plasma cutters

[SpringHollow]

Hi JJ,

I do not think they are using pure oxygen - one references 80% N2, 20% O2. I would be very concerned about large amounts of excess oxygen if O2 were the only gas used. Any oxygen consumed in the process would not be a concern. Lots of chemicals that normally are not a problem become big problems in a pure oxygen environment, especially when there is also an ignition source present. I also think there is a big difference between a production environment where conditions are somewhat controlled and where theoretically people have been trained in what to do versus some guy saying "oh, you can use O2" and grabbing a bottle and winging it. I mentioned it because i would want someone to seriously think about what they are doing before they do it.

Ken

 

[oughtsix]

When it comes to converting AC to DC via a rectifier single phase is a lot noisier (lumpier) than 3 phase. 120 times (100 times in Europe) a second single phase AC current changes polarity at which time there is 0 volts. With 3 phase current there are three different current waves each spaced 1/3 of a wave (120 degrees) apart. There is no time at which all three currents have a potential voltage of 0v all at the same time. Hence the rectified (pulsating DC current) is much smoother from a 3 phase source than a single phase source.

Capacitors store an electrical charge. By adding a capacitor or filtering circuit the the pulsating DC current you can smooth pulsations in the DC current. Since rectified 3 phase is smoother to begin with you don't need as large of a capacitor to smooth out the current.

3 phase welders use 6 diodes (2 for each phase) in the rectifier. Single phase uses 4 diodes in the rectifier so for a comparable current the diodes for single phase need to be bigger.

I believe the biggest advantage in single phase vs 3 phase when it comes to welding equipment is a much smaller circuit is required for 3 phase since the load is spread out amongst the three legs. 3 phase has a bigger advantage over single phase when you start talking about motors and torque. I have no personal experience in comparing 3 phase welding equipment to single phase.

There isn't really any two phase commercial electrical service. 200v vs 400v service (or USA 110v vs 220v) is just single phase service with double the amplitude (voltage).

 

[Scooby074]

There are some Plasmas that run pure o2 and air. Others are 20%o2 80%N. The pure O2 results in a less contaminated cut than the N2 mix, which enables the cut pieces to go directly to be welded. THe nitrogen in the 20/80 mix, mixes with the molten steel during the cut can cause defects in the finished welds unless its ground out before welding. Using O2 also results in a much cleaner cut than straight compressed air.

These plasmas running mixed gases are 200A + and are only used in the biggest shops, for cutting heavy steel. Usually on a CNC table. Using pure o2 in a machine not designed for it would be a bad idea IMHO.

 

[wushaw]

I have a Thermal Dynamics Cutmaster 52, I absolutely love this thing, it is so nice to just cut something, clean metal, dirty, painted, it don't care it cuts it, no flame to worry about when the cut is done, just lay the torch down anywhere before and after cutting.
The piece you cut you can pick it up with your bare hands

You need dry air and a good supply of it, I have a 80gal 7hp and it doesn't take long before it cycles.

If your wanting one of the coolest, fastest, easiest to use cutting devices made then plasma should be on that to get list.

 

[SpringHollow]

The pure O2 results in a less contaminated cut than the N2 mix, which enables the cut pieces to go directly to be welded. THe nitrogen in the 20/80 mix, mixes with the molten steel during the cut can cause defects in the finished welds unless its ground out before welding. Using O2 also results in a much cleaner cut than straight compressed air..

Hi Jason,

I read that. So when using compressed air, is the plasma cut metal edge good enough (with respect to nitrogen in the metal causing weld defects) for homeowner built stuff or should we be grinding our edges before we weld?

Thanks,

Ken

 

[Scooby074]

My understanding is that clean compressed air is fine for the majority of cuts. When people worry about nitrogen contamination issues, its for things like bridges, boilers, nuclear, stuff like that where a perfect weld is essential.

Of course, if there is any dross on the cut, it should be ground off before welding.

 

[SpringHollow]

Thanks Jason!

I have an inexpensive inverter one rated for 3/4" cut, 1" sever cut. I have only used it once with improper technique and very wet air. I cut an I-beam in two but it was not the prettiest. I bought a used membrane drier and hope to try it again soon. I will post how it goes - the unit is a stick and TIG (no pedal) welder as well.

Ken

 

[Renze]

There isn't really any two phase commercial electrical service. 200v vs 400v service (or USA 110v vs 220v) is just single phase service with double the amplitude (voltage).

I dont know, my compressor runs on 2 or 3, (cant remember, i just hooked it up according to the installation note the electric motor shop gave me) phase AND the null wire, so that gives 220V
The 180A welder (which we used to share, but will now move to my brothers new workshop) puts out 160A on 230V sinlge phase and 180A on 400V two phase, no null wire so 400V: When my compressor quits because one phase has a blown fuse, the welder just keeps on going (which can be very confusing to find the fault, if you dont know these things about cheating voltage or phases with a null wire)

Another funny thing i had, was that my 220V single phase drill press made a funny noise, as if a phase was blown: Its a single phase machine so thats impossible. Later i found that my brothers friend (who was an electrician) couldnt find a 5 wire cable, so instead used two 3 wire cables to supply the shop: Reason that the drill press made noise, was that it got its phase from one fuse group, and the null via another !!


Petrogen | Frequently Asked Questions

It looks like they burn just plain petrol... Looks cool to me, but 1000 bucks is too expensive for my use. Next to that, no spares available in Europe.

 

[Scooby074]

Thanks Jason!

I have an inexpensive inverter one rated for 3/4" cut, 1" sever cut. I have only used it once with improper technique and very wet air. I cut an I-beam in two but it was not the prettiest. I bought a used membrane drier and hope to try it again soon. I will post how it goes - the unit is a stick and TIG (no pedal) welder as well.

Ken

Ken you might also consider getting yourself one of those motorgard "toilet paper" filters that attach to your machine.Motor Guard Filters THese do a great job of cleaning up any last water or oil before it gets to your plasma. AS you liklely know, wet air is hard on consumables and can cause bad cuts.Your air can never be too dry for plasma or painting.

 

[jimgerken]

Renze, when shopping for your Plasma cutter, consider this: After the pig barns are down and you have some time, eventually you are going to want to add a computer controlled cutting table to your shop. I did this about 8 years ago and its the greatest thing I ever built. It opens the door to designing much more stuff than I could describe to you. With this toy, you will design in features that you'd never consider. Multiple idnetical parts are naturally the most attractive feature of NC control. My own table uses the standard hand-held torch that came wth my Thermodynamics cutmaster 81. Here's the kicker: When using the plasma on the NC table, it is always held vertical, and there is not enough intelligence in the programming to slow for curves, etc. SO, you basically loose some of the power capacity of your cutter when it is being controlled by the computer. Figure 50% reduction in capacity when under NC control. So I would buy now at least 60 amps, so later you can get good quality 1/2 inch (13mm) thick very trick tractor attachment parts out of the NC table. .......Trust me, you are heading to NC someday.