A brushless DC motor is a rotating electric machine where the stator is a classic three-phase stator like that of an induction motor and the rotor has surface-mounted permanent magnets. In this respect, the brushless DC motor is equivalent to a reversed DC commutator motor, in which the magnet rotates while the conductors remain stationary. In the DC commutator motor, the current polarity is altered by the commutator and brushes. On the contrary, in the brushless DC motor, the polarity reversal is performed by power transistors switching in synchronization with the rotor position. Therefore, brushless DC motor often incorporate either internal or external position sensors to sense the actual rotor position or the position can be detected without sensors.
The brushless DC motor is driven by rectangular voltage strokes coupled with the given rotor position. The generated stator flux interacts with the rotor flux, which is generated by a rotor magnet, defines the torque and thus speed of the motor. The voltage strokes must be properly applied to the two phases of the three-phase winding system so that the angle between the stator flux and the rotor flux is kept close to 90° to get the maximum generated torque. Due to this fact, the motor requires electronic control for proper operation.