The basic principle of electric resistance welded tube

1. The basic principle of electric resistance welded tube

The so-called high frequency is relative to the AC current frequency of 50Hz, generally 50KHz~400KHz high frequency current. When a high-frequency current passes through a metal conductor, two peculiar effects will occur: the skin effect and the proximity effect. High-frequency welding is to use these two effects to weld steel pipes. So, what are these two roles? The skin effect means that when an alternating current of a certain frequency passes through the same conductor, the current density is not evenly distributed on the cross-section of the conductor, but mainly concentrated on the surface of the conductor, that is, the density of the current on the surface of the conductor is high, and the density inside the conductor is small. So we vividly call it the "skin effect". The skin effect is usually measured by the penetration depth of the current. The smaller the penetration depth value, the more pronounced the skin effect.

This penetration depth is proportional to the square root of the conductor resistivity and inversely proportional to the square root of frequency and permeability. In layman's terms, the higher the frequency, the more concentrated the current is on the surface of the steel plate; the lower the frequency, the more dispersed the surface current. It must be noted that although steel is a conductor, its magnetic permeability will decrease with the increase of temperature, that is to say, when the temperature of steel plate increases, the magnetic permeability will decrease and the skin effect will decrease.

The proximity effect means that when high-frequency current flows in opposite directions in two adjacent conductors, the current will concentrate on the edges of the two adjacent conductors. Even if two conductors have another shorter side, the current will not flow along the shorter path, we call this effect: "proximity effect". The proximity effect is essentially due to the effect of inductive reactance, which plays a dominant role in high-frequency currents. Proximity effects increase with increasing frequency and with increasing spacing between adjacent conductors.

If a magnetic core is added around adjacent conductors, the high frequency current will be more concentrated on the surface of the workpiece. These two functions are the basis for realizing metal high-frequency welding. High-frequency welding uses the skin effect to concentrate the energy of high-frequency current on the surface of the workpiece; and uses the proximity effect to control the position and range of the high-frequency current flow path. The speed of the current is very fast, and the edges of adjacent steel plates can be heated and melted in a very short time, and the butt joint can be realized by extrusion.

2. The structure and working principle of high frequency welding equipment

After understanding the principle of high-frequency welding, it is necessary to have the necessary technical means to realize it. High-frequency welding equipment is an electromechanical system used to realize high-frequency welding. The high-frequency welding equipment is composed of a high-frequency welding machine and a welded pipe forming machine. Among them, the high-frequency welding machine is generally composed of a high-frequency generator and a feeding device. Its function is to generate high-frequency current and control it; the forming machine is composed of extrusion roller frame, its function is to extrude the molten part, remove the oxide layer and impurities on the surface of the steel plate, and completely fuse the steel plate into one body. High-frequency generator The high-frequency generator used on the welded pipe unit in the past has three circuits: high-frequency generator set; solid-state frequency converter; electronic high-frequency oscillator, which was basically improved to a single circuit later. There are many ways to adjust the output power of the high-frequency oscillator, such as autotransformer, reactance method, thyristor method and so on. The feeding device is used to deliver high-frequency current to the pipe, including electrode contacts, induction coils and impedance devices. Contact welding generally uses wear-resistant copper-tungsten alloy electrode contacts, and induction welding uses copper induction coils.

The main element of the resistor is the magnetic core, and its function is to increase the induction reaction on the surface of the tube to reduce the reactive current and increase the welding speed. The magnetic core of the resistor is made of ferrite, and its Curie point temperature is required to be not lower than 310°. The Curie point temperature is an important indicator of the magnetic core. The closer the distance, the higher the welding efficiency. In recent years, some large companies in the world have begun to adopt solid-state modular structures, which greatly improves welding reliability and ensures welding quality. For example, the WELDACG2800 high-frequency welding machine designed by EFD consists of the following parts: rectification control unit (CRU), inverter, matching and I compensation), DC cable between CRU and IMC, IMC to coil or contact element. The two main parts of the machine are the CRU and the IMC. The CRU consists of a rectifier section with main disconnect switch and full bridge diode rectifier (converts AC to DC), controller with controller and interface to external control equipment. The IMC consists of an inverter module, a matching transformer and a capacitor bank to provide the necessary reactive power for the induction coil.

The main power supply voltage (three-phase 480V) is sent to the main rectifier through the main isolation switch. In the main rectifier the mains voltage is converted to 640V DC and connected to the main DC cables via bus bars. The DC power is sent to the IMC through a DC transmission line consisting of multiple parallel cables. The DC cables are terminated on the IMC unit bus. The inverter module of the inverter part is connected in parallel with the DC bus through a high-speed DC fuse. DC capacitors are also connected together with the DC bus.

Each inverter module constitutes a full-bridge IGBT triode inverter. The driving circuit of the triode is on the printed circuit board in the inverter module. The direct current is converted into high frequency alternating current by the inverter. According to the specific load, the frequency range of AC power is in the range of 100-150KH. To adjust the inverters to the load, all inverters are connected in parallel with matching transformers. A transformer has several primary windings connected in parallel and a secondary winding. The turns ratio of the transformer is fixed.

The output capacitor consists of multiple capacitor modules connected in parallel. The capacitor is in series with the induction coil, so the output circuit is also series compensated. The role of the capacitor is to compensate according to the reactive power requirements of the induction coil, and through this compensation, the resonant frequency of the output circuit reaches the required value. The frequency control system is designed to make the triode always work at the resonant frequency of the system. The resonant frequency is determined by measuring the frequency of the output current. This frequency is then used as the time base signal to turn on the transistor. The transistor drive card sends a signal to each transistor on each inverter module to control when the transistor is turned on and when it is turned off.

The output power control of the induction heating system is controlled by controlling the output current of the inverter. The above control is accomplished by controlling the power control card of the triode driver. The output power reference value is given by the power reference potentiometer on the IMC control panel, or output to the control system by the external control panel. After this value is transmitted to the system controller, it is compared with the DC power value measured by the rectifier unit measurement system. The controller includes a limiting function that calculates a new output current set point based on a comparison of a reference power value to a DC power measurement. The output power setting value calculated by the controller is sent to the power control card, and the power control card will limit the output current according to the new setting value. The alarm system is based on the input signal of the alarm card in the IMC and the signals sent by various monitoring equipment in the IMC. and CRUs.
come to work. Alerts will be displayed on the workbench.

Control and Rectifier Unit (CRU) Inverter, Matching and Compensation Unit (IMC) DC Cable Output Power Bus, Coils and Contact Connections Cooling system mounted in a self supporting steel frame with all components connected as a complete unit. system package
Including: circulation pump with motor, heat exchanger (water/water), compensation container, output process side pressure gauge (secondary output), main inlet water temperature control valve, control valve and electrical cabinet. The main water inlet side heat exchanger uses untreated tributary water as cooling water, and the secondary side heat exchanger uses pure neutral drinking water as cooling water. The raw water is controlled by a thermostatic valve, which measures the temperature of the secondary output. The steel frame can be bolted to the door.

3. Main points of high frequency welding quality control

There are many factors that affect the quality of high-frequency welding, and these factors interact in the same system. If one factor changes, the others change with it. Therefore, in high-frequency adjustment, it is not enough to pay attention to local adjustments such as frequency, current or extrusion amount. This adjustment must be based on the specific conditions of the entire molding system, starting from all aspects related to high-frequency welding. Adjustment.