and high torque of the
wound-rotor induction motor and an improved power factor under load.
Several different forms of these
synchronous induction motors have been proposed, but they have not
shown any net advantage over usual salient-pole synchronous or
induction machines and are very seldom used in the United States.
FIGURE 20-44 Cross section of (a) a
conventional PM synchronous motor and (b) a reluctance synchronous
motor.
The PM synchronous motor is shown in
Fig. 20-44a. The construction is the same as that of an ordinary
squirrel-cage motor (either single or polyphase), except that the
depth of rotor core below the squirrel cage bars is very shallow,
just enough to carry the rotor flux under locked-rotor conditions.
Inside this shallow rotor core is
placed a permanent magnet, fully magnetized. The rotor core serves as
a keeper, so that the rotor is not demagnetized by removing it from
the stator. In starting, the rotor flux is confined to the laminated
core.
As the speed rises, the rotor frequency
decreases and the rotor flux builds up, creating a pulsating torque
with the field of the magnet, as when a synchronous motor is being
synchronized after the dc field has been applied. As the motor
approaches full speed, therefore, the ac impressed field locks into
step with the field of the magnet and the machine runs as a
synchronous motor. The absence of rotor I2R loss, the synchronous
speed operation, and the high efficiency and power factor make the
motor very attractive for special applications, such as
high-frequency spinning motors.
When many such motors are supplied from
a high-frequency source, the kVA requirements are reduced to perhaps
50% of those needed for usual induction motor types, with consequent
large savings.
If the rotor surface of a P-pole
squirrel-cage motor is cut away at symmetrically spaced points,
forming P salient poles, the motor will accelerate to full speed as
an induction motor and then lock into step and operate as a
synchronous motor.
The synchronizing torque is due to the
change in reluctance and, therefore, in stored magnetic energy, when
the air-gap flux moves from the low- into the high-reluctance region.
Such motors are often used in small-horsepower sizes, when
synchronous operation is required, but they have inherently low pull
out torque and low power factor, and also poor efficiency, and
therefore require larger frames than the same horsepower induction
motor.
The PM synchronous motor has superior
performance in every way, except possibly cost. A cross section of
the reluctance-synchronous motor is shown in Fig. 20-44b. These
motors are available up to about 5 hp.
If the number of rotor salients is nP,
instead of P, and if the P-pole motor winding is arranged to also
produce a field of (n - 1)P or (n - 1)P poles, the motor may lock
into step at a subsynchronous speed and run as a subsynchronous
motor. For the P-pole fundamental mmf, acting on the varying rotor
permeance will create (n + 1)P and (n - 1) P-pole fields from this
case, and these will lock into step with the independently produced
(n - 1)P- or (n + 1)P-pole field, when the rotor speed is such as to
make the two harmonic fields turn at the same speed in the same
direction.
It is difficult to provide much torque
in such subsynchronous motors, and their use is therefore limited to
very small sizes, such as may be used in small timer or instrument
motors.
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