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Thread: Squeezing more power from a DC motor

  1. #1
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    Default Squeezing more power from a DC motor

    I have a no-name treadmill motor whose name plate says 90v, 11.4A [1026w, which is 1-3/8 HP at 100% effic], & 1.6 HP. More power out than in - it's magic! To get the real scoop I set up a rig to measure output power (torque & RPM). At 90v & 11.4A in I got 1.5 ft-lb & 2800 RPM: 0.8 HP (60% effic) - 1/2 the name plate. I can't say that I'm surprised.

    DC motors don't have a fixed speed like AC do, so I increased the voltage to 120 and let the current go to 15A (+30%). I got 2-1/4 ft-lb and 3700 rpm: 1.6 HP. That's more like it.

    So now I'm wondering what limits how much power one can squeeze out of a DC motor. One should be able to increase the speed by quite a bit, limited by mechanical considerations, not electrical ones. In a brushed motor, would brush performance suffer at 10,000 RPM? There's probably no other mechanical limitation for 10,000 rpm.

    Torque, on the other hand, is a function of current and that would have significant limitations. The obvious one being the heat that current makes and its effect on the wiring's enamel insulation. But high currents would be tolerated if the duty cycle was low enough. The question being what duty cycle for how much over current. Unlikely that it could be answered without a bit of destructive testing. The 30% over current that I tested with is probably OK at anything less than 100% duty cycle. Current 100% over would probably require a pretty low duty cycle, and/or thermostatically based cooling.

    The other limitation on current would be the demagnetization of the field magnets (PMDC motors). I didn't get any help from Google on this question. My intuition is that the 30% over current that I tested with is probably OK.

    Anybody have any experience squeezing DC motors?

  2. #2
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    look up "Magnetic Field Distortion". As the speed changes so does the distortion. A brush motor is typically designed to run at a certain speed/voltage. This is the plane of maximum induction. If you had a way to vary the brush timing (like ignition timing in a car) you could possibly keep the motor at peek power for what ever voltage was applied. In stead of varying brush timing, designers use a variable field strength for some motors. But this still doesn't provide maximum induction for a given speed. (The brushes are still in the same place.) And it doesn't apply to permanent magnet motors.

  3. #3
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    SC motors DO have a fixed speed, the "base speed".

    That is the point where the rpm is just enough to balance out the applied voltage all except for just enough to drive the required current through the motor.

    Many treadmill motors are intended to run at higher rpm, because it takes less current (torque) for a given HP. From 3000 to 4000 is often the area that is optimal.

    You got nameplate power in that range with the expected 120V applied. So you found the correct speed. With a fixed field permanent magnet there are fewer options than with a wound field. So the speed you found is probably near the optimum, and was at a reasonable voltage.
    1601

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    Hashim Khan

  4. #4
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    While you're at it, look up 'treadmill horsepower'. Treadmill motors are almost always overrated. Maybe it has something to do with the 'pulsing' load imposed by a biped running/walking? In any case, you can de-rate one by 33% to start with, another 10-15% for chinese and...

  5. #5
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    Much of that is from folks having no clue that the rpm is near 4000 at rated HP......

    They try to slow it down on a direct drive, and winder why "chinese horses are so small"..... IOW they have no idea what they are doing.....
    1601

    Keep eye on ball.
    Hashim Khan

  6. #6
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    Yeah, I was shopping treadmill motors at surplucenter.com, and I noted that they were all rated around 4000 to 5000 RPM to get the claimed HP. Concluded that I would have to add something like 3:1 or 4:1 gearbox to be usable on a machine tool. Might be OK for a inverter/generator tho.

  7. #7
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    4-5k is still slow if you want to spin small mill/router cutters. What kind of machine are you building?

  8. #8
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    two of my machines with TM motors have countershafts/ intermediate pulleys (lathe, drill press), the mill simply reused the original motor pulley and junked the intermediate pulley. On the lathe I reused a roller pulley from a treadmill, which I think got me a 6:1 reduction. With the new 2 step pulley I made it should give me a 7(or 8):1 and a 3.5:1 ratio. All three machines can slow to a crawl (w/ back gear on lathe) and go up to ~2.4-4k rpm with a ratio change. The spread is usually about 1200-1500rpm in each ratio.

  9. #9
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    Quote Originally Posted by CCWKen View Post
    4-5k is still slow if you want to spin small mill/router cutters. What kind of machine are you building?
    At one time I was thinking to build a lathe, max of ~1400 RPM, but after doing the math I realized that I could just buy one with proper back gears for the same price (my target was $1500), and have all the functionality immediately. Probably better quality too. So instead of building, I've been looking around for a SB model B.

  10. #10
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    Quote Originally Posted by Bob Engelhardt View Post
    In a brushed motor, would brush performance suffer at 10,000 RPM? There's probably no other mechanical limitation for 10,000 rpm.


    Anybody have any experience squeezing DC motors?
    I have a dozen or so AC/DC universal brushed motors that live at 10,000 RPM all day. They really dont produce much torque but the speed is where they get their ummph.

    As for a real DC motor and jacking it up? Speed is not really where they shin. They are great for low RPM and great torque.

    Jerry and the other more proficient electron gurus could explain it better.

    I love DC motors for the torque curve at under 1700rpm numbers. Think automobile starter motor. JR
    My old yahoo group. Bridgeport Mill Group

    https://groups.yahoo.com/neo/groups/...port_mill/info

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