In which single-phase motor, the rotor has no teeth or winding?

Hysteresis Motor:

Hysteresis motor is a single-phase synchronous motor with a cylindrical rotor made up of ferromagnetic material and does not require any DC excitation to the rotor.

Construction of Hysteresis motor: Hysteresis motor consists of a stator and rotor.

Stator: The stator is wound with main and auxiliary windings so as to produce the rotating magnetic field. In some hysteresis motor, the stator can also be shaded pole type. The stator is designed to produce synchronizing revolving field from a single-phase supply.

Rotor:  The rotor is the smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity. The rotor does not carry any winding or teeth.​​

Working of Hysteresis Motor:

• When hysteresis motor is started it behaves as a single-phase induction motor and while running it behaves like a synchronous motor.
• When the stator is energized with a single-phase AC supply it produces a rotating magnetic field.
• The main and auxiliary, windings must be supplied continuously at the start as well as in running condition so as to maintain the rotating magnetic field.