Multiplaz-3500 Evaluation, Part 01: Plasmas and Plasma Torches
I work for a company that performs technical (and other) evaluations of many products and processes. We are doing an evaluation of the Multiplaz-3500 plasma cutter/welder for our possible use and I agreed to post the results on this forum. Our evaluation will try to include all (or as many as practical) of the factors that one would want to know before deciding whether or not to invest in this particular device. By the time that this evaluation is completed in a few months, I hope that you will have enough information to make an informed decision for yourself.
Let me emphasize that I will not be able to tell you whether the Multiplaz-3500, or any other piece of equipment, will be a good investment for you. Only you can decide that. My intent is to provide as much factual information as I can about the Multiplaz-3500 so that others in our company can make an informed decision about that. The company has no objection to my sharing the information with you as long as I leave their name out of it and make it clear that I am not endorsing any particular product.
Plasma, as the word is used in this evaluation, is defined as:
a. a hot ionized material consisting of nuclei and electrons. It is often regarded as a fourth state of matter and the most common form of matter in the universe.
b. the ionized gas in an electric discharge or spark, containing about equal numbers of positive ions and electrons.
Thus a plasma is present in all forms of welding that use an electrical discharge (carbon arc, stick, MIG, TIG, etc). In most of these welding methods, the plasma is unconstricted, that is, the shape and size of the plasma are determined only by the electric field between the electrode and the workpiece. In what are commonly called plasma cutting/welding torches, the plasma is mechanically constricted by forcing it to pass through a small orifice (hole) in a nozzle. The mechanically-constricted plasma torch was developed and patented by Robert M. Gage (USA Patent #2,806,124, filed 1955, issued 1957, assigned to Union Carbide Corporation). You can get a free copy of the patent athttp://www.freepatentsonline.com/2806124.html
It is one of the most comprehensive patents that I have ever read.
One of the results of this constriction is that the current density (amperes per square millimeter) in the plasma is greatly increased (10 to 100 times) compared to a more typical unconstricted plasma (welding arc). For the same power input, the apparent electrical resistance of the plasma increases with the current density. This means that the voltage drop across a constricted plasma will be much higher than the voltage drop across an unconstricted plasma. Whereas the voltage drop across a typical welding arc might be 20 V at 125 A (apparent resistance = 0.16 ohm), the voltage drop across a constricted plasma might be 125 V at 20 A (apparent resistance = 6.25 ohm) or 250 V at 10 A (apparent resistance = 25 ohm), depending upon how much the plasma is constricted and depending upon the composition of the plasma gas. The power into the plasma is the same in all three cases, but the higher voltage and lower current of the constricted arc offer some advantages and some greater dangers.
1. The voltage to the torch more closely matches the typical input voltage to the power supply.
2. The smaller current is easier and more efficient to generate and control.
3. The cable between the power unit and the torch can be smaller, lighter, and perhaps less expensive.
4. The constricted plasma is more focused, with much higher energy per unit area at the workpiece, and it cuts/welds better for some applications.
1. The higher voltage at the torch nozzle increases the danger of electric shock. Some plasma cutters have open circuit voltages of up to 375 VDC with transferred arc working voltages (nozzle to workpiece) from one-third to one-half of that.
2. If manual filler rod is used (welding), it must be grounded to protect the operator. The use of just gloves to insulate the operator from the rod may be inadequate. Even if the rod does not touch the torch nozzle, when it contacts the plasma it will carry the voltage of the plasma at that point, which may be up to 200 volts. The same is true for an unconstricted plasma, but there the voltage is only about 20 volts.
There is a limit to the current density that can be stably maintained in a transferred arc, particularly if the constricting nozzle is electrically conductive. If the current becomes too high, a “double arc” develops (one arc from cathode to nozzle and another from nozzle to workpiece). This is very detrimental to the nozzle and the orifice.
In the 55 years since the mechanically-constricted plasma torch was patented, the technology has greatly advanced and the cost has greatly decreased. Compressed air is now the most common plasma gas for cutting torches, primarily because compressed air is much less expensive than bottled compressed gases, such as helium, argon, or oxygen. The use of water vapor (steam) as the plasma gas for cutting has received more interest since the original Multiplaz cutting/welding torch came out in 1998. Water vapor offers advantages for some types of cutting (such as no nitriding of the cut surfaces, reduced oxide compared to air, and lower pollution).
In 2005 a second company, Fronius International in Austria (http://www.fronius.com
, then search for “TransCut 300”), introduced the TransCut 300 plasma cutter “using a liquid medium”. A more complete description, technical specifications, and price can be found athttp://www.oll-tools.com/product/transcut300
The Fronius unit differs from the Multiplaz unit in some details, but the AVERAGE plasma power is comparable. One difference is that the Multiplaz-3500 holds 60 mL of water in the torch itself; The Fronius TransCut 300 holds 1.5 L of proprietary liquid (a mixture of water and alcohol) in two cartridges inside the power unit and the liquid is pumped to the torch through a tube. As a result, the Fronius torch is somewhat smaller than the Multiplaz torch, but both are larger than a comparable compressed-air plasma cutting torch. Another difference is that the Fronius unit can operate at higher currents for a short time (30 A @35% duty cycle; 6 A at 100% duty cycle) while the Multiplaz unit has a higher continuous current rating (9.4 A @ 100% duty cycle).
A study of the effect of the composition of the plasma gas on the cutting of mild steel was presented at the 2011 International Symposium on Plasma Chemistry. The study used a modified TransCut 300 torch, but the conclusions should be valid for plasma cutting torches in general. You can get a free copy of the paper athttp://ispc20.plasmainstitute.org/my_ispc/papers/154.pdf
Although the composition of the plasma gas and the details of construction of the plasma torch do affect it somewhat, a general limitation on plasma input power is imposed by the amount of heat that can be dissipated from the torch itself. Using the evaporation and heating of the plasma fluid as the main cooling method of the copper constricting nozzle limits the power into the plasma to a maximum continuous power (100% duty cycle) of about 1.5 kW for a non-transferred arc and about 3 kW for a transferred arc. For higher power plasmas, additional gas and/or water cooling is usually required. No doubt there will continue to be advances in the design of water-vapor plasma cutting torches.
To be continued.
P.S. Depending upon the time that I have available, I plan to post test results for a given week on the following weekend. I will also try to answer any questions at that time.