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THEORY
OF THE HYSTERESIS MOTOR
The hysteresis synchronous motor is so named because it utilizes
the phenomenon of hysteresis to produce mechanical torque.
In its simplest form, the rotor of a hysteresis motor is a
smooth cylindrical tube of high hysteresis loss permanent
magnet material without windings or slots. It is placed within
a slotted stator carrying distributed windings designed to
produce, as nearly as possible, a sinusoidal space distribution
of flux. In single phase motors, the stator windings usually
are the permanent-split-capacitor type. The capacitor value
is selected to result in approximately balanced 2 phase conditions
within the motor windings: The stator then produces a rotating
field, approximately constant in space wave form and rotating
at synchronous speed.
 Instantaneous
magnetic conditions in the air gap are indicated in Figure
1 for a 2 pole stator. The axis SS' of the stator m.m.f. wave
revolves at synchronous speed, because of hysteresis the magnetization
of the rotor lags behind the inducing m.m.f. wave, and therefore
the axis RR' of the rotor flux wave lags behind the axis of
the stator m.m.f. wave by the hysteresis lag angle "a".
If the rotor is stationary, starting torque is produced proportional
to the product of the fundamental components of the stator
m.m.f. and rotor flux and the sine of the torque angle "a".
The rotor :hen accelerates if the counter torque of the load
is less than the developed torque of the motor. When the rotor
is turning at less than synchronous speed, each particle of
the rotor is subjected to a repetitive hysteresis cycle at
slip frequency. While the rotor is accelerating, the lag angle
"a" remains constant if the flux is constant, since
the angle "a" depends merely on the hysteresis loop
of the rotor and is independent of the rate at which the loop
is traversed.
 The
motor therefore develops constant torque right up to synchronous
speed, as shown in the ideal speed torque, Figure 2. This
feature is one of the advantages of the hysteresis motor in
contrast to a reluctance motor which must snap its load into
synchronism from the induction motor torque speed characteristic.
Hysteresis motors can synchronize any load they can accelerate,
regardless of the inertia. After reaching synchronism the
motors continue to run at synchronous speed and adjust their
torque angle to develop torques required by the loads.
There
are deviations from the ideal speed torque curves for several
reasons. In a single phase capacitor motor, a true two phase
condition occurs only at one load point. It is not always
possible to obtain a true sinusoidal winding distribution
tooth pulsation loss in the rotor etc., so that speed torque
curves A and B can be obtained. Therefore there is some flexibility
in design possible to obtain curves A or B; curve A, when
starting torque is not required; curve B, when a high starting
torque is required.
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