When the motor has a rotor that has an overall cylindrical outline and yet is toothed as a many-pole salient-pole rotor, it is a sub-synchronous motor. A typical rotor may have 16 teeth or poles, and in conjunction with a 16-pole stator will normally rotate at synchronous 450 r.p.m. when operated on 60 Hz. If this motor were temporarily overloaded, it would drop out of synchronism. Then the speed drops down toward the maximum torque point, and the motor will again lock into synchronism at a sub-multiple speed of 225 r.p.m. Hence the name of sub-synchronous motor.
This type of motor-starts and accelerates with hysteresis torque just as the hysteresis synchronous motor does. There is no equivalent of induction-motor torque as in the reluctance motors.
This type of motor in any given size will develop a higher starting torque but a lesser synchronous speed torque than a reluctance motor.
Fig. 27 shows a sub-synchronous rotor.