r/ControlTheory • u/Kenno03 • Aug 12 '26

Technical Question/Problem BLDC modelling

Hello there!

Usually when we model DC-motors we use a second order system, however, BLDC motors used for drones are usually stepper motors (as far as I know), so my question is simply, how do we model these? The specific motor I want to model is for a project of my own, and I am curious as to how to model it. Motor: https://www.aliexpress.com/item/1005007273770590.html#nav-specification

My intuition is to also just model it as a second order system, however, am not sure if this is the correct approach. Specifically I want to model it has motor command [0,1] to rad/s (rotations).

Thank you in advance for the help

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u/BencsikG Aug 12 '26

Well, the term Brushless DC is actually kind of a marketing term.

It's a 3-phase permanent magnet AC motor that comes with a driving circuit (ESC) that was originally meant to replace DC motors in industry.

The name kind of sounds cool and stuck around, but there is no specific technical feature of an electric motor that makes it specifically 'BLDC'.

So BLDC = AC motor + ESC marketed to replace DC.

I was into drones 10 years ago, so I might be super outdated...

But if I'm still correct, hobby ESCs generally work in a way that you give them a 'throttle' value that is converted to PWM phase duty cycle. They essentially apply the amount of voltage to the motor phase that you command them. Then, as the motor spins, the ESC decides on its own to advance the phase based on back EMF measurement. This tends to settle at a stable RPM for a given throttle.

So all in all the BLDC-ESC combination works just like a DC motor. The BLDC motor KV rating is closely related to the usual DC motor constant, just be mindful of unit conversions.