Re[TCML] NST rebuild good

[bartb]

Hi Marko,

Good you have a salient pole sync motor (at least we know the phase lock 
will always be in the same place). As long as you're not hearing the 
motor hunt while running, then everything should be good. If the value 
is .033uF, then charge time is almost right at 120bps (slightly faster 
at 7.6ms). This would charge the cap a little over your calculated Vp.

I think what you have going on is a battle between the safety gap and 
the main gap. The propeller begins firing, but as soon as the safety gap 
fires a couple times, the arc voltage at the safety gap is lowered and 
then it begins firing more often and eventually takes over the function 
(acting like a wimpy static gap in a thermal runaway condition). The 
sparks should be getting rather small when this situation occurs.

The remedy of increasing the safety gap will allow the main gap to see 
some rather high voltages that will be dangerous to the NST and possibly 
the caps. Going LTR would be nice, but ridiculously expensive since LTR 
for your system is 0.15uF for an SRSG and even 0.086uF for a static gap. 
An alternative is to double up on the bps and add 2 more rods 
perpendicular for 240bps (or 4 more stationary's with the existing 2 
rods). This will drop your pps in half at 4.17ms and give you some 
ability to contain the arc voltage to the main gap. But even in this 
application, the safety's will fire more often than using a static gap.

I would personally do as Gary suggested and build a static gap. This 
will clamp the voltage and allow running the 0.033uF size without 
problems. This is how I run my 12/200 NST and it works flawlessly every 
time and rarely does the safety gap fire. Static gaps can be built 
cheaply and robustly to handle your 3KVA system.

I highly recommend copper tube at 1.25" diameter and about 3" long (can 
be purchased ready to go at all hardware stores). Pick up a pvc coupling 
form about 4" deep. There is a lip in the inside center. I use 2 part 
high temp epoxy and place 1 tube edge down on the lip. After the epoxy 
is set up, I do the same for the next tube and place a .050 feeler gauge 
between the two pipes. I do this for 6 electrodes for 0.25" total gap 
ability. Install a hefty fan to suck through the bottom and you have 
yourself a nice static gap with a lot of surface area for cooling (and 
exact distances between each tube). Works great.

Here's an old photo but shows the basic idea. The coupler I install into 
it's larger cousin which is epoxied onto a fan. This allows me to pull 
out the coupler as needed for other electrode types or for cleaning. The 
copper tube is far superior to the brass stock I also show in the pic.
http://www.classictesla.com/photos/ba45/s2752.jpg

Take care,
Bart




mark olson wrote:
> Hi Bart,
>
>> Hi Marko,
>>
>> I'm having trouble sending out emails from my home pc. Server is 
>> having an
>> issue (I think). Anyway, thought I would send via web mail.
>> -------------------------------------------------------------------
>> Ok, I see this is a propeller gap. I take it the center tungsten rod is
>> the green stripe type?
>
> The tungstens are 2% thoriated, red tip.
>
>> I imagine this rod is spun around from a motor
>> shaft directly under and center to the rod?
>
> Yes, actually two rods so that the diameter described by the flying 
> electrode is 12"
>
>> Your gap spacing looks fine
>> on the main gap. The gap actually looks fine and I would think it should
>> be working ok. The only thing that I can see as a possible problem is if
>> the motor is synchronous and is not phased correctly or drifting.
>>
>> If this is a synchronous motor, is it AC or DC? Some motors do have
>> drift, so I wonder if that is happening? If your phasing, how are you
>> performing it (variac? physical?).
>
> The motor is a salient pole 1800 rpm.  I am controlling the 
> synchronization with a small variac wired as an
> inductor with a phase shift capacitor across the motor.
> When the rotary gap was tested with only the NST  connected,  the arc 
> in the gap could be changed
> from firing just before closest proximity to stretching the arc to 
> about 30 degrees after closest proximity.
> It appeared to control quite consistently under this test, which is 
> the only one I could think of.
>
>
>> Take care,
>> Bart
>
> I am really starting to believe that the capacitors that I have left 
> after blowing up half of them, simply are not up
> to the task, as the calculated voltage across the string should not 
> exceed 23kv. At 80v into the NST,
> calculated 11.3kv rms out,  Do see where I am going with this? Or do 
> you think I am off track?
>
> Thanks,
> Marko
> a
>
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