By constructing the secondary core of
an induction motor of hardened magnet steel, in place of the usual
annealed low-loss silicon-steel laminations, the secondary hysteresis
loss can be greatly magnified, producing effective synchronous motor
action.
Such hysteresis motors, having smooth
rotor surfaces without secondary teeth or windings, give extremely
uniform torque, are practically noiseless, and give substantially the
same torque from standstill all the way up to synchronous speed.
A hysteresis motor is a true
synchronous motor, with its load torque produced by an angular shift
between the axis of rotating primary mmf and the axis of secondary
magnetization. When the load torque exceeds the maximum hysteresis
torque, the secondary magnetization axis slips on the rotor, giving
the same effect as a friction brake set for a fixed torque.
Despite the interesting characteristics
of this type of motor, it is limited to small sizes, because of the
inherently small torque derivable from hysteresis losses.
Only moderate flux densities are
practicable, owing to the excessive excitation losses required to
produce high densities in hard magnet steel, and, therefore, about 20
W/lb of rotor magnet steel represents the maximum useful synchronous
power on 60 Hz.
Hysteresis motors have found an
important use for phonograph-motor drives, their synchronous speed
enabling a governor to be dispensed with and freedom from tone waver
to be secured.
The Telechron motor, which is so widely
used for operating electric clocks, also operates on the
hysteresis-motor principle. In the Telechron motor, a 2-pole rotating
field is produced in a cylindrical air space, and into this space is
introduced a sealed thin-metal cylinder containing a shaft carrying
one or more hardened magnet-steel disks, driving a gear train.
The 60-Hz magnetic field causes the
steel disks to revolve at 3600 r/min, driving through the gears a
low-speed shaft, usually 1 r/min, which merges from the sealed
cylinder through a closely fitting bushing designed to minimize oil
leakage.
Although the magnetic field has to
cross a very considerable air-gap length and pass through the tin
walls of the metal cylinder, the power required to drive a
well-designed clock is so small that sample output is obtained with
only about 2-W input for ordinary household-clock sizes.
The hysteresis motor has been displaced
for phonograph and tape-reel drives by the transistor-driven
brushless dc motor. It has been displaced for electric clocks by
solid-state circuits with digital readout.
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