Capacitor discharge welding

Capacitor discharge (CD) welding is a resistance welding process characterised by the way in which energy is generated. CD welding, as the name suggests, uses the energy stored in capacitors for welding. CD welding belongs to the short-time processes. Typical of the process are high welding currents and higher electrode forces than with other resistance welding processes. It is therefore mainly suitable for projection welding.

For welding, a pulsed direct current is used, which is generated by the discharge of a capacitor via the special welding transformer already mentioned above. The shape of the pulse is determined by the sum of the resistances in the discharge circuit. The energy W stored in the capacitor can be calculated as follows: W = ½ CU2.

Here, C corresponds to the capacitor capacity and U to the charging voltage of the capacitor.

The energy is introduced into the workpieces to be joined via the charged capacitors and the transformer. The charging current is several orders of magnitude lower than the subsequent discharging current, so that the procedure is characterised by a very low mains load and fuse protection. No high current peaks occur in the mains line because the capacitor is charged relatively slowly during the welding pauses.

In CD welding, the energy required for welding is switched from a previously charged capacitor bank via a switch (thyristor) to a welding transformer. Due to the abrupt discharge of the energy stored in the capacitors, the current in the secondary circuit increases very quickly. Due to the electrical resistance in the welding projection, the material heats up to the necessary welding temperature within a few milliseconds.

The rapid temperature rise heats the welding zone before the heat can dissipate, thus preventing the areas around the weld from heating up. After just a few milliseconds, the projection is welded. This effect is responsible for all the advantages and possibilities of CD welding.

This enables safe welding of high-strength steels and a wide variety of material combinations, including highly conductive welding partners of different dimensions.

The welding force parameter and the resetting of the welding system are other important parameters of capacitor welding machines.