john61ct wrote: ↑
Aug 19 2020 5:15am
That said, between the two of you, my case is:
big hills, heavy loads
cannot go too small a wheel because gravel / dirt roads and rough tracks
care nothing for "fun" nor top speed, pure utility transport, even 15-20mph would be fine
range / energy efficiency is only a medium priority
most important is torque, especially standing starts in stop-and-go city commuting, but also
when the load is extra heavy, and/or that long steep hill is not allowing much speed.
Yes, the limiting factor is shedding heat, yes gearing would be nice set those aside for now.
Question: slow wind or fast wind, makes any difference?
Assume I can choose the controller and pack voltage to suit
So your requirement is high torque (for climbing) and pretty good efficiency (for mitigating heat) at relatively low speed.
What you want is a motor with lots of poles, wide magnets, and a large diameter. These things maximize torque capability. But since your question is about windings, let’s assume you are specifically talking about the Leaf 1500W motor.
John is correct when he says that any winding of that motor is capable of the same maximum torque. A fast wind does it at low voltage and high current, and a slow winding does it with higher voltage and lower current. The maximum torque is determined by physical characteristics of the motor: diameter, magnet strength, copper fill, etc. No matter the winding, it will always make the same torque for the same heat, as long as copper fill is equal.
As e-bikers, we have practical limitations as to what voltage and current levels are widely available and economical, and what wire gauges are feasible to work with. These represent blanks in the equation we can fill in right away. Nobody wants to spend thousands on a kiloamp controller, or have to use #0000 gauge cable, if they can use cheap and readily available components instead. So the right, or at least most practical, way to approach it is to decide on a power level (in this case 1500W rated) and a voltage (the motor manufacturer assumes 48V, so I do too because it’s easy to work with).
Immediately, these choices allow us to fill in more blanks. Knowing the power and voltage means you’ll be using a 35A or 40A controller, and your battery must be able to support a current of that size.
You specified a not-too-small wheel, so let’s say 26”. The default winding of the Leaf 1500W motor has published performance data for it. Here’s the table.
So you see its unloaded speed at 48V is 600 rpm, and when loaded at full throttle, it makes 1500W at a little over 500 rpm and a little under 30 Nm of torque. 510 rpm on a 26 inch wheel is 39.5 mph. So you can see the rated power and speed won’t match even on a flat smooth road, let alone a steep hill.
I know (because I asked the folks at Leaf) that the unloaded speed of an 8 turn Leaf motor is 290 rpm at 48V. So... same motor, same efficiency, same battery, same controller. That means at the same power and voltage, you get more than twice as much torque. It will make its rated 1500W power at 60 Nm of torque and about 240 rpm, which is 18.6 mph for a 26” wheel.
Now these numbers aren’t exactly representative of what you might measure on a dyno, because the copper fill can vary by a small amount between different turn counts. But they’re not going to be wrong by much more than the difference in fill percentage.
You could in fact get functionally equivalent performance from the default winding by using 24 volts and 80 amps. But that’s a more exotic and costly controller, and a 6 gauge wire harness instead of 12 gauge. Remember the wires going into the motor are the same size in either case, so they’ll heat up 4 times as much at double the current. It’s not nearly as big a deal to raise the voltage and lower the current on a slow wind motor, to make it run at a higher speed.
In the case you’ve laid out, and assuming the Leaf 1500W motor because that’s our topic, I would decide what’s the lowest top speed that I could accept, and ask Leaf to make me a motor for that speed. I’d get a 48V controller from them too, because it’s sure to work and it plugs right in. And that way, I’d be assured of the most torque that my battery, controller, and operating parameters allowed. What I would not do is order the default hub and then figure out how to feed it less voltage and more current than it was designed for.