What's wrong with my plasma cutter design?

I was thinking about building a homemade plasma cutter. My plan would be to use two linked microwave transformers to step up wall voltage to ~ 50,000 volts. I would use welding carbons in the end of a copper pipe, nozzle, etc, so that an arc would be generated inside of the nozzle. I would then have a PVC assembly that would take compressed air and force it past the arc, and thus generate a spray of plasma? I KNOW how regular plasma cutters work, I was just wondering about this design out of curiosity, but I know there is likely some flaw (e.g. the arc isn't drawn to the work, etc.) that I haven't thought of. Advice is appreciated. Thanks! P.S. What, if this design works, would stop me from using this to weld? i know that the plasma is too hot, i would just keep the torch a distance away from the work. Or would the air contaminate the weld? Thanks! EDIT: I know how traditional plasma cutters work. I don't want to strike an arc with the workpiece. I want to create an arc in the device and used compressed air to blow the plasma onto the work. Also, i think i could use a smaller power supply, say 7.5 KV to generate that arc, without MOT's.

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twenglish18 years ago
also since you are going for high voltage maybe you will find this useful:

http://www.scribd.com/doc/11490949/378-50000-Vdc-Power-Supply
jesse_a_b5 years ago
You need a lot of kV to start the machine, but when the plasma arc is started you only need a few dozens of volts to keep it. The plasma arc in a welding machine is only around 50V.
Plasanator7 years ago
Most of what is being told here is rubbish. All you need is 240 volts dc at 13 amps and you can cut 3/16 steel. Rake it up to 25 amps and you can cut 3/8 steel. I know, I make plasma cutters for my friends and me and I sell the plans. http://theplasmacutterman.com/index.html
charley19577 years ago
If I were you I wouldn't even THINK about using Microwave Oven Transformers for this project.  The reason is, these transformers are not current limited, and they deliver AMPS of current.  Even the slightest contact with the output of one of these transformers will kill you.  It only takes a few thousandths of one amp to kill you, and these transformers are capable of thousands upon thousands of times that much. Please consider using an automotive ignition coil, or some other high voltage transformer that is much safer.  This project will require you to hold this high voltage in your hand, you would be much safer not using a microwave oven transformer. 
bylerfamily8 years ago
You don't want high voltage for a plasma cutter.You want high amps.Really high.So the arc will actually strike.
twenglish18 years ago
i found a problem with your design, plasma cutter require DC current and the 50,000 volts coming out of the transformers is going to be AC and you are going to need a pretty big rectifier for 50,000 volts
mad magoo (author) 8 years ago
I know I don't need 50kV for a traditional plasma cutter. I was thinking something that sprayed plasma from an arc out the tip of the torch. Similar to Atomic-Hydrogen welding, in which an arc is struck between two tungsten electrodes, and compressed hydrogen is sprayed between them. this atomizes the hydrogen, breaking H2 molecules into just H atoms. These atoms then recombine on the work into H2, heating the work very much.
You don't need 50kV for plasma cutter ! They use about 40V once the arc is struck. If you have an HF start, they use a Tesla coil arrangement to ionise the plasma channel, which is then ignited by the high current, low voltage transformer or inverter behind it. Compressed air is used to cut and to cool the electrodes, since,with steel cutting, the oxygen considerably speeds the cutting process.
lemonie8 years ago
Seen this?
http://www.youtube.com/watch?v=6W3tLDBdPQQ
Not great for instructions, but it might help a bit.

L
SuperCapMan8 years ago
How exactly would you get 50,000 V from two microwave transformers in series? They give about 1600V each, so you would have to use some kind of voltage multiplier, maybe CW cascade, or Marxgen... Do you need AC or DC to do plasma cutting? Also, I think you have to use Argon, and not compressed air, because the air will oxydize the electrodes and the piece being cut. Also, I think ( but am not very sure) that the heat of the arc would melt/burn the PVC, especially if you are using black PVC, because it absorbs radiant heat readily. That said, I know nothing about metalwork and plasma cutters, I have just a bit of experience with High Voltages.
Good luck with you design.

SuperCapMan
Is it rectified mains power? Is it a battery? No, it's SuperCapMan!
in reply to the original question, yes the air will cause some inclusions if you do manage to get it to work. and plasma cutters do use air not argon. argon is used in other welding applications though. a problem that I can see other than your power source is your air, aim not sure if the pvc will make the grade depending on how you connect the air and pressure regulator.
SuperCapMan8 years ago
Okay, now I understand.
However, I see a problem with your setup. Microwaves transformers are not current-limited, so if you short them, either your circuit breaker will trip(not so bad) or the wires in your home's wall will melt/explode(actually quite bad). A transformer's primary is pretty much a short(a few dozen feet of 22Gauge copper wire), so you should put a resistor to limit current flow between T1 secondary and T2 primary, a good beeefy resistor because it will dissipate a LOT of power(better hook up a heatsink too...). BTW, where do you live? Here in Canada the mains voltage is 120V, and all the MOTs I've seen are rated at 1600-1900V RMS. Maybe if you are in europe, then the voltage is higher?

Also, do you have some links to pages explaining the principles of plasma cutting? Seeing you question here made me realize that I knew nothing about them, and I'm always willing to learn High Voltage stuff!

SuperCapMan
Is it rectified mains power? Is it a battery? No, it's SuperCapMan!

mad magoo (author) 8 years ago
SuperCapMan: I would get 50,000 volts from two microwave transformers byplugging the primary of one into 110 mains power, then attaching that one's econdary to the primary of the second, thereby getting ~ 50,000 volts out of the secondary of the second. This would happen becuase all (the only) microwave transformer i've dealt with gives 2400 volts from mains. also, I know that plasma cutters use compressed air, so that isn't an issue. Also, You'll notice that i said that I would use a copper nozzle, etc. as the end output tip. Thanks for your comment!