OVERVIEW

AC machines are motors that convert ac electric energy to mechanical energy and generators that convert mechanical energy to ac electric energy. The two major classes of ac machines are synchronous and induction machines. The field current of synchronous machines (motors and generators) is supplied by a separate dc power source while the field current of induction machines is supplied by magnetic induction (transformer action) into the field windings.

AC machines differ from dc machines by having their armature windings almost always located on the stator while their field windings are located on the rotor. A set of three-phase ac voltages is induced into the stator armature windings of an ac machine by the rotating magnetic field from the rotor field windings (generator action). Conversely, a set of three-phase currents flowing in the stator armature windings produces a rotating magnetic field within the stator. This magnetic field interacts with the rotor magnetic field to produce the torque in the machine (motor action).

THE ROTATING MAGNETIC FIELD

The main principle of ac machine operation is this: A three-phase set of currents, flowing in an armature windings, each of equal magnitude and differing in phase by 120 , produces a rotating magnetic field of constant magnitude.

The stator shown in Fig. 5.1 has three coils, each 120 apart.

Figure 5.1: A simple three-phase stator. Currents in this stator are assumed positive if they flow into the unprimed and out of the primed ends of the coils.

The currents flowing in...