The cogging torque of a permanent magnet motor is the reluctance torque generated by the interaction between the cogging of the armature core and the permanent magnets of the rotor. Due to the existence of the stator cogging of the permanent magnet brushless DC motor, when the permanent magnet rotor magnetic pole and the stator cogging are in different positions, the permeance of the main magnetic circuit changes. Even if the motor windings are not energized, due to the effect of the cogging torque, the motor rotor tends to stop at several stable positions on the circumference. Therefore, there are literatures that the cogging torque is called the positioning torque. When the motor rotates, the cogging torque appears as an additional pulsating torque. Although it will not increase or decrease the average effective torque of the motor, it causes speed fluctuations, motor vibration and noise, especially at light loads and It is more obvious at low speeds.

　　This article is taken from ------"Permanent Magnet Brushless DC Motor Technology