Re: [TCML] Taming the Beast: Inductive Ballast vs. Variac, PFC

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi Jeremy

On Sunday 25 May 2008 11:12:52 pm Jeremy Scott wrote:
> Hello Coilers!
>
> I'm getting back into the 'hobby' after a few years of
> being out. A little rusty on my theory and math. I've
> accumulated some stuff since then and would like to
> build another coil using some parts from my old coils.
>
>
> At the heart of my system is a 3KVA GE Potential
> Transformer.

What is the input Voltage and  the output Voltage?

>
> Most of the things I'm confused about is how to build
> the proper controls to tame this thing without blowing
> breakers.
>
> INDUCTIVE BALLAST:
>
> In the past I've used a 500 foot 12 gauge wire spool a
> la Home Depot. The problem with this is that the heat
> it generates almost melts the spool it's on. I think
> this is because there is no cooling for all but the
> outer layer of wire.
>
> I'd like to build a more robust and controllable
> ballast.
>
> My idea is to wrap the 500ft of wire around a PVC pipe
> of a diameter that is the average (half) of the
> spool's diameter and then slide a chunk of metal in
> and out of the pipe to control current. That half
> diameter thing is just a guess. I have no idea what
> the reactance
> of the spool is or how to recreate it in a more
> heat-dissipation user-friendly adjustable version
> using common parts.

I have a spreadsheet designed just for that type of ballast.
It will calculate inductance and inductive reactance, as well as total length 
of wire, for each layer of a multi-layer solenoid using  Wheelers formula.
It calculates some other things as well, like power lost in the ballast due to 
resistive heating, etc. but those are only valid for 240V in 14400V out 
transformer. I need to change that. If you insert a core of iron wires, or 
ferrite type material into the ballast, it will increase the inductance 
considerably, but the spreadsheet only does the air core calculations. That 
will allow you to play "what if" and see what you can do with the wire you 
have. 

 http://deanostoybox.com/temp/

ballast.ods is the OpenOffice native format,
ballast.xls is for Microsoft eXcel. If you do not have either, OpenOffice is 
free and can be found at http://www.openoffice.org/

BTW there is an error in layer 20, but you will not get that far with 500 feet 
of wire, especially with THHN insulation.

> I'm also scared of turning my coil on full blast
> instantly. In the past I've used an expensive variac
> to dial the voltage up and look for problems, unwanted
> arcs etc, but I think it would be better to control
> the current instead. Perhaps both.
>
> I guess I'm looking for something that will limit up
> to 20A, so I can get all that the outlet will give
> without tripping a breaker.
>
> POWER FACTOR CORRECTION CAPS:
>
Here I will refer you to the recent thread:

Subject: Re: [TCML] PFC Question (again)

You are probably better off without them.

> Making the most of what I can get from the wall, I've
> got 6 oil filled 'can' capacitors wired in parallel
> across the primary. Each is 100uF @ 370VAC. I'm not
> sure if this is correct, I don't know the reactance of
> the transformer's primary to apply a formula. I did 6
> by trial and error, monitoring the current pulled from
> the outlet for the lowest value. It's probably
> completely wrong but it doesn't blow the breaker. :)
>
> Thanks in advance! -Jeremy

later
deano
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