Why BLDC Kv rating? Why not sine wave frequency?

[amasheep]

I understand that it means noload rpm per volt from the battery, but I cannot figure out how that really works..

If a common BLDC motor is really just a synchronous AC motor, then why is not the speed of the motor solely dependent on the sinewave frequency from the motor controller?

Is there some magic happening in these hobby motor controllers that when a higher voltage is applied, it can output higher frequency sine waves?

I am clearly lost here, and hope someone can point me in the right direction.

Best regards.

 

[Addy]

This is why you'll see "Max ERPM" ratings on BLDC motor controllers. It's the maximum frequency (not always a sinewave) that the controller can drive the motor with.

A BLDC motor produces a back-EMF voltage proportional to it's speed. At the unloaded maximum speed, the back-EMF from the motor is high enough to almost completely counter the voltage coming from the motor controller, so very little current can flow and there is not enough torque produced to spin the motor faster.

In this situation, the controller could still output a higher frequency, but the motor wouldn't be able to keep up. When you run the motor controller with a higher voltage, it has more voltage to overcome the back-EMF, so it can can supply current and spin the motor to a higher speed before the back-EMF balances things out.

 

[btdale]

This comment made my brain much larger. Thanks for the clear explanation!

Sent from my Phone 2 using Tapatalk

 

[amasheep]

Phenomenal explanation, Addy! Thank you very much.

This also explains why motors with less windings have higher Kv. Less windings means lower back EMF voltage per RPM and thus higher RPM per Volt.

Best regards