Greetings all,

I'm working on balancing my rotary converter and need
a sanity check.

(Basic statistics for the record. 1125RPM 15HP idler.
Feeds off a 125A breaker in the shop panel, through
a 100A fused disconnect. L1 and L2 from the disconnect
feed L1 and L2 in a 3p panel on the rails (I might pull
single-phase 220 from this subpanel for other uses in
the shop). The idler and start circuit feed out, and
back in through the 3P panel's original 100A Main breaker.
Starting is the standard push-and-hold auto starting setup,
with a Grainger voltage-sensing relay to cut out the
starter switch when L3 comes up. I've forgotten how
much starting capacitance I have in the box, but it
spins the (kinda chunky) 15HP idler up in less than 2
seconds. When running, it'll currently spin up a 5HP
high-pressure blower (Invincible vacuum), in about 5
seconds, pulling about 45A on L1 and L2, and 23A on
L3. Once spinning, it pulls about 25A on L1, 19A on L2,
and 5A on L3. I don't currently have voltages on it.)

Just for gits and shiggles, rather than just tweaking
the balancing caps with an amp-clamp and DVM, I thought
I'd build a full-blown current and voltage monitoring
system for it.

For current monitoring, I've stuffed 3x 100:5 current
transformers into the 3P subpanel, and brought them out
to current-sensing resistors.

For voltage monitoring I need voltage in the range -1V -- 1V
to match my scope inputs, so I need to build a voltage-divider.
This would be no big deal, except I just tossed myself a puzzler
when getting ready to solder the bits together.

One could build 3 bridges (say 2MOhm and 2KOhm roughly,
just to make the numbers reasonably neat), and bridge
L1-L2, L2-L3, and L3-L1, thereby being able to measure
the between-leg voltages. *Alternatively one could bridge
L1-C, L2-C, L3-C (and possibly tie C to ground/N). I've
doodled an ASCII version of this below. Hopefully it survives
being posted. L1, L2, and L3 are as traditional. L1-a is 2MOhms,
a-d is 2KOhms. The other legs are symmetric.


L1 L2
. .
_ _
| |
_ _
| |
_ _
. a b .
| |
_ _
| |
_ _
. d
\
/
\
. c
\
/
\
/
\
/