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random notes on motor design, cogging torque, Kv/Kt relations [misc]

BLDC motor torque production:

Before we move into things like cogging, let's just start w/ how a BLDC motor produce torque in the first place. Understanding this also helps us understand cogging and its effects.

Note Jan 15, 2021 (4)
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Above is a little write-up on how the brushless motor produces "torque." Essentially torque is produced "between" two magnets and such. We're mostly here to talk about cogging so let's get to that.

about motor cogging.

Cogging is an important thing to consider when designing a motor, we're just going to provide some basic info over here about the topic: https://www.radiocontrolinfo.com/brushless-motor-cogging-explained/

So cogging of a BLDC motor or any other permanent magnet motor is a phenomenon where the permanent magnets in the rotor attract the "slots" in the stator. The slots are usually made of a powder metal components generally consisting of iron, nickel, and cobalt. Sometimes they're made of steel as well. Essentially it's made of a magnetic material which increases the strength of the field generated by the coils themselves when the motor is on.

Download PDF • 1.78MB

What we can see from this PDF is the fact that cogging is more of a problem at LOW speeds rather than high speeds which make sense but also means it's more of a problem for us when we're mostly operating at lower speeds. It isn't a problem for very fast moving motors so it's something to take into account. It's a problem for us since we start and stop so often. Things that reduce cogging:

  • Skewing the windings, there's less "area" aligned with the magnets meaning less force is produced in general so lower cogging torque.

  • Use fractional slots per pole, (https://www.emetor.com/glossary/number-of-slots-per-pole-per-phase/) if the number of slots/number of poles/number of phases is greater than 1 this is called fractional slots/pole.

You could also use slotless/coreless motors but using those or reducing cogging torque with any of the above methods will cause a reduction in max torque able to be produced by a BLDC motor given a set current.

relations between Kv and Kt.

Another important spec when considering how to spec brushless motors, most of the time we can figure out the Kv rating of a motor and use that to generate the torque constant. Here's a simple relation between Kv and Kt (Kv is easier to calculate and measure).

KV Kt relation
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As makes sense, the higher the Kv the lower the Kt. We want motors with lower Kv's and we can convert to Kt by converting to SI units and taking the reciprocal and the derivation for that is above.

At some point we'll go back and review actually designing a motor based on windings but for now here's a good aggregate of info.

#motors #design #kvkt #forces #omgrobots

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