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Re: Re[TCML] NST rebuild good
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.
mark olson wrote:
I'm having trouble sending out emails from my home pc. Server is
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.
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?
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