A welding transformer is a two-winding transformer. To adjust the arc ignition voltage, the primary winding is equipped with tap changer, which is used to regulate the secondary side's no-load voltage with a tap switch. The primary and secondary windings are mounted on two separate core columns, which gives the transformer a high leakage reactance, causing the terminal voltage of the secondary side to drastically drop with the increase of current. An iron-core reactor is串联 in the secondary winding circuit for adjusting the welding current. By increasing the air gap length of the reactor, the current increases with the gap's growth.

Welding transformers come in mainly three types: iron-core type, moving-coil type, and tap-type. The basic principle of welding transformers has not changed much over the decades. Both domestically and internationally produced and used welding transformers are a type of special 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 combination of reactors with transformers, and the method of regulating current, the basic principle remains the same as that of general transformers.

W1 is the primary winding, while W2 is the secondary winding. Both W1 and W2 are wound on the same core. The primary winding transfers electrical energy to the core, generating an alternating magnetic field within it. The core then transfers magnetic energy to the secondary winding, causing it to produce induced electromotive force. This is the basic working principle of a transformer. The ratio k of the induced electromotive forces in the transformer's primary and secondary windings is equal to the ratio of their turns.

Welding transformers are widely used in production, essentially a type of special 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 rapidly decrease. Even if the secondary side is short-circuited (e.g., 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 aforementioned characteristics due to its unique structure, which differs from that of a standard transformer. The principle of the welding transformer is illustrated in Figure 1(a). Its primary and secondary windings are mounted on two separate cores. The secondary winding is串联 with a reactor, whose core not only has a certain air gap but also allows the adjustment of the air gap length via a rotating screw to achieve different welding current sizes. As the air gap increases, the current increases, shifting the characteristic curve to the right. Conversely, when the air gap decreases, the current decreases, shifting the characteristic curve to the left.