How to change the speed of a BLDC motor?

The direction of rotation of the BLDC motor is related to the direction of the current in the coil and the direction in which the coil cuts the magnetic lines of induction. The direction of the current in the coil changes the direction in which the coil cuts the magnetic induction line can change the direction of rotation of the motor; the direction of the coil cuts the magnetic induction line remains unchanged, and the direction of the current in the coil changes the direction of rotation of the motor. When the current through the coil changes, the rotation speed of the motor can be changed.

According to the left-hand rule, when one of the direction of the armature current or the direction of the magnetic field (that is, the direction of the excitation current) reverses, the direction of the armature conductor's force changes, and the direction of rotation of the motor changes accordingly. However, if the direction of both the armature current and the magnetic field change at the same time, the direction of rotation of the motor does not change.