Re: Transformer Ballasting

To: tesla-at-pupman-dot-com
Subject: Re: Transformer Ballasting
From: Tesla List <mod1-at-pupman-dot-com>
Date: Thu, 16 Oct 1997 21:31:28 -0600 (MDT)
Approved: mod1-at-poodle.pupman-dot-com


---------- Forwarded message ----------
Date: Thu, 16 Oct 1997 19:20:26 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Transformer Ballasting 



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Transformer Ballasting 
> Date: Thursday, October 16, 1997 5:18 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Thu, 16 Oct 1997 19:43:35 +0100
> From: Gwyn Zucca <gwyn-at-tcbod.demon.co.uk>
> To: tesla-at-pupman-dot-com
> Subject: Transformer Ballasting
> 
> Hello All,
> 
> I have tried searching the list archives for anything on the subject of
> current limiting power transformers, but I did not find exactly what I
> was looking for. The problem seems to be limiting the short out current
> without reducing the output voltage of the transformer. Obviously any
> resistance in circuit with the xfrm primary is going to develop a volt
> drop across it, and so the primary voltage will be lower than the rated
> output. The use of an inductive ballast, is going to have a similar
> effect. I have been experimenting with various arrangements, but have
> not yet achieved a satisfactory solution.
> 
> Any thoughts, experiences on this subject would be welcome.
> 
> See Ya...
> 
> Gwyn
> -- 
> Gwyn Zucca
> 

Gwyn,
The usual method for current limiting (or ballasting) power transformers
depends on the type of transformer being used.

Neon sign transformers are internally current limited by the use of built-in
magnetic shunts. So they don't require external current limiting at all.

Plate transformers, potential transformers, and pole pigs all require external
current limiting. As you note, adding a resistance in series with the primary
is one method, but it does create a voltage drop that is directly proportional
to current being drawn. By the way, you can also use ballasting techniques on
the *secondary*, but it is usually easier to ballast the primary, since the
voltage stresses there are much smaller. Of course, on the secondary the
*currents* are much lower... there are two sides to every coin.

To get around the problems inherent with resistive ballasting, you can use
inductive ballasting. This type of ballasting has the advantage that it kicks
in when the rate of *change* of current is high, and is relatively benign at
all other times. Since the moment of the gap firing is just such a moment, this
form of ballasting is well-suited to the coiling environment. Many coilers have
found that a bit of resistive ballasting in parallel with the inductive
ballasting can often help if you are experiencing a somewhat "bouncy" ride with
the inductance.

The inductance of choice for many coilers is the arc welder. You short out the
output of the welder by clamping the output cables together. The primary of the
welder is the "inductance" that you would then connect in series with the pole
pig primary (120 or 220). Used welders can be had for a reasonable price, and
they are hefty enough that they will take the abuse. You can normally select a
current rating with such arc welders, so there is a reasonable control
available.

Any inductor used with a pole pig is going to have to be capable of handling
the pole pig primary current. So they tend to be large and heavy. When not
coiling, you can always use the welder as a welder :)

Hope this helps.
Fr. Tom McGahee