I used a Fluke multimeter to measure the phase resistance of the 2208 Gimbal motor listed on the Supported BLDC Motors page. Measured 16.4 ohms. The exact motor I got from amazon is Fityle 2208 90KV 3mm Shaft Gimbal Brushless Motor for 100-200g Gopro3 RC Quadcopter
Just thought I’d mention it so the supported BLDC motors page could get updated with that phase resistance.
Question to you and forum in general. Most bldcs are star formation, some are delta.
If you have a star and measure 16ohms, should you divide by two as you are effectively measuring two phases?
Strictly speaking, yes. Given the following picture, if you measure resistance from A to B you are measuring phase to phase resistance. If you measure from A to N (which is exposed in some motors) you are measuring phase resistance.
Oh interesting, good point then. I just measured between pairs of leads, so yeah I measured phase to phase resistance. However, I’m using the SimpleFOC Shield V2.03 and it didn’t overheat or anything with this motor despite being under 10 ohms per phase with that new math.
Sorry to add to the confusion, but the 2208 90KV is advertised to have a resistance of 13.3Ω here and here.
I have two such motors. Measured resistance - 12.5Ω.
The BLDCs I’ve removed from old HDDs mostly had 4 pins - they are wye (star) wound with the center point is brought out. When measuring those you can see nicely that one of the pins has half the resistance when measured against the others. That’s the middle pin.
I guess for motor calculations you may need the phase resistance.
But for the purposes of calculating your voltage limit to prevent things burning, phase to phase resistance is fine.
I’ve bought quite a few motors by now for playing with SimpleFOC and figuring out which is best suited. I can say that for motors in that price range, I would neither trust the listings to be correct, nor would I expect a lot of consistency from the motors themselves. Sometimes I’ve found 1Ω or more difference when measuring the different phase-to-phase resistances - of the same motor!
I’m new in the community and new in the motor control area. I have been reading alot about FOC, but still have some fundamental questions that seems hard to find an answer for.
Btw, great community!
In relation to calculating the resistance (power/ current limitation perspective):
Why is it that the resistance is not calculated dependent on the commutation scheme?
I have been wondering about it because, during 6 Block commutation always only 2 phases are used, while when running FOC always all 3 phases are used.
Example in the case of FOC, the resistance would then be the phase resistance * 1.5, because two of the phases are in parallel.
Another question (more theoretical):
The q and d frame motor dynamic equation, includes the phase resistance.
Again, why is that? Because when running FOC all 3 phases are used → multiply phase resistance with 1,5
So I can only give a primitive answer to this, because my understanding of the motor physics is quite simplistic.
From a current limiting perspective, it is good to take the DC view - if you just divide voltage by phase resistance, this will give a upper bound on the current that can flow, and conversely, if you need to limit the current in software, it will tell you the maximum voltage you should allow the software to set.
From a motor dynamics perspective, the motor is an inductive load, and the PWM signal driving the motor can be considered an AC signal, so the motor’s coil exhibits impedance, not resistance. In effect this means the motor’s windings understand the quickly varying PWM signals as “analog voltages”, which depending on the settings and many other factors produce the commutation patterns. In rough terms the smoothed output of the PWM can be thought of as the sinusoidal commutation patterns you see in the pictures.
The phase resistance comes into the FOC algorithm when doing current control (torque control). I’d have to let someone more knowledgeable comment on what value to best use here, and how to best measure/obtain it.