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Ahh, so this is where the magic smoke comes from!

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  • Ahh, so this is where the magic smoke comes from!

    Buddy up the street comes down with a crude pencil drawing of a couple aluminum mounts for some automated machine he's working on at the plant. No sweat says me, I have a stick of 2"x3" 6061 on hand & I'll whip 'em out for you in no time (for the usual weekend special price of course.)

    About a 1/4 way through the 1st cut on my chop saw, it started smoking, I let off the trigger & it kept going with a crazed mind of its own. By the time I yanked the plug, the whole shop was full of very stinky magic smoke. I tore it down & the motor field windings are toast. I don't see any other damage at all. I wonder what happened. I know it's a wood cutting saw but I don't see any chips shorting any thing out. I guess it'll have to be scrapped since parts for chinese saws are probably non existent.

    Hmmm, it's got a 5 1/2 to one gearbox on it (planetary I think). I wonder if a treadmill motor could be grafted on to slow it down & have a poor man's cool (not cold) saw?

    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    Yep... burnt up the field winding. Did you bog it down with a dull blade?

    Happens... I had a high end Dewalt compound mitre saw that I was given like that, except the armature was burnt up too. $180 in parts and like new, but I just "had" to fix it
    Last edited by lakeside53; 04-08-2019, 12:10 AM.

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    • #3
      Interesting that the field has considerably thinner wire than the armature. It is very obvious in the picture. Yet, in general, the same current passes through both. Perhaps the coils are in parallel, which would have 1/4 the dissipation in each.

      The coil is also only half-burnt, with a section well toasted, and the rest appearing nearly untouched. For one of the coils, the center part is burnt, which one can understand, as tt would have less heat sinking, but the other coil is burnt in a different pattern.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Originally posted by lakeside53 View Post
        Yep... burnt up the field winding. Did you bog it down with a dull blade
        No, it had a fresh blade made for cutting aluminum & I was cutting at a steady rate without a lot of pressure...just letting the blade do the work. I'm guessing the field windings fretted through the varnish & then,POOF! It was given to me about 4 years ago so it doesn't owe me anything. I'd like to find another one of the same size so it'll bolt-on to the custom sliding mount I built for it. It goes into a cramped spot & stores out of the way but slides out for use easily.
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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        • #5
          Thinking a bit more about the failure, I think the main power section of the switch stuck "on". The saw has a dynamic brake circuit which has contacts in the switch that shorts out the field during coast-down, yes?. With power still being applied and the brake circuit kicked in, POOF!
          Milton

          "Accuracy is the sum total of your compensating mistakes."

          "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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          • #6
            I bet someone sells replacement parts, if it is a name brand.

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            • #7
              Whats the silver wire wrapped around the rotor wires by the commutator?
              Andy

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              • #8
                Originally posted by DICKEYBIRD View Post
                Thinking a bit more about the failure, I think the main power section of the switch stuck "on". With power still being applied and the brake circuit kicked in, POOF!
                Clumsy bastard.

                Interesting failure mode. I think most of us would have reacted similarly, and wouldn't have thought to keep the trigger pressed while trying to unplug the skitzo tool. For some reason, I think a switch failure causing this would piss me off more than the outright failure of a cheap winding.

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                • #9
                  Originally posted by vpt View Post
                  Whats the silver wire wrapped around the rotor wires by the commutator?
                  String.

                  It is there to help keep the wires and commutator from flying out at high RPM. Many have several times that much wrapping.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    Why are you guys so fascinated with treadmill motors????

                    -D
                    DZER

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                    • #11
                      Originally posted by Doozer View Post
                      Why are you guys so fascinated with treadmill motors????
                      Speaking for myself, because I have one, they're (relatively) light & powerful, easily speed-controlled & in this case run much slower than the original motor (intended use: metal cutting) and the most important reason, CHEAP.
                      Milton

                      "Accuracy is the sum total of your compensating mistakes."

                      "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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                      • #12
                        Originally posted by vpt View Post
                        Whats the silver wire wrapped around the rotor wires by the commutator?
                        Not typically wire but strong thread and varnished.
                        I always keep an eye out for local T.M.'s that in some cases are offered free as users often want them gone if they quit.
                        Max.
                        Last edited by MaxHeadRoom; 04-08-2019, 10:55 AM.

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                        • #13
                          Originally posted by DICKEYBIRD View Post
                          . . .I don't see any chips shorting any thing out. . .
                          Chinese motor magic smoke is encapsulated at extremely high pressure and contains a catalyst that increases that pressure by the day, sometimes by the hour. Given time it will burst the containment element regardless of what you or anyone else does.
                          Last edited by chipmaker4130; 04-08-2019, 11:47 AM.
                          Southwest Utah

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                          • #14
                            Originally posted by DICKEYBIRD View Post
                            Thinking a bit more about the failure, I think the main power section of the switch stuck "on". The saw has a dynamic brake circuit which has contacts in the switch that shorts out the field during coast-down, yes?. With power still being applied and the brake circuit kicked in, POOF!
                            If the field winding was shorted out as a brake, with AC power still applied, the current from the supplywould flow through the short, and likely blow a breaker.

                            I don't think it would affect the field winding at all.

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                            • #15
                              If you had a failure that let all the smoke out then cam't you just buy a bottle of Liquid Smoke and top it up?
                              The shortest distance between two points is a circle of infinite diameter.

                              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                              Southwestern Ontario. Canada

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