The dragged member of an AC synchronous motor is its rotor, which has either a permanent magnet or an electro-magnet. The motor rotation is synchronous with the AC line frequency because the rotor is locked to the rotating magnetic field which is synchronous with the line frequency. The dragged member of a DC motor is its stator poles. The dragged member of an induction motor is a rotor winding in which the rotating field induces a current. That current reacts with the rotating field to produce torque.
Slip varies with load torque.
Increasing the load slows the motor until the increased current in the rotor produces an equal torque. The dragged member may use magnetic hysteresis, or non-uniform shape, or other effects to generate torque between itself and the rotating magnetic field. Speed High speed motors are wound with two poles in their rotating field, North and South, at opposite ends of a diameter. Synchronous motors with a rotating field with two poles run at 3600 RPM with 60 Hz power. Such motors run at 1800 RPM, synchronous, with 60 Hz power.