On Jan 11, 2:03*am, "DoN. Nichols" wrote:
On 2009-01-11, willray wrote:
Greetings all,
I'm working on balancing my rotary converter and need
a sanity check.
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.
....
I hadn't originally thought of using the "Wye connected" meter
setup, but I can't currently think of a downside. *I'm
wondering if anyone who does this stuff more regularly might
know some reason not to do this, that I haven't thought of.
* * * * Yes -- one very good reason -- have you tried measuring the
voltage from ground to the center point? *I'll bet that it is a nasty
range above ground, since you have no isolation between the incoming
power line and the rotary converter.
Hi DoN - always nice to see familiar knowledgeable faces when I have
time
to drop in!
I haven't measured the center point, but I'm assuming it's off center.
The "Wye Connected" bridge network was my solution to let me monitor
the center - but of course you're right - if it's off center, that
screws up the leg-leg measures.
Filament transformers it is. I probably have enough identical ones
in a box somewhere here to put that together from spare parts.
Finding
that box, could take a while, so maybe I'll order some more...
* * * * * * * * * * * * * * * * * I'm suspicious that my
scope inputs don't have isolated grounds.
* * * * Most don't. *The probe ground is connected to the safety ground
pin on the power cord.
I let the smoke out of the first scope I discovered that on... I
"manually float" most scopes that cross my path now. I'm pretty sure
my Tek. scopemeter has completely isolated inputs, but I don't
trust that "pretty sure" enough to bet the magic smoke on it, and
I'm less-sure about the 4-channel Tek that I'm planning to use for
monitoring the 3P converter.
Ok. Back to the barn to search for transformers...
Thanks much!
Will Ray
* * * * You want the isolation that the current transformers give
you,
and similar isolation on the voltage side with the filament
transformers. *(Those used to be cheap enough and common enough so you
should be able to pick up three identical ones.
* * * * Good Luck,
* * * * * * * * DoN.
--
*Email: * * | Voice (all times): (703) 938-4564
* * * * (too) near Washington D.C. |http://www.d-and-d.com/dnichols/DoN.html
* * * * * *--- Black Holes are where God is dividing by zero ---