Cogging torque of electrical motors is the
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
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
Rotor may refer to:
Science and technology
Engineering
*Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element so called the stator
* Helicopter rotor, the rotary wing(s) of a rotorcraft ...
and the
stator
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric mot ...
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
Counter-electromotive force (counter EMF, CEMF, back EMF),Graf, "counterelectromotive force", Dictionary of Electronics is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF c ...
and so reduce the resultant running torque.
A slotless and coreless permanent magnet motor does not have any cogging torque.
See also
*
Torque ripple Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum and minimum torque over one complete revolution, ge ...
*
Dual-rotor permanent magnet induction motor
A dual-rotor permanent magnet induction motor (DRPMIM) is a motor having two rotors within the same motor housing. This rotor arrangement can increase power volume density, efficiency, and reduce cogging torque. Some dual-rotor permanent mag ...
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