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Re: [TCML] Ballast Inductor Question



Hi Stefan,

I will leave the fancy mathematics to others more qualified to discuss the matter, but I believe the bottom line is that if you are trying to process too many kVAs through a limited cross section of iron, you start running into core saturation
issues, where the core can no longer take in and process
any more Guass' of magnetism. Once you reach the iron
core's magnetic saturation point, any additional current or
voltage will only saturate the core and the current draw
will tend to "go through the roof" at this point. However,
by applying this formula to my ballast in my control panel,
it would appear that even my ballast is undersized for my
use to control the ~15 kVA to my Green Monster spark
gap driven coil system. (~11 sq. inches yields around 4.7
kVA, or roughly 5 kVA). However, I assume that since my ballast core basically consist of 2 "U's" of iron with about 1/4" of space between the opposing faces of the 2 "U's", and I have an equal sized coil of wire on each "leg" of the enclosed "Us", I'm "assuming" that the power hand- ling capacity is basically doubled.? The fact is that I've never felt my ballast core or the coils on it get notable warmth above ambient temperature, though and believe me, I've put that homemade ballast through its paces. ;^)

The short answer to your question is that your core does
seem pretty "slender" for the power that you're proposing
to control with your ballast (25 - 30 kVA!). Plus, you are on 50 Hz there in Europe as opposed to 60 Hz here in North America, so that would even further increase your need for more iron by just a bit, for a given power level.
I've managed to salvage old large cores from discarded
x-ray transformers before.

David Rieben




----- Original Message ----- From: <teslalabor@xxxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, February 25, 2010 6:45 AM
Subject: [TCML] Ballast Inductor Question


Hi,

I want to build a new ballast inductor for my big coil. It runs on 400V
2phase at arround 60-80A, 50Hz (Germany) using a bank of 12 or
16 potential transformers, voltage controlled via variac.

I allready have a core for the inductor, but I'm concerned about it's
size. Is it big enough for that power? The core is an U-I type with
adjustable air gap. Its cross sectional size is: 3,15" x 2,36" = 7,4
square inch.

What is the general difference between a small core, which needs more
windings for a given voltage and a bigger core, which needs therefore
less windings? In both cases the current is nearly zero when the gap is
closed and rises, if opened. So why need big cores? I have allready read
here the tumb rule "1kW - 2,5 square inch" or the formula P =
(A/0,16)^2. Following this, for 400V / 60A I would need an VERY VERY BIG
core! So I wonder if I could use my "small" 7,4 sq.inch core.

Coiler Greets from Germany
Stefan



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