The same question came up on another forum just a week or so ago....here is a description of different starting methods used by plasma torches over the years. In a nutshell: 1. Contact or Blowback start with a pilot arc is the latest technology in air plasma torches used for hand held cutting as well as entry level and light industrial CNC cutting machines. 2. High Frequency start with a pilot arc is used in all liquid cooled 100% duty cycle industrial plasma cutters.
A plasma torch needs a method of heating the gas (air in an air plasma torch) heated up to its ionization temperature......when this occurs the gas becomes electrically conductive, which allows the DC energy from the plasma power supply to superheat the air into a plasma gas, which creates the high temperature cutting process.
There are two popular ways to ionize the gas in a torch...."High Frequency", high voltage discharge, and the "Contact start" or blowback start method.
High frequency starting uses a high voltage (around 15,000 volts) high frequency AC electrical source to create a high intensity spark inside the torch to heat the plasma gas. This discharge, similar to the power used to fire spark plugs in an internal combustion engine, is developed using circuitry in the plasma power supply, and is transmitted through wires in the torch leads to the torch. High frequency plasma systems (as well as tig welders with high frequency) have been known to cause issues with sensitive electronic devices. Many cnc machines that use a standard PC as their controller often have issues with electrical noise interference that is caused by this starting method. Industrial CNC machines often are designed with complex grounding and filtering that allows high frequency plasma systems to be used without issues.
1.Some high frequency torches are designed to fire a "pilot arc" which is a high intensity arc that essentially will fire in the air. These types of torches work well with transferring the arc to rusty or painted surfaces. Once the pilot arc makes electrical contact to the plate being cut..the main plasma power ramps up and the cutting process begins.
Many older technology air plasma systems uses this high frequency/pilot arc starting method. All high end industrial plasma systems currently use this method of starting....as it allows for better torch accuracy (cut quality) and starting reliability in high duty cycle applications.
2.Other high frequency torch designs (generally very low cost copies of old plasma technology) use high frequency to create the arc through making physical contact with the plate. These torches, often know as high frequency start or scratch start must actually have the nozzle in extremely close proximity to the plate, or on dirty, rust plate must scratch through the surface of the plate in order to allow the high frequency discharge to make contact and ionize the gas.
This technology is generally reserved to very low cost hand plasma cutting systems. It is difficult at times to get the arc to transfer to the plate, and is not generally good for mechanized cutting applications as the nozzle must be in very close proximity to the plate in order to transfer tha arc....which causes molten metal to blow back and damage the nozzle orifice.
3.The contact start or blowback start torches (developed by Hypertherm, but used by many companies today) use a moving electrode inside the torch. The main DC power is activated in the plasma torch, then moments later the gas flow starts in the torch. The gas flow causes the spring loaded electrode to slide back away from the nozzle, which creates a short circuit spark that ionizes the gas flow through the torch. These torches will fire in the air, and will penetrate through rusty, dirty, painted surfaces as well as the first method of High frequency with a pilot arc.
The majority of air plasma systems use this technique today for hand and mechanized cutting applications. This innovation allowed for smaller inverter based plasma technology systems to become easier to use, and lower priced. Since there is no high frequency discharge, these types of systems can be used on many entry level cnc machines with no need for additional grounding and filtering, and since there is a pilot arc, they can transfer to the plate from a distance...allowing for piercing on thick plate with no nozzle damage.
Jim Colt Hypertherm