HYSTERESIS MOTORS BASIC INFORMATION AND TUTORIALS



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.

No comments:

Post a Comment

PREVIOUS ARTICLES

free counters