How Does A Brushless DC Motor Work

Brushless DC motors have a unique back emf waveform that behaves like a brushed DC motor (BDC). According to the Lorentz law of force, a force is created when a conductive wire is placed in a magnetic field. As a result of the reaction force, the magnets also experience opposing forces. In brushless DC motors, the current-carrying wire is stationary while the permanent magnet moves. When the stator winding is energized by the power source, it becomes an electromagnet and begins to create a uniform magnetic field in the air gap. The power source is DC, but the switch produces a trapezoidal AC waveform. The interaction between the electromagnetic stator and the permanent magnet rotor keeps the rotor spinning. As the coils transition between high and low signals, the corresponding coils are energized as the north and south poles. The permanent magnet rotor with the north and south poles are aligned with the stator poles and the motor is running.

BLDC motors cannot be operated directly from a DC voltage source. Rotors with permanent magnets and wound stators and electronic commutators are available. Usually, three Hall sensors are used to detect and convert the rotor position based on the input from the Hall sensors. The motor is controlled by a square or trapezoidal voltage swing and a specific rotor position. To achieve maximum torque, the voltage swing must be properly aligned between phases to keep the angle between the stator and rotor fluxes close to 90°.