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Stepper motor driving- Unipolar Vs Bipolar

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

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  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.
     
  2. Richard

    Richard Guest

    Hi

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

    Thanks
    Richard
     
  3. John Jardine

    John Jardine Guest

    Yes.
    A bipolar drive benefits from less winding resistance and can give upto 40%
    more torque.
     
  4. ....for the same copper losses.
     
  5. John Jardine

    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 :).
    regards
    john
     
  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.
     
  7. Tauno Voipio

    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.

    HTH

    Tauno Voipio
    tauno voipio @ iki fi
     
  8. John Jardine

    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.

    regards
    john
     
  9. John Jardine

    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.
    regards
    john
     
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