There is a little trick you can do that will give the "ok to move" signal to the THC and the cutting machine with any plasma system. This signal is simply a contact closure that indicates that the arc has transferred to the plate and there is current (Amps) flowing through the work cable (ground clamp). This signal is properly called "arc transfer"....although it is also known as "arc good", "machine motion"....and probably has a few other names.
Her is the trick: Go to an electronics supply store and find a magnetic reed switch...these are usually in the form of a small (maybe 1/4" dia.) glass tube with a couple of wires coming out of one end. These switches activate when a magnetic field is near.... Next, find about a 4" long piece of iron or steel pipe that the reed switch slides snugly inside...with the wires sticking out one end. Epoxy the switch inside the pipe. Wrap about 2 to 4 turns of your work ground cable tightly around this steel or iron pipe and tye wrap the cable in place. When the plasma arc transfers to the plate...the magnetic field created between the iron or steel pipe and the electrical current from the ground cable will activate the reed switch.....and you will have a nice arc transfer signal.
I have used this quite a few times over the last 30 ur so years...it works very well!
The "adjustable" divided arc signal: This is an input required for all arc voltage based THC systems. It is a divided ratio of the actual arc voltage as measured between the electrode and the plate you are cutting. (of course this voltage can be found inside the plasma power source..it is the negative and positive DC output). The simplest THC systems use a simple voltage divider circuit which has a fixed voltage division of 25:1,
50;1 or 100;1...or some other ratio that provides feedback voltage to the THC electronics...as the raw arc voltage is too high (typically between 70 and 200 volts DC depending on the thickness and type of material). These simple dividers generally work with newer style air plasmas with non high frequency start.
With high frequency start systems....there is a high voltage (as much as 15,000 volts AC) spike that is directly coupled between the nozzle and the electrode at the beginning of every cut. This high voltage (high frequency) will blow the resistors right off the circuit board in a simple voltage divider circuit. For these types of systems...specially filtered voltage dividers are used that filter out high voltage AC...and only allow DC. Some of the more sophisticated circuits also incorporate a high voltage isolation relay that does not allow monitoring of voltage during the brief high frequency start signal....saving the voltage divider and the THC electronics from early failure. You can compare the high frequency to a lightning strike on a power line that feeds your house...something will probably get damaged.....and this happens every time the arc fires.
High end industrial THC systems (Hypertherm, Kaliburn are two manufacturers) are designed to work with high frequency...many of the lower cost THC systems are designed to work with non high frequency start systems. I would rely on the THC manufacturers recommendations! For similar reasons...rely on recommendations from cutting machine manufacturers regarding the use of high frequency start plasma...in this case the high frequency is not directly coupled (as it is in the THC)...but can couple inductively between the plasma system cables (torch leads especially) and the drive and encoder signal wires that often run in the same cable trac.
Hopes this sheds some light on the subject. Sometimes it is cheaper to find another plasma system that does not use high frequency start! Hypertherm was the leader with non high frequency start with its Powermax systems...but uses high frequency exclusively on its large industrial systems.
Jim Colt  |