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. =A0Alternatively 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 \ / \ / \ / . L3
This setup appears allows metering of both the between leg voltages by metering across both middle .1% resistors on a leg pair ( ie, a to b, b to c, c to a) and the Leg->Neutral voltages if the d is tied to ground/N.
I hadn't originally thought of using the "Wye connected" meter setup, but I can't currently think of a downside. =A0I'm wondering if anyone who does this stuff more regularly might know some reason not to do this, that I haven't thought of.
Hmm - while I'm at it, another question -- Is there any reason that I can't tie together one leg of each current-sensing resistor for the current monitor part, and use this as a "common", for metering purposes? I'm suspicious that my scope inputs don't have isolated grounds.
Many thanks for your time, William Ray