50-AMP remote meter needed

[Dimitrij Klingbeil]

"John Fields" <jfields@austininstruments.com> wrote in message
news8gfrvs60f39tqd06rbpflur27okcf10pk@4ax.com...

On Sun, 16 Nov 2003 17:55:42 +0000, Fred Abse
excretatauris@cerebrumconfus.it> wrote:

On Sun, 16 Nov 2003 09:43:10 -0600, John Fields wrote:

There is no such thing as a "current to voltage converter" transformer.

What you're describing is using what's called a "current transformer"
and
a "burden resistor" to generate an AC voltage which is proportional to
the
current flowing in the primary, which is the single wire running
through
or looped through the center of the toroidal core.

Sorry, John, beg to differ here. Any transformer having current in the
primary will induce a voltage in the secondary. There is no need for a
burden resistor. OK, I know there's no such thing as an infinite
impedance
measuring device, but a 200 meg input DVM is reasonable approximation,
and
I've used one of those with Rogowski coils before now .......

---
I suspect that if you were to use a current transformer with an unloaded
secondary in this application you'd be in for a nasty surprise!

Let's take, for example, a current transformer with a 1000 turn
secondary and a single turn primary (an unlooped wire going through the
center of the toroid) with 50A going through it. Since the turns ratio
of the transformer is 1000, 50A flowing through the primary will try to
induce 50/1000 = 50mA in the secondary. So, since E = IR and the
unloaded secondary looks like an open circuit, the voltage induced in
the secondary will be E = IR = 0.05A*infinite ohms = infinite Volts!

Looking at it a little more realistically, with your 200 megohm load on
the secondary the voltage would only rise to 5.0E-2A * 2.0E11 ohms =
1.0E11V = 100,000,000,000V, but it might still be a good idea to load
the secondary down a little more heavily to get the voltage down to
where your meter can measure it on a more conveniently available range.

Sorry, but this makes no sense at all. The maximal voltage a trafo's
secondary can produce at a point in time is proportional to its number of
turns times the first derivative of the primary's current. It is quite
obvious that none of these factors can be infinite (apart from spikes when
that thingie is switched on and off, but 2 zeners or this like will handle
that). The assumption that 50A in the primary will (in this case) try to
induce 50mA in the secondary is completely wrong. It will induce a limited
maximal voltage (depends on the transformer, the limit may be higher than
guessed, so take care) at zero current. It will go down a little with a
substantial load, but a DMM will hardly make any change (at least not in the
range it can measure). The 50 mA you mentioned will be there if (and only
if) the secondary is shorted, it will be the maximal current ever reachable
in the secondary with this setup at zero V out. You can easily test it by
connecting any trafo with a very small number of turns on the primary in
series to a lightbulb and checking the secondary's voltage. It will never
exceed a certain limit that is proportional to the primary's current
(Warning: This limit may be high!).

Dimitrij

 

[ltj]

I've found a 4-20mA transmitter for $140:
http://www.davis.com/showpage.asp?L3ID=1075

and a digital panel meter with dual setpoint for $290:
http://www.tmgp.com/cgi-bin/nph-tame/laurel/laureate-dpm.tam

Are these good choices?
thx

"ltj" <ltj@yahoo.com> wrote in message
news:3fb4f006$0$89724$a0465688@nnrp.fuse.net...
We have a large industrial motor that draws up to 50 AC amps that we
need
to
monitor in the control room 100 feet away. Looks like a current
trransormer takes our 50A and drops it to a 5A (full scale) level, which
we
would run to a panel meter. I'm not sure this is a good way to go.
Perhaps
a
"permanent" clamp-on amp meter that outputs something besides mA-A
signals
would be better? Looking for suggestions...thx