Brushless Motor Cogging Explained – Not what you may think!

Brushless DC motors used in radio control applications contain windings just like every other electric motor. However, what is more unique about them verse an AC induction motor, is that they contain a permanent magnet. The permanent magnet in the brushless motor has the opportunity to interact with the make up of the winding section of the motor. This is where the cogging begins.

Cogging in Brushless Motors used in RC

Cogging in Brushless Motors used in RC

What is Cogging in a Brushless Motor

As mentioned above, we know cogging of a brushless motor starts with the interaction of the permanent magnet on a brushless motor and the windings. Looking more specifically at the winding section of the motor, cogging occurs in a slotted stator. A slotted stator is made up of thin steel lamination’s that are stacked up and the motor windings are placed in the created slots. It is these iron slots that create the cogging torque on brushless motors.

The permanent magnet as it is rotated by the motor shaft, passes by these slots. The magnet then interacts with the steel lamination’s creating that bump that you know of.  If you don’t know what I am referring to here, you can grab any slotted brushless motor and try it out. Grab a brushless motor and slowly rotate the shaft. Depending on the motor, you will feel the strength of the cogging torque applied by the interaction of the iron core and the permanent magnets. I have noticed that 6 Pole inrunners tend to have the strongest cogging torque.

Does Cogging Exist in a Sensored Motor?

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.

How Does Cogging Torque Effect Performance?

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. Most ESC’s have no problem overcoming the effect of cogging torque applied. Once the motor is rotating, cogging does not effect the performance any longer.

Can I see Cogging Torque acting on my RC

Yes, you can. If you have an RC car that is geared very high, it will be easier to pick out. (On Road Application) The easiest way that you would be able to see cogging in action is to get your RC car rolling under its own power and then let off the throttle completely. As the car begins to slow to a crawl you can hear cogging in action. You will only be able to hear it just before the car comes to a stop.

Another way to hear cogging is to push your RC car very slowly along the ground. The car should sound very smooth as you push it across the ground. However, where there is cogging present, you will hear rattling and backlash of the gears as positive and negative torque values are applied to the motor shaft as a result of cogging.

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