Brushless DC motor

[JohnCU]

I can't find a good article on brushless DC motors, how does is the rotor energized if there are no brushes? not induction, so how can you connect it to a circuit without brushes or slip rings?

 

[bobsmith1492]

I'm pretty sure it still has brushes or slip rings to physically connect to the coils, but the difference is the rotation of the motor never breaks/remakes contact. The controller has to switch the current flow from phase to phase as the motor rotates. I'm thinking big 3-phase brushless motors, though; I'm not sure about the little ones like in a computer case.

 

[PottedMeat]

According to Microchip AN885 App Note BLDC motors use permanent magnets in the rotor.


Microchip - Brushless DC Motor Fundamentals - AN885 ( PDF )

Lots of interesting details in there.

 

[pm]

In a nutshell, the commutation system in a brushless motor is electronically controlled. The coils are energized by an electronic controller.

From the wikipedia entry at http://en.wikipedia.org/wiki/Brushless_Motor

In a BLDC motor, the brush-system/commutator assembly is replaced by an intelligent electronic controller. The controller performs the same power-distribution found in a brushed DC-motor, only without using a commutator/brush system. The controller contains a bank of MOSFET devices to drive high-current DC power, and a microcontroller to precisely orchestrate the rapid-changing current-timings. Because the controller must follow the rotor, the controller needs some means of determining the rotor's orientation/position (relative to the stator coils.) Some designs use Hall effect sensors to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall effect sensors, and therefore are often called "sensorless" controllers. (The BLDC motor has a trapezoidal backemf, while a brushless AC motor has a sinousoidal backemf.)

 

[Mark R]

Essentially, a brushless DC motor is an AC synchronous motor (i.e. permanent magnet rotor), where the DC supply is electronically commutated (to provide a pseduo AC to the stator coils).

 

[pm]

Great description, Mark R.