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Thread: Squeezing more power from a DC motor

  1. #11
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    Default Oop. P.S.

    I just happened to be watching battle bots and reading your post.

    Look at the motor these folks are using. And they are the top in their field. A team out of Brazil.

    Those motors don't look like they are wired to draw much current. So that got me thinking. The DC motor is very basic, what we all learned first.

    I love DC motors for several reasons including torque, speed control and noise.

    A regular 90-120VDC motor has its speed limit. I wouldn't chose a DC motor for speed unless you are going with a large motor and speed enhancers (multipliers?).

    Two basic ways to change the way a DC motor motivates is Voltage and Resistance, I did say they are basic.

    So you can have a low resistive motor like what is seen in the RC would. Or a very high resistive motor like what you might find under your car to start the main engine. Used to call them Pony Motors, but the were engines

    Neither one can stand much prolonged use due to the current draw and heat associated with it. That is where these 90-120DC motors came into play. Its a packaging issue. DC motors are big.

    AC? I can talk about AC motors all day. Good ones? 440/400AC, real tiny are scary strong.

    Anyway. Solly JR

    Forgot the pic...

    My old yahoo group. Bridgeport Mill Group

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

  2. #12
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    Quote Originally Posted by JRouche View Post
    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.

    Think automobile starter motor. JR
    That is why a series field motor is used for Automotive starter, very high torque, the down side is when unloaded, they operate in a run away condition, as do the other series (Universal) motors.
    Max.

  3. #13
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    There ar two basic kinds of DC motors....

    1) "shunt" type motors, which I will include with permanent magnet motors. These have a "fixed" field strength, fixed either by the field structure (windings and iron structure) and the input voltage, or by a permanent magnet.

    Shunt motors have a fixed "base speed", but can be varied around that speed by the applied armature (rotor) voltage. They will not run away, but they also have a fixed torque limit.

    2) Series motors". These have a variable field strength, because the armature current produces the field as well, by passing though a heavy field winding, and consequently have a variable base speed. When loaded heavily, they have a large current flow, which increases the torque per ampere, and also reduces the base speed. As a result, the motor has an effect similar to being "geared down" for low speed and heavy torque. The opposite occurs on light load, the base speed can rise to very high rpm. It may be that the base speed rises higher than the mechanical limit of rpm, so the motor may be damaged.
    1601

    Keep eye on ball.
    Hashim Khan

  4. #14
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    Quote Originally Posted by JRouche View Post
    I just happened to be watching battle bots and reading your post.
    Look at the motor these folks are using. And they are the top in their field. A team out of Brazil.
    Those motors don't look like they are wired to draw much current.
    -They probably aren't. Those are the drive motors, and while some power is nice for those, these days most teams pile the power and weight into an active weapon.

    Although it's also worth noting I pulled what the nameplate called a four HP DC motor out of a professional gymnasium-quality treadmill many years ago. The thing was about 30% larger than a coffee can, but I'm pretty sure the power cables were no bigger than 12 gauge. I'm no electrician, that's for sure, but I was told it had to do with the voltage vs. current or something.

    On the Battlebots thing, I do recall from back in the old Comedy Central days, that one of the more popular motors to use were Briggs & Strattons that would produce something like 12 HP at 60V. Virtually everyone overvolted them by at least 100%, getting 24 HP at 120V. One of the dominant 'bots back then had two such motors powering it's weapon, putting nearly 50 HP (!!) into the rotor.

    And that was more than 15 years ago. I'm sure the technology today could nearly double that.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  5. #15
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    On The subject of series/shunt fields, back when large DC motors were used in CNC spindles, Crane motors etc, the wound shunt field was always fitted with a field loss circuit to shut the motor down in case for field loss, as the result was that if the motor lost its field during normal operation, it can, and often did, ran away to destruction when lightly loaded.
    Of course, this cannot happen with a P.M. field.
    Max.
    Last edited by MaxHeadRoom; 07-14-2019 at 09:24 AM.

  6. #16
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    There were a number of replies (thanks) about the speed. The name plate gives the speed as 2600 rpm at 90v which is about what I got (2800) fully loaded. But that only gave 0.8 HP, not the name plate 1.6. To get the 1.6 HP I used 120v & the loaded speed was 3700. The 120v no load speed was 4800 rpm. Those speeds are about what other posters have described as normal.

    That the 120v speed dropped from 4800 to 3700 is disconcerting - if I wanted to use this for variable speed, I'd want to know that I'd get what I set it for. I was just using a variac, so I suppose that a controller that sensed the rpm and adjusted its output would work. Is that normal - fixed voltage results in large speed drop with load & a controller is needed to maintain speed?

    I was thinking about using this motor to convert a 12" wood-cutting miter saw to metal cutting (the wood saw speed is much too high). The idea was to use a 2 or 3 to 1 belted speed reduction. And just use a bridge on 120v AC for 120v DC, i.e., not use a controller. I suppose having a blade that slowed from 1600 to 1200 under load would be OK, but I'm leery. Usually a big speed drop means that you're overloading the tool & should back off. How would I know when the load became too much? Put an ammeter on it? That sucks.

    I am glad that I tested the motor & didn't just forge ahead based on the name plate.

  7. #17
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    All motors slow when loaded, it is part of how they work, and is normal, a pat of the design. The 3700 vs 4000 is really only about 7.5%, and is not a big deal. A regular induction motor is often rated to slow down by about 4% (1725 rpm vs 1800) because they run very close to synchronous at no load.

    Yes, a speed sensing controller can hold speed closer, but it is questionable if it is worth the effort. If you listen to a cutoff saw, you can clearly hear the saw slow down as you cut, and the operation is not affected by that. Same with most other small machines.
    1601

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    Hashim Khan

  8. #18
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    Quote Originally Posted by Bob Engelhardt View Post
    Is that normal - fixed voltage results in large speed drop with load & a controller is needed to maintain speed?
    Feedback is needed as in servo motors etc, one guy on the CNCzone developed a PID controller for the typical Universal Motor Router when used in a gantry application etc.
    Uses a simple triac and Keeps amazing speed accuracy.
    http://www.vhipe.com/product-private/SuperPID-Home.htm
    I have also used a 3ph motor and a VFD with encoder feedback for pretty good control.
    Max.
    Last edited by MaxHeadRoom; 07-14-2019 at 01:51 PM.

  9. #19
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    Quote Originally Posted by J Tiers View Post
    All motors slow when loaded, it is part of how they work, and is normal, a pat of the design. The 3700 vs 4000 is really only about 7.5%, and is not a big deal. A regular induction motor is often rated to slow down by about 4% (1725 rpm vs 1800) because they run very close to synchronous at no load.

    Yes, a speed sensing controller can hold speed closer, but it is questionable if it is worth the effort. If you listen to a cutoff saw, you can clearly hear the saw slow down as you cut, and the operation is not affected by that. Same with most other small machines.
    Exactly. And not only with brushed motors like DC and universal brushed motors either. A bench grinder is likely a good example of this. Very little drag when free running. But the motor loads down when you push a piece of metal firmly into the wheel. And if you were running an AC current meter in the line while doing this you'd see the current rise in accordance with the reduction in RPM from the load. Without any form of feedback to compensate for this any motor will slow down from the "rated RPM" when loaded down. It's the lag from the rated free running RPM that causes the motor to pull more current.

  10. #20
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    Default

    best method to get more power out of a dc motor is to give it dc current.

    not rectified ac mains.

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