Theory of Operation

Owned by Kevin Li
2021-08-15
2 min read

Motor operation is based on the attraction or repulsion between magnetic poles. Consider the three-phase BLDC motor shown in the figure below:

Three-phase BLDC Motor Operation

The process begins when current flows through one of the three stator windings in a particular direction and generates a magnetic pole that attracts the closest permanent magnet of the opposite pole. The rotor will move if the current shifts to an adjacent winding. Sequentially charging each winding will cause the rotor to follow in a rotating field. To amplify rotation, another winding may be energized at the same time with current flowing in a particular direction to create a magnetic pole that repels the permanent magnet of the rotor in the desired direction.

For three-phase BLDC motors, each commutation sequence has one of the following:

  • A positively energized winding (current enters the winding)

  • A negatively energized winding (current exists the winding)

  • A non-energized winding

Torque is generated because of the interaction between the magnetic field generated by the stator coils and the permanent magnets of the rotor. Ideally, the peak torque occurs when these two fields are orthogonal or at a 90° angle to each other. In order to keep the motor running, the magnetic field produced by the windings should shift position as the rotor moves to catch up with the magnetic field of the stator. In general, torque is governed by the following factors:

  • Current amplitude

  • Number of turns on the stator windings

  • Strength and size of the permanent magnets

  • Air gap between the rotor and the windings

  • Length of the rotating arm