I'm designing an electronic speed controller for an electric bike and I recently mounted my wheel and performed some tests. The wheel (an old 24V brushed DC motor) works pretty much exactly how I expected it to EXCEPT there is a large amount of mechanical resistance both when I attempt to turn it by hand and when I hook up my ESC. Here are the values of a preliminary test with the bike mounted (no load):

Preliminary Test Results

The weird part is that there is a little bit of "give" (low resistance) when the wheel is turned by hand for about 6 inches or so. I uploaded a video to youtube of me moving the motor by hand to show this clearly (sound on!). The video doesn't exactly show this but the housing of the wheel is not rubbing anywhere on the fork of the bike.

Does anyone know if the mechanical resistance of the motor might have something to do with the brushes? Or maybe how the wheel itself is mounted? I appreciate any and all answers!

  • $\begingroup$ There is something strange going on there. The motor should turn at much less than 75% of 24 V. You're putting 24 V x 2.5 A = 60 W into the motor and getting no output. Either your measurements are incorrect or your motor is partially seized or has a partial short circuit (which could also explain the resistance to rotation). The free motion indicates excessive play somewhere in the gears. It might be time to strip it down. It's not too difficult but photograph as you go ... $\endgroup$
    – Transistor
    May 7 '18 at 22:40
  • 1
    $\begingroup$ Unless I'm misreading something, when you turn a motor's shaft by hand, you're turning it into a generator, thus generating a voltage. This should create some resistance, since generating a voltage requires energy. $\endgroup$
    – BillDOe
    May 8 '18 at 0:55
  • $\begingroup$ If regenerative braking is enabled in the controller (shorts the DC motor terminals when switched off) then you will find it very hard to turn the motor fast, but possible to back-drive it slowly. If it's geared, it may not be possible to turn at all. Is the resistance present when the motor is completely disconnected? $\endgroup$ May 8 '18 at 14:16
  • $\begingroup$ I'm voting to close this question as off-topic because it's too narrow a topic. Apologies if that's not a rule on this site. $\endgroup$ May 8 '18 at 15:45
  • $\begingroup$ If your question would closed as off-topic, you could re-try on the electronics.stackexchange.com . $\endgroup$
    – peterh
    May 8 '18 at 21:19

Instead of looking at a bunch of numbers, look at a plot to understand the data better:

Note that torque is proportional to current, shown a amps on the X axis. This shows that it take considerable torque to overcome something to initially get going. After that, the speed is roughly proportional to the torque, as expected.

It looks like this is showing there is something in the motor causing significant friction. It may be something is mis-aligned, causing rubbing, a bad bearing, etc.


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