To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices.
Synchronous motors turn at a speed exactly proportional to the frequency.
The very largest motors are synchronous motors with DC passing through the rotor.