Power-To-Weight Ratio
Much of the fascination with RC Brushless motors of late has to do with the very favorable power-to-weight ratios they possess. However, if you look at some of the more leading edge AC Induction motors out there they deliver performance that even makes these RC motors look tame:
http://www.acpropulsion.com/tzero/index.php
Now, AC Propulsion produces its unique propulsion technology, tzero technology, for other electric cars. The AC induction motor produces up to 220 horsepower but weighs only 110 pounds. It provides all of the tractive effort, and most of the braking, too. Highly effective regenerative braking recaptures kinetic energy from deceleration, returning it to the battery for later use.
Now let's do that math on this...
220 hp = 165,000 watts (165 kW)
Dividing... 220 hp / 110 lbs =
2 hp / lb
So an equivalent RC Brushless motor would need to weigh about one pound in order to equal the power-to-weight ratio of the bigger AC Induction motors in order to get two hp. For the case of the ebike with it's one horsepower limitation you would "in theory"
only need a motor weighing one half of a pound!. However... it's also true that efficiency of AC Induction motors drops quickly at about 1 hp, so it's not a direct comparison.
I keep looking at different motors, but the industrial preference in design is to use these massive cast iron frames in order to help dissipate the heat. All that cast iron acts like a huge heat sink... which makes sense on some shop floor, but not on an ebike.
So I'm still at the place of "I just don't know yet"... (will it work well or not?)
-----------------------------------
Thinking in the "larger perspective" the only real difference between the RC brushless motor and the AC Induction motor as far as the design is that the RC motor uses magnets for the rotor. Permanent magnets have plusses and minuses... the plus is that they respond instantly to magnetic fields created by the stator and that means instant and synchronous torque. The negative side is that since the response is directly related to the magnets themselves and the magnets create a backEMF it means there is a fixed top speed to the design.
Switching out the magnets and putting an AC Induction motor solid iron or copper squirrel cage as the rotor means that torque is not glued to the stator fileds, but is asynchronous and "slips" behind all the time. So the negative is that torque lags somewhat compared to the instant response you get with permanent magnets. But the plus is huge... on the positive side the lack of permanance in the rotor reaction means that you can just keep driving and driving the rotor faster and faster. The big advantage of AC Induction motor is powerband width.
So it really comes down to:
For peak power within a narrow powerband you use permanent magnets.
For the widest powerband you use a solid rotor and induction based fields.
...obviously the power legal permanent magnet motor desperately needs to use multispeed gearing and the induction motor does not. Given the laws about ebikes it kind of pushes us towards the AC Induction motor because the laws are all about peak power and not about how broad the powerband can be.
It's all chaos at this stage I guess... (design is still in a state of flux)
We've come a long way from when the only choice was a hub motor.