Heat is always our motors' limiting factor. Power is torque X rpm. Current creates torque and the heat from current is primarily from copper losses, which is resistance X current squared. Heat is also created on the rpm side in the form of iron core losses. The alternating magnet field creates heat in the stator steel and to a lesser extent in the magnets, and it increases with frequency, so the faster those alternating pole magnets pass the stator teeth and the faster the current changes direction in the copper windings the more heat is created. While the iron core losses increase in a generally linear fashion with rpm, as opposed to copper losses that increase geometrically with current, the iron losses occur whenever the motor is spinning whether you're on the throttle or coasting down a hill, so the heat accumulates continuously.
Since the wiring and magnets in our motors are limited to relatively low temperatures compared to the parts in gas burning engines, how quickly we can dissipate the heat created in the motor to the outside world is especially important. That's why you see members going to such extremes to modify our motors.
