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RE: Ballasting the secondary side of transformers



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

Well, first, I have a 120V input. I live in the US and have no available 240V outlets. Here are my calculations for the capacitive ballast, assuming the transformers are perfectly efficient:
P=IV=(15A)(120V)=1800W
1800W=I(4000)   I=450mA
4000V=(450mA)Z   Z=8889Ohm
X=Z=8889Ohm
X=1/(2(pi)fC)  at f=60Hz, C=.298uF

Maybe I did this wrong, but I'm not sure where you got 166uF. I don't have many resources at the moment - almost all the components I have came from disassembled equipment - so I just roughed the capacitance by connecting 3 microwave capacitors in series. As far as the transformers drawing 7A with open circuit outputs, I think it's probably just because they're cheap transformers. However, today when I plugged in the PSU, even with the spark gap connected, the whole thing only draws about 6A. There's something fishy here, probably some loose connection or something erratically arcing, but I'll try to fix it. Also, yesterday, while browsing some TC sites for some research, I came across Greg's Garage (http://hot-streamer.com/greg/) . He apparently had the idea first to use microwave caps as hv ballasts and his site has some great material. You may want to take a look.

Chris



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Sun 4/8/2007 8:56 PM
To: tesla@xxxxxxxxxx
Subject: Re: Ballasting the secondary side of transformers

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Chris,

Good point! Capacitive ballasting the hv side could be practical for
some setups. Considering your 4kV output on the MOTs with say a setup
of 240 Vac input with a 15 amp limit, your looking at about 80 ohms
of reactance which puts the cap at about 166uF. What value did you
use on your hv side and how did you connect it? (curious why the 7A
draw open circuit).

A pig limiting to 40 amps would use 6 ohms of reactance, so about
440uF. Of course, it's a fixed ballast, but that's not always a bad
thing (certainly can be done). Too bad, I once had about eighty 100uF
1.5KV caps lying around. Might have been able to series-parallel for
test. Only have one now. Sadly, I disposed of them and I didn't have
can crushers and coin shrinkers in my mind those days (dimensions
were around 4"x5"x9"). When you move from Minnesota to California,
the burden of heavy garage junk makes for rash decisions.

Take care,
Bart

Tesla list wrote:

>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:
>
>
>
>