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VESC with really high KV

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bengce
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VESC with really high KV

Hi,

i got a 2250KV motor, a 3S LiPo battery and a 4.12 VESC. This is for an rc-car so no massive torque is required. Before i start to roll i just want to make sure that this should theoretically work.

These are my specs:
4.12 VESC min V: 3S
4.12 VESC Amp limit: 50A
4.12 VESC ERPM limit: 60,000

Battery voltage: 3S (11.1V) ~12V
Battery Amps: 180A

Motor: 2250KV
Motor Amps: 5.4A without load
Motor magnets: 8
Motor poles: 4 (-> 2 polepairs)
---

Staying below the ERPM limit (check): 2250KV * 12V * 2 = 54,000
Staying below the VESC Amp rating (check): 5.4A way less than 50A
Supplying VESC min voltage (check): 3S LiPo

Any thoughts on this setup and whether it might or might not work are much appreciated.

hexakopter
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I think it should work fine, although I haven't tested such a high kv motor with a VESC myself. Benjamin himself used the VESC in a rc-car some years ago with a 2200kv 4 pole motor referring to the comment under his video.

https://www.youtube.com/watch?v=3puKPFVUfdc 

 

maxpla
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I tested the VESC 6 on a 2200kV motor. Runs without any problems so far. There is technically no reason why it should not run "high-kV" motors.

I'm also planning to use the VESC in an RC monster truck.

spork
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I thought ERPM = actual_rpm x magnetic_pole_pairs (not electric pole pairs).  No?

 

maxpla
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How are "electric pole pairs" defined? bengce apparently has a 2-pole brushless motor, which is very common for high-kV RC car motors. I don't see a problem in his calculations.

I wanted to note, that the current may be much higher than 5.4A. It all depends on what type of car you drive and how you drive it. Is it an off-road truck or an on-road competition car? What scale?

bengce
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I intend to use this setup on a 1:8 rc-car for autonomous driving so mainly on clean floor and maybe the road, no off-road tracks. I assume that the current could easily double but as long as it stays below 50 the ESC should be fine. Problem is that most other 1:8 high KV motors work on 100+A which would blow the esc.

spork
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How are "electric pole pairs" defined?

You just count up the sets of windings and divide by two.  For magnetic pole pairs you count the magnets and divide by two.  For ERPM I thought you multiply the true RPM by the magnetic pole pairs.  Perhaps I'm wrong about that(?)

 

 

spork
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Looking at a typical brushless motor, the electric poles (numbered in red) come in sets of 3.  So you can end up with pairs, but as in this case you might not.  The magnetic poles (numbered in white) always come in pairs.

https://www.dropbox.com/s/18h4lbj4igwkuo4/pic.jpg?dl=0

But I have to confess I'm confused now by his original post.  He indicates that he has 8 magnets, and 4 poles (or 2 pole pairs).  I'm not sure what the 4 poles (or 2 pole pairs) refers to.

 

bengce
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To be honest im getting confused aswell.

So the datasheet states that this motor "[..] uses a 4 pole/8 magnets (staggered pole) rotor technology [...]". Hence i thought: 4 poles -> 2 pole pairs but apparently there is way more to it and everyone means a different number.

However, 2 pole-pairs would fit the setup (staying below 60,000 ERPM) whereas 4 pole-pairs would be too much already. But then there is always the option to limit the motor voltage.
Assuming 4 pole-pairs, since the motor is rated 2-6S i could limit the voltage and thus the ERPM to 2250KV * 7.4V * 4 = 66,600. Now this is just my understanding so correct me if i'm wrong but those 66,600 ERPM would be the top speed limit right? Now i'm planning to use speeds from 60 RPM to 8,000 RPM which would be way below than what the motor would be capable at 7.4V (2250KV * 7.4V = 16,650 RPM) and thus i should not hit the ERPM limit at any time.

spork
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I'm wondering... is this the motor...

https://www.hobbywing.com/goods.php?id=530

I haven't before heard of "staggered pole rotor technology", so I'm not really sure how many pole pairs this would represent.  I did find an article on it, but haven't yet given it a close look.

 

bengce
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Actually the motor you mentioned seems nice except that it does not feature hall sensors.

The one i have in mind would be this one: https://www.hobbywing.com/goods.php?id=516

Oh and apparently the "pole count" of the manufacturer means the pole pairs already so i got 4 pole pairs not 2.
Source: https://www.motioncontroltips.com/faq-what-is-pole-count-and-why-does-it...

spork
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Oh and apparently the "pole count" of the manufacturer means the pole pairs already so i got 4 pole pairs not 2.

I suspected that's what they meant, but I couldn't find a reference that made it clear.  That is some seriously bad nomenclature.  I'm glad you found the answer.

EDIT:  I just read your link.  I didn't see any reference to staggered poles, so I'm not entirely sure what your pole-count would be.  I did find this...

https://www.semanticscholar.org/paper/Reduction-of-Torque-Ripple-in-Consequent-Pole-Using-Li-Wang/1cbed36dffaf208e653faaaaad4500a58e13590a

 

 

bengce
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Interesting paper you found there, thanks for sharing.

I contacted hobbywing support so maybe they can clear some things up for me.

spork
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I considered doing that as well.  Please let us know what you learn.

 

bengce
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So apparently the motor is not suited for this project. In an answer to my questions i was told that the max current for this motor is about 120A which would be way too much for the ESC.
However, i'm still waiting for a reply on the "staggered pole" technology.

Meanwhile I decided to use this motor: http://www.castlecreations.com/en/sensored-1406-1900kv-four-pole-brushle... with an average wattage of 425W (so at 3S 35A which is fine) and max 850W (70A - inside VESC peak limit). Doing the math the ERPM would be a little too high again but i can limit this in the VESC settings so i don't exceed and damage the DRV chip, right?

spork
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So 1900kv with 3S should give a max RPM (unloaded, with fully charged LiPo's) of 1900 * 4.2 * 3 = 23940.  And 4 poles would suggest 2 pole-pairs.  But again, there doesn't seem to be a fixed nomenclature.

If it is indeed 4-poles, 2 pole-pairs... your max ERPM should be 47,880, which shouldn't be a problem for the VESC.  If by "4 poles" they really mean "4 pole pairs" then your ERPM is more like 95760.  I don't recall the max ERPM for the VESC, but I think it's 60,000(?).

I also don't think that would damage your DRV.  I think it simply means that it will not be able to turn the motor as fast as it should turn with 3S - but I could be wrong about this.

 

bengce
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Found this picture in the Castle product description: https://www.castlecreations.com/img/product/description/images/SM-magnet... but tbh i can't really make out the pole(s)/ pairs.
VESC 4.12 max ERPM is 60,000 yes. Seems like i'll just have to experiment and see if it works or not.

For my application i'm looking for speeds within 60 RPM to 8000 RPM so by simply limiting the speed, current and voltage i should be fine regarding the ERPM anyway.

spork
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To me that looks like an in-runner with 12 electrical poles and 4 magetic poles (2 pole pairs).  And that would certainly be the proper way for them to have described it (in my opinion).  It just seems insane for people to use "poles" as shorthand for "pole pairs".  So I think with 3S your VESC should be fine with this up to its max RPM.

 

 

bengce
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Quick update. All components are now finally here and it does work. Setting the current, rpm and watt limits works just fine so the setup turns out to be okay.

However, i have a really hard time setting up the VESC tool so that the motor will turn smoothly at very slow speeds - but that's something for a different post i guess.