From smoking to crackling

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fred ungewitter
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Re: From smoking to crackling

Postby fred ungewitter » Sat Jan 07, 2017 5:19 am

I wanted the LO 60w as well, but for me, out of stock means look elsewhere. Domestic wins over foreign especially as I've been bitten so far by the chinese tube failure after so few hours. Thanks for the clarification on the power supply.
fred

Redsail clone 60w 700 x 500
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Hans Nieborg
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Location: Central Florida - USA

Re: From smoking to crackling

Postby Hans Nieborg » Sat Jan 07, 2017 12:16 pm

Hello Fred, I'll respond to these thoughts in the order they were posed.

I've removed the questionable glass tube from my machine after performing another test, ensuring I had good connectivity on the ends. Your suggestion that I ensure the insulators on the anode and cathode raises a question. The wiring is securely attached and the failure came halfway through a test job of perhaps 30 minutes total. The first portion was working well, the second ended with lightning bolts at the HV end.

It's good that the mechanical connections are 'tight', rather than the wires just twisted around the pins and glued. If the failure point was not from the anode or cathode, there are two conditions that typically occur. The first is from tracking, which is when the HV finds a conductive path across the surface of an insulator. The second is a capacitive distribution effect, which is where the HV can couple through an insulator, due to having too low of an insulation resistance to the applied voltage and no clear (easy) or direct path for the HV to take. The voltage builds up in something of an ion 'cloud' at the point of weakness, then when the air is ionized sufficiently, the voltage can cascade through to the grounding path.

I've had discussions in the past regarding high voltage, unrelated to laser cutters, but certainly relevant here. The voltages were somewhat lower but the currents many magnitudes higher. This particular team informed me, when I ignorantly questioned them about insulation, that the best insulator is dry air and distance.

It is true that the best available insulating situation is air and distance, which is what power distribution relies upon for delivering electricity to consumers. Unfortunately, we don't have the luxury of the voltage-to-ground spacing necessary at the voltage our laser machines run at, so we have to rely on glass, wiring and connector insulation to transport the HV from the laser power supply to the laser tube.

Allowing for the fact that one can not easily change distance within the cabinet and at these currently levels, don't really have to worry about that, the dry air part comes into play. At the levels of these cutters, adding a silicone plug in a piece of tubing is not really going to reduce the insulation level appreciably either. Would you agree? If that's the case, then not having a silicone plug in a piece of tubing isn't going to increase the insulation level either. I suspect the silicone plug inside the tubing is more a safety feature and minor mechanical securing method than any insulation factor.

It is true that when you are forced into fixed spacing regarding an air & distance situation, then the quality of the air (or isolation from the air) becomes important. This is why on a humid day, you can hear the power lines buzz and crackle, where the moisture suspended in the air is creating a partial tracking condition across the standoff insulators holding the cables off of the power pole.

The silicone tubing and silicone plug has value if it COMPLETELY seals off the wire / pin connection from the air, but if it was applied more for the purpose of holding the twisted wire in place rather than as a high voltage insulator. Of significant note also, is the type of silicone glue that is used, let alone the silicone rubber tube. Regular (inexpensive) RTV silcone releases acetic acid when it cures, which smells something like vinegar. Since it is acid based, it stands to reason that it conducts HV. Instead, the silicone should be specific for use with electrical applications such as Momentive's RTV162. It is a little pricey, but can be bought for around $20 for 2.8 ounces at Amazon. Here is a link to their datasheet https://www.momentive.com/products/show-technical-datasheet.aspx?id=10306. You can see in the electrical properties that the dielectric (insulation) strength is 19kV per millimeter of thickness, which is definitely more than enough for our purposes. I didn't trust the Chinese glue and connections, so I took care of the connections, cleaned out all of the original glue, placed the original silicone tubing over the connection, then applied RTV162 by completely filling the silicone tubing and ensuring it had spread out uniformly onto the glass tube underneath it.


Onward from there, the problem probably originated from the excess condensation on the tube, due to strong cooling and low, infrequent power use. I might have been at 30 percent and running for too short of a time to "burn off" the condensation. I'm going to strongly suggest that burning off condensation is a poor practice as well. It wasn't in my operating philosophy and won't be.

I've added the high volume high speed muffin fan to the cabinet and during the one test I performed, a half hour of cold water produced zero condensation, while almost immediately after shutting down the anti-evap fan, I had condensation haze on the tube.

Wouldn't you allow that having a nice thick layer of water over the HV end of the tube is bad practice, and that it would permit these wonderful lightning bolts to travel to ground? I suspect it is/was the water, as the spark began at the top of the tube, near the HV contact point and traveled along the outside to ground. This explanation fits in with the perceived curvature of the spark I saw as it was zapping away during the scan process.


It's very possible that the issue could be tracking due to condensation, but I have had condensation on my tube here in 'always-humid' Florida and haven't had a problem (yet). I keep the laser tube outlet fluid temperature of mine between 25 and 35 degrees C. To be honest though, pure condensate water itself is not very conductive and as such, requires impurities to make it conductive enough to move significant amounts of electricity. If the tube has something on it, such as from being handled bare-handed, coated with dust, smoke residue or several other similar possibilities, the voltage will track down the contamination / wet surface and along the tube to a point that is closest to ground, then it will leap across from there.

Also, electricity prefers to take the straightest path in its movement through open air, which is also known as taking the path of least resistance, so if it had curvature during the arc it is usually due to atmospheric conditions acting on the arc, such as movement of air or heating of the air caused by the arc. If it was from the arc heating the air, the arc tends to bow upwards as the hotter air rises and pulls the arc with it.


Of course, when the replacement tube arrives (or is it "if"?), I will take additional care to ensure that there are zero bubbles in the tube. Draining and removing the existing tube shows me that it is a simple enough process.
Ke Hui 7050, 60W (53W Real), 70x50cm, 2" FL Only, 6442G Controller, RDWorks V8.01.19, Refrigerated Chiller, In-Home Install, Home = TR.
Retired Design Engineer For High Power Motor Controllers. Now, I Make Items For Geocachers & The Cons (MegaCon, MetraCon, ComicCon, etc).
Hans

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fred ungewitter
Posts: 220
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Location: East Central FL, USA

Re: From smoking to crackling

Postby fred ungewitter » Sat Jan 07, 2017 12:48 pm

Wow, what a wealth of information!

I'm hopeful that your reference to the RTV silicone means it's the optimum choice. I had used "ordinary" silicone to seal off the connection during the troubleshooting part, but had no sparks and no laser output either. I'll be placing an order forthwith for the recommended silicone, encouraged by the insulation specification. A couple mm of goop should give me protection from impurities surrounding the area, although I'll probably leave the evaporation fan in place. Error in previous post calling it an anti-evaporation fan. It's an anti-condensation fan!

After reading your valuable information, I considered for a moment to re-install the tube and attempt to make it work, once the better RTV silicone arrived, but then I realized that my last test was without anything on the wiring, strong mechanical connection and no arcing or output. That points to a dead tube, which means my efforts would be mostly a waste of time.

Thanks again for your advice. It is appreciated greatly.
fred

Redsail clone 60w 700 x 500
home upper right
RDWorks 8.01.18
Windows 7

Emblaser A3 4w diode laser
BCN3D Sigma Dual Extruder 3d printer
LMS Mini-mill
Cube 3rd gen 3d printer (works great, for sale)
Under construction: OX CNC 1000x750 router - out of the box, still in baggies!

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Hans Nieborg
Posts: 30
Joined: Mon Nov 28, 2016 8:39 am
Location: Central Florida - USA

Re: From smoking to crackling

Postby Hans Nieborg » Sat Jan 07, 2017 5:02 pm

Yes it is the optimum choice, especially at that price point, but you shouldn't use acetic acid based RTV on electrical equipment because it is both conductive and corrosive. There are other silicones out there that out-gas methanol during curing that would be OK, just as long as it is allowed to fully cure before firing up the laser, or it could catch fire. A little tricky there...

I used the RTV162 product when designing, building and testing 15kV, 10,000HP motor starters. It was used on circuit boards within the live components area that switched the 15kV to the motor and we never had a tracking or insulation breakdown issue, when properly applied. Other unusual materials used for voltage insulation within our area of interest are, Kaptan tape (a little thickness goes a long way) and PTFE or Teflon tubing. There were times I would run a silicone jacketed, UL3239 rated wire inside a piece of Teflon tubing to isolate it from opposing AC voltage phases within the equipment and it was another 'little bit goes a long way' product. Stuff isn't cheap though.

If the laser tube power supply is still functioning, as indicated by your arc-test, I am thinking that the tube structure has failed. With it having failed so abruptly, it is likely the gas has escaped, either via breaching the bubbles in the glass I had mentioned or even the fittings on the tube lost their seal. Unfortunately, we don't any good way of detecting gas viability though, other than it works / doesn't work.
Ke Hui 7050, 60W (53W Real), 70x50cm, 2" FL Only, 6442G Controller, RDWorks V8.01.19, Refrigerated Chiller, In-Home Install, Home = TR.
Retired Design Engineer For High Power Motor Controllers. Now, I Make Items For Geocachers & The Cons (MegaCon, MetraCon, ComicCon, etc).
Hans

User avatar
Hans Nieborg
Posts: 30
Joined: Mon Nov 28, 2016 8:39 am
Location: Central Florida - USA

Re: From smoking to crackling

Postby Hans Nieborg » Sat Jan 07, 2017 5:31 pm

Hello Fred, I'm pleased that I can be of help.

I spent almost 20 years in the motor controls industry, designing equipment, providing training seminars for customers and utilities on how to work with our equipment and also medium voltage safety (note: Medium Voltage = 1000V to 36,000V here in the states). I was traveling the world over, spending an average of 21 days of the month on the raod and then the last year I accepted a position as a general manger (and engineer) of a high voltage equipment re-manufacturing facility. After a good friend lost his life during a motor control startup at a jobsite and then watching fellow engineers slowing burning out in this high-intensity, high stakes business, I decided to retire before something bad happened to me... And, here I am!

I've taken my interest in power electronics, motor controls and CNC type machines and turned it into a hobby. Now it is developing into a small business that is both interesting and fun and with basic safety protocols, I should be much better off and safer than I was before I bought my machine.
Ke Hui 7050, 60W (53W Real), 70x50cm, 2" FL Only, 6442G Controller, RDWorks V8.01.19, Refrigerated Chiller, In-Home Install, Home = TR.
Retired Design Engineer For High Power Motor Controllers. Now, I Make Items For Geocachers & The Cons (MegaCon, MetraCon, ComicCon, etc).
Hans

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fred ungewitter
Posts: 220
Joined: Tue Aug 16, 2016 12:55 pm
Location: East Central FL, USA

Re: From smoking to crackling

Postby fred ungewitter » Sat Jan 07, 2017 6:13 pm

Your reference to lost gas brings to mind a video "out there" that a friend showed me. It's a monster laser unit with "folded tubes" totaling a distance of 24 meters over I think 12 individual tube with adjustable mirrors at each bend or fold. It used a power supply the size of a compact car and had a flowing gas mix of Helium, Argon and Carbon Dioxide. I've forgotten the rated power but I'm sure it was substantial. Seeing the video made me think that it would be useful to have the ability to "re-load" these tubes as needed. Of course, I also recognize the impracticality of such a practice for the hobbyist.
fred

Redsail clone 60w 700 x 500
home upper right
RDWorks 8.01.18
Windows 7

Emblaser A3 4w diode laser
BCN3D Sigma Dual Extruder 3d printer
LMS Mini-mill
Cube 3rd gen 3d printer (works great, for sale)
Under construction: OX CNC 1000x750 router - out of the box, still in baggies!

User avatar
Hans Nieborg
Posts: 30
Joined: Mon Nov 28, 2016 8:39 am
Location: Central Florida - USA

Re: From smoking to crackling

Postby Hans Nieborg » Sat Jan 07, 2017 6:36 pm

Yes, the monster laser you describe can have a gas change-out system where they have the gas in tanks and feed it into the tybe, then there is a vacuum evacuation system that will remove the old gas after a week to a month of use, then they pump in a new gas mix.

We had a shop across from one of the places I worked where they had a 10kW CO2 cutting laser that used this system. I used to visit them every so often to have them produce a special piece for my designs. What an amazing beast!

Unfortunately, at our level, the tubes in our machines are like a disposable lighter, relatively inexpensive and not worth repairing. At least, it doesn't look that way. Maybe you're onto something...
Ke Hui 7050, 60W (53W Real), 70x50cm, 2" FL Only, 6442G Controller, RDWorks V8.01.19, Refrigerated Chiller, In-Home Install, Home = TR.
Retired Design Engineer For High Power Motor Controllers. Now, I Make Items For Geocachers & The Cons (MegaCon, MetraCon, ComicCon, etc).
Hans


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