Brushless DC Motor

  Where brushed DC motors have a very long history, BLDC motors only came into their own in the 1960s. BLDC
motors are considerably more efficient than brushed DC motors; they last a lot longer; and they deliver more torque
per weight. By moving the permanent magnets to the rotor and driving the field coils with transistors you can eliminate the brushes in DC motors along with their disadvantages (inrunner configuration, the most common). Alternatively the armature coils can form a fixed core with the permanent magnets revolving around them and driving the motor shaft (outrunner). In either case the coils are stationary. BLDC motors are referred to as electronically commutative motors

  (ECMs) in contrast to mechanically commutative brushed motors. BLDC motors need sophisticated electronic control
circuitry as well as some way to continuously determine the position of the rotor. The position of the rotor can be
determined either by a Hall effect sensor or by measuring changes in the back EMF (BEMF) at each of the armature
coils as the motor rotates. Whereas the speed of brushed DC motors is determined by the applied voltage, the speed of BLDC motors is determined by the frequency at which it is switched.