Re: generate 3 phase delta?

On Fri, 16 Dec 2005 11:40:25 -0800, BobG wrote:

> BE:
> This is a four pole hysteresis synchronous mortor, B-2700 series. It
> runs at
> 1800 RPM on 60Hz single or three phase, 115VAC. It gives one ounce-inch
> of
> torque at 1800 RPM, 12 watts.
> To run single phase connect the AC line to the Blue and Green leads.
> Connect
> a 2uF, 200VAC capacitor between the White and Green leads. To reverse
> direction, connect the capacitor between the White and Blue leads.
> ===================================
> Hi Bob. I saw where one could run it from single phase with a cap....
> but I wanted to see if I could drive it with a microcontroller
> somehow...wondering how to generate 3 phase. I noticed it put out a sin
> wave between any pair of wires.... that's why I thought it had a perm
> magnet in it. Sounds like a lot of work. There's a dozen or so of em at
> Skycraft.

Generating 3-phase in a uC isn't hard at all - just have three look-up
tables of 3 sine waves 120 degrees out of phase from each other, and
run 3 DACS to give you the three phases. Or, you could use one LUT and
three scrolling indices 1/3 of the way apart from each other, but that'd
be nasty ugly because your LUT would have to be 3N entries long. ;-) You
probably wouldn't need very big look-up tables; it's surprising how
quickly harmonic content drops off with just a few samples.

That also gives you absolute speed control, without relying on a
cap, but mind what John Popelish said about making the drive voltage
inversely proportional to the frequency. (at least, I _think_ that's
what he said - it's been a couple of posts ago. ;-) )

I don't know how to turn three, ground-referenced outputs into a
delta, however. I wonder what would happen if you just used three
half-H's, which, I guess, amounts to 3x totem-pole outputs? My intuition
says, "of course", but I haven't done the grunt work yet.

Good Luck!
Rich

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