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Re: Ballast choke questions

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Weazle, 
           I won't repeat what others have said but a point worth 
noting for anyone wanting to use an existing transformer as a ballast.
When the airgap is set to a minimum (max L), you can safely assume 
that the number of turns in the original transformer primary is the 
minimum number of turns you want in your ballast *as long as the 
inductance so formed does not resonate with the primary cap*. So a 
few more turns is better. Now you can easily calculate how heavy the 
wire can be to fill up the winding window with just that number of 
turns (i.e. secondary windings now absent). There should be around 
50% more window to fill up.

        I concur with the idea of varying the gap rather than tapping 
turns which is a messy approach. There are pros and cons to consider 
for both approaches however:

Setting L by gap variation:

Pros - gives continuously variable inductance (a con for the tapped   
       winding approach)
     - leaves no unused turns for any L setting (a con for the tapped 
       approach because tapping turns out leaves an autotransformer   
       with elevated voltages across the unused portion of the        
       windings and this requires extra insulation from the core)
     - doesn't require a tapping switch with attendent contact ratings
     - allows a single wind (easier)

Cons - same wiregauge for all currents which is highly inefficient in 
       its use of the winding window (a pro for a variation on the    
       tapped approach - something I've pondered in relation to       
       transformer applications and is currently submitted as a       
       design idea to a British Electronics magazine).

      Without the variation mentioned (which hopefully will appear in 
print in due course), the most efficient way of using the window is 
to grade the copper size for each tapping to minimize I^2.R losses 
for all settings.


On 8 May 01, at 22:50, Tesla list wrote:

> Original poster: "J. B. Weazle McCreath by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <weazle-at-hurontel.on.ca>
> Hello Coilers,
> I've spent the last two evenings disassembling a large transformer
> that I acquired with the intent to rewind it as a ballast choke to use
> with a 5 KVA pole pig I'm getting.
> The stack of E and I laminations measures 3.5 inches thick, with
> external dimentions of 7 inches by 7.75 inches.  The center most
> leg, or the middle of the E measures 3.5 inches by 2.25 inches.
> The two "windows" measure 5 inches by 1.5 inches.
> Now for the questions to you transformer and choke experts:
> 1) How much power should a core of that size be capable of?
> 2) When re-assembling the core, should the laminations be
>    put back interleaved as originally found, or should all
>    of the E's be put together facing the same way?
> 3) If all E's are aligned, should there be an insulator put
>    into the gap between the E's and the I's?
> 4) I'm going to put multiple taps on the choke windings for
>    selecting different currents.  Should the first 79 turns
>    as per the table below go nearest to the core, or on the
>    outside of the windings?
> Quoted from a post by Bert Hickman:
> >The example below assumes a 250 turn ballast
> >and a 10:1 current range in 10% increments.
> >
> >  
> >           Multiplying   Turns
> >            Factor:      at Tap
> >
> >   Nmax      1.00        250    Total number of turns (Max. L)
> >   N(90%)    0.95        237
> >   N(80%)    0.89        224
> >   N(70%)    0.84        209
> >   N(60%)    0.77        194 
> >   N(50%)    0.71        177
> >   N(40%)    0.63        158
> >   N(30%)    0.55        137
> >   N(20%)    0.45        112 
> >   N(10%)    0.32         79
> >
> Any and all comments are welcomed.  I want to do this once and do it
> right the first time.  Thanks in advance.
> 73, Weazle, VE3EAR/VE3WZL
> Listening: 147.030+ and 442.075+
> E-mail:    weazle-at-hurontel.on.ca
>            or ve3ear-at-rac.ca
> Web site:  www.hurontel.on.ca/~weazle