Adaptive control of the welding frequency for roller seam welding
The resistance roller seam welding system is used for continuous connection of two workpieces. The workpieces for welding are usually moved at constant speed between two roller electrodes which are pressed together with a defined electrode force.
Roller seam welding is in theory no different to a row of spot welds in which the spot welding electrodes are replaced by roller electrodes.
Adaptive control circuits can clearly improve the welding success rate (patent no.: 102005058588.4-34).
Welding with 3-phase DC or MF:
The DC produced from a three-phase AC grid or the rectified 1000 Hz MF current provides energy continuously to the piece for welding. This method is used to good effect at high, fixed welding speeds.
To achieve good welding results, a minimum ripple of the welding current is required" (DVS 2906). Sealed, volume or fixed seams are achieved by controlling the welding current with a variable mark/space ratio.
The decisive disadvantages of welding with DC are:
- The welding current ripple can only be limited to a degree
- Owing to the Peltier effect, there is a risk of different heating of the electrodes and the welding specimen alloy.
Welding with AC and single phase DC
If the AC over the entire length of the seam is connected without interruption, we refer to constant AC. Each half wave generates a welding spot. The spot interval depends on the welding speed. At a mains frequency of 50 Hz, for example, at a speed of 6 m/min, the result is a spot interval of 1 mm.
This is true only if the full half wave(180°) of the mains voltage is available, which is only the case at maximum welding current. With the phase cutting control normally used to regulate the welding current, its pulse length is shortened so that discontinuities arise in the welding current cycle.
A reduction of these "discontinuities" in sealed welding is achievable only by reducing the speed of advance. Advances speeds greater than 3 m/min are therefore usually achievable only for fixed and volume seams.
Servo inverters in resistance press welding systems
Using servo inverters in welding systems offers a way out: The disadvantages of DC, such as the lack of ripple and the
alloy susceptibility, as well as the disadvantages of the limited advance speed at 50 Hz AC can be fully compensated with this type of current.
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