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Re: Capacitors as current limiters

Hi Jim,
    Be careful with this one.  Maxwell uses this method on their 10 kW
+- 100 kv power supply.  It worked fine until I hooked a three phase
variac to it.  At a certain setting on the variac I got 0.1 Hz
resonance.  Every 10 seconds the variacs and transformer would emit a
loud hum.  I disconnected the Marx from the power supply during one of
these periods and got twin 15 inch arcs.  Huge overvoltage.  It blew a
phase in the rectifiers (200 1n5402s in series).  I learned a stiff
lesson there.  Oscillators dont have to be tuned to exactly the same
frequency to pick up energy (0.1 vs 60 Hz!).  Those low frequency
harmonics need to be considered very carefully when using capacitors
for current limiting.  If you bring the system up with a variac,
monitor the output voltage with a scope and a high voltage probe at
about every 10 volts VAC input.  Clamp the output with a spark gap for
safety (the transformer's and your's).  You may have to tune the
system.  You'll have to do the same thing all over again when you
connect the primary tank capacitor to the transformer output.  It'l
change the resonant frequency of the system again.  A nice light
design, iron-wise, but needs more care in the implementation.

Barry

----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, October 27, 1999 4:31 PM
Subject: Capacitors as current limiters


> Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
>
> It occurs to me that the function of the ballast inductance (i.e.
the
> welder) as a current limiter is to provide a reactive impedance in
series
> with the load (i.e. the pole transformer).  Has anyone tried using a
series
> capacitor to do the same? The power factor will be horrid, of
course, but it
> is the same with inductive ballasting, just of the opposite sign.
>
> A quick calculation shows that capacitance values in the range of
300 to 10
> uF would be required to limit the current in the 1 to 50 amp range
(from a
> 240 V line). These would need to be motor RUN caps or PFC caps, of
course,
> and would be quite expensive if bought new. Surplus is readily
available,
> and at least it would be potentially a lot lighter weight than an
inductor.
>
> There might be problems with interaction with the series inductance
of the
> transformer, particularly when lightly loaded.
>
>
>
>
>