The disadvantages of the Brushless DC Motor over the AC motors are:
higher maintenance (due to brushes)
more expensive
In addition to the above the Brushless DC Motors have the same advantages over AC motors but they also have more advantages over Brushed DC Motors
lower maintenance (no brushes)
even higher starting torque ( Due to rare earth magnets)
even higher efficiency (over 90%) on sub fractional motors
even more compact design
better heat dissipation
lower noise
The voltage supplied to the stator windings of a brushless DC motor is pulse-width-modulated by a circuit which interrupts the energization thereof for a predetermined period of time whenever the DC line current supplied thereto exceeds a current reference. The frequency and off/on duty cycle of the applied voltage is relatively high near zero speed to produce smooth motor starting torque and decreases with increasing motor speed. The pulse-width-modulation effects regenerative braking whenever the stator winding energization sequence is changed to reverse the direction of motor rotation.
