Jim,

Yes, you certainly can - PROVIDED the center taps of the two phases are not
connected together inside the motor.  Generally speaking, if the motor has
six wires coming out of it you can use a bipolar drive.  If it has five
wires coming out you probably can not.

As far as torque is concerned, that is determined by the current in the
windings and how many windings are energized at a time.  When a motor is in
a unipolar drive setup there will be current in only one half of the winding
for a phase at any given time.  When a motor is driven in a bipolar
configuration you will get the same holding torque from the motor for a
given current by connecting the bipolar drive between the center tap of a
phase and one end as you would get in a unipolar situation.  If you connect
the bipolar drive across both halves of the phase windings you may get even
more more holding torque from the motor than in a unipolar setup (depending
on the amount of iron in the motor and the strength of the rotor magnets).
BUT you WILL almost always get considerably more running torque - at least a
low speeds - in this configuration than with a unipolar drive.

This is a vast oversimplification.  There are many more factors to consider.
But go ahead and do it because a bipolar drive on a stepper will almost
always get you more torque from the motor, other things being equal.

Bill K.
-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jim Tellier
Sent: Wednesday, December 03, 2003 7:48 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE:] Stepper Motor question


Can anyone tell me if it's OK to use a "unipolar" stepper (i.e. one that has
center-tapped coils) *as* a "bipolar"?   i.e., just ignore the center tap
and use the two series-connected coils as one.  I realize that this might
not be as low of a resistance value as what I might see with an actual
"bipolar" single-coil device, but the question is... will it work?   My
instinct is that it should, but perhaps with less startup torque.  What say
you all ?
Thanks!
Jim

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