Cogging torque
Encyclopedia
Cogging torque of electrical motors is the torque
due to the interaction between the permanent magnet
s of the rotor
and the stator
slots of a Permanent Magnet (PM) 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.
Almost all the techniques used against 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.
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
due to the interaction between the permanent magnet
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, and attracts or repels other magnets.A permanent magnet is an object...
s of the rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
and the stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
slots of a Permanent Magnet (PM) 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:- Skewing stator stack or magnets
- Using fractional slots per pole
- Modulating drive current waveform
- 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
The counter-electromotive force also known as back electromotive force is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is the effect of Lenz's Law of electromagnetism...
and so reduce the resultant running torque.
A slotless permanent magnet motor does not have any cogging torque.
Model railway motors
Model railway motors usually have a 2-pole permanent magnet and 3, 5 or 7 poles on the armature to reduce 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".
- the cogging torque