AC vs DC?
 

Ok, so I've decided to redo my bike project. I want to do an external motor and use gearing advantage (even though motors are efficient, it seems like a lot of these motors peak out in RPM and energy is wasted in torque). But my question is, I see some of the higher powered bikes here using AC motors. It would seem to me that it's a waste of energy to invert DC batteries into AC (10-15% loss) just so I can run an AC motor. Is there some torque advantage of using an AC motor that makes up for it?

Even DC motors `convert' DC to AC -- they just do it mechanically, via their brushes and commutators. I'm guessing that the AC motors you're referring to are also called brushless motors?

If so, they're good because you don't have any losses in the brushes and the ESC can do things like adjust the timing of the motor on the fly to make it more efficient. Ultimately, brushless motors tend to be more efficient than brushed motors. It's not a huge difference for good quality motors, but it's significant.

Brushless motors are MORE efficient than brushed motors (a.k.a. DC motors), not less.

For the most part the size and weight of an e-bike motor determine how much power you can put in to it, because they determine how much heat it can dissipate. (overheating is your limiting factor, except when brushes on brushed motors are the weak link instead).

This also means that more-efficient motors can accept more input power without overheating. (a more efficient motor is one converts more power into motion and less power into heat)

DC brushless - switches DC current to create a rotating magnetic field to pull the permanent magnet rotor around
AC brushless - forms AC current waveforms electronically from a DC bus to create a rotating magnetic field to pull the permanent magnet rotor around

Our motors (for power) are DC brushless. AC brushless requires accurate rotation angle feedback and is only required where smooth and accurate positioning is required - i.e. brushless servo drives

Brushless are not only more efficient but have a wider operating power range. They are not limited by low and high current density capability of commutator brushes and some (particularly sensorless) adjust timing on the fly for different current levels.