Plasma Cutting & Gouging

Transcription

1 Plasma Cutting & Gouging By Leif Andersen, Technical Product Manager Welding, WSS Plasma cutting & gouging can be performed on all current carrying materials simply by utilising the compressed air onboard the vessel. Flame cutting processes like Oxy Acetylene and Oxy Propane cutting and gouging can be performed on steel only. Background Way can flame cutting processes only be performed on steel? In order for a metal to be cut by flame ( oxyacetylen/ oxypropane) the following must apply: 1) The melting point of the metal must must be above the ignition point of the metal. This is ok for steel that can be ignited at 900 ºC and melt at 1550 ºF. This is not the case for Cast Iron that will melt before ignitin point have been reached. 2) The metals oxides ( all metals form an oxide with oxygene in air that we refer to as rust) should melt before the metal itself. This is not the case for Aluminium that melts at 658 ºC (1216 ºF) but the aluminium oxides melt at 1926 ºC ( 3500 ºF). 3) The heat produced by the combustion of the metal with oxygene must be sufficient to maintain the flame cutting operation. Aluminium and copper are good termal conductors that fast transport the heat away from the cutting location. 4) The termal conductivity of the metal must be sufficiently low to bring the material to its ignition temperature. The same problem as 3) apply. 5) The metals oxides formed during cutting must be sufficient fluid so as not interupt the cutting operation. Cast iron have a thick sluggish silicate oxide that makes a oxyacetylene cutt difficult to maintain. Nonferrous metals such as aluminium and copper also have refractory oxides (oxides that are chemically and physically stable at high temperatures) coverings, which phohibit normal flame cutting. Stainless steel cannot be flame cutt with standard flame cutting equipment and technique because of the refractory chromium oxide. The Solution So how can we overcome natures laws for a cutting process to work on on a given metal? Simply by changing process from chemical (Flame cutting) to an electric (Plasma cutting) process. In Plasma cuttig only one condition must apply in order to perform cutting: 1) The metal must be electrically conductive.

2 The Plasma process Plasma is the fourth state of matter. We normally think of the three states of matter as Solid, Liquid and Gas. For the most commonly known element water, the three states are Ice, Water and Steam. The significant difference between these states relate to the energy level. If we add energy in the form of heat to ice, the ice melts and the result is water, a liquid. If we add more energy to water, it vaporizes to steam, a gas. If we add more energy to the gas, it becomes ionized gas, Plasma gas. The gas has become electric conductive. At a temperature of between 2000 ºC (3600 ºF) and ºC (18000 ºF) a process of ionization and dissociation of the gas molecules takes place. The molecules are split in molecular and atomic ions and free electrons. When this happens, the gas, which has now become plasma, is electrically conductive because free electrons are available to carry current. A plasma torch uses an alloy copper nozzle to constrict the ionized gas stream to focus the energy to a small cross section. The principle is the same as using a magnifying glass to concentrate the sun's energy to create intense heat. The gas flowing through the nozzle also serves as a medium to remove the molten metal heated by the ionized gas. Approximately 30% of gas is actually ionized (under optimum conditions) while the remaining 70% of the gas stream is used for material removal and cooling. What happens There are three major components inside the torch body: Electrode Gas Baffle (Swirl Baffle that rotate the plasma gas) Nozzle

3 These items are called consumables. They are consumed over time during the plasma process and must be replaced. The electrode is connected to the negative side of a DC plasma power source. The nozzle is connected to the positive side but is electrically isolated by means of a normally open relay. The Plasma Cutter Power Source Inverter plasma cutters rectify the mains Alternating Current (AC) supply to Direct Current (DC), which is fed into a highfrequency transistor inverter between 10 khz to about 200 khz. Higher switching frequencies allow smaller transformer resulting in overall size and weight reduction. The transistors used are for most plasma machine manufacturers IGBTs.(InsulatedGate Bipolar Transistor). This is power semiconductor device primarily used as an electronic switch. Note that plasma cutters work with Low Amperage and High Voltage and therefore need special safety features. The torch connection to the front of the plasma cutter should be according to EN standard that states that a torch connection should be made so that there is no risk for electrical shock and no possibility to remove the torch without a tool. The torch connection, which is the negative voltage, will if exposed be very dangerous given that the open circuit voltage often exceeds 300VDC and the cutting voltage is approx. 100VDC. The risk to the operator or other personnel is extremely high. TP Lead The torch must also have the necessary safety features for CE marking like safety circuit to prevent accidental activation of arc when removing the torch consumables. In Unitor plasma cutters the circuit will be broken if the operator unscrew the head to change the consumables (note safety pins that break the circuit in the moment the shield cup is removed).

4 OCV/Ignition Electrode Pilot arc Electrode There are in general two different start/ignition system for plasma cutters. The Blow back system or High Frequency system*. This is in general what happens when a start input is given to the plasma system: The main contactor within the power supply energizes and placing a high negative voltage on the electrode (negative polarity). Gas begins to flow to the torch and is swirled (rotated) by the baffle ring. The normally open contact in the nozzle circuit close, providing a path to the positive side of the power supply. A high frequency generator provides a high frequencyhigh voltage potential between the electrode and nozzle. This causes a small spark to jump between nozzle and electrode, ionizing a path through the gas. Along this ionized path, a larger DC arc begins to flow between the electrode and nozzle. This is called the pilot arc. The pilot arc is blown out of the nozzle by the gas flow, and contacts the work piece. The main arc is created when the pilot arc transfers to the work piece (if the torch is close enough). The nozzle relay opens, removing the nozzle from the circuit. A transferred arc condition has been established. The main arc increases to cutting amperage after the nozzle relay is opened. Due to the design of the torch tip this arc and the high velocity flow of free electrons and ionized particles called the plasma jet is constricted to a very small cross section with highenergy concentration. In the impact zone the high inherent energy jet, consisting of heat, ionization energy and dissociation energy is released creating temperatures up to 28000ºC (50000ºF). The high velocity air plasma jet will efficiently melt and blow away practically any electrically conductive material, and provide a narrow smooth cut. The plasma cutter electrode. The shiny button at the centre of the electrode is an insert made of pure hafnium (Hf). This is the source of the electric arc that makes a plasma cutter work. Hafnium is an element and is used because it releases electrons to the air very easily, while at the same time resisting the attack of oxygen from the air. When the hafnium insert is consumed, the electrode must be replaced. * The ignition system in UPC1040/1041/1041TP is high voltage low frequency 5Hz so it will never be anywhere close to the Radio Frequency bandwidth.

5 Types of plasma cutters: The basic plasma cutting equipment consist of a constant current DC power source, an arc starting circuit (High frequency or air type) and a torch. Plasma machines will normally be divided in capacity by relating to their max Amperage. The Unitor range consist of a 30 Amp and a 100 Amp plasma cutter. UPC310TP Technical Specifications: Current adjustment 530 A Power supply V *, 1~50/60 Hz TP Function V Mains fuse 3216 A slow blow Maximum power 2,85 kva Process power 95 V, 530A Duty 40 ºC 110 V 230 V Power factor 0,95 Open circuit voltage 424 VDC Protection class IP 23 S Cooling system Forced Temperature class F Maximum cutting capacity steel/stainless 10 mm Aluminium 8 mm Supply pressure 49 bar Setting on machine 3,5 bar consumption 115 l/min 6,9 m3/h Weight 10 Kg Width 190 mm Height 320 mm Length 440 mm Approval marks RoHS NonAsbestos declaration CE Yes Yes Product no UPC1041TP Technical Specifications: Current adjustment A Power supply 400/440 V *, 3~50/60 Hz TP Function V/ Phase detector Mains fuse 25/20 A slow blow Maximum power 14,7 kva Process power 120 V, 20100A Duty 40 ºC 100A 80A 60A Power factor 0,95 Open circuit voltage 240 VDC Protection class IP 23 S Cooling system Forced Temperature class F Maximum cutting capacity 40 mm Quality cut 25 mm Supply pressure 67 bar Setting on machine 5 bar consumption 190 l/min 11,4 m3/h Weight 15 Kg Width 190 mm Height 320 mm Length 560 mm Approval marks RoHS NonAsbestos declaration CE Yes Yes Product no

6 How to maintain the plasma machine UPC1041/1041TP and torch: Grind location where return clamp is connected to the work Also important: Keep longest possible distance between plasma cutters machines front and location where plasma cutting takes place. The machine need cooling and dust from the cutting can be drawn into machines interior if it is placed too close to cutting location.

7 How to maintain the plasma machine UPC1041/1041TP and torch Replace the electrode if center has a pit deeper than 2mm (1/16 ) Replace nozzle if opening is deformed or 50% oversize. The air entering the plasma cutter must be absolutely dry and free from oil. Make sure that location where return clamp is fastened to base material are grinded to secure good electric conductivity. Every ½ year make sure to remove the plasma machines cover plate and blow through with dry compressed air. This is in order to remove dust and dirt from electric components. Also, keep in mind to change the filter in the filter cartridge at regular intervals.

8 In depth plasma information Gas Swirling Swirling (rotating) the gas assists cutting in several ways. Swirling increases cooling. The unionized gas atoms are heavier/cooler and are thrown to the outside of the spinning gas stream. This cool barrier provides protection for the copper nozzle. As amperage is increased, the amount of ionization increases (changing the 30/70% ratio) and cooling decreases, shortening the life of the nozzle. Nozzles are designed to operate within a specific current (amp) range. Swirling gas improves cut quality lf the plasma gas is not swirled, results would be a bevel on both sides of the cut. By swirling the gas, the arc is distributed evenly along one side of the cut. lf the swirl is reversed direction (CW to CCW), the square side will switch. As the ionized gas (plasma arc) is swirled, the electrical arc will attach itself evenly to the leading edge of the cut. These multiple attachment points provide a more even power distribution through the workpiece. This equalizing of power from top to bottom results in a squarer side. The other side having a 5 to 8 degree bevel. Double Arc A double arc is a condition, which allows the nozzle to stay in the plasma circuit. As described above, the nozzle should only be in the circuit during the pilot arc phase. lf left in the circuit, the nozzle will carry cutting amperage, which will destroy it. Double arcing is caused by: Standing pierce. The torch has to be positioned close enough to the workpiece to allow the pilot arc to contact the plate, so the main arc can transfer. Pierce spatter is ejected at a shallow angle during the initial pierce. As the arc penetrates the material, the spatter becomes vertical. This debris may connect the plate and nozzle, keeping the nozzle in the circuit even when the relay opens to remove it. This scenario may damage the front end of the torch. Pilot arc malfunction. This can occur if the pilot arc relay circuit fails to disconnect the nozzle. This can happen either with a shorted relay or resistor. Again the nozzle is left to carry more current than intended, damaging it.

9 Ignition system UPC310TP 1. A start input signal is sent to the power supply. The OCV is generated but since there is contact between electrode and nozzle it is practically 0V (no risk for operator). OCV Electrode 2. Gas start to flow pushing the electrode upwards separating it from the nozzle and creating an electrical spark, energy transferred from the spark to the gas causes the gas to become ionized, therefore electrically conductive. This electrically conductive gas creates a current path between the electrode and the nozzle, and a resulting plasma arc is formed. The flow of the gas forces this arc through the nozzle orifice, creating a pilot arc. Ignition Electrode 3. Assuming that the nozzle is within close proximity to the workpiece, the pilot arc will attach to the workpiece. Current flow to the workpiece is sensed electronically at the power supply. As this current flow is sensed, the pilot arc transistor is opened. Gas ionization is maintained with energy from the main DC arc. Pilot arc Electrode The temperature of the plasma arc melts the metal, pierces through the workpiece and the high velocity gas flow removes the molten material from the bottom of the cut kerf. At this time, torch motion is initiated and the cutting process begins. Main arc Electrode

10 Ignition system UPC1041TP 1. A start input signal is sent to the power supply. This simultaneously activates the preflow of gas. Preflow Electrode 2. After the gas flow stabilizes, the open circuit voltage and ignition circuit is activated. The high voltage breaks down between the electrode and nozzle inside the torch in such a way that the gas must pass through this arc before exiting the nozzle. Energy transferred from the high voltage arc to the gas causes the gas to become ionized, therefore electrically conductive. This electrically conductive gas creates a current path between the electrode and the nozzle, and a resulting plasma arc is formed. The flow of the gas forces this arc through the nozzle orifice, creating a pilot arc. OCV/Ignition Electrode 3. Assuming that the nozzle is within close proximity to the workpiece, the pilot arc will attach to the workpiece. Current flow to the workpiece is sensed electronically at the power supply. As this current flow is sensed, the ignition is disabled and the pilot arc transistor is opened. Gas ionization is maintained with energy from the main DC arc. Pilot arc Electrode The temperature of the plasma arc melts the metal, pierces through the workpiece and the high velocity gas flow removes the molten material from the bottom of the cut kerf. At this time, torch motion is initiated and the cutting process begins. Main arc Electrode