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Stepper motor driving- Unipolar Vs Bipolar

Discussion in 'Embedded' started by Bryan Hackney, Feb 26, 2004.

  1. When you energize a coil in a unipolar motor, that's 25% of the motor,
    but when you energize a coil on a bipolar motor, that's 50% of the motor,
    so yes, you get more output.
    Bryan Hackney, Feb 26, 2004
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  2. Bryan Hackney

    Richard Guest


    Is it true that driving using bipolar method will provide more torque for
    most motors as compared to unipolar method?

    Richard, Feb 26, 2004
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  3. Bryan Hackney

    John Jardine Guest

    A bipolar drive benefits from less winding resistance and can give upto 40%
    more torque.
    John Jardine, Feb 26, 2004
  4. ....for the same copper losses.
    John Popelish, Feb 26, 2004
  5. Bryan Hackney

    John Jardine Guest

    Argghhh, Copper losses ...
    John .
    You're well au fait with the steppers. Any morsels you can cast in my
    direction? ...
    I finished a unipolar chopper design last month (1/2 to 6amps). Ran tests on
    a collection of (11) motors and noticed they -all- overheat. This time round
    I used a real temp' probe as against the usual spit and was surprised to see
    temps of 90deg+ (and rising).
    The makers usually specify a max case temp rise of 55degC over ambient. This
    seems only qualifiable when a single motor coil is fed with the rated
    voltage and -not- the rated current (=spiralling I^2.R loss due to +3900ppm
    wire tempco). I've got no sense out of makers as to how they actually do
    their motor temp' rating tests. Is there some kind of 'industry standard
    constant-current derating factor', I've missed?.
    (I'm worried that I can't in honesty say to a customer "this chopper will
    drive a motor at it's rated current", knowing it could blow smoke if they do
    so :).
    John Jardine, Feb 27, 2004
  6. Losses in stepper motors depend a lot on the details of the driving
    circuit (pwm frequency, peak voltage, spike clamping, as well as
    average current.)

    I seldom run any stepper at its full rated current. If I really need
    the full rated torque, I prefer to move up a frame size to keep the
    temperature under control.

    Can you send me a copy of your schematic. I might be able to suggest
    some efficiency improving changes. But as a generality, using only
    half a winding at a time is not the way to keep temperature rise low.
    John Popelish, Feb 27, 2004
  7. Bryan Hackney

    Tauno Voipio Guest

    You may have met the other major loss: eddy and hysteresis losses in
    the iron of the motor. For a fast PWM, you may get an advantage over
    the high-frequency losses by using external filter chokes with
    core material and structure fit to the chopper speed.


    Tauno Voipio
    tauno voipio @ iki fi
    Tauno Voipio, Feb 27, 2004
  8. Bryan Hackney

    John Jardine Guest

    Thanks John. The points are appreciated. Nothing speaks better than the
    voice of experience ;-)

    As mentioned to Tauno, this aspect repeats at DC. Maybe it's why the motor
    makers seem rather coy about clearly specifying their test ratings.
    But If I may, I'll scan the circuit and send it through. A second pair of
    eyes would be welcome.

    John Jardine, Feb 27, 2004
  9. Bryan Hackney

    John Jardine Guest

    Indeed, ... These items were my first port-of-call but I lost further
    interest having noticed the same temperature rise occurs at DC.
    I.e apply rated voltage from a DC Power supply. Meter DC current. Sit
    looking bored for 30 minutes whilst occasionally tweeking the supply voltage
    to hold the current at it's rated value.
    John Jardine, Feb 27, 2004
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