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I have a box full of DC motors that I've been scavenging from old electronics. Most of them have no labelling whatsoever, so I'm lost at determining what voltage and or current would be adequate to drive them.

I connect them to my power supply and increase the voltage to get the minimum, and after they start spinning, I can keep increasing the voltage and see them spin faster, but, how can I know if I'm within the range of the motor and it's not going to burn if I keep that voltage for a while?

Although the questions is generic, I'm talking small motors here, so voltages from 3V maybe up to maximum 12V or 18V.

Is there a test or rule of thumb or relationship between voltage applied and current drawn that I could use? I'm testing them without load but I guess could test with load if needed.

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    \$\begingroup\$ You can’t actually “know” because nobody can know unless the actual designer happened to live on your same street and recognised the motor. You can guess of course but, in my long standing experience as an EE, my only option would be to regard them as junk and try and recycle them through the normal local services available to you then, buy what you need for the job in hand. Your profile says you are CTO at Telefonica so spending a few quid on a new motor shouldn’t be a worry plus, as CTO, you should really know better. \$\endgroup\$
    – Andy aka
    Nov 20, 2019 at 16:46
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    \$\begingroup\$ this has to be the least constructive comment ever. Electronics is a hobby for me, I don't do this to save pennies in components. Believe it or not I actually find satisfaction in disassembling and reusing stuff, and ... wait for it.... learning stuff!! \$\endgroup\$
    – palako
    Nov 20, 2019 at 16:50
  • \$\begingroup\$ Well, learn this, a pro EE wouldn’t reuse an unmarked component whether it’s a motor or a resistor, unless there was a threat from a death star and the component in question might just get the Death Star busting laser gun working in double quick time. If you don’t think that’s helpful so be it. \$\endgroup\$
    – Andy aka
    Nov 20, 2019 at 16:58
  • \$\begingroup\$ As Andy says, there is no way to know aside from making note when you disassemble the motor. \$\endgroup\$ Nov 20, 2019 at 17:36

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The voltage and current rating of a motor, relay, solenoid, etc. is based on a variety of factors:

  • wire gauge used for each winding
  • insulation on windings and connectors
  • materials used in construction
  • allowed temperature increase (which is related to chosen materials)
  • duty cycle (energize-time versus off-time)
  • expected lifetime and abuse (used in an office appliance? a commercial kitchen? an industrial machine?)

For example, if you're dealing with a small motor salvaged from an office printer, it may only operate "once in a while" and the printer manufacturer chose a motor with relatively low duty cycle. This could imply that it can be driven with some voltage and current for a short time, but wouldn't survive lengthy or continuous application. The motor manufacturer might have built it to specification, which might mean a relatively small wire gauge, less cooling/heatsinking, and so on — to meet some price point.

An inexpensive fan motor might be designed for continuous operation, but has little torque and no cooling method (the designer knew the attached fan will provide airflow).

I have lots of salvaged motors and I use them from time to time in unimportant hobby projects. My method is to slowly increase voltage on a motor and measure the current and temperature. Not precisely, mind you, but in general to assess whether I think the motor housing is getting too hot, or if the motor sounds abnormal. There's nothing very scientific about the approach, it's just experience and best-guesses. As long as you aren't using the motors in anything important, where failure is no big deal, you should be able to obtain some good results and experience. But keep a fire extinguisher near!

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  • \$\begingroup\$ Thanks JYelton. That looks similar to my current approach. Very small motors, so no fire ext needed hahaha. I already burnt one by just testing it with a 9V battery, and all I have is one less motor. Very informative about the different usages of motors, thanks! \$\endgroup\$
    – palako
    Nov 20, 2019 at 17:36
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When you salvage, you should study the original equipment to see if you can determine the voltage. You might try to salvage the circuitry that controlled the motor. You can estimate the maximum power by measuring the motor and comparing motors of the same size. One problem is that motor size is more related to the torque rating than anything else.

Increasing the voltage with no load, will increase the speed, but not necessarily make the motor get too hot. With excessive speed, it is more likely to vibrate excessively and be damaged mechanically particularly by bearing failure.

Increasing the load torque will make the current increase. Excessive current will make the motor get too hot. To some extent, what is too hot could be determined by comparison with motor of known ratings.

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  • \$\begingroup\$ Thanks Charles. Yes, sometimes I also kept the circuits that drove the motor. Others, like is the case with a current project, is not even salvaged, is a mini water pump that I bought for a plants watering system, and left it there for years. Now I've decided to finally build it, and I didn't keep notes of the specs of the pump. But I take from your answer that is definitely best to test under load. It makes all the sense, thank you! \$\endgroup\$
    – palako
    Nov 20, 2019 at 17:38

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