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Author Topic: stepper motor, bipolar vs. unipolar, best torque  (Read 1138 times)
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Hi All,

I'm experimenting with 2 stepper motors.

1) 5VDC, bipolar, Torque - Holding (oz-in / mNm): 1.5 / 10.6 
http://search.digikey.com/ca/en/products/26M048B1B/403-1005-ND/417812

2) 12VDC, unipolar, Torque - Holding (oz-in / mNm): 1.3 / 9.2
http://search.digikey.com/ca/en/products/26M048B2U/403-1004-ND/417811

My drive circuits are exactly as per the examples show on the Stepper library page http://arduino.cc/en/Reference/Stepper?from=Tutorial.Stepper

4-pins used in both circuits.

I wasn't expecting much difference in the torque with these two configurations, but there is a HUGE difference.  The bipolar works fine while the unipolar has very little torque, if I touch it lightly with my fingers it stops.

This is the Darlington I'm using with the unipolar:
http://search.digikey.com/ca/en/products/ULN2004ADR/296-15067-1-ND/562615

Any ideas about why the unipolar setup is so weak would be much appreciated.

Thanks,
Francis
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A unipolar will generally provide less torque than a bipolar since it only uses half of a winding when driven in unipolar mode. You don't have to drive your unipolar motor in unipolar mode -- just treat it like a bipolar motor and drive it using the "outside" winding terminals. The holding torque for the unipolar motor was probably specified as a "salesmanship number", i.e., as large as possible, when driving the motor using the full winding (bipolar mode).

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Thanks, thats helpful.  The only issue I have with running the 12v unipolar in bipolar mode is that I won't be able to get the full rated current (torque) at 12v because of the extra resistance from using the whole coil...  I guess there is no way around that short of increasing the voltage?
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