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Stepper motors

Discussion in 'Electronic Basics' started by Periproct, Oct 29, 2007.

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  1. Periproct

    Periproct Guest

    I'm setting up a small CNC mill to educate myself before taking the plunge
    on a larger machine and I have a few questions about the stepper motors and
    stepper driver boards.

    Firstly, the motors have what looks like 7/0.2 wires attached and according
    to the people I bought the machine from they are rated at 2 amps. Surely
    that is very fine wire for so high a current.

    Secondly, the driver boards have an potentiometer to adjust the current. My
    simple mind says the motors will take the current they require or is this
    some kind of current limiting.

    Lastly, would it be sensible to adjust the boards to a slightly lower
    current?
    I've set all three boards to 2 amp and already killed one board.
     
  2. Tim Wescott

    Tim Wescott Guest

    Stepper motors always need to have excess current, so that they have
    torque when they need them. This is different from how a DC motor works,
    and that's probably what you're thinking of.

    As long as your steppers are working correctly you can turn down the
    current -- you're setting the available torque, if you've got what you
    need plus a safety factor then you're doing OK.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  3. john jardine

    john jardine Guest


    Not an unusual size. The stepper driver can still force 2 amps through it,
    even with many ohms of internal/external wire resistance.

    It -would- take the current it needs, if only you supplied the motor with
    it's normal working voltage, which may only be about 2Vdc.
    I'll bet though that your controller is being fed from something like a
    24-40Vdc supply.
    The idea is for the drive PCB to force a fixed (usually marked on
    nameplate), controlled current through the motor winding, that will not be
    affected by connecting wire resistances, or the evil, increasing coil
    impedance which is a big problem at faster and faster stepping rates.
    One problem is, that as the driver PCB has so much voltage headroom to play
    with, a careless setting of the current adjust pot will happily force say 2
    Amps through a 1/4 amp motor. A minute later and the motor is toast.
    The driver PCB doesn't mind what's on the end of it's wires.
    Yes. Set the current lower. You can always wind it up later if more
    performance is needed. I'd start at maybe 1/2 amp (or less) and see how it
    goes.
    At continuous full rated current, the motors bodies can run -very- hot. This
    is most noticeable with the motors stopped or doing little work.
    Because of this waste of power some drive boards allow entering a
    low-current/sleep mode, if no steps have been sent for a couple of seconds.
    This can be about 10%-50% of nominal current.
     
  4. Periproct

    Periproct Guest

    Thanks to you both for your replies. I can certainly confirm that last bit.
    I had to go out in a hurry and left the whole lot powered up for about three
    hours and the motors were very hot when I got back. I'm now a lot more
    careful about turning everything off.
    I'll also wind the current down and see how I get on.
    Thanks again
     
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