How to Choose the Right High-Frequency Welding Equipment?

High-Frequency Welding Equipment

High-frequency welding equipment is primarily used for pipe manufacturing. As depicted in Figure 5-6, a high-frequency welded pipe mill unit consists of a horizontal guide roll, a high-frequency generator and output transformer, extrusion roll, external burr remover, polishing roll, frame, and various auxiliary mechanisms and tools.

The high-frequency generator and welding auxiliary devices play a key role in the quality and productivity of the welded pipe.

Figure 5-6 High-Frequency Welded Pipe Mill

1 – Horizontal Guide Roll, 2 – High Frequency Generator and Output Transformer, 3 – Extrusion, 4 – External Burr Remover, 5 – Polishing Roll, 6 – Base

1. High-Frequency Generator

There are three types of high-frequency generators used for pipe manufacturing: a motor-generator set with a frequency of 10kHz, a solid-state frequency converter, and an electron tube high-frequency oscillator with a frequency up to 100~500kHz, the latter being the most widely used. The typical power range for high-frequency oscillators is 60~400kW.

The basic circuit of an electron tube high-frequency oscillator with a frequency of 200~400kHz is shown in Figure 5-7. The power grid supplies power to the step-up transformer and rectifier through the circuit switch, contactor, and thyristor regulator. The step-up transformer and rectifier convert the grid frequency AC into high voltage DC for the oscillator.

To ensure the voltage fluctuation coefficient is less than 1%, a filter device must be installed at the output end of the high voltage rectifier.

The oscillator converts the high voltage DC into high voltage high-frequency electricity for the output transformer, which then converts the high voltage low current high-frequency electricity into low voltage high current high-frequency electricity and directly delivers it to the electrode (sliding contact) or induction coil.

Methods to adjust the output power of the high-frequency oscillator include the autotransformer method, thyristor method, transistor method, and saturated reactor method.

Figure 5-7 Basic Circuit of High Frequency Oscillator

1 – Circuit Switch, 2 – Contactor, 3 – Thyristor Regulator, 4 – Step-up Transformer, 5 – Rectifier, 6 – Filter, 7 – Output Transformer, 8 – Oscillator

2. Electrode

The electrode contact is an important device for supplying power to the tube blank. It must conduct high-frequency current under high temperature conditions and high-speed sliding friction with the tube wall. Therefore, it should have high conductivity, high-temperature strength, hardness, and wear resistance.

Alloys such as copper tungsten, silver tungsten, or zirconium tungsten are commonly used and can be made into a composite structure, as shown in Figure 5-8. The dimensions of the contact block 2 are: width 4~7mm, height 6.5~7mm, length 15~20mm. The contact block is brazed to the contact seat 1 made of copper or steel using silver brazing.

This electrode can conduct a welding current of 500~5000A, and the pressure on the tube wall is 22~220N.

3. Induction Coil

The induction coil is a crucial component of the high-frequency induction welded pipe machine. Its structure and dimensions significantly impact energy conversion and efficiency. A typical induction coil structure, shown in Figure 5-9, is usually made up of a single turn or 2~4 turns of a metal ring made of pure copper square tube, round tube, or pure copper plate, wrapped with an insulating glass tape and cast with epoxy resin to ensure inter-turn insulation, with internal water cooling.

Figure 5-8 Schematic Diagram of Electrode Structure

1 – Contact Seat,
2 – Contact Block,
3 – Brazing Seam,
4 – Cooling Water Hole

Figure 5-9 Typical Induction Coil Structures

a) Multiple Turns of Square Tube,
b) Multiple Turns of Round Tube,
c) Single Turn of Plate
HF – High Frequency Power Source,
T – Cooling Water Pipe

4. Impedance Device

The impedance device is a crucial auxiliary device during high-frequency welding. The key component, the magnetic core, serves to increase the impedance on the backside of the pipe wall to reduce ineffective current, increase effective welding current, and enhance welding speed.

The magnetic core is made of ferrite material (like MXO or NXO type) with a high Curie point and high magnetic permeability. The structure of the impedance device is shown in Figure 5-10. The magnetic core is composed of magnetic rods with a diameter of Φ10mm, and the casing is made of fabric phenolic or fiberglass. In situations prone to damage, stainless steel or aluminum can also be used.

The interior of the impedance device should allow for water cooling to prevent heat during welding from affecting magnetic properties. The length of the impedance device should be adapted to the pipe diameter:

  • for welding tubes with a diameter less than Φ38mm, the length of the impedance device is 150~200mm;
  • for welding tubes with a diameter of Φ50~Φ75mm, the length of the impedance device is 250~300mm;
  • and for welding tubes with a diameter of Φ100~Φ150mm, the length of the impedance device is 300~400mm.

In addition to the high-frequency welded pipe mill unit, a high-frequency welded pipe production line also includes other related equipment, such as the uncoiler, straightener, leveler, mandrel, straightening machine, end-milling beveling machine, flying saw machine, and shear welding machine. These factors should be considered when selecting equipment.

Currently, the main technical parameters of the high-frequency induction heating equipment used for welding metal pipes domestically produced are as follows.

  • Power stages:60kW, 100kW, 200kW, 300kW, 400kW, 600kW, 700kW, 800kW;
  • Frequency stages: 200~300kHz, 400kHz, 600kHz, 1~2MHz;
  • Welded pipe diameter: Φ(8~48)mm, Φ(20~45)mm, Φ(20~76)mm, Φ(60~114)mm, Φ(90~219)mm, Φ(114~273)mm;
  • Welded pipe wall thickness: 0.5~1.0mm, 1.25~2.75mm, 2.75~4mm, 3.5~12mm; Welding methods: seam welding, spiral welding, induction welding, resistance welding.

Safety Techniques for High-Frequency Welding

During high-frequency welding, the most significant factor affecting personal safety is the high-frequency welding power source. The voltage in the high-frequency generator circuit is extremely high, and improper operation could lead to severe accidents if an electric shock occurs.

Therefore, to ensure personnel and equipment safety, in addition to the protective devices already installed in the power supply equipment, the following measures should typically be taken:

1) The high-frequency generator casing and output transformer must be properly grounded. The grounding wire should be as short and straight as possible, and the grounding resistance should not exceed 4Ω. Around the equipment, especially at the operator’s position, insulating rubber mats with a pressure resistance of 35kV should be laid.

2) Operating the equipment with the door open is prohibited. An interlock door switch should be installed on frequently opened and closed doors, ensuring that the equipment can only be started and operated when the door is securely closed.

3) When power is off for maintenance, the main distribution switch must be turned off, and signs indicating “Operator at Work” and “Do Not Close Circuit” should be hung. After opening the machine door, discharge rods should be used to discharge each capacitor group. Only after discharging can specific maintenance operations begin.

4) Live maintenance is generally not allowed. If necessary, the operator must wear insulating shoes and gloves, and a supervisor must be present.

5) When starting and operating the equipment, the cooling water system should be carefully checked. Power can only be turned on to preheat the oscillating tube when the cooling water system is functioning properly.

Furthermore, because the high-frequency electromagnetic field affects the human body and surrounding objects (causing nearby metals to heat up and human cell tissues to vibrate, leading to fatigue, dizziness, and other symptoms), all high-frequency conductors exposed outside the high-frequency equipment casing need to be shielded with thin aluminum or copper plates.

This measure ensures that the electric field strength in the workplace does not exceed 40V/m.

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