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Stepper motor question

Discussion in 'Electronic Basics' started by vorange, Oct 16, 2007.

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  1. vorange

    vorange Guest

    1) I have a stepper motor in mind :

    http://tinyurl.com/2qr4d9

    I'm guessing since it has 6 wires that it has 2 coils. But should
    that not mean it has 4 phases. Why do they list it as a "2 phase
    stepper motor"?

    2) The motor above is listed as a "bipolar (in series) and unipolar"
    as well which is kind of confusing. Is it possible to drive it as a
    bipolar motor (i.e. without using the two center taps)? Is there any
    downside to driving a unipolar motor as a bipolar motor (or is this
    question totally ridiculous).

    3) Can you suggest a driver chip (IC) i can use to drive the above
    stepper motor? Would the following driver be ok :

    http://www.onsemi.com/pub/Collateral/MC3479-D.PDF

    4) Is "load voltage" the voltage rating of the stepper motor? (in this
    case 4V)

    Thank you for your help
     
  2. Yes. 6 wires indicates two windings and a center tap on each.
    2 phases means that the coils are placed in two magnetic
    locations. You get 4 effective phases by driving each with
    current in one of two directions.
    Bipolar means that the driver circuit applies voltage in
    either direction across the whole winding (the two halves of
    the center tapped winding, both halves in series, carrying
    the same current). Unipolar means that the driving circuit
    can force current in only one direction, so you get the
    reversal by applying it to only one half of the center
    tapped winding at a time, both currents either toward or
    away from the center tap. The effect is one each half
    magnetizes the poles in one direction and the other half
    winding magnetizes the poles in the other direction. The
    efficiency is lower, because only half of the copper is
    being used at any time.
    An excellent question. It is perfectly reasonable to drive
    a unipolar motor with a bipolar driver, and it will be more
    efficient because the whole winding will be in use. Having
    the current pass through twice as many turns also produces
    about the same torque as twice the current from a unipolar
    driver. The down side is that the motor will require 4
    times the supply voltage to max out at the same speed,
    because twice as many turns produce 4 times the inductance.
    You can give up the current efficiency and recover the
    full speed by driving only half of the center tapped winding
    with the bipolar driver, but you will be wasting half of the
    copper, so the temperature rise will be worse.
    Allegromicro makes many:

    http://www.allegromicro.com/en/Products/Categories/ICs/motor.asp
    I would pick a dual bipolar driver with two or three times
    the series current rating of the motor if you want full
    torque from the motor. Perhaps the AD3992, because it is
    available in a DIP package for easy prototyping.
    http://www.allegromicro.com/en/Products/Part_Numbers/3992/index.asp
    It is a bit wimpy, with a current rating of only 0.35 amp,
    which I would not get close to, for good reliability. It
    would probably turn the motor but would not get much torque
    from it.
    The chip is rated for up to an 18 volt motor supply, but it
    puts out about 3 volts less than the supply because of the
    voltage drops of the power switches. It also has no
    provision for current regulation like the AD3992 has.

    Current regulation allows the supply voltage to be well
    above the DC rating of the motor, so that there is extra
    voltage available to overcome the generated voltage as the
    motor moves faster. Basically, the motor torque is
    proportional to the coil current, but the voltage it takes
    to produce that current is proportional to the speed, plus a
    fixed amount to overcome the winding resistance.
     
  3. vorange

    vorange Guest

    Thank you John.

    As always, you are an encyclopedia of knowledge. You really should
    write a book on electronics for beginners to intermediate. I predict
    it would sell like hotcakes.

    Just a quick comment :
    Yes I noticed that too. I had planned to put transistors on its
    outputs to boost the current. But the Allegro suggestion is looking
    good.

    Cheers!
     
  4. BRW

    BRW Guest

    My favorite stepper motor tutorial:

    http://www.cs.uiowa.edu/~jones/step/

    BRW
     
  5. (snip)

    Micro stepping is used with both bipolar and unipolar
    drivers, but I have seen it most often with bipolar drivers.
     
  6. Jamie

    Jamie Guest

    unipolar steppers are niffy with the correct controller on it.
    the CT Fields give you the option of micro stepping. That is
    if you have the correct controlling electronics. what this does
    is locks both fields on . You don't get as much holding torque as
    you would on a single phase position but it's nice where you would
    have lets say a 100 PPR stepper and emulate a 200 PPR stepper.
    In some cases, this is used as a soft position index to reduce
    shock movement between poles. But then again, it depends on the
    controller.
     
  7. Jamie

    Jamie Guest

    It works smoother with unipolar but with less torque.
    the bipolar gives you more torque but is a little less
    stable in the micro step. In many cases, more torque is more
    favorable but does require more current to operate it while in the
    split phase index.
    I wrote a little code in an AVR to operate a unipolar motor
    using PWM to perform soft movement of the rotor. I gotta say, it
    was an interesting experience. Parameters had to be tweaked to
    account for the induction sang in the coils. When it was all done
    how ever, the motor it self was able to turn so smooth between
    indexes, you couldn't tell it was a stepper motor.

    We had a project where we needed smooth movement but be able to
    lock the rotor once it stopped and index 2 notches . the locking
    position didn't have to be exact! but the motion to get it there
    needed to be smooth.
    it was a toss up between using a stepper or a brushless DC motor.

    I guess either one would work, just a matter of how you would want
    to approach it.
    And bipolar is more common due to it's simplicity in design.
     
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