[SOLVED] High voltage rated inductor 5uH at 30KV

[Freddybaby]

I need to build an air wound coil of 5uH. This will be in series with a 5K resistor and a 30KV pulse.

I was going to wind in sections and make the length about what a Globar 36KVPK rated part
length is, about 6".

The application will draw an arc across a short gap so the full voltage will be across the inductor for a short time (500pF
capacitor discharge)

Is this a good approach or does anyone have a better idea ?

 

[mtwieg]

Could you draw a schematic? I'm having trouble seeing the point of a 5K resistor when trying to create an arc.

For high voltage coils, you'll want to make sure it's wound around a proper form. Fiberglass and even PFC work well, but you have to make sure they are clean, and you should coat the windings in some lacquer or epoxy afterward.

 

[Freddybaby]

The 5K ohm is part of the pulse shaping network. The 5uH is called out in the specification so
I must put it in but I suspect this was left in from an estimated or measured value of the entire
assembly.

I also realize I left out some mention of insulation rating on the mag wire. I need about 44 turns
on a .75" form. The mag wire I.R. is better than 9KV (at DC) so I hope it will fair well at high
frequencies. I think the volts per turn will be sufficient, it's the end to end withstand thats
in question.

Yes I agree, a good coating of varnish (I have some good conformal coating) is key.

 

[FvM]

5uH/5k would results in an incredible low time constant of 1 ns, which in other words says, that the full voltage will never appear across the inductance, if it's actually a series connection. So you are probably doing something pretty useless.

 

[Orson Cart]

you'll need a series string of resistors to avoid flashover on too few resistors and they should be rated for the peak current expected - spacing the turns on the former will help keep the capacitance down and help with turn to turn volt rating. Regards, Orson Cart.

 

[mtwieg]

The 5K ohm is part of the pulse shaping network. The 5uH is called out in the specification so
I must put it in but I suspect this was left in from an estimated or measured value of the entire
assembly.

Still not seeing how a 5K resistor can shape a pulse except by killing it entirely.

I also realize I left out some mention of insulation rating on the mag wire. I need about 44 turns
on a .75" form. The mag wire I.R. is better than 9KV (at DC) so I hope it will fair well at high
frequencies. I think the volts per turn will be sufficient, it's the end to end withstand thats
in question.

9KV is insanely high for magnet wire; I've never heard of something even close to that except for very large taped wire for large motors. And for very fast rise times (like <1us) the breakdown voltage will effectively be lower. I'd check exactly what the wire specs are and what the test conditions are.

At 36KV across 6", you're looking at over 2KV/cm, which is pretty high; enough to create streamers through air if you have any unrounded edges. You should look into getting some insulating plates for the ends.

 

[FvM]

Without knowing the real circuit, it's likely to jump into conclusions.

 

[Freddybaby]

I agree FvM with your estimation of "actual" voltage across the inductor, this was evident in my simulation as well as my low voltage prototype. But the requirement was for a 25KV rated inductor so this was my design criteria. Not being an off the shelf part I decided to make it myself based on the specification.

mtwieg, the mag wire hi-pot was at DC and yes I was surprised at the breakdown as well. I realize this is not a true dielectric withstand at high frequencies but it was a good start. As FvM pointed out, It will probably never see this kind of stress.

Another reason for making a physically large part was it works well for my high voltage grading, this is a 25-40KV application and a tiny molded inductor might
give me corona problems leading to HV breakdown. So a large diameter inductor sized the same as my HV buss is beneficial.

The "output" of this circuit is across a 1/16" needle gap so the initial risetime may be quite fast.

Thanks for the input everyone, it was greatly appreciated. I will proceed with my design even if it seems a useless venture, at least I'm hourly and not contract

 

[FvM]

Of course it does no harm to achieve excess voltage ratings in some places. Nevertheless, I would like to see a principle schematic of the generator to understand the component requirements.

 

[Freddybaby]

Sorry, I should have posted this sooner. Here is a simplified schem. View attachment New Bitmap Image.bmp

 

[mtwieg]

I don't think that circuit works the way you think it will... as is, the resistor will dissipate like 99.99% of the energy in the capacitor, and practically nothing will go to the spark gap. The inductor serves no purpose at all because its impedance is nothing compared to the resistance. You could use a tiny air core inductor and it wouldn't matter.

I think you need to reexamine whatever design you're basing this off of.

 

[FvM]

The maximum inductor voltage will strongly depend on the contact switch-on time. Do you know it?

 

[Orson Cart]

Is the idea to create an equivalent ESD strike? - if so the closure rate of the switch must be very high - Regards, Orson Cart.

 

[Freddybaby]

Yes Orson, this is to simulate an ESD event to evaluate ammunition and explosives packaging for accidental ignition. It's based on a standard test method (MIL STD 1576). However I don't see the inductor in the spec's schematic but is required by the contract.

The HV switch has large sphere contacts to reduce pre-strike. No FvM, I don't know the actual on time as it's a variable distance point gap.

After several tests at various gap lengths I decided I am probably over-engineering this part (as suggested by many). I will focus more on the sizing of the part for good high voltage practice (won't change my basic design much anyway) and will still give a good coating of super corona dope just to be safe.

I'll post a picture and test results when I get some time. Thank you all for the input, very helpful.