Single-phase cylindrical (non-salient-pole] synchronous-induction or shaded-pole motors are classed as hysteresis motors. A hysteresis motor has neither a salient-pole rotor nor direct excitation, but nevertheless it rotates at synchronous speed. This type of motor runs into synchronism and runs on hysteresis torque.
Hysteresis-type lamination, shown in Fig. 26, are usually made of hardened, high retentivity steel rather than commercial, low retentivity dynamo steel.
Working. As a result of a rotating magnetic field produced by phase splitting or a
shaded-pole stator, eddy currents are induced in the steel of the rotor which travel across the two bar paths of the rotor as shown in Fig. 26. A high-retentivity steel produces a high hysteresis loss, and an appreciable amount of energy is consumed from the rotating field in reversing the current direction of the rotor. At the same time the rotor magnetic field set up by the eddy currents causes the rotor to rotate. A high starting torque is produced as a result of the high resistance (proportional to hysteresis). As the rotor approaches synchronous speed, the frequency of current reversal in the cross-bars decreases, and the rotor becomes permanently magnetized in one direction as a result of the high retentivity of the steel rotor. Consequently the motor continues to rotate at synchronous speed.
- An extremely important use of this type of motor is for the rotation of gyroscope rotors in inertial navigation and control systems. Here the requirement is for as near absolute accuracy as can be achieved. One major component of the instrument accuracy that contains the gyroscope is that the gyroscopic moment be absolutely constant. This constancy requires a synchronous motor that is driven by a regulated constant-frequency source.