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Re: Ballast choke questions

Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>

Hi Weazle, all,

> 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.

This core sounds ideal.  About a year ago I got two ballast inductors
made for TC duty by a local transformer manufacturer.  These units use
laminated E-I cores that are similar in size to your core.  Each unit
allows the current to be adjusted over a range of 1 Amp to around 40
Amps by varying the air gap.

They perform very well in TC service with no noticeable saturation or
bucking problems,  and are one of my better purchases for this hobby ;-)

> 1) How much power should a core of that size be capable of?

My units have a similar core cross section and each is good for
ballasting up to 10kva.  The air gap takes care of core saturation,
so the main concern is using the thickest wire that you can fit into
the winding window.  This keeps heating in the winding to a minimum.

> 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?

Not interleaved.  Stack all of the E's together.  Try to get a nice
smooth surface where they meet the block of I's.  This ensures you
can get down to the lowest possible current settings by minimising
any remaining air gap.

> 3) If all E's are aligned, should there be an insulator put
>    into the gap between the E's and the I's?

YES, definitely !  Its job is not to insulate electrically,  but to
act as a non-magnetic gap in the core.  This air gap is essential to
get the correct operating behaviour,  otherwise the iron core will

> 4) I'm going to put multiple taps on the choke windings for
>    selecting different currents.....

I was advised against tapping the winding,  as it does not make very
efficient use of the available winding window,  and gives a fairly
limited range of control.  (I can explain this further if required.)
You need the air gap anyway to prevent saturation,  so adjusting the
gap to control current seems like a simple and effective method.

Basically,  you wind enough turns of wire onto the core to prevent
saturation with NO AIR GAP and FULL RATED VOLTAGE applied. This gives
you the maximum inductance (minimum current) end of the scale.  The
non-magnetic gap is then made progressively wider to decrease the
ballast inductance and increase the current flow.

It is practically impossible to saturate the core in this design,
since the maximum current occurs with the biggest air gap in the
magnetic path.  I also like the fact that there are no moving brushes
or switches which have to handle large inductive currents.

> Any and all comments are welcomed.  I want to do this once and do
> it right the first time.  Thanks in advance.

Some info about my inductive ballasts can be seen here:


Click on the link to see plots of the V/I characteristics.  I can't
promise this design will work perfectly in all situations,  but I do
know that several other coilers have reported excellent results using
this method too.

Let me know if you need any more information.  I will try to help.