Synchronous & Hysteresis Machines

A synchronous rotor carries a field from permanent magnets, DC excitation, or reluctance saliency and rotates at the stator field speed. In steady locked operation there is no induction-motor slip; torque is associated with angular displacement between rotor and stator field and, for salient machines, reluctance alignment.

Machine families #

• PMSM/brushless AC: permanent magnets provide rotor flux; inverter controls stator current.
• Wound-field synchronous: controllable rotor excitation adjusts field and reactive-power behavior.
• Synchronous reluctance: saliency produces torque without rotor windings or magnets.
• Hysteresis motor: a hard-magnetic rotor develops hysteresis torque during acceleration and locks synchronously.

Torque angle and pull-out #

For a simplified cylindrical synchronous machine, electromagnetic power varies approximately as $\sin\delta$, where $\delta$ is the load angle between fields. Increasing load advances the angle until stable torque reserve is exhausted; beyond pull-out the machine loses synchronism. A controller must limit current and angle before this physical boundary.

Inverter control #

Rotor electrical angle allows phase currents to be transformed to $d$–$q$ coordinates. In a surface-PM machine, $q$-axis current primarily produces torque while $d$-axis current manages flux; negative $d$ current enables field weakening above base speed at the cost of current and demagnetization margin. Interior-PM and reluctance machines also produce saliency torque, so optimum current angle differs.

Hysteresis mechanism #

A hysteresis rotor's magnetization lags the rotating field. The enclosed $B$–$H$ loop represents energy dissipated per cycle and produces torque over run-up, after which remanent poles can lock to the field. This offers smooth, quiet torque but lower efficiency and specialized use; it should not be generalized to modern inverter drives.

Verification #

Check phase order, rotor angle polarity and offset, current sensor polarity, no-load current, back-EMF constant, demagnetization/current limits, field-weakening voltage margin, and loss of synchronism response. For a line-start or hysteresis machine, verify actual starting load and supply condition, not only synchronous steady state.