Synchronous AC motors

In modern AC motors, as in modern AC generators, the current is applied to the stationary part of the motor and the magnet is on the rotor. The varying current in the stator generates a rotating magnetic eld (via Am- pere's law) which interacts with the rotor magnetic eld, pushing the rotor around. AC motors in use now come have several options for generating the rotor magnetic eld. In some the rotor has permanent magnets; in others the rotor carries an electromagnets driven by an external DC power source; in still others (induction motors) the rotor merely picks up induced magnetic fields from the magnetic field of the stator.

Synchronous AC motors
  A synchronous AC motor really is essentially the same as the AC generators described in a previous reading. As before, the rotor speed and the frequency of the alternating current are irrevocably tied together. The motor must turn at the exact frequency of the alternating current that drives it (modulated only by the number of magnetic poles). What are the benets of a synchronous AC motor? One very large benet is that you can avoid brushes, with their short lifetimes. You will need either a permanent magnet in the motor or slip rings to pass current to an electromagnet, but those currents are smaller than the drive current, and slip rings don't spark like brushes and so are longer-lived in general. 2 In
practice, most small AC synchronous motors just use permanent magnets. That produces a sturdy, reliable, if not readily controllable motor. But if you just want to turn something, and you're OK with letting our 60 Hz grid system constrain that turning rate (think: household fans), an AC motor is just fine.