RE: Ballasting the secondary side of transformers

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Breneman, Chris" <brenemanc@xxxxxxxxxxxxxx>

Are a resistive ballast and an inductive ballast the only 
options?  As I said in my last email, I'm trying out a capacitive 
ballast on the hv side, and it appears to be working.  A capacitive 
ballast is much more feasible on the high voltage end because the 
capacitance doesn't have to be nearly as large.  Also, I think there 
might be another plus for the hv ballasting, considering my setup in 
particular.  Previously, I was using a MOT with the hv winding 
shorted as a lv ballast in series with two other MOTs with the 
primaries in parallel.  Assuming that the MOTs have approximately the 
same inductance (particularly when the hv side is shorted), the 
ballast MOT will drop about 2/3 of the mains voltage, significantly 
decreasing the input voltage on the other transformers, and 
decreasing the output voltage.  This is important in a system where a 
mere 4000 volts is the ideal output voltage.  With a hv ballast, the 
impedance required to limit current to a sane amount should cause 
much less of a voltage drop.  This appears to be the case with 
me.  With a setup like this, there is however an additional 
disadvantage.  With the hv end open circuit, it draws about 7A from 
the mains, whereas with the lv ballast, it only drew around 3A.
Also, considering a hv inductive ballast, could an ignition coil be 
used?  The insulation should be sufficient for the hv, and it should 
have a high inductance, but I'm not sure what the current-carrying capacity is.

Chris



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Sat 4/7/2007 1:53 AM
To: tesla@xxxxxxxxxx
Subject: Re: Ballasting the secondary side of transformers

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Bart,

Ballasting on either side should be equally effective at limiting the
current.  Of course, on the HV side, the inductance value needs to be
the turns_ratio (n) squared times larger because it has n times the
voltage to deal with and needs to limit the current to 1/n times the
current on the primary.  This assumes an ideal transformer between
the LV and HV ballast points.

LV ballasting:      + smaller inductance needed, lower voltage stresses.
                           - larger current means larger guage.
                           - core is needed to get the inductance and
saturation needs to be considered.

HV ballasting:      + smaller current.
                           - HV insulation needs to be considered.
                           - inductance needs to be n^2 larger.
                           - core is needed to get this larger
inductance and saturation needs to be considered.

These are all of the plusses and minuses that I could think of.  If
others, maybe someone else could chime in and comment on what is said.

Gerry R.


>Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
>
>Can someone please tell me why we are still ballasting on the LV
>side of Pigs and PT's? This should be easy enough to do for a fixed
>current limit. The costs associated with a LV ballast almost demands
>we do this. The LV side is starting to appear very silly to me at
>the moment. Granted, there are HV concerns, but is it really a big
>deal? I get the feeling LV ballasting is simply convenient. However,
>it is also expensive (unless one builds a ferrite ballast).
>
>Just curious is anyone else has contemplated a high side ballast.
>
>Take care,
>Bart
>
>Tesla list wrote: