Hi Steve,
As Bert has already pointed out, pole transformers, i.e. 'pole pigs'
are not totally impervious to being damaged by the HV transient
"nasties" that are always generated by the active resonant circuit of
a spark gap driven TC. That being said, pole pigs are certainly the
transformer of choice that is available to serious coiling hobbiests
that crave plenty of power and their robust design certainly renders
them VERY resistant to sustaining damage due to the aforementioned
phenomenon.
Although you didn't mention the resistance rating of your wire wound
resistors, just the POWER rating (100 watts), I assume that they are
also rated at 100 Ohms, from the figures that you plugged into your
I*2 x R formula? If that's the case, then yes, your setup should be fine.
I originally ran my big coil (up to around 18 kVA with 12 to 13 ft.
arcs!) with ZERO protection (and NEVER had a mishap with the
transformer), but eventually decied that I should at least attempt
some form of protection. Since all of my protective horn gap attempts
for a pole pig output usually failed to extinguish once it "fired",
due to the heavy current arc that formed, I personally have been using
distribution arresters for this purpose. (Yup, the devices that your
local utility opower company use to protect their equipment from line
transient spikes, due to switching surges and lightning strikes). I
place them in parallel with the output of my pole pig power supply
(also a 10 kVA, 14,400-120/240 volt unit) to address this issue:
https://www.ebay.com/itm/HUBBELL-OPTIMA-DIST-ARRESTER-2137157314-PDV100-HD/272071557889?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D44039%26meid%3D5a3f17733d7642709f701c4fa0ba8f8f%26pid%3D100005%26rk%3D1%26rkt%3D1%26sd%3D272469016073%26itm%3D272071557889&_trksid=p2047675.c100005.m1851
With a little persistence, these can sometimes be found on eBay for <
$50. I originally started with an 18 kV rated unit, but it eventually
died as a short circuit. An 18 kV rated arrester is generally rated
for 15.8 kV MCOV (maximum circuit operating voltage - I believe) and
my varaic stack was able to provide up to 280 volts input, which would
translate to 16.8 kV out. My local power company uses 18 kV arresters
on their 13.2 kV primary line circuits. After this incident, I
upgraded to a 27 kV rated arrester and have been running without a
hitch ever since. I also more recently located some 21 kV rated
arresters and have replaced one of the 27 kV rated ones with this. (I
use the higher rated one right at the (Async)SG assembly and the lower
rated one at the transformer's output).
I have personally had far more issues with the nasty transient
kickbacks on the primary LV side of my pole pig than the secondary HV
side. As stated before, although the HV windings of a pole pig are not
totally bullet proof to these kickbacks, they are much more robust
than the fragile NSTs or OBITs and are actually quite unlikely to
sustain debilitating damage from this issue. The MUCH bigger problem,
for me at least, has been what the transformer sends down stream to
the components on the LV voltage input to the transformer. I use two
industrial line filters that are rated to handle the amperage (and a
small line filter at the input of each of my two cabinet cooling
fans), as well as three paralleled 1 KW rated, 1 ohm, ribbon style,
wire wound power resistors (for 0.33 ohms total) in series with my
inductive ballast, AND a pair of 150 Vrms MOVs between each 'hot' leg
of the incoming 240 volts and the ground to combat this problem!
https://www.ebay.com/itm/Panasonic-ERZC20EK241-150Vrms-Metal-Oxide-Varistor-Chassis-Mount-3-pcs/160706951833?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649
Finally, I typically run my coil with about 75 to 80 amps draw with
variac set for about 200 volts input to the transformer, so that
translates to around 15 to 16 kVA and my transfomrmer never even
breaks a sweat processing this rate of power. So you definitely have
plenty of room for "expansion" with your power!
Hope this helps,
David
----- Original Message ----- From: "Steve White" <steve.white1@xxxxxxxxx>
To: "Tesla Coil List" <tesla@xxxxxxxxxx>
Sent: Friday, February 09, 2018 5:37 PM
Subject: [TCML] "Terry" filter and safety gap questions for pole
transformer
My spark gap coil has been running for about 6 months with a 10 KVA
pole transformer for power. I pull about 4.8 KW from the wall. The
only transformer protection that I am currently using is a horn gap
in parallel with the spark gap. I have had no trouble with it but
lately I have become concerned with damaging the pole transformer.
Because of this concern, I am in the process of building a low pass
RC ("Terry") filter for additional protection. I have 2 questions
regarding the resistors. I see that people are using standard
wire-wound resistors. I am surprised that the 14.4 HV RMS does not
flash across the windings in the resistor. Is this type of resistor
indeed adequate for such high voltage? The second question concerns
the power rating of the resistor. Since my worst-case RMS current
from the pole transformer is no more than 0.5 amperes, the power
dissipation should be no more than about 25 watts.
Power = (I**2) * R = 0.5 * 0.5 * 100 = 25 watts
Therefore I assume that a pair of 100 watt resistors, one on each HV
lead, should be more than adequate even for a pole transformer. Is
this correct?
I also have a safety gap question. My safety gap is just a simple
2-terminal horn gap connected in parallel with my RSG. I see that the
safety gaps used for NSTs consist of 3 terminals: 2 for the high
voltage leads and a central ground terminal. For pole transformers,
is there any advantage favoring the 3-terminal gap vs the 2-terminal
gap?
Steve
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