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Basics: What is the cogging torque?
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The cogging torque of electric motors is the torque which is produced due to the interaction between the rotor’s permanent magnets and the stator slots of a permanent magnet machine. It is also known as “no-current torque” or detent torque”.
Cogging torque
The cogging torque is position dependent and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator. It is an undesirable component for the operation of such motors, since it leads to fluctuating torques and uneven running – especially at low speeds. At high speeds the motor’s moment of inertia filters out the effect of the cogging torque.
The cogging torque therefore makes it more difficult to master challenging servo tasks, for example, micro or nanometer-precise positioning or tasks with crawl speed. In such cases, servo motors with minimum or without cogging torque are required.
The cogging torque or rather its effect, is not desirable in most electrically operated machines. However, it is used purposefully in stepping and reluctance motors.
How strong is the cogging torque?
The strength of the cogging torque depends on multiple factors. On the one hand the magnetic force of the permanent magnets and the number of poles. In a two-pole motor with three coils and pole shoes, for example, the relationship between the magnets and pole shoes is non-linear. This leads to a smaller cogging torque than in drives with a linear relationship – for example, in a four-pole motor with six coils and pole shoes.
On the other hand, the cogging torque is also dependent on the size of the air gap between the permanent magnet and the pole shoe. The larger this gap is, the smaller the cogging torque. Other influencing factors are also the shape and size of the magnets and the pole shoes. If these have an angular shape and are roughly the same size, the detent torque is also stronger.
How can the cogging torque be minimized?
The cogging torque can be reduced on the one hand by design measures, for example, by arranging the slots in the stator with a slight skew. The transitions between the slots and the magnet edges are therefore not parallel, which results in a smoother transition.
On the other hand, the cogging torque can also be compensated for by control measures. To this end, Baumüller offers a special function in its servo controllers, which is called cogging torque compensation.
The high-precision servo motors of Baumüller’s DSH series offer virtually zero cogging.
Two types of cogging torque-free drives
There are fundamentally two types of cogging torque-free motors:
- motors whose winding does not have an iron core, e.g. three-phase asynchronous motors
- motors whose winding has a back iron that is not slotted
Iron-free AC asynchronous motor
Permanent magnet synchronous motor PMSM / brushless DC motor BLDC
In an iron-free drive, or rather in its stator, no eddy currents are produced, which is why it is able to run faster. It has a lower torque density, however, this can be balanced by a higher power density. Iron-free drives are also characterized by their lower weight.
Cogging torque-free drives with slotless back-iron offer a higher torque density compared to iron-free drives, however, they cannot run so fast due to the iron losses.
You want to know more about servo motors and their cogging torques? Then get in touch with us!
Matthias Beetz
Training Engineer Academy I Baumüller Nürnberg GmbH