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Due to certain circumstances I need to run my brushed DC motor in reverse (positive to black wire, negative to red wire).

What I noticed is that when set up this way running at no load requires 1.5A. Running it forwards, as I assume is supposed to be run, requires 0.9A and judging by the difference in pitch I'd say is running quite slower than in reverse.

I did a quick search and found out that this is some hardware setting on the motor which increases torque in one direction at the expense of torque in the other direction. Since I'm not all into that I wouldn't want to disassemble the motor and could live with difference in torque but I'm having trouble figuring out which direction is set up with the higher torque.

AFAIK higher current means higher torque, so in my case my motor will have higher torque in the reverse direction, which is even better for me because that's how I need to run it. However the motor was sold to me originally as an e-scooter motor, which doesn't make much sense, why would they increase the torque in reverse on such a motor which will barely ever be run that way given it's original purpose?

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  • \$\begingroup\$ Your question title says it's faster in reverse, but the question body says slower. Which is it? \$\endgroup\$
    – Hearth
    Commented Oct 30, 2022 at 14:09
  • \$\begingroup\$ Does it have a built-in cooling fan? It might be designed to run in only one direction. \$\endgroup\$
    – Mattman944
    Commented Oct 30, 2022 at 15:22
  • \$\begingroup\$ @Hearth Idk it's probably my wording or something, as Transistor also got it this way. I tried to say that it's slower when rotating forwards. \$\endgroup\$
    – php_nub_qq
    Commented Oct 30, 2022 at 18:49
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    \$\begingroup\$ Can you tell us the motor's model number and specs, and/or post a photo of it? \$\endgroup\$
    – Bruce Abbott
    Commented Oct 31, 2022 at 3:50
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    \$\begingroup\$ As an alternative to flipping the motor around (which you say requires removing another component), could you instead just offset it slightly and use a pair of small gears to reverse the spin? \$\endgroup\$
    – Darrel Hoffman
    Commented Oct 31, 2022 at 17:30

3 Answers 3

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I did a quick search and found out that this is some hardware setting on the motor which increases torque in one direction at the expense of torque in the other direction. Since I'm not all into that I wouldn't want to disassemble the motor and could live with difference in torque but I'm having trouble figuring out which direction is set up with the higher torque.

This is called phase advance. Some brushed motors have a mechanical phase advance which optimizes in running in one direction over the other. Normally, it looks like a disc on the motor backplate that is held captive by some screws in some curved slots that allow limited rotation. You loosen the screws, rotate it some amount and then tighten it again. Being in the middle is the neutral phase advance where it runs the same in both directions.

"AFAIK higher current means higher torque, so in my case my motor will have higher torque in the reverse direction, which is even better for me because that's how I need to run it. However the motor was sold to me originally as an e-scooter motor, which doesn't make much sense, why would they increase the torque in reverse on such a motor which will barely ever be run that way given it's original purpose?"

But this would not be expected of phase advance. The optimized direction should consume more power, produce more speed, and be capable of more torque.

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    \$\begingroup\$ Some cheap (i.e., "can") brushed DC motors have the phase advance built in, and you can only adjust it by hacking the backplate. \$\endgroup\$
    – TimWescott
    Commented Oct 30, 2022 at 17:54
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    \$\begingroup\$ @php_nub_qq Yeah. How are you defining reverse to begin with? Is forward just the direction you want and nothing labelled on the motor or in its spec sheet? Have you tried running it under load? Like with a fan or friction wheel or whatever? \$\endgroup\$
    – DKNguyen
    Commented Oct 30, 2022 at 19:05
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    \$\begingroup\$ If this is a used motor, it's possible that the brushes have worn more in one direction, and when reversed this could cause phase advance or lag. If you can access the brushes, take a look at how they are worn, and you might be able to turn them 180 degrees to favor the other direction. \$\endgroup\$
    – PStechPaul
    Commented Oct 30, 2022 at 20:35
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    \$\begingroup\$ @DKNguyen I don't see any labels on the motor. It has a red and black wire. I'm assuming + to red - to black is forwards and the opposite is reverse. I'll be able to load it only after I mount it and I'm trying to figure out whether I can mount it in reverse via this question. Otherwise I'd have to remove another component, that is in the way, which I wouldn't want to do, thus I mentioned I need to run the motor in reverse. \$\endgroup\$
    – php_nub_qq
    Commented Oct 30, 2022 at 22:14
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    \$\begingroup\$ "...is optimized to run in reverse? Since it goes faster..." In this context faster isn't better. All else being equal, a lower torque constant means less efficient operation. Just because the no load speed is faster doesn't mean the motor will put out as much actual power at a given speed in reverse as it can going forward. \$\endgroup\$
    – TimWescott
    Commented Oct 30, 2022 at 22:30
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AFAIK higher current means higher torque

Well...

For a motor whose torque constant is, well, constant, that is the case.

For a motor whose commutator timing is not neutral, I'm almost certain that the effective torque constant is higher in the "preferred" direction (forward) and lower in the reverse direction.

Because a motor's torque constant is closely related to it's back-emf constant (in an ideal DC motor they're equal), and because (usually) a faster no-load spin means a lower back-emf constant, the motor spinning faster in reverse actually indicates that it'll generate less torque at speed in reverse.

Just to make life more complicated, this whole commutator timing bias thing really is about timing, so the impact it has on motor performance is speed dependent. At low speeds, I would expect that a motor with non-neutral timing will operate pretty much the same in both directions. It's only as the speed goes up, and the armature current starts lagging the commutation events, that the effect of a timed commutator will start showing up as a change for good or ill in the motor performance.

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  • \$\begingroup\$ Well that seems right, and is the opposite of what DKNguyen is saying, if I'm understanding correctly. \$\endgroup\$
    – php_nub_qq
    Commented Oct 30, 2022 at 22:17
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AFAIK higher current means higher torque, so in my case my motor will have higher torque in the reverse direction, ...

In your case you are running unloaded so it's telling you that it requires more power to run in reverse.

... which is even better for me because that's how I need to run it.

I don't think so.

... why would they increase the torque in reverse on such a motor which will barely ever be run that way given it's original purpose?

They haven't. You've misinterpreted your readings.

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  • \$\begingroup\$ Oh.. I missed the obvious, obviously. But then again I'm pretty confident it is running at a noticeably higher speed in reverse, so it only makes sense to require more power? \$\endgroup\$
    – php_nub_qq
    Commented Oct 30, 2022 at 10:52
  • \$\begingroup\$ I read your post as saying it was slower in reverse. Remove the "accept" for a day or two to encourage better answers. \$\endgroup\$
    – Transistor
    Commented Oct 30, 2022 at 11:43

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