Cogging torque of electrical motors is the
torque
In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...
due to the interaction between the permanent
magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
s of the
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 the cogging torque
A summary of techniques used for reducing cogging torque:
* Skewing stator stack or magnets
* Using fractional slots per pole
* Optimizing the magnet pole arc or width
Almost all the techniques used against cogging torque also reduce the motor
counter-electromotive force and so reduce the resultant running torque.
A slotless and coreless permanent magnet motor does not have any cogging torque.
See also
*
Dual-rotor permanent magnet induction motor
*
Magnetic reluctance
*
Torque ripple
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"
External links
*
. Hanselman{{ webarchive , url=https://web.archive.org/web/20110719232621/http://www.eece.maine.edu/motor/BPMM_Ch9.pdf , date=July 19, 2011 , title=Brushless Permanent Magnet Motor Design
Electric motors
Torque