Complete re-design of a second-hand motor

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May 16, 2019
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I'm new here, but have been on this site before for information on EV conversions and such, due to an electric conversion for my 1979 Puch Maxi Sport. The goal is a 12kW hub motor, 6kWh battery pack, custom carbon frame and swingarm, full motorcycle suspension, and full electronics/features to be road-legal. But that's a slow project, and the question for today is for a mini-bike conversion to electric power. It's a Monster Moto MM-B80, and I would like to rip out the stock engine to implement an electric mid-drive system. I've been looking at motors online, and they are far too expensive for this kind of project (just looking to experiment and get some practice before finalizing my plans for the moped).

My knowledge of electric motors is limited, but still more extensive than the average guy in life, and I was wondering if I could take a used motor from Ebay, and rewind it for a totally different voltage and amperage. I would like to take the cheapest foot-mount motor I can find of sufficient size, strip out all the windings and such, and wire it for 48-72VDC, and something like 35-50A. If this is possible, would it matter if the motor was originally AC or DC, brushed or brushless (installing hall sensors isn't a problem afaik), 1/40hp or 4,000hp, single-phase or three-phase? I'd like something like 2000-3000W of power from it, and any RPM that doesn't require absurd gearing. I have my eye on a 115VDC 6.6A motor and I'd like more current and lower voltage. An even better one, if possible, would be a 130VDC 1A motor, but it's a bit small and completely enclosed so I would understand if cooling would be too much of an issue for higher amps.
 
You have to use the same kind of motor that you want to end up with, and it needs to be in the same general power level.

Rewinding doesn't change the way the motor operates, it just changes what voltage it will take to spin it a certain speed, under a certain load.

For example, if you start with a brushed motor, and want to make a brushless out of it, you might as well just buy all the raw materials and tools to design and build a motor from scratch, because that would be easier and faster and probably no more expensive. There's so many differences in the way they work you'd end up replacing almost everything in there. Same for other motor designs--not many are similar enough to convert one to the other at all, much less easily.

(I did once try converting a cieling fan induction motor to a BLDC, and it sort of worked, but would've needed a lot of work to get it into a usable powerful motor).

If you pick a motor that was meant for say, 300w, and need to get 5000w out of it, you'd probably end up needing to liquid cool it to do so. Complicated, expensive, etc. For instance, a 130v 1A motor is only 130w capable. 115v 6A is 690w, so that's way better, but still nowhere near big enough for your needs.


It's much easier, simpler, and cheaper, just to get the motor you want in the first place--there are LOTS of motors out there, used and new, and I can almost guarantee you can find one that is close enough to do the job you want as-is.

If not, close enough that a rewind would fix the kV/kT to make it how you want it to be--but even a rewind of a motor is a messy, somewhat complicated, process, sometimes requiring several attempts to get it right (nicking insulation on the wire and shorting to stator, or just being unable to pack the amount of turns you need in there, etc).


So, if you can't find a $200-$400 motor (or less) that will do what you want, without rebuilding it, I'd guess you might not be looking in the right places. ;) I don't know what "foot-mount motor" means, so if that's a non-negotiable requirement, you'll have to explain it for us to help you find one.


One very easy way to do what you want is to take an ebike hubmotor, put a sprocket on the disc mounts, and drive your chain (or belt) with that. The hubmotor is already intended to run near the RPM your wheel needs to go, so no rewind is needed. Just have to find a motor that can take the power you need. Plenty of threads around here on using hubmotors as middrives.
 
amberwolf said:
long quote
I suppose I definitely am not looking in the right place then. On another note, if you were to run a liquid cooled motor with a powerful water pump that circulates ice cold coolant through the motor at a sufficient rate, is it theoretically possible to crank up the volts and amps a really considerable amount? Like taking a 36V 21A motor and pushing like 50A of 120V through it? Maybe with a rewind for thicker wire? Is heat management really the only issue with high power through smaller motors?
 
By the time you've done any of that you simply could have bought a 4.8kw Cyclone and controller. The batteries will cost the most anyways.
 
Another limitation with a brushed motor is the brushes themselves. You can rewind for a different voltage but the brushes can only handle so many amps. And rewinding a motor is not trivial.

Water cooling has limitations. You can run a lot more power but something will still fail at some point.

You might look at treadmill motors.
 
There are a few posts on here about the Neu 80xx series motor, specifically the 8057. I have tested the 8019, 8038, and 8057. They are pretty inexpensive and ideal IMO for "just getting some practice".
 
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