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Any way to determine the design voltage of a random miniature DC motor?

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  • Any way to determine the design voltage of a random miniature DC motor?

    I have a box of little permanent magnet DC motors I've been collecting forever, some bought at surplus stores, some pulled from equipment, none have usable markings to determine voltage. I suppose I could run them up till they burn out, but that kinda defeats their usefulness. Any magic tricks?
    Location: Jersey City NJ USA

  • #2
    A perm magnet dc motor works as a generator if spun with another motor. If you know the rpm of the motor in question and spin it at that rpm it will generate very close to its rated voltage. Of course you need to know the rated rpm and have a means of measuring it when back driving it to use this method.

    Also, if its a gear reduction type motor then back driving it will not work.

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    • #3
      That's about the best. Estimate 10% more supply volts and it should come out right.

      Shunt and PM motors have a "base speed" that they naturally hold, it is where the rpm volts and supply volts balance for the specified load.
      2730

      Keep eye on ball.
      Hashim Khan

      Everything not impossible is compulsory

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      • #4
        Originally posted by Sparky_NY View Post
        If you know the rpm of the motor in question...
        RANDOM box of motor! They can be anywhere from 3k to 10k, or possibly more. Some of those little bastards are fast!

        Location: Jersey City NJ USA

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        • #5
          Originally posted by Sparky_NY View Post
          A perm magnet dc motor works as a generator if spun with another motor. If you know the rpm of the motor in question and spin it at that rpm it will generate very close to its rated voltage. Of course you need to know the rated rpm and have a means of measuring it when back driving it to use this method.

          Also, if its a gear reduction type motor then back driving it will not work.
          IMO Easier just to start with lower speed and use tachometer.
          design rpm is probably also unknown but typically 5000 to 20000 rpm for small dc motors like the size in cordless drills.
          You can extrapolate some info also from manufacturer data. Mabuchi is one very common in coedless drills and toys: https://product.mabuchi-motor.com/search.html?method=4
          Location: Helsinki, Finland, Europe

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          • #6
            Originally posted by gellfex View Post

            RANDOM box of motor! They can be anywhere from 3k to 10k, or possibly more. Some of those little bastards are fast!
            The voltage they generate is proportional to the speed they are spun at. So, for example if you spun the 10K rpm one at 2500 with your drill, it would generate approx 1/4 of its rated voltage.

            Alternately, if you know the rpm of the motor and can measure it, slowly increase the voltage with a bench supply until it reaches its rated rpm, that's the rated voltage !

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            • #7
              Originally posted by MattiJ View Post

              IMO Easier just to start with lower speed and use tachometer.
              design rpm is probably also unknown but typically 5000 to 20000 rpm for small dc motors like the size in cordless drills.
              You can extrapolate some info also from manufacturer data. Mabuchi is one very common in coedless drills and toys: https://product.mabuchi-motor.com/search.html?method=4
              Yes, the volts per 1k rpm should be constant, so measuring anywhere is OK.

              You still need to know a speed to know voltage intended.
              2730

              Keep eye on ball.
              Hashim Khan

              Everything not impossible is compulsory

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              • #8
                I'm in the same boat, except I have a couple of very nice Maxon motors with encoders, and one without. The numbers are an OEM spec and don't show up at Maxon. On the one I've used (for the head servo on a home built EDM), I just ran it at the speed I needed and the current was miniscule, around 30ma. It'll run all day and not get warm but still has loads of torque. That one is actually a gear motor and it doesn't have the encoder.
                Southwest Utah

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                • #9
                  When you say "little motors" are we talking the sort in toys where the motors are about the size of a golf ball? Or the bigger can sizes similar to the size found in a lot of the brushed motor cordless power tools?

                  If it's just the little ones then be warned. The same size cases can hide a wide variety of internals. In the end it might be easier to get some fairly strong batteries like C or D cells and try the motors on 3 then 6 volts. You're looking for a happy high pitched smooth buzz without it shrieking at you. And those that don't run up to a nice smooth fast sort of speed you can try out on the car battery. I know that's not very technical but we're talking about ball parking motors with no markings. So getting an idea of what they run like is the best you can do.

                  I'm also not sure that running such motors off a hand drill to act as a generator will tell the whole story. The higher torque motors will have fewer turns of heavier wire. The lower torque motors will generally have more turns of finer wire. The greater the number of turns the more voltage they will generate when driven at a given RPM. But both might be rated to run on the same voltage. Comments from those that suggest the generator approach are sought on this. But what I just wrote is how I understand this to work.

                  When it's running fairly fast try stalling the output shaft with your fingers or with the aid of a bit of leather. If they are made for light torque applications you'll stall them easily. If they are wound for heavier torque applications it won't stall or you'll need a strong pinch to stall them. Needless to say don't hold them stalled for long.

                  With 3, 6 and 12 volts to work with you should get a pretty good idea of the running voltage and the torque each of them can produce. You're still not out of the woods though. If you want to load down the torque'y motors a lot you may find you are pulling more current then they are rated for. If so the motors will get more than warm in use as a warning that they are not suitable.

                  If you have a few of the power tool size motors then look in through the cooling ports. Odds are good that you can see some of the windings and get a feel for the number of turns. Fewer turns of heavier gauge generally means lower voltage with higher current potential. Some with very fine wire might in fact be intended to run of 50 or more volts.
                  Chilliwack BC, Canada

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                  • #10
                    Given the information supplied so far, it sounds like the most useful thing would be if you know an rpm at which you'd like it to run. Then increase the supplied voltage until you get there. Unless it it an extremely high speed you're looking for (> 10k?) chances are this will do the trick. You could also monitor the temperature as you slowly raise the voltage. I'm guessing it would noticeably overheat before it actually burned out or suffered serious heat damage.
                    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                    • #11
                      I have no idea the rated RPMs of these things, and I'm talking toy sized, cases from .6 to 1.5" diameters. Running them up till they seem unhappy seems a better test of bearings than voltage, no?
                      Location: Jersey City NJ USA

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                      • #12
                        Originally posted by gellfex View Post
                        I have no idea the rated RPMs of these things, and I'm talking toy sized, cases from .6 to 1.5" diameters. Running them up till they seem unhappy seems a better test of bearings than voltage, no?
                        Run them up slowly until they begin to get warm, then back off a bit.

                        Same principle as with tightening bolts: tighten until they feel funny, then back off a little.
                        There are no stupid questions. But there are lots of stupid answers. This is the internet.

                        Location: SF Bay Area

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                        • #13
                          Originally posted by gellfex View Post
                          I have no idea the rated RPMs of these things, and I'm talking toy sized, cases from .6 to 1.5" diameters. Running them up till they seem unhappy seems a better test of bearings than voltage, no?
                          Take a look on the mabuchi website link above for typical rpm and after that measure the rpm/volts (near)constant.
                          Location: Helsinki, Finland, Europe

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                          • #14
                            Originally posted by gellfex View Post
                            I have no idea the rated RPMs of these things, and I'm talking toy sized, cases from .6 to 1.5" diameters. Running them up till they seem unhappy seems a better test of bearings than voltage, no?
                            If they seem unhappy at some RPM then it's likely that you are running up past their rated speed anyway. That's my logic behind running them at a high speed but where they still sound unstrained.

                            For example, a fairly popular small Mabuchi motor size is their 280 frame. HERE'S A LINK to part of the 280 general family page. Note the 6, 9 and 12volt operation for various versions and the RPM varying from just over 10K to 14K. And this is only one of many sizes that Mabuchi makes. For the most part you'll find that the generic small motors in the size range you have will tend to be up around 10K to 15K rpm where they are happy. Much more than this and the simple brushes they use start to spark too much.

                            And by way of further example showing how size doesn't mean much compare these stronger torque 280 can size motors intended for use at powering RC model airplanes.






                            Chilliwack BC, Canada

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                            • #15
                              You could just measure the resistance of the motor, make a guess about the wattage according to size, and thus determine the maximum current. You can safely increase the voltage on the motor until it either reaches that maximum current, or reaches the highest speed you want. You can also rig up a torque sensor, perhaps with a pulley and some weights or a spring scale, and see what kind of torque is obtainable at the maximum current as determined by the test above. Then maybe determine the output power using the HP = torque * RPM / 5252 (with torque in lb-ft). You might expect about 50% efficiency so perhaps you can get 1/2 watt mechanical output with 1 watt electrical input. I have not actually done these tests, but I think they should give you a good idea of the specs for a small DC motor.

                              Another thing you can look at is the number of slots on the commutator. A fast motor may have only 3 or 4 slots, whereas a lower RPM will have a lot more.
                              Last edited by PStechPaul; 10-11-2021, 10:15 PM.
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

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