When using thyristor modules as power stages in resistance welding with mains frequency technology, the power or current setting is realised via the so-called phase angle control. Current control is made possible by firing the thyristors in an alternating path circuit. The ignition timing delay allows the power or current setting to be metered. This ignition timing delay is called phase angle. The current flow angle in which the welding current flows results from the phase angle. From an electrical point of view, the phase angle or the current flow angle is given in electrical degrees.
The current flow angle can theoretically be between 0 and 180 degrees, in practice values between approx. 30 and approx. 150 degrees are set. As always, you can find details on this at your Harms & Wende Partner or in Mr Krause’s reference book. However, we will not delve deeper into electrical engineering here. It is important for the resistance welder that the current set on the welding control unit corresponds to this phase angle in percentages or parts per thousand (parts of the scale). With a high scale division value, i.e. high current, a small phase angle is output, i.e. the thyristor is ignited immediately after the voltage crosses zero.
The reverse is true for small current values (small scale division). A large phase angle is set there, i.e. a large ignition delay. The thyristor is only ignited here shortly before the next voltage zero crossing. This can be explained very well in graphics. The graph here is only intended to give an overview. Our training courses focus more on this topic. Please contact us or your Harms & Wende partner. In the next edition of the encyclopaedia, the current setting in the medium frequency technology (pulse width modulation) will be discussed, which is increasingly used today.