Re: current transformer winding wire help please

"Rich Grise" <richgrise@xxxxxxxxxxx> wrote in message
news:pan.2005.04.29.20.23.39.98707@xxxxxxxxxxxxxx
> On Fri, 29 Apr 2005 03:19:57 -0700, Winfield Hill wrote:
....
>> No, no, why don't you first try the second part of my advice, "keep
>> the load resistor small." That's a simple quick fix.
>
> Maybe you can help me with a bit of brainlock here. If I put a CT, and
> short the secondary, how does that reconcile with the idea of an ordinary
> transformer with a shorted turn in the secondary? Is it just that the
> current will be 1/N, and the voltage will be trivially small, based on
> the resistance of the wire itself?

Exactly. It differs from your shorted turn only in
that the secondary current is available at the
terminals instead of looping inside the transformer.

> My only experience with a CT was when I built a 24VDC/120VAC, 1.2KVA
> inverter from a "kit", following instructions exactly: "7 feet of
> #24 teflon wire, wound on the toroid with about 7" left over for leads."
> The thing was, there were two different cores in the "cores" drawer of
> the parts bin, and I spent hours trying to figure out why the current
> limit circuit wasn't getting activated - then finally, in exaxperation,
> I wound another toroid, but with a _blue_ core, and it worked like a
> charm. Apparently, the core characteristics have a _lot_ to do with it.

One nice thing about current transformers is that, due
to the typically low voltages at the terminals, shunt
inductance plays only a small role in the response.
The downside is that magnetizing current directly
subtracts from the output current, unlike the situation
with a voltage transformer where, (approximately),
the output voltage is Ns/Np times the input voltage.
If core characteristics are affecting the output of a
current transformer very much, the core is too small
or lossy.

> I've always been kind of afraid of things magnetic, but the guy I was
> working for at the time designed his own ferroresonant transformers.
> We'd be standing at the bench, looking at a new ferro design, and it
> wasn't to spec, and he'd say stuff like, "Oh, let's just add a lam or
> two" or "Hmmm - maybe use 14-lam shunts" or "Maybe a 15 uF ferro cap"
> - in other words, Black Magic. ;-)

I knew a fellow like that once. When questioned,
it turned out he had well internalized the various
formulae that I still had to deal with on paper.

> But we always got it, usually within only a few hours, and with those
> exact #lams, #shunts, and cap, went right into production.
>
> Spooky. ;-)

Sometimes.

--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


.

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