Stepper motor driving- Unipolar Vs Bipolar

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Hi

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

 Thanks
Richard





Re: Stepper motor driving- Unipolar Vs Bipolar
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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.

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Re: Stepper motor driving- Unipolar Vs Bipolar

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Yes.
A bipolar drive benefits from less winding resistance and can give upto 40%
more torque.



Re: Stepper motor driving- Unipolar Vs Bipolar

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...for the same copper losses.
--
John Popelish

Re: Stepper motor driving- Unipolar Vs Bipolar

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40%
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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




Re: Stepper motor driving- Unipolar Vs Bipolar
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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

Re: Stepper motor driving- Unipolar Vs Bipolar
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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


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