The Key to Eliminating Cogging Torque with the Right Motor Technology

Cogging torque, also known as detent torque, no-current torque, or simply “cogging”, is an undesirable effect observed in electric motors, particularly in permanent magnet motors. The primary cause of cogging is the magnetic attraction between the rotor's permanent magnets and the edges of the stator teeth. As the rotor turns, these magnetic interactions cause a periodic variation in torque, contributing to the torque ripple, increasing vibrations and noise, and negatively affecting the smoothness of motor operation, especially at low speeds, making precise control more difficult.

Having a low cogging torque motor is a critical consideration in applications requiring precise and smooth motor operation, such as in robotics, medical applications, aerospace, and various types of precision machinery. Understanding and reducing cogging torque is essential for improving motor performance in these sensitive applications. For example, motor-driven prosthetics (Fig.1) require smooth and natural movement to mimic human motion accurately. Cogging torque can cause uneven movements, hindering the functionality of the prosthetic.

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In conventional iron-cored motors, reduction of cogging can be achieved by several measures, such as skewing the windings or the magnets (Fig.2), choosing certain slot/pole combinations, shaping magnetic poles, and applying advanced control algorithms. However, all these measures, while reducing cogging torque, also lower the overall performance of the motor.

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In slotless motors, there are no stator teeth, thus the magnetic interaction mechanism that could result in cogging is simply not present. So, slotless motors have better performance in terms of smoothness of operation and control precision. They can be made almost vibration-free and silent.

Qualitative representation of theoretically possible torque curves is given in Fig. 3 for an iron-cored (slotted) motor, where the curves include the combined cogging and spatial harmonic effects. In this case it is assumed that none of the cogging-reduction measures, compromising the performance of the slotted motor have been implemented. The torque ripple for a slotless motor is an order of magnitude lower than for the slotted one, as in the slotless motor, the cogging torque is zero and the spatial harmonic torque ripple is never higher than 2%.

References

[1] A. Krings, C. Monissen. (2020). Review and trends in electric traction motors for battery electric and hybrid vehicles. 10.1109/ICEM49940.2020.9270946.