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Plasma Cutting Features

Due to the concentration of plasma arc energy, high temperature, great mechanical impact and stable arc, plasma arc cutting has the following characteristics:

  • All current metals can be cut. Including ferrous metals, non-ferrous metals and various high-melting point metals, such as stainless steel, heat-resistant cast iron, tungsten copper, titanium copper, aluminum and their alloys. Cut stainless steel, aluminum, etc. up to 200 mm or more. The non-transferred plasma arc can also cut various non-metallic materials, such as refractory bricks, concrete, granite, silicon carbide, etc.

  • Fast cutting speed and high productivity. For example, cutting a 10 mm thick aluminum plate, the speed can reach 200 ~ 300 m/h, cutting a 12 mm thick stainless steel plate, the speed can reach 100 ~ 130 m/h.

  • High cutting quality. The incision is narrow, clean and tidy, the deformation and heat-affected zone of the incision are small, and the change in hardness and chemical composition is small. Usually, it can be welded directly after cutting, and there is no need to process and clean the groove.

  • The ability to cut thick plates is not as good as that of gas cutting, and the incision width and bevel angle of the cutting surface are larger. However, when cutting thin plates, special cutting torches or processes can be used to obtain near-vertical cutting surfaces.

Plasma cutting equipment

The composition of plasma arc cutting equipment is shown in Figure Composition diagram of plasma arc cutting equipment. It usually consists of power supply, high frequency generator, gas supply system, cooling water (gas) system, control system (control box), cutting torch and cutting table and other devices and components. The functions and composition of its main devices and components are listed in Table -Functions and composition of main devices and components of manual plasma arc cutting equipment.

plasma cutting equipment
Functions and composition of main devices and components of manual plasma arc cutting equipment

Plasma cutting equipment—power supply

Basic requirements of plasma cutting equipment for power supply

Plasma arc cutting adopts DC power supply, which should meet the following basic requirements:

  • It has a steep external characteristic curve.

  • It has high no-load voltage and working voltage.

  • When igniting the main arc, the current rise cannot be too fast.

Typical external characteristic curve of power supply for plasma arc cutting

According to the static characteristics of the plasma arc and the frequent fluctuation of the arc length during the cutting process, in order to maintain a stable plasma arc, the external characteristics of the power supply should have a curve that is steeper than that of the electrode arc welding power supply and close to the constant current characteristic. Figure—Typical external characteristic curve of power supply for plasma arc cutting shows the typical external characteristic curve of the power supply for plasma arc cutting.

As shown in Figure-Influence of arc external characteristics on arc stability, if the external characteristic is a slow-falling characteristic, once the flow rate of the working gas increases and the arc voltage rises, the arc static characteristic curve may move up to the top of the power supply external characteristic curve and arc extinguishing occurs. In addition, during the plasma arc cutting process, the anode spot jumps up and down frequently in the incision, causing the arc length to change continuously, and the change in the height of the cutting torch will also cause the arc length to change. In the case of slow-down characteristics, the current fluctuation value ∆I2 is relatively large , easy to make the cutting process unstable. In the case of steep drop characteristics, the current fluctuation value ∆I1 is very small (Fig.Influence of arc external characteristics on arc stability), which can ensure the stability of the cutting parameters, and can also reduce the possibility of double arcs caused by the sudden increase of the current, which is beneficial to prevent the electrode nozzle from of burning loss.

Influence of arc external characteristics on arc stability

Since diatomic gases such as nitrogen, oxygen, and hydrogen are usually used for cutting, these gases must absorb a large amount of decomposition heat before ionization, and the ionization voltage is relatively high. In addition, the diameter of the plasma arc is small and the current density is extremely high, so the arc column unit length The voltage drop is also large, so the working voltage of the plasma arc is higher than that of the arc welding, which is 100~200V. In order to maintain the stable combustion of this high-voltage arc, the no-load voltage of the power supply should generally be twice the working voltage, usually between 150~400 V (depending on the working gas used), and the large thickness used for cutting large thickness plates. The power supply requires a no-load voltage of up to 500V.

Types of Plasma Arc Cutting Power Supplies

The existing power supplies for plasma arc cutting are:

  •  Three-phase magnetic saturation amplifier silicon rectifier power supply.

  • Three-phase moving iron split magnetic rectifier power supply.

  • Saturated reactor rectifier power supply.

  •  Thyristor bridge rectifier power supply.

  • Flux leakage transformer plus tap reactor rectifier power supply.

  • Transistor inverter power supply.

The main types of plasma arc cutting power supply are: transistor inverter power supply for working current below 100 A; thyristor bridge rectifier for current 100~700 A; saturated reactor power supply for current greater than 700 A.

When it is necessary to use high current to cut thick parts, two plasma arc cutting power supplies of the same type can be used in parallel.

Plasma Cutting equipment——control box

The electrical control box mainly includes program control relays, contactors, high-frequency oscillators, electromagnetic valves, water pressure switches, etc. The plasma arc cutting program diagram is shown in Figure-Plasma arc cutting program diagram, it is controlled according to the required program, and its action points are as follows:

  • Before cutting, the gas flow and trolley speed can be adjusted (during automatic cutting), and the concentricity of the electrode and the nozzle can be checked by high-frequency discharge sparks.

  •  Gas supply in advance and gas stop after delay to protect the electrodes from oxidation.

  • Use a high-frequency oscillator to ignite the non-transferred arc, and automatically cut off the high frequency after the non-transferred arc is established.

  • Reliably transition from non-transferred arc to transferred arc, once the transferred arc is established, the non-transferred arc will extinguish itself.

  • When the cooling water is not connected, the flow is insufficient or the water is cut off in the middle, the cutting machine cannot be started, and the machine that has been started should stop working immediately to prevent the nozzle from burning out.

  • When the cutting is finished or the arc is extinguished due to other reasons, the control circuit can be automatically disconnected.

  • When the cutting power supply is short-circuited or overloaded, the protection device can automatically cut off the network power supply.

Plasma arc cutting program diagram

Plasma Cutting equipment——water system

The nozzle and electrode are cooled with water, and the water-cooled resistance and water-cooled wire that limit the current of the non-transferred arc are also cooled to ensure the stable and continuous operation of the cutting torch. According to the way of cooling the electrodes, there are usually two types of cooling water circuits: indirect water-cooled electrodes and direct water-cooled electrodes.

Figure-Schematic diagram of the indirect water-cooled electrode water system is a schematic diagram of the water system of the indirect water-cooled electrode. Cooling water enters from the lower part of the nozzle, and after cooling the nozzle, it passes through the upper cavity and then indirectly cools the electrode. The water flow rate should be controlled above 3 L/min, the water pressure should be 0.15 ~ 0.2 MPa, and the cooling water can be tap water or circulating water.

Schematic diagram of the indirect water-cooled electrode water system

The high-power plasma arc cutting torch that requires strong cooling must cool the nozzle and electrode separately to prolong the service life of the electrode and nozzle. The water flow rate should be above 10L/min. At this time, a water pump is needed for circulating cooling. Generally, a water pump with a lift of 41.3 m and a flow rate of 4 m3/h or similar models can be used. Figure-Schematic diagram of direct water-cooled electrode water system is a schematic diagram of the water system of the direct water-cooled electrode.

Schematic diagram of direct water-cooled electrode water system

In order to prevent nozzles and electrodes from burning out when there is no water flow or the water flow rate is too small during work, a water flow transmitter should be installed in the waterway system so that it can automatically cut off the power supply and cannot ignite the arc or perform cutting.

Plasma arc gas cutting has certain requirements on the water quality and cleanliness of cooling water. The sundries in the unclean water will accumulate in the waterway and cutting torch, affect the cooling effect, and corrode the interface of cables and pipelines. In the general plasma arc cutting, the water inlet pipe should use a transparent hose, so that no floating objects or other colors in the water can be seen visually. In order to prevent ionized substances in the water, the method of measuring resistivity can be used to check, and the resistivity is greater than 5kΩ·cm.

Plasma Cutting equipment——Air system

The role of the gas is to compress the arc as the medium of the plasma arc, prevent electrode oxidation and protect the nozzle from being burned out. Stable and continuous supply of gas is one of the important conditions to ensure stable plasma arc cutting, so it is necessary to ensure that the gas system is unimpeded.

2-24-25 plasma cutting equipment

Depending on whether the working gas is a single gas or a mixed gas, the gas supply system has two types: a single gas circuit (Figure-Single gas gas path system diagram) and a mixed gas circuit (Figure-Mixed gas gas circuit system diagram). The purpose of setting the air storage tank in the gas circuit is to reduce the impact of the air flow before starting the cutting, so as to facilitate the arc ignition; and to delay the gas stop at the end of the cutting to protect the electrodes from being oxidized. Gas mixing is used to mix two gases evenly. lose

The pipeline for sending gas should not be too long, and hard rubber tube can be used for the gas delivery pipe. The gas working pressure is generally adjusted to 0.25 ~ 0.3 MPa. The flow meter should be installed behind each air valve. The selection of the flow meter depends on the cutting thickness and the usual flow rate. Generally, a larger margin should be selected. The flow rate used should not exceed half of the full scale value of the flow meter used to avoid electromagnetic gas. The moment the valve is connected, the impact will damage the flowmeter.

When using hydrogen, it is even more necessary to prevent pipeline leakage. It is best not to pass the hydrogen pipeline through the control box, so as to avoid mixing hydrogen and air in the sealed control box when the gas leaks. When the volume of hydrogen reaches 4% ~ 73%, the relay will trigger The sparks generated by the point cause an explosion.