Power Electronics Circuits
Which of the following correctly describes a brushless DC motor?
- ABrushes and a commutator switch the direction of the winding current mechanically, so no drive circuit such as an inverter is needed and it runs simply when connected to a DC supply
- BThe rotor position is detected with sensors or other means, and an inverter switches the current in the stator windings electronically
- CIt simply runs at the supply frequency when connected directly to an AC supply
Show answer and explanation
Answer: B. The rotor position is detected with sensors or other means, and an inverter switches the current in the stator windings electronically
In a brushed DC motor, the brushes and commutator mechanically switch the direction of the winding current as the motor turns. A brushless DC motor has permanent magnets on its rotor, and an inverter switches the current in the stator windings according to the rotor position (electronic commutation). The position is detected with sensors such as Hall elements or from the back EMF induced in the windings. With no brush wear or sparking, it is efficient and long-lived and is widely used in fans, drones, home appliances and cars.
- A. This describes a brushed DC motor. In a brushless DC motor, the drive circuit takes over switching the current.
- B. Correct. Switching the current electronically eliminates the brushes and commutator.
- C. Connecting directly to a supply and running at its frequency is how synchronous and induction motors are used. A brushless DC motor is run by a drive circuit.