Over the last couple of years I have been experimenting with direct-drive (DD) hub motors on my bikes, building them into ISO 559 wheels (26" MTB wheels). Since I live in an area where there are many hills, I have found the advent of Statorade, Hubsinks, and motor braking to be beneficial.
My recent experiments have been with the Edge1500 hub motor as sold by LA E-bike. I laced one into a 559 wheel, then tested it on a specific loop ride in its stock configuration, a second trial with Statorade added, then a third trial with Hubsinks added. My fourth trial has taken another instance of this motor and laced it into a smaller ISO 406 (20", BMX) wheel, running the same test course and conditions. The details of my experiments using this motor can be read here:
https://mrbill.homeip.net/hybridBike.ph ... 500Testing
In summary, the net efficiency of this motor laced into a 406 wheel appears to be about 20% greater, and motor overheating, even with the insufficient amount of Statorade in my 406 wheel, has not yet occurred. Some of the of the efficiency gain is due to the motor operating at a higher electrical RPM, but most of the efficiency gain appears to be from the greater amount of regenerated energy that can be captured by the system.
In a larger wheel the motor spins more slowly, leading to lower energy recapture with a given controller. To achieve the same braking force at slower speeds, the controller transitions from regenerating to plugging as speed slows from 30 to 12 kph. Using a 559 wheel, motor braking at speeds over 30 kph is regenerative, while motor braking at speeds under 12 kph is mostly plugging. Plugging consumes energy from the battery, converting it into heat in the motor instead of charging the battery.
That leads me to believe that a motor with, say, 29 pole pairs laced into a 559 wheel would appear to the controller indistinguishable from the 23 pole pair motor laced into a 406 wheel with both configurations enjoying the same improved efficiency and lower operating temperatures.
My experiments suggest a couple of questions that I submit to the E-S community:
1) Why do we not see a wide selection of DD hub motors with more than 23 pole pairs, e.g. 28-30 pole pairs, that allow such a DD hub motor in a 559 wheel to enjoy the same efficiency and lower operating temperatures one sees with an otherwise identical 23 pole pair motor laced into a 406 wheel?
2) Why does there exist a lower practical RPM limit (~20-30kph in a 559 wheel) at which a controller can recapture regenerative energy yet still maintain high braking force without plugging and generating heat? Is this a controller design limit, a device design limit, or a Law of Physics?