Re: [TCML] PFC Question (again)

["DC Cox" <resonance@xxxxxxxxxxxx>]

Bart & John:

I don't use variacs anymore.  I just use a larger ballast and a motor drive
to slowly remove the laminated core over a period of 15 seconds.  It gives
precise control of 220 Volt current over a wide range.  Be sure to meter the
current.

When the contactor closes 220 V is applied across the pole xmfr, however,
due to magnetizing current (typical 3-4 Amps), with the core all the way in
there is not enough current to charge the cap bank, so no operation as this
low current can barely provide enough current to magnetize the xmfr core.
Then the core is slowly pulled out and the current increases to the value
you require.  Of course, you have to take care to wind the ballast with
really large wire such as 6 AWG, and we use small 3/16 x 3/16" phenolic
spacers between winding layers.  I usually wind around 280 turns on the core
box (takes more because a Variac itself, with it's inductance, also operates
as a limited semi-ballast).  With a pole xmfr this gives (short circuit)
approximately 3-4 Amps.  I use a small reversable DC motor drive (Grainger)
with a lead screw to pull the core out and I can set any value from 3 Amps
to 70 Amps without an expensive variac!  Typical core with 3 x 4.5 inch
cross section (larger for 15-20 kVA systems) seems optimum.

Variacs are really a thing of the past as current control is necessary and
voltage control totally unneccesary in most applications involving PTs and
pole pigs.

Dr. Resonance




On Tue, May 27, 2008 at 2:18 PM, <FutureT@xxxxxxx> wrote:

>
> In a message dated 5/26/2008 9:02:15 P.M. US Eastern Standard Time,
> bartb@xxxxxxxxxxxxxxxx writes:
>
> >But,
> >you say that you are surprised that coilers haven't  done tests in this
> >area. I'm not so surprised myself. This area is not  well known. To some
> >degree it is, but little has been done regarding  testing.
>
> Bart,
>
> Well the reason I said I'm surprised more testing wasn't done is
> precisely because not much is known.  There's always coilers  who
> "want to know", so it would make sense for folks to do testing in
> this area.  Maybe more tests will be done soon.
>
> >I run STR with a 10kva pig, but I also use a variable current  ballast. I
> >have both voltage and current control for any coil using my  control
> >cabinet, but there is a limit. Two 4th of July's ago I smoked  my variac
> >ballast. No short (very lucky) so I was able to clean up and  continue to
> >use. However, I am more cautious these days. I can't go  over 7 kva
> >without ballast issues.
>
>
> In one test I ran a PT powered 600 watt ballasted 120 bps coil.   Using
> my homemade ballast the coil drew 600 watts, but using a 20  amp
> variac as a ballast, the coil drew 1200 watts.  That's when Dr.  Resonance
> suggested cutting a slot in the variac to prevent saturation of the  core.
> I think with STR operation the slotting may not quite as  important?  It
> may be very important at 120 bps.  (I'm not sure if every 120 bps  coil
> will
> behave the same in this sense.)  Slotting seemed to be very  much needed
> because only a few turns of the  variac were needed at 120 bps.
>
> Cheers,
> John
>
>
> >Take care,
> >Bart
>
>
>
>
>
>
>
>
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