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Trying to run a treadmill motor without the controller

  1. Sep 4, 2012 #1
    Hi, all; I see that there some pretty knowlegeable people here so maybe someone can advise me:

    I'm trying to run a dc permanent magnet 100v treadmill motor from house current via a bridge rectifier, not through the usual pwm controller. I'm using the choke that came with the treadmill and tried a 1200 mfd cap across the output of the rectifier to smooth the current, but I'm still getting too much sparking and a little smoke at the commutator. The rectifier shows 108v output, which I wouldn't think would be too excessive (I only intend to run the motor for about 15 seconds at a time). I also tried a good sized resistor to cut the voltage a little, but no difference. I already fried one motor. It lasted awhile (it's to raise and lower a milling machine table) but obviously something wasn't right. Any thoughts? Thank you very much.

    Rick A
     
  2. jcsd
  3. Sep 5, 2012 #2

    NascentOxygen

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    Hi Ricka, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    You are rectifying 110V AC? This has a peak value of 155V, and if the choke is a good choice it averages out to 99V. The capacitor probably won't contribute much in the way of smoothing here. (Unless you have the cap on the wrong side, then you might have nearer to 150V out.) This might explain why the meter would show 104V.

    The inductor may be the cause of the sparking, because each time the current in an armature coil breaks there will be a high emf. A capacitor should ameliorate this, if it has a suitable value. Can you try a 0.22 μF polyester cap instead of (or as well as) that electrolytic?

    Also try omitting the inductor, i.e., short it out. The inductance of the motor might be sufficient.

    Perhaps the motor is not rated for 100V?
     
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  4. Sep 5, 2012 #3

    jim hardy

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    i have run DC treadmill motors from bridge rectified household power (120V in US) with no ill effects.

    it sounds to me like like you have AC across the motor, perhaps from a defective or mis-connected bridge?

    what is rating of your bridge compared to your motor's starting current?


    old jim
     
  5. Sep 6, 2012 #4
    NO:

    We actually have 117v house current here, hence 108vdc. I was having the trouble I've described before I added the choke, so that may not be the problem, but I can try a cap as you suggest.

    OJ: Could be a bad rectifier, I guess. It's rated at 35a as I recall; I used six of them in parallel to rectify my AC buzzbox welder and they work OK for that. The motor is rated at 100v, 21a according to the sticker on it, pretty standard treamill motor.

    Thank you for the suggestions; tomorrow I'll follow up on them.

    Rick A
     
  6. Sep 6, 2012 #5

    NascentOxygen

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    As you plan to use it, will you need to run the motor at full speed? If not, a series resistor could be tried. I know you said you used "a good sized resistor" but that sounds a bit vague. So under load the motor could draw up to 20A at 100V? At the moment are you running it with a load, or on no load, and seeing excessive sparking? What current is it drawing as you are currently using it?

    If you are measuring 108V at the motor, I don't think there could be a faulty rectifier.
     
  7. Sep 6, 2012 #6

    jim hardy

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    i'd think a 35 amp rectifier would do the job.

    simple things first - make sure neither of the motor's leads is connected to frame and earth, for that would bypass half your bridge. Draw it out- housepower has neutral tied to earth at panel, so if motor is earthed internally then you have a sneak circuit around half your bridge.

    You could try running it from your rectified buzzbox output, that'd give you transformer isolation.

    Another simple test, if it's mechanically feasible - spin that motor with an electric drill and see whether it generates DC voltage , a permanent magnet machine should make voltage directly proportional to speed. Perhaps you could light a car headlamp with it, that'd check its ability to make current at the low speed you can get from an electric drill.
    If it sparks when spun i'd be suspicious of shorted turns in the armature.
    Does it smell burnt?

    Nascent's questions are right on track for good troubleshooting.
     
  8. Sep 7, 2012 #7
    Some more good suggestions. I tried a different rectifier today, no help.

    I'm running it no load, but this is the second motor I've tried here. I used the first on the mill for quite awhile, raising and lowering the table OK, although it seemed to spark too much. Finally it started smoking and gave up the ghost. Raising the table was a considerable load, but I don't think that's what cooked the motor. If anyone's wondering, it takes 20 turns of the crank to raise the table one inch, so it gets tiresome cranking it by hand!

    I guess it'd run off the welder, albeit slowly at 30-40 volts. We used to use an electric motor to run the traction motors on a diesel locomotive when we set up a portable lathe to turn the wheels. I'll try the electric drill trick, might tell me something. Don't really smell burning, although I'm only running it momentarily, not long enough to heat it up 'til things start melting.

    I'll let you know what I find out tomorrow. Thanks again.

    Rick A
     
  9. Sep 7, 2012 #8

    jim hardy

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    check your welder it is probably more like 70 volts open circuit and drops to 35-ish when loaded. Welders are built that way, high voltage to strike the arc but intentionally "just awful" voltage regulation so voltage drops to barely enough to maintain arc when it's going good. With appropriate caution and care, you can use your rectified buzzbox output on low current setting as a quick charger for large truck batteries.

    anyhow - something is not right and here in US it is a common enough mistake to attempt using a bridge with unfortunate combination of housepower and an earthed load. The result is always a fried bridge and smoke. So it's something to check for , is all i was thinking. Probably you knew that already.

    old jim
     
  10. Sep 8, 2012 #9
    Didn't get a chance to do more than spin the motor with a drill today, but it put out current as predicted. I'd think the welder open curcuit voltage wouldn't matter, it's still below the motor's rating. Hope to try that tomorrow. Thanks.



    Rick A
     
  11. Sep 8, 2012 #10

    jim hardy

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    one last thing

    the treadmill motors i tinkered with were fairly high speed, like a few thousand RPM.

    One of those would have to be geared or belted down to drive a handwheel at reasonable speed....
    horsepower is (Torque X RPM X 2pi)/33,000
    a 2hp(?) treadmill motor rated at 5000 RPM is capable of not very many ft-lbs of torque,
    it needs a gearbox to deliver 2hp at 50 RPM and 100X more torque...
    without one it'd be overloaded and soon burn up.

    let us be careful that we both have similar mental images in place....... it's those simple assumptions that foul us up!

    i hope you have the reduction gear train from the treadmill as well as its motor.

    old jim
     
  12. Sep 9, 2012 #11
    Jim:

    Yeah, they turn at about 6600 rpm. Knowing that, I built a worm and pinion reduction gearbox, plus there's about a 3:1 reduction from the treadmill belt and pulleys. It worked well until the first motor fried. It turned the handcrank a lot faster than I could by hand. It takes 240 turns to move the table a foot. The only limitation I ran up against is, it'll raise the table with vise OK, but with vise and rotary table or dividing head it's a little too much.

    Rick A
     
  13. Sep 9, 2012 #12

    jim hardy

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    aha

    so if you can measure the current you'll know how hard the motor is working....

    its rated current should be about

    (rated hp /746) gives rated watts

    (rated watts / rated volts) gives rated amps, approximately


    a clamp-on meter around AC wire going to bridge would be easiest.
     
  14. Sep 9, 2012 #13
    Jim:

    Yeah, I can do that; have a clamp on meter. Although, don't know if the classic hp formula applies to a treadmill motor- they seem to rate them HIGHLY optimistically!

    Rick A
     
  15. Sep 14, 2012 #14

    jim hardy

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    yes that's common practice in audio industry to rate a piece of gear for its peak instantaneous power capability.

    on AC motors i am used to a "cont" in the duty spot meaning continuous duty.

    treadmills might be not capable of continuous operation at max rating, i dont remember specifics of the nameplate on mine, but they should run until armature wires overheat. Your 15 seconds sounds okay provided there's a couple minutes between run cycles to cool down.

    still i'd be curious to know how much current the motor is drawing.
    An overloaded permanent magnet motor will be susceptible to something called "armature reaction" which is the high armature current distorting the internal magnetic field, and that'd cause sparking and reduced torque which would aggravate the situation further. "Interpoles" were invented around 1890 to get around that problem but the only treadmill motor i ever took apart didnt have them.

    So - measure that current and if it's much more than result of the simple calc above, increase your geardown ratio to get current down closer to max rated.... shoot for <1.5X, that'll produce 2.25X rated internal heating; and surely they left that much margin in the field magnets.

    old jim
     
  16. Sep 15, 2012 #15
    Jim:

    That sounds like a logical explanation for my problem- asking too much of the motor. In theory, you'd think lifting the table on a 9" X 42" mill at the rate I do wouldn't tax a motor like that. Before I tried the treadmill motor, I had a .5 hp AC wash machine motor, which lifted the table OK, but a starter winding gave out. I'm guessing it wasn't made to start under load. Worked well for quite awhile, though.

    I recently changed out the starter on a friend's Dodge PU and am toying with the idea of trying the old one (just the solenoid was bad) on the mill. If I can rewind a microwave transformer to get the 12v, I may be in business. How does that idea strike you?

    Rick A
     
  17. Sep 15, 2012 #16

    jim hardy

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    Car starters make incredible torque, so much that you must never energize one on the workbench without clamping it down. It'll throw itself across the bench if your current source is big enough .

    Your idea ought to work fine
    but you might shop Ebay for a 200 amp bridge,
    or place about 0.01 ohms in series with each 35 amp bridge to force current sharing.
    An automotive 30 amp blade fuse should be about 0.002 ohms, so a fuseholder plus a few feet of #12 wire in series with each bridge ought to make them share current.

    There' s plenty of hobbyist sites with good directions for re-winding microwave oven transformers , which are called MOT's,
    but i must say two things:

    1. Remove the High Voltage winding VERY FIRST THING.
    NEVER ENERGIZE A MOT WITH ITS HV WINDING STILL THERE , for it is lethal.
    The HV winding is spot welded to the metal transformer core and that is a grave danger. You do NOT have the protection of transformer isolation until you completely remove the HV winding and replace it with an insulated LV winding..

    2. Don't cut the metal core, you can never get it back with zero air gap.

    You will find both precautions on any decent hobbyist site and i cannot overstate the importance of #1. I cringe at the kids who post their MOT experiments on youtube, and several have got killed or maimed..

    Doubtless you are aware but there may be less experienced readers about.
    Stay safe my friends!

    old jim
     
  18. Sep 15, 2012 #17
    Yeah, even these treadmill motors will give a pretty good twist; I clamp them in the vise. Wasn't aware of the idea of resistances on the rectifiers. I'd read about the problem of unequal load sharing but haven't observed any problem with my welder yet.

    I've fooled with MOTS enough to know what they can do at 2000v. Have to remove the hv winding anyway to fit the 12v winding in- it'll be pretty big wire. FWIW, a dandy use for a MOT is a homemade soldering gun. You use the small winding in the middle, makes about 5v, and ground one end to the work and the other to a carbon electrode. It makes instant heat and gets hotter than an ordinary soldering gun.

    Thanks for the ideas. I'll let you know how it unfolds.

    Rick A
     
  19. Sep 16, 2012 #18

    jim hardy

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    Thanks to you as well !

    that sounds like it might make a spot welder with suitable clamp....

    do you use a plain carbon torch electrode for soldering?

    old jim
     
  20. Sep 17, 2012 #19
    You can make a spot welder out of one:
    http://www.5bears.com/welder.htm
    I looked for the website that told me how to make the soldering gun, but no luck. I used a piece of 1/4" carbon arc gouging rod- it's cheap at a welding supply store. I just extended the small middle winding, put an alligator clip on one for the ground and attached the other end to the carbon rod, about 3-4" long, and encased the rod in a piece of snug-fitting rubber air hose to make a handle. The rod gets pretty hot. I also mounted an on/off switch on the MOT. I also tapered the end of the rod to a dull point. Pretty simple and works well, only down side is having to attach the ground clamp, usually to the other end of the wire I'm soldering on.
     
  21. Sep 17, 2012 #20
    Old Jim:

    Could you also answer another question? I've been looking on E-bay for a rectifier big enough to run this starter motor, and most of the bigger ones are three phase. The internal wiring looks similar; do you know if one would work with single phase? Thanks.

    Rick A
     
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