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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Overall, my first trial went well. I was able to get to a mains input voltage of 100v and my streamers were about 4 1/2 to 5 feet. First, the problem is that when I got between 90v and 115v the TC would just stop. The spark gap stopped firing and the coil's streamers abruptly stopped. I would then lower the variac to almost zero and then when I powered up again everything worked out well.
Now, the setup. I have a 20A variac going into a RFI filter with a volt and ammeter. Two 15kv/60ma Franceformers. One of them has a GFI and self-diagnostic; the other one has nothing. There is about 270uf of capacitor correction attached to the NSTs. There is a "terry filter" consisting of a spark gap, 225w/1k resistors x 2 and two doorknob capacitors. The primary capacitor is 0.05uf/50kv and the primary coil is 1/4 copper tubing forming an inverted cone with about 8 1/2 turns. There is a strike rail going to ground. The secondary is 29 1/2" 20 awg magnet wire on a 6" acrylic form. The toroid is about 2 1/2 to 3 feet in diameter (I don't remember the exact measurement). The spark gap is a sychronous motor that is controlled by a phase-shift controller that I have next to my variac.
So, I start it up and begin to get spark-gap firing at 50v. I tune the phase-shift controller to get the best spark gap firing. Then I start cranking up the variac and the streamers begin to sing. Soon, they are making 4 foot stikes onto a metal ladder. Sometiimes thick white ones hit the strike rail. The current draw is about 12 amps. After each strike I can see a momentary jump in the current draw. Suddenly, the spark gap stops. The rotary motor is still going. I drop the variac down to zero and start it up again. Everything works just fine?!
So, has anyone had an issue like this? Could it be that my rotary motor is getting interferance and is loosing sychronization? If this is the case, should I keep the spark gap far away? I guess I could eliminate this possibility by getting rid of the controller. Could the GFI on one of the NST be kicking it offline? The other thing I can think of is that the spark gap on my Terry Filter is being activated, but I can't get close enough to see. Here, I guess I could change the positioning to get a better look.
Any thoughts and help would be appreciated because I am sure someone out there has encountered this before.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
First off: Cool! Sounds like you're getting some decent sparks there.
Now on the problem: Doesn't sound like the GFI. If it were that, I'd expect that you'd have to push a "reset" button to get it going again.
Could be your Terry filter, if something is wrong with it...or maybe one of the NSTs is arcing internally? Can you still draw a nice arc off of each of them? Hmmm...
Aaron, N7OE
EDIT: You might see what kind of current (if any) is being pulled when the coil is in this pseudo-off state. And try without the Terry filter in place, if just briefly. Best to eliminate as many variables as possible.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I was going to do that...start eliminating components. The problem with "drawing arcs" or getting close to see what is going on is that I don't want to be next to the TC if it just fires up on its own. I'd be caught in the middle of 5 foot streamers and I still have things to do in this world.
My fear about removing the filter is what if it is actually protecting the NST and by removing I subject the NST to big stress? I will look at the current draw next time. Maybe I'll dig up an AC voltmeter and connect my 1:1000 divider so I can monitor the NST output voltage at the same time.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
IamSmooth wrote ...
I was going to do that...start eliminating components. The problem with "drawing arcs" or getting close to see what is going on is that I don't want to be next to the TC if it just fires up on its own. I'd be caught in the middle of 5 foot streamers and I still have things to do in this world.
I wasn't suggesting drawing arcs off the NSTs with the rest of the coil hooked up! I was just saying it couldn't hurt to make sure your NSTs can still make a nice Jacob's ladder. In fact, just running them open-circuit as you dial up the variac and measuring the current is often a fine way to spot internal arcing. If you see a big current spike as you dial up the primary V with nothing connected to the secondary, that would be a problem!
IamSmooth wrote ...
My fear about removing the filter is what if it is actually protecting the NST and by removing I subject the NST to big stress? I will look at the current draw next time. Maybe I'll dig up an AC voltmeter and connect my 1:1000 divider so I can monitor the NST output voltage at the same time.
A short run with just a safety gap--provided all else is in good order--probably isn't going to fry anything, however as a first test, you might just disconnect everything on the "output" side of the Terry filter and run your primary voltage up to make sure your filter can take a full 15kVAC.
Also, a convenient way to measure the output of a NST is with another NST. If you can assume that they're not both bad (may or may not be a valid assumption!), then since the secondaries are in parallel, just supply 120V to the primary of one NST and measure the voltage at the other. It should be 120V or really, really close. If not, well, that would be bad.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You probably ought to try changing your gap phasing. I've seen people write on the Pupman list about a ferroresonant type of effect that happens when the input voltage is increased to the point where the NST core starts to saturate.
IIRC, the result is that the leakage inductance starts to fall, which changes the optimum gap phasing, and now the phasing is out, the voltage resonates up and saturates the core even more, so you go into a vicious circle.
If this was what was happening, the line current in the "pseudo-off" state would be very high (and the NSTs at risk of burning out) The cure would be to move the gap phasing (earlier?)
It's probably not a good idea to leave it in this state for any length of time. Even if the line current doesn't seem excessive, there could be huge reactive currents in your NSTs that may be hidden by the PFCs you added.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I tend to think that things usually have a simple explaination. I'm going to start with the guess that it is the filter. Maybe, the caps I have are under-rating their voltage breakdown. I think they are 20kv apiece - one for each 7500v leg of the NST. I could try putting two in series to double the voltage breakdown.
If I wanted to monitor the NST output voltage would putting a few hundred-meg resistor on each terminal and then connecting them to an analogue AC voltmeter work without frying the meter? What have the rest of you done for this? 500meg/300watt? 1000meg/300watt?
EDIT: I just remembered that I have one bleeder resistor across my two 0.025uf/50kv maxwell capacitors. It is over 100meg but I don't know its voltage rating. If its rating is under 30kv (2 x 15kv nst voltage) then this might be shorting my capacitor bank. this will be the first thing I try.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
You'd want a voltage divider, so just putting resistors in series with the NST output wouldn't do the trick. In fact, it would probably fry your meter...perhaps worse! You'd probably want to run a string of resistors from each HV leg to the case, then measure across the resistor closest to the case. Multiple the result by the number of resistors to get the output voltage. I've never done it this way, though, and I'm not sure of the "practical" voltage rating of small 1/4 and 1/2-watt carbon resistors. Generally people seem to push this alot, e.g., using only one or two in series for bleeders on multi-kV caps. I don't know what you'd consider "safe" for this (i.e., to be certain you don't toast your VOM).
If you think you'll be measuring kV's a lot, getting a HV probe for your VOM or scope is probably a worthwhile investment. I'm not sure of their internal construction; they may be resistive or capacitive dividers. Utilities generally use capacitive voltage dividers for instrumentation.
All that said, if you have more than one NST, the NST-to-NST measurement trick I described above is a popular way to do it.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I was going to put a few HV resistors on each leg and get the amperage down into the microamp range and then measure the volts across the ends.
THe more I think about my loss of power problem the more I'm thinking I'm shorting my bleeder resistor across my capacitor. It is just one tigerstripe 100meg resistor. Does anyone know the voltage rating of a tigerstripe - 15kv, maybe? Anyone have a good source for them other than the few I've seen on Ebay?
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First light. Almost perfect and need some help
I was able to get to a mains input voltage of 100v and my streamers were about 4 1/2 to 5 feet. First, the problem is that when I got between 90v and 115v the TC would just stop. The spark gap stopped firing and the coil's streamers abruptly stopped. I would then lower the variac to almost zero and then when I powered up again everything worked out well.
I was just saying it couldn't hurt to make sure your NSTs can still make a nice Jacob's ladder. In fact, just running them open-circuit as you dial up the variac and measuring the current is often a fine way to spot internal arcing. If you see a big current spike as you dial up the primary V with nothing connected to the secondary, that would be a problem!