Re: Two related questions, Sherlines and Steppers

Alan,

That is a common misconception. You would not change the inductance
one iota; you would only halve the resistance.

Rewiring the motor in parallel would have no effect on the number of
turns of wire the current passes thru and thus the inductance.

Think of it this way. Does it make any difference if a winding is
made from a solid wire or a stranded one? In your case it would have
two strands.

Torque is proportional to ampere-turns. This simply means torque
equals the amount of current multiplied by the number of turns of
wire it passes thru. The upper limit here is iron saturation; when
100% of the magnetic dipoles in the iron align with the flux path,
there can be no further increase in torque. Normally the iron is 80%
saturated at the motor's rated current, so there is nothing to be
gained here either. To get a majority of that last 20% would require
doubling the current because the iron's saturation curve will be
nonlinear at that point.

Using a current source instead of the motor winding resistance may
help some but it will be at the expense of considerable heating in
the current source pass transistor. Also mind the fact the motor also
is a dandy 2:1 step-up autotransformer as well; at 75VDC your phase
transistors will see 150VDC or more.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Alan Rothenbush <beer@s...> wrote:
> Mariss;
>
> Thanks for the reply.
>
> Given your guesstimate of 30 oz-in, I may elect to not use any
reduction
> pulleys, which should help the speed. The table only has 3 inches
or so of
> travel, so these small numbers are less troubling than first
appears.
>
> I could run it unipolar with a genuine constant current source on
the top
> end ( as opposed to an R/L drive ). I should have some LM783s
around .. good
> for 75 volts and a single resistor turns them into a CC source.
I'd just have
> to find some high voltage driver transistors .. out of an off-line
switching
> power supply, perhaps.
>
> But it would seem to me that if wired in bipolar mode, I'd cut the
inductance
> in half. With a suitable drive, that should get me double the
speed and twice
> the holding power ( more or less ) as in unipolar mode. I
understand that
> reality might well rear its ugly head, but this SHOULD be an
improvement, no ?
>
> And that being the case, any thoughts on how would I rewire the
motor ?
>
> Alan
>
>
>
> > Speed certainly won't be an issue with those motors because there
> > won't be any particularly with a 4:1 reduction.
> >
> > What you have there are some size 17 motors, probably about 30 in-
oz
> > in holding torque. I'm guessing you have a 20TPI leadscrew so you
can
> > calculate "push" in lbs as being 2pi * 20TPI * 4 * 30 in-oz /16
or
> > 942 lbs (neglecting screw efficiency). The problem will be speed;
at
> > 24VDC those motors may manage 3 revs per second at best. Figuring
80
> > revs per inch (TPI * reduction ratio of 4:1) you will get 3/80
inches
> > per second or 2.25" per minute.
> >
> > There isn't much point in rewiring the motors because you will
still
> > have way to much inductance to make it worthwhile to run them
with a
> > switching type drive.
> >
> > You may wish to think about re-motoring with higher current
motors.
> > If you insist on running them anyway, plan on using an R/L drive.
> >
> > Mariss
> --
>
> Alan Rothenbush | The Spartans do not ask the number
of the
> Academic Computing Services | enemy, only where they are.
> Simon Fraser University |
> Burnaby, B.C., Canada | Agix of
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