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 selector switch to regulate the secondary side's no-load voltage. The primary and secondary windings are mounted on separate core columns, which gives the transformer a higher leakage reactance. Consequently, the terminal voltage of the secondary side will drastically drop as the current increases. 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: moving-iron, moving-coil, 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 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 basic principle remains the same as that of general transformers.
W1 is the primary winding, and W2 is the secondary winding. W1 and W2 are wound on the same core. The primary winding transmits electrical energy to the core, creating an alternating magnetic field within it. The core then transfers magnetic energy to the secondary winding, which induces an electromotive force. This is the basic working principle of a transformer. The ratio k of the induced electromotive force in the transformer's primary and secondary windings is equal to the ratio of their turns.
Welding transformers are widely used in production, essentially serving as a specialized step-down transformer. Their working principle is similar to that of a standard transformer, but their performance differs significantly. The characteristics of a welding transformer include the requirement for a sufficient arc voltage (60-75V) on the secondary winding before welding. During the welding process, as the welding current increases, the secondary voltage can rapidly decrease. Even if there is a short circuit on the secondary side (e.g., when the welding rod touches the workpiece, the secondary voltage drops to zero), the secondary current will not be excessively high. This means that the relationship between the output voltage u2 and the output current i2 of the welding transformer is as shown in Figure 1 (b).
The welding transformer possesses these characteristics due to its distinct structure from a typical transformer. The principle of the welding transformer is illustrated in Figure 1(a). Its primary and secondary windings are mounted on two separate cores, with the secondary winding in series with an inductor. The inductor's core has not only a certain air gap but also a rotating screw that can adjust the length of the air gap to achieve 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.
