Re: 6 wire motor running bipolar

Hi Gerry,

I was saying that my drive can put out a maximum of 2.5A/phase.
Not quite the 3 amps per phase that you mentioned originally, but
resonably close. I was also saying that if you used those
motors in bipolar series, you would use about 1.5A/phase, which my
drive could easily handle, but with this you sacrafice torque at the
higher step rates due to increased inductance (twice the number of
turns of wire around the core). Thus the best solution is generally
to run in half-coil if you are using a bipolar drive.
The motor's biggest concern for "holding" torque is how much
current is running throught the coils and how many turns of wire
there are in the coils. If the motor is rated at 3
amps unipolar, you certainly do not want to apply more than this.
Applying 2.5 amps (half-coil) will only give you ~83% of this
torque. But, while you may loose some power in running at a lower
current level, you do get some power return if you run in
microstepping mode as power isn't wasted in the motor oscillations
you might see when running in full step mode. This becomes important
because you really want your torque while you are stepping,
not "holding". While stepping rather than just "holding", another
factor comes into play, and that is voltage. A higher voltage allows
you to change the current levels in your motor coils faster, thus
achieve faster step rates, while maintainging proper torque. For
many small systems (Sherline/Taig), 24V is a good voltage to run at
(my drives like this voltage just fine). When you start getting into
bigger machines (Bridgeport etc), your start needing higher
currents/voltages.

As mentioned, a safe operating voltage for my drives is 24V, and is
also a common voltage that smaller stepper motors are often rated
when torque charts are produced. See if you can find a torque chart
for your motors running at 24V at varying amperage. You should get
an idea if my drive will fit your needs, or if you need to move to
something that can handle higher voltages and currents like the
Geckos.

It is easy to look at motor rating tables and see torque ratings and
amps/phase rating. By going from a three amp motor to a two amp
motor and maintaining or gainging torque (as it looks like you've
done) should clue you in that
something else has to change too. Most likely, in the table will be
a coulmn with a "mH" or "inductance" heading. You should notice that
the inductance jumped up when going from 3 amp to 2 amp (and
maintaining or gaining torque). This increased inductance also leads
to an increase in the amount of time it takes to charge the coils.
The way they have a smaller amperage but similar or more torque is
that the wire used is thinner. It can't handle as much current, but
because it is thinner, you can put more turns inside the motor. At
some point you end up with a similar configuration as you would in
series-mode discussed above. Again, at a "holding" or slow step rate
this doesn't matter to much, but as you start to increase your step
rate, your high end torque will start lagging with the higher
inductance motors when the coils can't be fully energized before the
next step pulse comes in. There's really is no free lunch. You need
to pick out the motors that will give the the torque you need at the
speed you need, and then pick the driver to handle that. Of course
if the motors are really cheap, and you can sacrafice a little speed,
then maybe you can fudge a little on the drive too.

Jeff
http://www.xylotex.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "notoneleft" <ger21@c...>
wrote:

> Thanks Jeff. So, what your saying is, if I'm going to use 6 wire
> motors, I'll get the best performance if the motor's unipolar

rating

> is as close to 2.5a , and the bipolar half winding will have the

same

> rating. Will I get the unipolar rated torque, or less, or more?

Sorry

> for all the questions, just want to buy the best motors to use for
> your driver. I can get a good price on the vexta's, so it looks

like

> the PK268-2A will work better, as it's rated at 2 amps with more
> torque. I'm looking for the most torque in the 200-300rpm range, if
> that helps. Thanks again,
>
> Gerry
>
>
>
>
>
>
>
>
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp" <xylotex@h...>
> wrote:
> > Hi Gerry,
> > With the Xylotex drive, and 6-wire motors, you have two
> choices.
> > Bipolar series winding, and bipolar half-winding.
> > With bipolar series winding (end to end, center tap unused),
> when
> > running in microstepping mode (preferable), you actually want to
> use
> > HALF the rated unipolar rating, as you will be driving DOUBLE the
> > number of wires turns. The probelm with this winding mode is

that

> > you increase the inductance, thus the time it takes to fully
> energize
> > the coils. Becasue of this, you can end up loosing torque at
> higher
> > step rates
> > With bipolar half winding you use one end and the center tap.
> You
> > would then run the drive at full amperage (2.5A/phase). Although
> > this is not quite the full rating of the motor, you can still get
> > good performance at lower speeds, and MUCH better performance

than

> > series winding mode at higher speeds.
> > You never want to run a motor at more than it's specified
> amperage
> > rating, but almost always run it at higher than the specified
> voltage
> > rating. The most common voltage used for the Xylotex drives is

24V.

> >
> > Jeff
> > http://www.xylotex.com
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "notoneleft" <ger21@c...>
> > wrote:
> > > I'm building a small CNC router and I have a question about 6
> wire
> > > steppers. I plan on using a Xylotex driver. I've been doing a

lot

> > of
> > > research, as this is my first CNC project and I've come across

a

> > > conversion chart for unipolar and bipolar ratings. I'm looking

at

> > > some Vexta PK266-03B motors, rated at 3amps 2.3V unipolar. If
> what
> > > I've read is correct, when running these on the xylotex
> board "end
> > to
> > > end", do the motors then actually draw 2.1 amps @ 3.22V? And do

I

> > see
> > > a 40% increase in torque. Thanks.
> > >
> > > Gerry