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  • Choosing a DC motor. Question about amps

    I need some schooling on DC motors. Thinking about building an Arduino winder and choosing the right DC motor and motor controller has me confused. Especially when it comes to power. As an example of my confusion I tried to test the current draw of my winder. When I have it running it seems my power supply meter says ~2A. The kill-a-watt meter I have the PS plugged into says ~.5A and when I measure the amps at the PWM it says ~.02A

    of course when you look at motors on Aliexpress you get all sorts of strange data. The motor I was looking at says it’s a 200w motor that can operate from 12-40v.
    https://www.aliexpress.com/item/3273...c00xOVzXD&mp=1

    is that max wattage or averaged? Who knows???

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  • #2
    Hi Starry,

    The digital multi meter needs to be in series to properly read the load. It looks like it's in parallel. I would trust the DC power Supply meter between that and the kill-a-watt for accuracy. The PWM is the way to go for DC motor speed control. What voltage does it accept? You may be able to use something like a doorbell transformer and a rectifier instead of that DC power supply.

    The motor from ALI-EX is 200w max. That thing will go 12,000 rpm! Wow! It may be overkill. But the chuck/collet is nice. Is that what you're after?

    If you put your digital multi meter (DMM) on the output of the PWM and read voltage, you will see an "average" voltage reading. The reality is the PWM puts out the full voltage but due to the nature of the off/on signal it will show a different voltage. ( you may know that, just thought I would say it anyway)

    I was up in your part of the world last summer. I could have brought you a winder. If I come up again, I may be able to bring you some "stuff"!
    Roadhouse Pickups

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    • #3
      I was inspired to get my PWM winder out. I made this several years ago and wound many many pickups on it. It uses a dedicated DC power supply mounted in the bottom. The DC motor I found also had an encoder on the end of it (black box) that I modified slightly for a 50% duty cycle which I sent the counter signal through. The PWM had its control pot mounted to the board so I just stuck it through the side. I put a volt meter on it which I "tick" marked for certain speeds which I used a non contact RPM meter to calibrate. It worked very well. The round piece on the end of motor outside the box was something I had a local machinist make. It mounts on the shaft and has differently spaced threaded holes to hold different pickups.

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      Roadhouse Pickups

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      • #4
        Hey Ken!
        thanks for chiming in. Perhaps the kill-a-watt is calculating load from 120v and the PS calculating load from 17v. I didn't know the DMM had to be in series, thanks! Strangely I put the probes across the fuse while the motor was spinning and didn't ready any current at all. Not sure what's going on there. That's a slick winder. You PMW module looks much more robust than mine.

        Anyway, I figure the 200w winder i linked to should be an ok choice. If I = P/V then I'm looking at 200w/12v=16amp. I assume that's the motor running at maximum which it says is 5000rpm at 12v.. I could also opt for a higher power supply voltage to bring the amps down but I don't need crazy speed.

        this controller probably is fine for the build:
        https://www.ebay.ca/itm/Cytron-13A-5...r/114238976496

        hey, if you're in my neck of the woods next time lunch is on me.
        cheers,
        Tony

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        • #5

          Perhaps the kill-a-watt is calculating load from 120v and the PS calculating load from 17v.
          Correct and it measures mains input (AC) current, not motor DC current.

          If I = P/V then I'm looking at 200w/12v=16amp. I assume that's the motor running at maximum which it says is 5000rpm at 12v..
          Motor current draw doesn't depend on rpm but on motor loading.

          Strangely I put the probes across the fuse while the motor was spinning and didn't ready any current at all.
          If you left the fuse in, there will be no current through the meter as the fuse is a short and shorts the meter.
          Last edited by Helmholtz; 06-16-2020, 08:41 PM.
          - Own Opinions Only -

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