TRANSFORMER CURRENT LIMIT
Subject: TRANSFORMER CURRENT LIMIT
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Thu, 13 Apr 1995 18:43:00 GMT
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* Original msg to: Esondrmn-at-aol-dot-com
Quoting Ed Sonderman:
ES> I fired up my new pole transformer last night. It seems that
ES> the inside tap is set at 14,400 volts. I apply 10 volts and
ES> get 600 out. 60:1 turns ratio.
That sounds perfect!
ES> I tried several experiments to limit the current into the
ES> primary. Right now all I have is two 120 v 10 amp variacs
ES> which I'm using in series.
This will not cut it on a pole pig. Especially in series. Some
people place two variacs in series; using the first variac to
control the voltage; and the second variac to control the
current. John Couture has diagrammed control circuit wiring in
his books showing this exact same layout. I STRONGLY ADVISE
AGAINST THIS PRACTICE! Variacs are not at all suitable for
current limiting without major core modifications. When
attempting to control shell wound transformers, such as pole
pigs, run the variacs in parallel (if at all possible) to help
them handle the current load, then use some other circuit
component(s) for ballast (resistive, inductive, or both).
ES> My goal is to limit the primary current to 10 amps without
ES> dropping too much voltage across the ballast. 10amps in at
ES> 220 volts would give me 2.2kva. This should be about 14kv
ES> at 160ma. Using my old neon sign transformers with the
ES> secondaries shorted as ballast, I was able to limit the
ES> primary current to 15amps at 50% on the variacs but I had
ES> about 100volts dropped across the neons.
I thought this might be the case. The problem is that the core of
the neon is inefficient due to the placement of the shunt plates.
Just as when you use a neon by itself for a power supply: without
correction the efficiency is limited to around 50%. Apparently
this is also true when the same core is used as a ballast.
ES> I set the pole transformer up as a jacobs ladder so I would
ES> have some current draw on the secondary. I never did get any
ES> more than a pitiful small spark - like a 12kv 30 ma neon.
ES> At first I tried both neons in parallel and then in series
ES> with the line side of the variac. I slowly turned up the
ES> variacs and when the jacobs ladder finally fired, it drew
ES> 45amps and burned a rough spot in my variacs.
Yeouch! You need to figure some smoother method of ballasting
this pig down, and until you get it all figured out I would put
some "safety" resistance, such as my old favorite the electric
oven element, in series to prevent surges such as this.
ES> My later testing was done with the ballast between the
ES> variacs and the pole pig.
My experience shows that if your are correctly ballasted, it is
preferable to place the ballast between the breaker box and the
variacs. For protection of the power cabinet and variacs I also
put in a replaceable fuse or breaker (and I use both) between the
ballast and variacs. This would have prevented your scorching the
ES> I'm sure at this point that I will need to go buy that used
ES> 240v 40amp variac that I've had my eye on.
Sounds like a winner!
ES> The big problem now seems to be how to control the primary
ES> current. I have Henry Transtrom's book Electricity at High
ES> Pressures and Frequencies on order at the library but I
ES> don't know when it will come in.
Lindsay Publications has copies in print for around $10.00. It is
a decent reference book (worth owning) and they can have a copy
to you in a week or so.
ES> Is it realistic to expect to limit the primary current to 10
ES> or 20 amps with only 10 or 20 volts dropped across the
Yes. But not at first. When the coil is being tuned in and the
control circuits are being matched to the power supply and tank
capacitance, and everything is settling down, your ballast will
make the coil managable, but at a cost in voltage. It will save
you ruined components and blackouts.
Later however, when everthing on the coil, including the control
circuits and spark gaps, are matched, tuned, and operating a full
power. It is possible to reduce or remove the ballast entirely.
You just kind of "open everything up" and the tiny bit of ballast
remaining really just smooths the coil operation out, it does not
Another question you ask yourself is: is all of my voltage drop
due to ballasting? No. High current draw by the Tesla tank
circuit will bring the voltage down even if no ballast is present
in the control circuits.
ES> Do I need to use an arc welder? I have one at work that I
ES> could take home and try.
They are nice because they are rated to efficiently limit current
with this same input voltage and amperage.
ES> This is a 5kva transformer. With 240v input, that means I
ES> need to limit the primary current to 21 amps. Assuming the
ES> ballast will drop at least 20 volts, then I can input 23
ES> amps at 220 volts for 5kva. Will it hurt this transformer if
ES> I allow the primary current to go up to 30 or 40 amps?
Nah, not a problem. You can run a pig at 100% over it's rated
current output for 3 - 5 minutes before it begins to warm up.
... If all else fails... Throw another megavolt across it!
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