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Twosome, threesome, forsome...

Inq

Well-known member
#1
I'd like to understand the fundamentals of these cheap 30A ESC that come with the "kits" below. I think I understand the three leads going from the ESC to the motor are the different phases and each has a sine wave signal going across it to drive the motor. Does the ESC have circuitry that "senses" the motor's position or something back through the same wires? I've heard the term EMF feedback, but don't really know what it is or if that has anything to do with this or not.
1671891338075.png

I guess what is driving my Inquisitiveness, is how is the same ESC able to work on these 2212 motors that come in different flavors of 1000, 1400 and 2200KV. How does the ESC know what's hooked up? How does it know what rate to send the sine wave frequency depending on how the prop is loaded climbing and diving or even what prop is attached?

Now... for where I was heading with the title... I'm thrashing something around in my head about making a gearbox so I can swing scale WW2 type propellers. Big gear on the propeller shaft, small one on motor shaft - easy-peasy. As I sketched it up, I realized, there was nothing mechanically limiting me to the number of motors I attach...
Gearbox.png


Not that I would do this, but could I... say, I'm using 2, 3... 6 of the same 2200KV motor. They'd all be geared at the same ratio, so they would always be turning at the same speed. If I had enough amp capability, could I use one ESC on multiple motors running in parallel off the same 3 output wires?

If you need to throw out the big EE terms, I can do the research and I have a couple of EE's in the family who would be glad to use small terms to explain it to me. :ROFLMAO:
 

Foamforce

Active member
#2
FWIW, my WL-Toys F949 has an arrangement of brushed motors like that. It has a cluster of three of them driving a big reduction gear on the prop shaft.

https://www.aliexpress.com/item/4000529890378.html?pdp_npi=2@dis!USD!US $7.99!$7.99!!!!!@211b446516718974834241485e3c72!10000002695180347!btf&_t=pvid:ad160645-0c74-4f81-ae51-0b2999db826b&afTraceInfo=4000529890378__pc__pcBridgePPC__xxxxxx__1671897483&spm=a2g0o.ppclist.product.mainProduct&gatewayAdapt=4itemAdapt

My understanding is that brushless motors have three different windings. During each phase, one of them has a positive charge, another has a negative charge, and the third is open. The positive and negative push and pull on a fixed magnet. Meanwhile, the open winding actually generates a small electrical charge as the winding passes over the permanent magnet. The ESC senses that electrical charge and uses it to time the switch to the next phase. I think it would be fine to do what you suggest, and what the F949 does, but with a brushless motor. If one of the motors was a little weaker than the others, the others would just work harder but nothing would get out of sync. You would need a separate ESC for each motor though.

Edit: I see you were actually asking about using a a single ESC. Huh. I would say practically not. Maybe if you were able to “time” all the motors very precisely, but I doubt you could do that in practice. There probably aren't enough cogs on the little gears to allow the precision that would be needed, and then if any of them skipped a gear it would all go to pot.
 

Merv

Legendary member
#3
...Does the ESC have circuitry that "senses" the motor's position or something back through the same wires?...
I’m no expert so take my comments with a grain of salt.
Yes, I think the ESC gets some kind of feedback from the motor.

...How does the ESC know what's hooked up?...
I don’t think the ESC knows the Kv of the motor. I think the Kv is determined by how the motor is wired. It’s the number of polls, number of magnets and number of winds each poll has that determines the Kv. I’ve been told you can change the Kv of a motor by rewinding it and changing the number of winds.

5-10 years ago the ESC timing was something you could change. The timing was either high or low and you selected it based on the Kv of the motor. Now all of the ESC’s have automatic timing.

...could I use one ESC on multiple motors...
No, you need a separate ESC for each motor.
 
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Tench745

Master member
#5
Aircraft ESCs do not have a dedicated position sensor; they are relatively open-loop systems. Propellers put a relatively low initial load on the motor so it can accommodate slight mis-matches in motor speed relative to the ESC.
Most ESCs now days have an automatic timing adjustment, so they must be able to sense something, but I don't understand how that works. KV and timing are otherwise not related to one another.
That said, I have had a relatively heavy prop that the motor couldn't overcome the inertia and spin up as fast as the ESC told it to. This leads to a terrible grinding, whining sound as the ESC gets off phase with the motor and the motor twitches erratically. Reprogramming the ESC to a lower timing solved the issue.
As I understand it, brushed motors for RC cars do have some kind of position sensor to keep the ESC and motor in time with one another because they have a relatively high initial load on the motor.

I think one ESC could be used for multiple brushless motors geared together but have no experience with it. Depending on how the ESC sets auto-timing, multiple motors together may confuse it and require you to set the timing manually. Experimentation is probably required. Turning a larger prop will mean higher amp draw and multiple small ESCs may be more economical than a single high-amp rated ESC.

If you want an in depth discussion of KV I found this article. It is not information you need to know to be successful in the RC hobby by any means, but if you're the kind of person who likes to go in depth and try to understand everything, dig in.
https://www.tytorobotics.com/blogs/articles/how-to-calculate-motor-poles-and-brushless-motor-kv
 

Piotrsko

Master member
#7
@Foamforce has the answer. Easy peasy to detect the back emf or the unpowered phase, then it's just a matter of timing to the powered phase but that could even be done with hardware or discretes...

If the multi motor setup has fixed shafts ganging the motors, then one esc can be made to work but you'll need adjustable housings to time everything. Testing the timing is easy, just go for biggest hand powered cogging. All the experiments I have seen says it's just easier to wire one motor to one esc, not like ESC cost a weeks paycheck any more
 

tamuct01

Well-known member
#8
While the idea of attaching multiple smaller motors to a single larger prop looks interesting, I think that first, you would definitely need one ESC per motor as others have suggested. There would be periods of rotation where one or more motors would be fighting the others. It would be much, much simpler to utilize a single, larger motor for the given propeller. The second would be cost, complexity, and weight. Once you buy 6 ESCs and motors you could probably have just bought one larger motor and ESC. The complexity would likely lead to a troubleshooting nightmare and the weight of such a system would likely be more than a single motor setup.

As for why a simple ESC (no feedback mechanism) can drive different KV motors is that the programming in the ESC doesn't care about the pole arrangements in the motor. The ESC just cycles its 3 phases around in a loop and hopes the motor keeps up. Generally, a higher KV motor will have fewer poles so that one ESC phase loop completes more of a revolution than one with more poles.
 

quorneng

Master member
#9
The conventional brushless ESC does indeed have feed back from the motor. It senses the voltage generated from the unpowered coil to 'time' when to change to power the unpowered coil. This voltage is small compared to the power volts so the 'quality' of the motor connections is significant.
There is no problem using 6 brushless motors each with its own ESC geared to a single shaft to drive a big prop but a big single motor can have a low kV so could drive the big prop directly so saving the gearing losses which can be as much as 20%. Taking this into account it is quite likely a single big motor would be lighter too.
A geared set up only becomes the lightest option if the prop is very large indeed, like a helicopter, where a direct drive motor would require an impossibly low kV.
 

Tench745

Master member
#10
Here's a video by Tom Stanton where he powers an e-bike with a racing drone motor. It's a good exploration of gearing down a small, high KV motor.

And here's a video where a guy tests/demonstrates why motors shouldn't share an ESC.