Hysteresis motor with overexcitation and solid-state control

The overexcitation for a short period of a hysteresis motor when running synchronously gives a reduction in stator current and an increase in pull-out torque. The physical concept of the phenomenon is briefly discussed and an empirical equivalent circuit incorporating a residual flux voltage is established which gives reasonable agreement with measured values. Methods of implementation of the technique are described, including delta-star starting and two solid-state-control devices. It is shown that, with short-duration overexcitation, harder magnetic materials may be used for the rotor, thus increasing the machine rating for a given frame size.