Z-Force Motor/Limits

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The following regards what engineering constraints limit the power and RPM of the (IPM) Z-Force motor. Pre-IPM motors generated more heat for a given amount of power, so cooling was usually the first concern, but the IPM design pushes the frontier further so the topic is worth addressing.

RPM
The Z-Force motor is design-limited to 6,000 RPM as follows.
RPM is primarily limited by the inductance of the motor windings.
At low RPM, under full throttle, the current through the driven phase(s) of the motor ramps up from 0 to the maximum allowed by the controller pretty quickly, and you get full torque because you've got full current.
But the faster the motor turns, the faster the motor controller has to cycle the phases of the motor on and off, and eventually, you reach a point where there's not enough time (because of the inductance of the motor windings) for the current to ramp up to maximum before that phase has to be turned off and the next phase cycled on.
At that point, power starts to drop off, so very shortly after that, the motor won't have enough torque or power left to drive the load.
The mechanics of the motor could easily have been designed for higher RPM, but there's no point since you'll be current-limited anyhow.
Torque / Power
The easiest way to get more torque, and thereby more power (since power is proportional to torque times RPM), is to pump more current through the motor.
Several things can bottleneck that effort:
  • First, the motor windings have a DC resistance because they're copper wires with fixed gauges and lengths.
  • The bike's wiring external to the motor also has some DC resistance in it.
  • If you apply more voltage to a fixed DC resistance, you'll get more current, but the batteries only give you ~110VDC to play with....and of course, the battery has a current limit of its own.
  • You'll need to be sure the controller can handle the current as well; you won't go very fast once you smoke the controller.
  • Since the motor rotates and therefore the windings have to be commutated (driven on and off in the proper sequence and speed), the motor's inductance also comes into play as discussed above about RPM.
  • You could change some of this stuff fairly easily (higher-gauge wiring is pretty simple, though might be tough to fit in some of the tight confines the Zero's wiring is routed through), but other things are more difficult.
  • To reduce the motor winding's DC resistance, you'd want to use heavier wire, but you wouldn't be able to get as many turns onto the same coil form.
  • Fewer turns would create less inductance, which also helps allow higher current levels, but fewer turns also results in a weaker magnetic field for the same current, so you might actually reduce the torque the motor puts out.
  • Then you'd also need to be sure you don't over-current the controller, or the batteries, and cooling everything involved becomes an additional topic.
It's kind of like asking what's the highest VO2 max an athlete can put out.
  • It's not just about his lungs, it's about his heart, his muscles, his metabolism at a microscopic level, even about his mental attitude.
  • Tweak any one thing and the system's whole balance shifts.
  • That's why engineers make the medium-sized bucks!
Speed
Vehicle top speed can only be altered by the gearing (sprocket ratios).
Power output is not affected by gearing; only torque is.
So, a ratio yielding a higher top speed will result in lower torques throughout the range, resulting in slower accelerations from start.
References
What is the limit of a Zero motor?