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Bipolar drive of unipolar stepper motor?

Discussion in 'Electronic Design' started by Don Lancaster, Mar 16, 2006.

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  1. I'm trying to puzzle out the stepper drive on a programmable wire cutter
    I am refurbing.

    The medium power fist sized stepper is clearly six wire and intended for
    unipolar drive.

    Typical 1.5 ohm low voltage windings, one on either side of a center tap.

    The drive is a L298, which is a fancy high voltage 38 volt switchmode
    current drive bridge.

    The center taps of the stepper windings do not seem to be connected
    anywhere else. And clearly not to each other.

    Can you simply bridge drive BOTH bifilar windings at once by ignoring
    the center tap, causing the bipolar stepper to become a unipolar one?

    Presumably of double power.

    The "wrong" current direction (caused by the left half bridge versus the
    right half) through the second winding should aid the first one.

    Can't seem to find any mention of what might be obvious anywhere else.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  2. Tim Wescott

    Tim Wescott Guest

    AFAIK, yes, you can.

    To avoid overheating the motor you'd need to drive it at sqrt(2) of the
    unipolar drive voltage, so you'll only increase torque by 40% rather
    than doubling it. Depending on how the motor is designed you may or may
    not have funny saturation problems. Of course you can always run the
    motor at the unipolar current for double the power dissipation in the
    motor -- if it's intermittent use you may even be able to get away with it.

    I've never done this myself, so I can't lend anything more than some
    ramblings based on theory.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See
  3. Guest

    Actually, it's pretty low end for serious motion control.
    Yes. This is quite common. One catch through is that you increase the
    inductance, and for this reason you may want a higher power supply
    voltage to supply your chop-mode current regulator if you need high
    step rates. Good motors may be 8 wire so that you can run the half
    windings in parallel rather than series (using high current drivers of
    Not necessarily. With a good driver the limiations may actually be
    either heat dissipation in the motor, or the field strength at which
    you risk demagnetizing the permanent magnets in the motor. (BTW, don't
    ever pull the rotor out of a stepper motor without using a flux keeper

    Try asking on alt.machines.cnc or the yahoo group CAD_CAM_EDM_DRO - the
    hobbyist CNC gang does a lot with stepper motors from those using L298
    type drivers up to ones using discrete H bridges of N MOSFETs with
    floating high-side date drivers, etc.
  4. A Schleuniger?
    Yes. Maybe a NEMA 34 frame, depending on how big your fist is.
    Old-fashioned bipolar dual H-bridge design, gets really hot with
    substantial current. ST makes them, Mouser and Digikey should carry
    replacements if it's shot. It's only switchmode if you drive it so
    (eg. with the sidekick sequencer chip).
    That's fine.
    Yes, you can use the unipolar as a bipolar by ignoring the center taps
    on each coil and driving the ends with an H-bridge.
    Yes, but you are getting twice the heating with two coils energized.
    Not sure the relationship is between torque and driving method, but
    motor data sheets should have this information.

    Best regards,
    Spehro Pefhany

  5. Unit is a Schuleniger UC 3750 and will be up for sale once I complete
    the refurb.

    Presumably the original engineering with regard to heat and current
    etc... will still be valid once I replace the driver. I'm just trying to
    find out exactly what they were intending to do.

    Besides being frozen and locked up (already repaired with new bushings,
    etc), some junior G-Man plant electrician managed to replace a bridge
    rectifier 180 degrees out of position, thus converting the +42 volt
    supply to a MINUS 42 volt supply, and raising all holy hell in the process.

    Most of the refurb is done. Blades appear perfect and rollers repolished
    beautifully. I suspect the slightly smaller rollers might affect the
    exact wire size programmed in, though.

    Discovered after a dozen false starts that the way you replace an oilite
    bushing is to use a Hex Allen Cap Screw! Between the 20 threads per inch
    and another 18:1 or so of wrench gain, two or three pounds of twist
    gives you half a ton of insertion/withdrawal force.

    Sure has been an adventure restoring an obvious basket case.
    Also willing to sell as is at 90 percent refurbed.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  6. Sjouke Burry

    Sjouke Burry Guest

    For unknown 6 wire stepper motors,I temporarely glue
    a stick to the shaft, then touch a mall voltage to each
    of the four coil ends. the stick moves a bit for each ending,
    and it shows you the order in which to step them(watch out
    for bouncing across whole step(s)).
    to control the motor then ,the centre taps go to the supply,
    and you step through the four endings in succesion.
    I get a higher momentum when i step (a+b b+c c+d d+a .......),
    watch motor temperature when you do that(but the holding torque
    is very high).
  7. Tim Williams

    Tim Williams Guest


    Thought that stuff hasn't been used for ages.

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