1

On one DIY eBike I currently have a 36v 250w rear hub kit. It's a "cassette" version, not "freewheel". Problem is, the internal freewheel mecanism of the wheel is very damaged (still barely works for now, but at start I have to pedal before using the throttle, otherwise the freewheel doesn't "grip", and it makes heavy noises, more and more over time...). Sellers had warned the "cassette" version is weaker then the default "freewheel" version...

So, I'm considering to just replace the wheel and see how long the new one works.

The thing is, I would like to take the opportunity to upgrade to a motor a bit more powerful. A 48v 500w would be nice, but my other eBikes are currently 36v, I would have to additionally buy 48v batteries (which would be very costly as I ride a lot and need a lot of capacity). Also I would have to manage 36v and 48v bikes and batteries, which I know is tedious. Also, 48v controllers are larger and I would have to adapt the bike. Finally, I already own a lot of spare 36v components (controllers, displays, etc) but not much 48v components.

I considered 36v 350w motor, but it wouldn't be much of an improvement (if any...). I could also pick a 36v 500w motor, but I don't feel 36v is suitable for 500w, 48v/500w would be a much better choice.

Thus, I thought of buying a 48v 500w wheel, and use it with my current 36v controller and batteries. The limiting part would be the controller, and I could upgrade the bike over time, first by trying a more powerful 36v controller, then by doing the switch to 48v controller and batteries.

Would it work? Am I right stating a motor with bigger voltage and wattage is simply sturdier, and can handle lower voltage and wattage just fine, the limiting part being only the controller?

4
  • Note that, in some cases, a 48v motor is less powerful than a 36v motor. Jul 28, 2021 at 18:25
  • Maybe, but I'm just interested in replacing a 36v/250w motor with a 48v/500w one, keeping my current 36v 6-mosfets controller and 36v batteries at a first step. On paper, such a motor could handle the current provided by my current controller, while having room for speed up by upgrading to 48v power source. I'm pretty confident such motor is more powerful (actually, I see a lot of ebikes around me, these 48v/500w are damn fast already, and everything above is just insane). Jul 29, 2021 at 3:04
  • 36v/250w is 6.9 amps. 48v/500w is 10.4 amps. How much can your battery and controller supply (without burning out)? Jul 29, 2021 at 12:31
  • The controller provided with my 36v 250w kit is (supposedly) rated 10A, max 20A. That seems a bit high. I guess they bundle the same controller with 350w and 500w motors. It's possible the stated values aren't that legit, the same controller can be found on the market with various stated currents. It's also possible my 250w motor has been overpowered by the controller, electrically it was working but mechanically it overstressed the wheel. That's another reason I'm considering a 48v/500w wheel, it's possible it has a sturdier conception. Jul 30, 2021 at 1:32

1 Answer 1

4

You’re quite likely to burn out the new wheel by running it on a lower voltage than what it is specified for.

Power dissipation is I^2 * R

For a given motor power output the amount of current (I) is increased by a factor of 1.33 (48/36). The amount of heat generated by the motor is increased by about 1.77 (1.33^2)

If the windings can’t dissipate or tolerate that extra heat you’ll end up with an impromptu mobile barbecue.

The controller might deal with the situation by reducing available power.

1
  • If it depends only on intensity, a 48v 500w motor can handle about 10A (and probably more in practice...). At 36v, 350w runs at a bit less intensity, and 250w runs at a tiny 7A. According to your explanations my plan should work, I just have to rule out a too powerful 36v controller. For the time being, I could run the 48v 500w wheel with my tiny 36v controller, while being ready for a future 48v controller+battery upgrade. Jul 28, 2021 at 13:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.