Cogging torque is an inherent problem in permanent magnet motors. When the motor rotates, cogging torque manifests as an additional torque pulsation component. While it doesn't increase or decrease the motor's average effective torque, it causes speed fluctuations, motor vibration, and noise, especially noticeable under light loads and low speeds. Vibration and noise are amplified when the frequency of this pulsating torque equals the stator or rotor frequency. Therefore, cogging torque must be reduced in the design and manufacture of servo motors.
Generation of cogging torque
Cogging torque is the torque generated by the interaction between the permanent magnet and the stator core when the windings of a permanent magnet motor are not energized. It is caused by the pulsation of the tangential component of the interaction force between the permanent magnet and the stator teeth. When the motor rotor rotates, the magnetic permeability of a small range corresponding to the stator teeth on both sides of the permanent magnet changes significantly, causing a change in the stored energy of the magnetic field, thereby generating cogging torque.
The dangers of cogging torque
① It affects the positioning accuracy and servo performance of the motor, especially at low speeds;
② It affects the smoothness of motor operation, easily causing torque pulsation, which in turn leads to speed fluctuations;
③ Resonance causes the motor to vibrate and produce noise;
④ A larger starting torque is required to start the motor.
Maixin servo motors employ an excellent electromagnetic solution.
① It adopts a 12-slot, 10-pole design, which results in lower cogging torque compared to a 12-slot, 8-pole motor;
② The fractional-slot concentrated winding and the integer-slot distributed winding result in higher motor efficiency;
③ Reduce motor harmonics, thereby reducing motor vibration and noise.
Common methods to reduce cogging torque
① Employ the optimal slot pole number combination;
② Change the stator slot height, width, and tooth shape;
③ It adopts a helical tooth slant pole design;
④ The rotor magnets are designed with segmented and staggered alignment;
⑤ Optimize the shape of the magnet;
⑥ Unequal thickness air gaps.
