Resistance welding is a process in which two workpieces are welded together by heating them due to electrical resistance and applying pressure. No welding consumables are used, resulting in a clean and fast joint.
The most common resistance welding processes include
- Spot welding: Individual spots are pressed together by electrodes and welded.
- Projection welding: Pre-stamped projections or elevations on the workpieces are fused together.
- Flash butt welding: The workpieces are joined by controlled burning of the contact surfaces and subsequent pressing together.
- Butt welding: The workpieces are welded together by direct contact and pressure.
- Roll seam welding: Continuous weld seams are produced by rotating electrodes.
The direct current (DC) process uses a constant direct voltage. Direct current produces uniform heating, which results in a more stable weld. The power source can be provided by a medium frequency rectifier (MFDC), which converts alternating current into direct current.
The welding transformer converts the AC mains current into low voltage with high current, which is required for resistance welding. In spot welding, projection welding and seam welding, the transformed current is fed directly into the welding point.
A 3-phase rectifier converts three-phase alternating current into direct current. This technology enables more precise control of the welding current and ensures uniform heating, which is particularly advantageous for flash butt welding. It also reduces mains fluctuations and improves energy efficiency.
Medium frequency rectifier units (MFDC) operate at a frequency of approximately 1,000 Hz, producing a more stable and efficient weld compared to traditional alternating current (AC) methods. MFDC systems are smaller, lighter and have lower energy consumption as there are fewer losses due to magnetisation of the welding transformer.
The alternating current (AC) method is typically used in traditional resistance welding systems, such as spot welding or projection welding. It offers a cost-effective solution, but is less efficient than modern MFDC systems as there is a zero point at each half-wave of current, which can lead to energy loss.
Roll seam welding uses rotating electrodes to create a continuous weld seam. This technique is ideal for long and tight joints. In combination with an MFDC welding system, constant heat transfer is ensured, which increases the quality of the weld seam.
In flash butt welding, the two workpieces are held under tension and heated to such an extent that material melts and separates at the contact surfaces. The workpieces are then pushed together under pressure, creating a high-strength welded joint. This process requires precise control, often using 3-phase rectifiers or MFDC systems.
The main advantages of a water-cooled high-current transformer are
- Efficient heat dissipation: High temperatures are dissipated quickly and efficiently, keeping the transformer stable even at high power output.
- Longer service life: Cooling prevents overheating damage, which significantly extends the service life of the components.
- Continuous operation: Ideal for applications that require a constant, high welding current, such as roll seam welding or flash butt welding.
The typical design of a water-cooled high-current transformer includes
- Primary winding: The winding that absorbs the mains current.
- Secondary winding: The winding that delivers the converted voltage and the high current.
- Iron core: This is used to increase the magnetic coupling between the primary and secondary windings.
- Water cooling system: Channels for the water that is circulated by pumps to cool the components.
- In versions with a rectifier, a rectifier unit is also integrated.
Water cooling is achieved through cooling channels that run around the most important components such as the windings and the iron core of the transformer. Water is continuously pumped through these channels to effectively dissipate the heat generated by the current flow. This ensures that the transformer remains stable and reliable even during continuous operation.
A water-cooled high-current transformer with rectifier also contains a rectifier unit that converts the alternating current into direct current. This type of transformer is used in direct current (DC) processes such as flash butt welding or seam welding. In contrast, a transformer without a rectifier generates pure alternating current (AC) and is used in classic welding processes such as spot welding.
The use of 3-phase AC rectifier units enables a more even power supply compared to single-phase AC systems. This increases the welding quality, especially with thicker materials or complex components, such as those used for butt welding or flash butt welding.