[Home][2018 Index] Re: [TCML] "Terry" filter and safety gap questions for pole transformer [Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [TCML] "Terry" filter and safety gap questions for pole transformer



Hi Phil,

The type of spark gap really doesn't matter. All types of short air spark gaps break down within nanoseconds, and in a pole-transformer TC, typically at 20+ kV. The abrupt voltage and current transitions excite high-amplitude, multi-MHz oscillations within the stray capacitances and inductances in nearby HV supply wiring, AND within your unprotected transformer's HV winding. That pole transformers usually survive this environment is really a tribute to their robust insulation system.

It's a crap shoot. Maybe one in 100 pole transformers may eventually fail under the stress. Older transformers, with age or heat-degraded insulation, might be more susceptible. But it's painful if YOURS happens to be the failing one - particularly when prevention is relatively inexpensive and simple.

Bert

Phil wrote:
All,

I have always run my pig (~10kva) without any protection in the belief that a Synchronous rotary significantly reduces the likelihood of HV transients occurring, with ARSG's and statics being the usual candidates. Am I correct in this assumption, or as I'm now wondering, totally wrong?

Phil

www.youtube.com/user/follytowers


On 10/02/18 13:32, David Rieben wrote:
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
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla






_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla