A basic synchronous machine is sketched in Fig. 1. The stator contains a three-phase armature winding. When a source of three-phase ac is connected to this winding, a magnetic field of constant amplitude is produced within the machine; in a two-pole machine, this field rotates at frequency equal to the frequency of the applied ac. The rotor contains a field winding that is excited by dc; this winding behaves as an electromagnet, producing a field of strength proportional to the applied field current that is aligned with the axis of the field winding. Alternatively, the field winding may be replaced by a permanent magnet