What is cogging in a brushless motor

While these two terms are sometimes used interchangeably, cogging torque generally refers to the phenomenon that causes torque variations, and torque ripple is the effect these variations have on motor performance.

In other words, when the poles of the rotor line up with the teeth of the stator, a force is required to break the attraction, and this force is referred to as cogging torque. Cogging torque is position-dependent, according to the location of the stator teeth relative to the permanent magnets, as the magnets constantly search for a position of minimum reluctance.

Cogging torque occurs in permanent magnet motors, including brushed and brushless DC motors and synchronous AC motors. Sensored motors have been designed as a result of sensorless motors not being able to determine rotor position.

It is critical for the ESC to know exactly where the rotor is relative to the windings to accelerate the motor from a stop. As a result of a sensorless motor, the motor stutters on takeoff.

It does seem that most would contribute this phenomenon to cogging. In fact this is not the definition of cogging. We described cogging as the result of the interaction between the slotted stator and permanent magnets in a motor. A sensored motor only adds a sensor to aid in determining the rotors position.

Therefore cogging does in fact exist in a brushless sensored motor. The only area in our RC applications where cogging torque can effect performance is in the startup from zero RPM. This is more than likely where confusion of the term cogging all began. In a sensorless application the ESC must overcome the additional torque caused from the magnetic field interacting with the stator slots. The ESC must do this as it is figuring out the position of the rotor.

As the ESC sends a pulse to the windings of the motor and moves the rotor as a result, the cogging torque applied changes the position of the rotor slightly. This slight positional change is something that the ESC must overcome and figure out. You can click here for contact details for us and if you have any questions please ask. No products in the basket. What is brushless DC motor cogging and how do I get rid of it? What are the options for eliminating cogging? Firstly, it has to be noted that sensorless motors are much more likely to have this issue than sensored motors.

Therefore, a simple fix may be to look at introducing a sensored version of the same motor rather than a sensorless motor. If this is the case then please read on as there are a number of potential options to reduce cogging with sensorless motors.

Switching to a brushless motor with more poles — the more poles a motor has the smoother it will perform at lower speeds where most cogging occurs. This will typically make startup easier and reduce cogging. If you are using a sensorless motor then a trapezoidal type controller will have less chance of starting up as required than will other types of controller. Check current settings. One common cause of cogging is that the startup current settings are not set appropriately.

This is especially true in applications where startup occurs under load. Use a carefully optimised brushless motor controller.