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Electric bike motors

  1. Apr 7, 2016 #1
    I have been reading up on electric bike motors, there are a few things regarding size and wire gauge that have me confused.
    I have seen disassembly videos of around 300 watt electric bike motors with planetary gears, and I can't see how anyone can push through 300 watts through the wires without it overheating. The wire gauge seems to be around 16-20. There doesn't seem to be nearly enough wire length to offer sufficient resistance as well.
    What am I missing here?

    One of the videos:
     
  2. jcsd
  3. Apr 7, 2016 #2

    Merlin3189

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    300W is power. You push current through wires. What current did you have in mind?
    Heating is caused by I2R, so in a motor, you do not want resistance. The less resistance the better.

    What you are missing is that the useful power output comes from the product of back emf and current. The product of resistance and current squared is the wasted power.
     
  4. Apr 7, 2016 #3
    I presume the voltage is around 24v while current js around 12A. What I am confused about, is how the wires shown in the video don't overheat, or short circuit the battery. In order for the battery not to short, the the wire resistance must equal the calculated resistance for the circuits, but there doesn't seem to be enough wire to provide that resistance, assuming only one coul is energized at the same time.
    Are all/some of the coils energized at the same time? That may account for the seemingly high current and voltage. In that case, how is current and voltage distributed to each coil?
     
  5. Apr 7, 2016 #4

    Merlin3189

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    I think you're right about the current and voltage, but I think that would be when the motor is running at full load.
    That would be so only when the motor is stalled. When it is running at a normal operating speed, most of the applied voltage is opposed by the back emf. Much less of the voltage is used to overcome the resistance and much less of the electrical input power is being dissipated as heat in the windings. If the motor were stalled and the full voltage applied, then it could indeed draw much more than the 12 amp and the winding would overheat.
    I know very little about these motors and the control units, but I would suspect there is some current limiting by a switch-mode controller. Also I think on some bikes the motor cannot be energised unless the bike is moving and you are pedalling: so power should not be applied to a stalled or near stalled motor.
    These days I would not be surprised to find the controllers are very intelligent and only provide safe levels of drive under all operating conditions. The 300W at 12A would be the maximum possible output when peak power is being supplied. At higher speeds power and current supplied would reduce due to increasing back emf with speed and at lower speeds I would expect the current to be limited by the controller.
    Presumably there are multiple windings which are, as you say, energised in sequence (when the motor is rotating), but I don't know any specifics. I shall have to see what I can find out about both motors and control units.
     
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