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On 4/28/16 11:14 AM, David Boyle wrote:
Hi, my TC uses 2 nst's, each rated at 15kv and 60ma. The outputs are
paralleled to produce 120ma. We travel around with it and demo it at
different locations. The problem is that it pulls about 33 amps at 120v
from the ac line. This is too much and I have to add more pfc caps.
I'm thinking what if I used 2 power lines that plug into different
outlets that gets me 240 volts - then each line would only pull 14 amps
each (with increased pfc) and we could take it just about anywhere. I
think I would add a 240 volt relay which would only pull in if both
phases were present. That way I disallow the state of only 1 transformer
on at a time. The outputs would be in parallel just as before and all
our problems are solved - yay!
I've done this kind of thing. It's harder to make work in a bulletproof
way than you think, but it's worth pursuing. If *you* the operator are
careful, you can have some indicator lamps that tell you what
configuration to use.
First off, you'll be bringing in two AC 120V lines. THere are four
possible scenarios:
1) They're fed off the same phase, and through the same breaker - so the
breaker will trip.
2) They're fed off the same phase, and through different breakers -
excellent, this works great.
3) They're fed off different phases, by definition they'll be different
breakers. Maybe good, maybe not.
3a) they're fed off the two sides of a standard 240 V (120-0-120)
circuit: you reverse the wiring to one of your transformer primaries,
so now the inputs are out of phase, and the output is in phase: GREAT
3b) they're fed off different phases on a 3 phase system (208/120Y) so
they're not exactly 180 degrees apart, but, rather, 120 degrees apart.
The voltage between the "hot" legs is 208V, not 240V
3B is the troublesome one. In light industrial and office parks,
208Y/120 is real, real common. What can you do:
a) cobble up some sort of array of capacitors to get the additional 60
degrees of phase shift
b) hook your two NST primaries in series, and connect it across the 208V
(ignoring the neutral), and use your variac to boost it to 240V (or live
with the lower voltage)
So the trick is, you need some sort of test rig that will tell you
"reliably" what the configuration you've got is. An AC voltmeter is enough.
Then, you need some reasonably safe way to reconfigure your primaries in
various ways. I'm sure some combination of switches can do it, but make
sure you can't inadvertently short things out.
I think you could do it with a "polarity reverser" switch (a DPDT) on
one power cord, followed by a switch that connects the NST primaries in
series or parallel.
As always, when you have a system with TWO AC plugs, you MUST, MUST
figure out some way to make sure that if one of the plugs is plugged in,
and the other is not, that the unplugged cord is not live.
In particular, you don't want some sort of scheme where backfeeding
through the transformers or a series connection is possible. You plug
your two cords in, set everything up, and someone pulls one of the plugs
out.
(I tried the "relay to detect the 240 when both are connected" and found
that with one cord disconnected, the relay still didn't drop out)
Good luck.
But things are never so easy so I'm asking for advice here first. Any
thoughts or experience with this? Thanks in advance!
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