What are Brushless DC or Permanent magnet motors?

Answer by Hugh Falkner (Atkins)

Brushgear refers to the electrical sliding contacts or “brushes” that transmit electricity to the copper rings or segments found on the rotating part of some motor types, for example DC or Universal motors. 

The term “brushless DC” is often given to Permanent magnet motors, as it describes the key advantage over conventional DC motors of not requiring any brushgear. This is possible because the permanent magnets in the rotor create the rotor field without the need for any external power supply, eliminating the ongoing maintenance required to keep brushgear in good condition.

Because there is no need for rotor windings (or bars) and their attendant losses, the efficiencies of these motors can be exceptionally high. 

As with a conventional DC motor, the stator of a Brushless DC motor requires a controller for operation.   Some conventional induction motor VSDs are also configurable to run these motors.

Many low power fans use this type of motor to achieve very high efficiencies.  An interesting feature of these is that the motor is “inside out”, with the permanent magnets embedded in the external fan rotor itself. These are sometimes known as Electronically Commutated Internal rotor (ECI) motors.  Although there are not any formally agreed IEC test methods, these motors are available commercially at an efficiency equivalent to IE4.