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RE: Bargain Pole Pigs



Bryan Zublin wrote:

> with the large ceramic insulators.  We later switched to a smaller, lighter 
> unit called a "butyl rubber potential."  It is a transformer, used (I 
> believe) for power factor correction.  It has a 100:1 turns ratio.  We 
> supply it with 120VAC, output is 12KV.  The power rating is a few KW at 
> least.

These are used in metering applications.  They are basically the same as any 
other transformer except the turns ratio accuracy is very high on these units
and of course they have smaller VA rating because they only power meters
and associated equipment.  The secondary windings on these are almost always
rated 120 volts.  They call them potential transformers just to differentiate
them from current transformers which are used to measure current (of course).
The current transformers are usually rated xxx:5.  For example on a 400:5
CT if 400 amps is flowing on the primary side then 5 amps will flow in the
secondary circuit.  These things can be quite dangerous.  Even on a low 
voltage circuit the secondarys of these can produce tremendous voltages if
the circuit to be monitored is energized while the secondary current loop
is open. It usually destroys the CT and causes a lot of damage to the 
equipment being monitored.  They do build CT's that look like the high
voltage PT's, but you can avoid a mistake if you remember that PT's are
usually marked xxx:1 and CT's are marked xxx:5.

These can be hard to come by because they are rather expensive new,
about $900 for a 100:1 1500 VA unit, (if you think neons are expensive
go price these), and if good ones are taken out of service they are usually
kept as spares.
 
> We also bought some capacitors there.  They are used for power factor 
> correction.  Unfortunately, the capacitance is quite large for a Tesla coil, 
> around 2 to 4 uF.  This would require lots of power to charge.  We lowered 
> the capacitance by removing dielectric layers (what a mess - I'd never do it 
> again), but the capacitance was still too high.  Finally ended up buying 
> some other capacitors (0.02 uF, 45 KV) which worked great.

Aside from the the capacity being too high as you mentioned, these caps
are not rated for pulse discharge and probably wouldn't last too long
in a Tesla tank circuit.  Where did you get the the 45 KV caps?

-Mike