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Stepper motor specs

Discussion in 'General Electronics' started by sami, Feb 3, 2004.

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  1. sami

    sami Guest

    Hi all,

    I've been reading about stepper motors and need some help to understand
    some of the specs and how to choose the step motor that is best for my

    1- What is the difference btw Detent torque and Holding torque ? Which one
    is of more importance?

    2- I need to get two step motors for my robot (as wheels). The robot is
    about 3-4 lbs. I don't need it to go fast, but i want the step motors
    to be strong enough to move the robot. I think a reasonable voltage
    is btw 9-12v. I don't want to use more than 12 v. I'm not sure about
    the Amps i found step motors with 40 mA, 80 mA, 500 mA, ..etc.

    How do I know what Amps is best for me? Also what type and value for
    torque will i be needing ( 200 g-cm, 400 g-cm, 1000 g-cm, ...)?

    3- I've read about Unipolar and Bipolar. What is the major difference btw
    them? what things can I do with one that i cannot do with the other?
    Which one is easier to use?

    thanks in advance
  2. BobGardner

    BobGardner Guest

    1- What is the difference btw Detent torque and Holding torque ? Which one
    OK, I'll guess on this one... someone will correct me if I'm wrong... detent
    torque is with coils off, and holding torque is with coils energized?
    Get a fishin scale and see how many pounds you need to pull it at the speed you
    want. Torque is this force x the wheel diam (adjust units from SI to English as
    I always get these confused... unipolar has 2 centertapped coils (6 wires) and
    you hook the centertaps to the batt and step the coils with transistors.
    Bipolar has 4 wires... 2 coils... and you energize them with either an H-bridge
    (one side goes up to +v, the other goes down to gnd), or +-volt supplies. The
    1st type only has half a coil energized, 2nd type has the whole coil
    energized.. more torque, but more complex drive. 1st type can 'half step'
    though, doubling the steps. Tradeoff.
  3. Rich Webb

    Rich Webb Guest

    It does seem counter-intuitive, with the "uni-" having more wires and
    more individually-addressable windings. What worked for me was to
    silently replace the -polar with -directional, since during operation
    the current goes two directions in each bipolar winding but only one
    direction in each unipolar.
    Nothing to say that you can't step with two coils in a unipolar setup;
    instead of A - !B - !A - B, one would use A/B - B/!A - !A/!B - !B/A.
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