speedmd wrote:Ten percent torque increase is what in current and heat transfer? Looks impressive to say the least. Reading the link a bit makes me wonder if it can not be dialed in so that it works best (moves into the gap) at higher current levels and spins mostly out of the way as the revs increase via centrifugal force and current also drops . Mildly magnetic may keep it from wanting to find a way out also. Looks like something worth exploring a bit.
Speed doctor, the paper is written that rpm/volt decreases. Whatever % Kv decreases is the same as saying Kt (torque per amp) by that same %, because Kt in Nm/amp = 9.549/Kv in rpm/volt. This simple and direct relationship isn't commonly understood, because it means motors with identical Kv's make the same torque per amp regardless of size. It's just that for the same Kv a larger motor will have thicker copper, so it can handle more current, and a bigger stator for more torque making magnetic force before reaching saturation. I apologize if you already understand it, but I repeat this stuff as often as possible, because less than 1% of members understand the implications of these relatively simple facts.
speedmd wrote:Ten percent torque increase is what in current...?
It's an increase in torque with no increase in current. The only "cost" for the increased torque is the added frictional losses called windage, because the ferrofluid has higher viscosity than air. I'm thinking that I'd want to fill in the gaps between the stator teeth with a non-magnetic material to make the stator perimeter as smooth as possible to minimize these losses.
While the author mentions a 500rpm limit of good benefits, he used a small low pole count motor, and I wonder how that would apply to our hubmotors. Is rpm a good guide, or should we look at operating frequency or velocity in the air gap instead?
In terms of heat transfer, I don't think we'd want to use the minimum liquid required to obtain the torque benefit, because that would create a low resistance to heat transfer only between the stator and magnets. I for one don't want to pass the bulk of the heat loss through the magnets. That means more oil fill to obtain a good heat pathway to the side covers, where I'd add more outside surface area to move heat faster to the environment for lower temperatures. That's were every oil fill I've seen falls down, because getting heat to the side covers quicker is mostly meaningless as the system reaches a stabilized temperature, since to remove heat faster either the outside surface needs to be hotter or surface area needs to be greater.
