A brushless DC motor is an AC synchronous electric motor that from a modeling perspective looks very similar to a DC motor. In this motor the stator coils are fed from a DC source through a polyphase inverter generating rectangular current waveforms, which produce a trapezoidal shaped rotating field in the rotor/stator gap. This provides a smooth torque irrespective of speed, but requires a rotor position sensor to synchronise rotor and stator fields. This motor can be considered to be an inside-out DC machine with the rotor coils transformed to the stator position and a permanent magnet providing the field in the rotor position. No electrical connection to the rotor is required, and the characteristics looking at the DC bus feeding the inverter are those of a DC shunt motor, hence the name brushless DC motor.

Brushless resolver comprises exciting signal transmitting means for transmitting a resolver exciting signal from the stator side to the rotor side in a non-contact way and a resolver part for modulating the resolver exciting signal according to the angle of rotation to be detected. The resolver part also serves as exciting signal transmitting means. The resolver part is constituted of a pair of a rotor composed of a rotor iron core having slots and wrapped with a rotor winding and a stator composed of a stator iron core having slots and wrapped with a stator winding.