A welding transformer is a two-winding transformer. To adjust the arc ignition voltage, the primary winding is equipped with tap changers, which are used with a tap switch to regulate the secondary side's no-load voltage. The primary and secondary windings are mounted on two core columns, giving the transformer a significant leakage reactance. As a result, the terminal voltage on the secondary side will drastically decrease with an increase in current. An iron-core reactor is串联 in the secondary winding circuit for adjusting the welding current. By changing the length of the air gap in the reactor, the current increases as the air gap lengthens.
Welding transformers mainly come in three types: iron-core, coil-core, and tap-type. The basic principle of welding transformers has remained largely unchanged for decades. Both domestically and internationally, the welding transformers in use are a special type of step-down transformer with a steep external characteristic. Although they can be categorized into several types based on the method of achieving the steep external characteristic, the way the reactor is combined with the transformer, and the method of regulating current, the fundamental principle is still that of a general transformer.
W1 is the primary winding, and W2 is the secondary winding. Both W1 and W2 are wound on the same core. The primary winding transfers electrical energy to the core, creating an alternating magnetic field within it. The core then transfers magnetic energy to the secondary winding, generating an induced electromotive force. This is the basic working principle of a transformer. The ratio k of the induced electromotive force between the primary and secondary windings is equal to the ratio of their turns.
Welding transformers are widely used in production, essentially being a special type of step-down transformer. Their operating principle is the same as that of a standard transformer, but their performance differs significantly. The characteristics of welding transformers include: before welding, the secondary winding must have sufficient arc voltage (60-75V), and during welding, as the welding current increases, the secondary voltage can quickly drop. Even if the secondary side is short-circuited (such as when the welding rod touches the workpiece, the secondary voltage is zero), the secondary current will not be excessively high. In other words, the relationship between the output voltage u2 and output current i2 of the welding transformer is as shown in Figure 1(b).
The welding transformer possesses the above characteristics due to its structure being different from that of a general transformer. The principle of the welding transformer is shown in Figure 1(a). Its primary and secondary windings are mounted on two separate cores, with the secondary winding in series with a reactor. The reactor's core not only has a certain air gap but also the rotating screw can adjust the length of the air gap to obtain different sizes of welding current. As the air gap increases, the current increases, shifting the external characteristic curve to the right. Conversely, when the air gap decreases, the current also decreases, shifting the external characteristic curve to the left.
