The lag angle exists because the metal of the rotor has a large
hysteresis loss. The angle, by which the rotor magnetic field lags the
stator magnetic field, depends on the hysteresis loss of the rotor. At the
synchronous speed, the stator flux stops to sweep across the rotor,
causing the eddy currents to disappear and the rotor behaves like a
permanent magnet. At that time, the developed torque in the motor is
proportional to the angle between the stator and rotor magnetic fields
that is determined by the
hysteresis of the motor.
Consequently, a constant
torque (indicated as the
hysteresis torque) exists from
zero up to the synchronous
speed