Re: Sherline driver voltage was Re: Electrical wizards

>Are you talking chopper drive? L/R? "Appropriate" as in 60 oz.in. or
>125? Or 5v,1a vs. 3v,2a(and several others)?
>
>Because in all these cases ,there is a large speed difference between
>18 and 34volts, empirically determined. L297/298 chopper, and 5804
>unipolar. Back EMF itself will re-shape the torque curve, and when
>the mill is operating so "close" to the abilities of the stepper
>power available, that change in the torque curve translates
>into "meaningful" pretty quickly, IMO.

I was referring to what I think is about a 60 oz-in motor. The ones I'm
familiar with are, IIRC, 1v 2a. ( It's been a few years though )

I was also referring to a chopper drive when I referred to a "modern"
controller. In this case, it was a MOSFET bridge being driven by a PIC.

I did build an old fashioned constant current drive for this person once.
LOTS of heat, of course, even for these small motors. In quick testing it
performed at least as well as the chopper drive and I would not be the
least bit surprised to find that it might perform better in the real
world.

Certainly, it would be less sensitive to varying motor inductances. I
<have> had to alter chopper frequency with some motors. On one of my
machines, they're running at 10khz. At the more typical 20 khz, they really
don't work at all. Mind you, they are unipolar motors run bipolar, and
never designed to be run in a milling machine.

At any rate, with the PIC/FETs, there was very little difference in top
speed using either of these two power supply voltages. This test was done
with the motors on the machine and vise mounted on the table. Now, we didn't
approach the problem in an absolutely scientific manner, but did spend some
time tweaking acceleration curves.

However, I really don't care how fast the motors will run. How fast a machine
can do NOTHING is not that interesting to me as a hobbyist at home. Of more
interest is how things move when doing SOMETHING, e.g., removing metal.

In even less scientific testing of metal removal as a function of supply
voltage, we couldn't see any difference. We simply took deeper and deeper
cuts at low speeds, then returned to zero at medium speed. "Badness" was
deemed to occur when the return to zero error was outside of our arbitrary
spec. Again, supply voltage didn't seem to make much of a difference. Whether
flex in the Sherline or motors running out of poop, I can't say. Doesn't
really matter to the owner though, he's got a limit and some guidelines.

Alan

--

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 Sparta