Glyn wrote: ↑
Dec 17, 2017 11:21 am
Reading between the lines in your above statement, are you saying that the motors windings dictate it's optimal rpm? This would suggest that the windings can be modified for higher torque/lower rpm?
Yes, but as Chalo pointed out it's easier to lower the voltage and make the wheel smaller, which effectively does the same job regarding speed.
It doesn't change the torque, though; you'd still need either a wider stator (and magnets), or larger diameter rotor/stator, or both, to increase the torque.
(the more wire that runs parallel to the magnets, vs the end-turns, the more work the current thru the wire gets to do, vs how much work is wasted at the ends.
the larger the diameter of the stator/rotor, the more leverage it has so the more force is multiplied for higher torque, all other things the same.)
Having already purchased some motors (too late to cancel now - already been shipped) how easy would it be for a novice to modify the windings?
It's not techically difficult to wind a motor--but figuring out what windings you would need, including what thickness of wire (and/or how many strands of it) will fit in teh stator with the winding pattern you need, and then actually doing the winding, is not necessarily that easy.
As a novice, I've tried to rewind a couple of motors, and I gave up the first time because it was actually really hard to pack the wire in there correctly (all I was doing was a repair on a burned winding on a brushed motor, so no math was involved
). The second time I eventually gave up because I kept nicking the wire and shorting it to the stator (and itself thru the stator). It also hurt my hands to keep pushing and pulling the wire tight enough. :/ I keep envisioning building/rewinding some motors to do exactly what I want on my trike, but it's a lot of work that I don't know for sure would work when I was done (if I screw up or short windings, etc).
But it's definitely possible; you can look up threads where people have done this. "rewind" would be the search term I'd use first.
It's just not the first thing I'd do to fix the problem--I'd use the brushed motors with gearboxes, mostly because I like to overbuild things, not necessarily because it is the best option. Most likely, custom-wound motors would be the "best" solution, but not necessarily the most practical.
FWIW, I'd probably try the stuff you already have first, just to see how it performs. You never know--it might be "good enough" to work, even if it gets hot and wastes power (requiring more battery than you'd like to get the range you need). I tend to try what I've got at hand, learn from what it doesn't do right, and what it does do right, and go from there--but I'm more of an iterative-artistic type than an engineer type.
Before you act on any of my information, you should look thru my build threads and see if you're willing to go thru that kind of process.
I've seen posts on this forum where people have requested custom windings from the manufacturers. Does anybody know how willing they are to do this?
Depends on what minimum order they have; almost certainly you'd have to buy some dozens (or hundreds) of motors to get a custom winding.
If they happen to already have motors wound for the speed you want, you could just buy those.
Or, as Chalo pointed out, get motors (that already have the torque you need) that if put in a small enough wheel at a low enough voltage would spin at the correct speed. So...if you need them to spin at 5MPH in a 20" wheel at 24v, for instance, then get motors wound for xxMPH in a xx" wheel at xxV. The three "xx" parts you can work out proportionally from what's available from a vendor vs what you need it to do. (sorry I'm terrible at math so I cant' give you a formula).
Would changing the windings be able to produce a hub motor which is suitable for my application?
Yes...but probably not *just* changing the windings, unless the motor were already capable of producing the torque you want. The goal of changing the windings is just to reduce the full-loaded-speed of the motor to what you want out if it. It still has to be able to create the torque to start with.
(one way of creating more torque is run a motor at a *higher* voltage than it was listed for, and apply higher throttles to get the higher torque--but this also means it's wasting more power as heat than a motor designed to create that torque with less current, *and* it's going to spin faster at that higher voltage, if left at full throttle).
Motor relationships are not quite as complicated as they seem, but it's easier to see how they work when playing with simulators like Justin's at http://ebikes.ca/simulator
and comparing different ones or the same one with different parameters, using the Compare System A vs System B options.
I see nothing that says what speed it is intended to go at what voltage. You need to know both of those to know if it will work for you, because you'd want it to have a very slow max speed at the voltage you will run it at.
It also says nothing about how much torque it has (foot-lbs or newton-meters) under what conditions. You need to know that because you need higher torque to climb the hill.
are there companies willing to do it?
You can probably find motor-rewinding companies even in your local area (wherever that is), but it will be expensive.
I guess the primary reasons for using a hub motors are space saving (fits within a conventional fork) and simplicity. Buying a hub motor means I don't have to fabricate complicated mechanisms for transferring power from the motors to the wheels.
Its definitely simple, and is the primary reason I'm using hubmotors now (I had various problems using motors to drive the chainline, mostly because I didn't know how to build frames to prevent twisting; I do now, though I still haven't gone back and done the chaindrive thing yet. Is in the plans for the future, eventually).
My thoughts about fabrication apply here. For motors with an output shaft to drive the wheel directly, these seem to be intended for much smaller caster-type wheels. If I attached the 20" wheels I'd lose a lot of torque.
Unfortunately, yes, to both torque and complication of fabrication.
How is this achieved? fans? liquid cooling?
With brushed motors, the easiest way is to put a fan on one end of the motor that sucks air thru it, and put slots on the opposite end for intakes. You can look at various DIY car /etc conversions using forklift motors to see the types of fan rigs used on those, and emulate something similar in smaller format.
So, let's say I was using 15ah 3.2v Headway LiFePo4 cells. I'd need to wire 15 cells in series to achieve 15ah at 48v.
You'd actually want 16s (not 15s), for a typical LiFePO4 "48v" pack.
You'll also probably have to parallel a number of these packs to support the current draw you are likely to need. (or make parallel groups of multiple cells, then series those).
Each cell has a Max Discharge Current (Continuous) of 5C. Wired in series, am I correct in thinking that they have a combined discharge rate of 5C, not 5C per cell?
The C-rate applies to cell or a set of paralleled cells exactly the same, but not seriesed cells. If one cell has 15Ah, then 5C is 75A. If you parallel two, then there's 30Ah, which at 5C would be 150A.
My personal take on Headway stuff in general based on memory of posts around the forum is that Headway overrates them, and the voltage sag you get at the "rated" C-rate is high enough that I wouldn't use them at that rate. But I have no direct experience with them. There are members that have builds using them, though, so you can look up their build threads and see what they have to say. (lots of posts about Headway)