Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the electric rotor and the stator slots of a Permanent Magnet machine. It is also known as detent or 'no-current' torque. This torque is position dependent and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator. Cogging torque is an undesirable component for the operation of such a motor. It is especially prominent at lower speeds, with the symptom of jerkiness. Cogging torque results in torque as well as speed ripple; however, at high speed the motor moment of inertia filters out the effect of cogging torque.
Reducing cogging torque
A summary of techniques used for reducing cogging torque:
- Use odd number stator coils and even number magnets
- Skewing stator stack or magnets
- Using fractional slots per pole
- Modulating drive current waveform
- Optimizing the magnet pole arc or width
- Place magnets asymmetrically by 1/2 slot pitch
Almost all the techniques used against to cogging torque also reduce the motor counter-electromotive force and so reduce the resultant running torque.
A slotless permanent magnet motor does not have any cogging torque.
Footnotes and References
- Islam, M.S. Mir, S. Sebastian, T. Delphi Steering, Saginaw, MI, USA "Issues in reducing the cogging torque of mass-produced permanent-magnet brushless DC motor".
- D. Hanselman
- Reduction of cogging torque in permanent magnet machine by K. Chandrashekar
- Cogging and Torque Ripple Questions by Hurley Gill