Re: progress report, and question

Tesla List wrote:
> Original Poster: Finn Hammer <f-hammer-at-post5.tele.dk>
> I asked around for neons, and would have to buy a new one at 125$ for a
> 8000V 50mA unit. Since I have the impression that neons are whimpy and
> fragile transformers, and I could get a 3kW core for the same money, + a
> bit, I bought just that. It is a BLUM modular core, which means it is 4
> separate sticks of transformer laminations. These laminations  are glued
> together, and ground to a fine finish where they mate, to form a U-I
> style core. The section is 2" by 2", and the winding window is 6" by 2
> 3/8". Voltage per winding: 0.88V.
> I will wind a transformer, 15000V, 0.2 A on this core, and now the
> question:
> I wouldn`t attempt to start the coil up, without a variac, so I`l buy a
> 16 A variac, (any European members got one?) but are there other ways
> that I should controll this beast. I have read about ballasting, but am
> not quite clear about, what it means.
> In the readme file of rotjit, the author mentiones that it is
> necessacary to use an inductor upstream, and I could wind a choke if
> needed be, but which walue should it have, then?
> Cheers, finn


Sounds like you are off to an excellent start! Good luck on the
home-brew transformer. As you indicate, the variac is essential, since
it permits a "soft start", and allows you to smoothly ramp the power
going to your system. A ballast is simply a current-limiting element
which allows you to control the amount of short-circuit current from
your HV transformer. The current-limiting element can be a high power
resistor bank or electrolytic resistor, a high-current inductor, a
capacitor bank, or some combination of these. A NST does not require
extenal ballasting, since it's internally constructed with magnetic
shunts. This causes the NST to have a very high leakage current, which
in turn self-limits its short-circuit current. 

Most coilers using potential or distribution (pole pig) transformers
tend to use inductive ballasting, since it is relatively easy to do, is
quite efficient, and provides excellent performance. Typically, the
input of a commercial AC arc welder, with the output short-cicuited, is
connected in series with the variac and non-limited HV transformer
primary. Maximum current is controlled by adjusting the welder's
adjustable current setting (effectively adjusting its inductance). Other
coilers have successfully used short-circuited microwave transformers
(good for short runs only...), defective NST's, commercial tapped
reactors, and some have even wound their own home-brew ballasting
inductors. An air-gap is usually inserted in the magnetic path such that
so that the ballast inductor does not go into hard saturation under full
load, and the wire size must be sufficient to handle the maximum current
you expect to be pulling off the mains. 

Typically, the leakage inductance of a distribution or potential
transformer is very low (Less than 1 mH) compared to the inductance of
your ballast, and can typically be neglected. You can approximate the
amount of loaded ballasting inductance you'll need:

   Let V = Mains input voltage
       f = Mains frequency 
       I = Maximum desired Mains Current (RMS)
       Z = Desired Inductive Reactance
       L = Required inductance (H)

       Z = V/I = 2*Pi*f*L
       L = V/(I*2*Pi*f)

For example, for 3 KVA and 240 VAC at 50 Hz, the resulting RMS current
would be 3000/240 = 12.5A. Solving for the required inductance:

       L = 240/(12.5*2*3.1416*50) = 61 mH

If you do decide to wind your own, make provision to either adust the
inductance or provide taps off the winding so that you can later select
other values for greater power control and higher-power operation.

Hope this helps!

-- Bert --