Wire gauge for rewinding - Very confused

[kkEdlund]

I have an SK3 6354 outrunner that I would like to rewind, mostly for the experience. From what I have read here and on the RCTech forums, there are two competing schools of thought. Some claim that numerous parallel strands of smaller wire are superior to fewer, larger strands due to increased surface area while others maintain that having fewer strands means less space wasted as insulation and air gap, improving copper fill. I understand that increasing copper fill is generally beneficial to motor performance, and is a good thing. I don't really understand the concept of stator saturation, but I guess there is a theoretical limit beyond which more copper does not really help... Need to read more about that...

Anyway, having read the wikipedia article on skin effect it is my opinion that our controllers do not switch fast enough (maybe 12 - 24 khz?) for skin effect to matter, and you would be better off with larger wire strands. Does anyone have practical experience to share regarding how large becomes too large (ie hard to bend, etc...), or even supporting the theory of using lots of tiny wires? I am going to order some wire from McMaster or gobrushless, and I am pretty sure I can follow the documentation here and on other sites to wind it properly. I'm just not sure how to figure out what size wire I need to get.

Thanks,

Kristoffer

 

[Dauntless]

Once you start deviating from the conventional wisdom with an opinion of your own, all anyone can do is either restate the conventional wisdom or say "Sounds like a kewl experiment." Is skin effect relevant in DC?

I would think people could better theorize pushing the envelope if you told what you hope to accomplish with the rewind. How hot to you expect the motor to get?

I guess it's safe for me to add that the RC world feels that bigger gauge/fewer winds-strands works better, while industrial used leans to the highest copper fill. I learned this be reading about the work of others, when I was a kid rewinding slot car motors I'm sure I did what some magazine article told me to do, I don't recall how I made those decisions.

http://en.wikipedia.org/wiki/American_wire_gauge

http://www.easa.com/sites/files/resource_library_public/EASA_AEMT_RewindStudy_1203.pdf

http://www.csiro.au/outcomes/energy/powering-transport/in-wheel-motor-for-solar-powered-electric-vehicles-technical-details

 

[Dauntless]

I just learn so much more reading about what other people want to do than trying to research ideas of my own.

I find it interesting on the 6354 that the kv215 has 22 turns, the 245kv has 20 and the 260kv has 18. Not the most linear progression.

Here's from an RC board discussion on kv decisions.

I thought you were at 12 turns per arm DLRK, is that not right? The math for changing the Kv would be:

winds_new = winds_old * Kv_old / Kv_new

If your Kv is 1000 now:

12 turns = 1000 Kv

If you want to change it to 500 the math would be:

12 * 1000 / 500 = 24

And a how to.

http://www.rcgroups.com/forums/showthread.php?t=1249657

 

[Thud]

Here is a great tool for comparing motor winds & determining how much wire of a given size will actually fit your stator:
http://www.drivecalc.de/

use the English version & in the "tools" section you will find the stator section to make the compare charts that look like these:

http://endless-sphere.com/forums/viewtopic.php?f=30&t=20618#p300796

 

[drebikes]

@OP: I'm doing finite element simulations now to choose the wire gauge for a electric motor design; not ebike, but the reasoning is similar. Basically both schools of thought are right - more copper is better IF the frequency is low enough, but also more copper surface is better if the frequency is high enough. The compromise is something to this effect: one large wire may be bad, two better and up with the count until you hit the sweetspot, then adding more wires while reducing the gauge and diminishing the filling ratio degrades performance.

This was to explain a bit the "why"; about the "how", first thing to notice is that all things being equal, the switching frequency doesn't matter. What does matter is the fundamental frequency of the motor. Say the inverter switches @24kHz, but the motor is an 28 pole pair 500 RPM, then the fundamental frequency is 500/60*28 = 233.3Hz

Above you have the numbers, but how to interpret without finite elements? I found in the motors I'm working on that under 1kHz fundamental it's ok to use think wires, even bars - so if by any chance you're under 1kHz, then just put as much copper as you can. Another thing to maybe consider is that thick wires are harder to wind, so you may end up with larger and heavier end-turns; last thing: if the motor is thermal constrained, more copper filling means better heat transfer towards the casing because copper transfers heat well radially, while varnish and air insulate.

cheers

 

[kkEdlund]

Once you start deviating from the conventional wisdom with an opinion of your own, all anyone can do is either restate the conventional wisdom or say "Sounds like a kewl experiment." Is skin effect relevant in DC?

I would think people could better theorize pushing the envelope if you told what you hope to accomplish with the rewind. How hot to you expect the motor to get?

I am mostly trying to understand what conventional wisdom says, and why exactly that is the case. This is a motor for a longboard, and its performance characteristics are adequate in current form. However, reducing the kV from its current 260 to the range of 190-210 would make it easier to have appropriate gearing and I think this is a reasonable goal. My expectations for rewinding are to better understand the theory behind what makes a given motor more/less efficient than any other, and to reassemble mine such that it is operational.

Here's from an RC board discussion on kv decisions...

And a how to.

http://www.rcgroups.com/forums/showthread.php?t=1249657

I am familiar with those formulas, and have read several how-tos. The one you linked was new to me, however, and I've never seen a motor wound like that.

For a total amateur such as myself, I found this thread very informative:
http://www.rcgroups.com/forums/showthread.php?t=666877

Here is a great tool for comparing motor winds & determining how much wire of a given size will actually fit your stator:
http://www.drivecalc.de/

use the English version & in the "tools" section you will find the stator section to make the compare charts that look like these:

http://endless-sphere.com/forums/viewtopic.php?f=30&t=20618#p300796

Very cool links. I will look through both. Thanks.

...Basically both schools of thought are right - more copper is better IF the frequency is low enough, but also more copper surface is better if the frequency is high enough. The compromise is something to this effect: one large wire may be bad, two better and up with the count until you hit the sweetspot, then adding more wires while reducing the gauge and diminishing the filling ratio degrades performance.

I guess that is the question I am asking. I don't have the benefit of experience to know where that sweet spot may be, and I am looking for a good starting point. Between the 2 schools of thought, tiny wires vs huge ones, I have no frame of reference to decide for myself where to begin.