The stored energy welding power supply, commonly called a Capacitive Discharge or CD Welder, extracts energy from the power line over a period of time and stores it in welding capacitors. Thus, the effective weld energy is independent of line voltage fluctuations. This stored energy is rapidly discharged through a pulse transformer producing a flow of electrical current through the welding head and workpieces. Capacitive discharge power supplies are rated in accordance with the amount of energy they store and the welding speed. The energy stored, expressed in watt-seconds (joules), is the product of one-half the capacitance of the capacitor bank and the square of the applied voltage. The energy delivered to the electrodes is considerably less than this value because of losses in the primary and secondary circuits. Some power supplies provide a “Dual Pulse” feature which allows the use of two pulses to make a weld. The first pulse is generally used to displace surface oxides and plating, and the second pulse welds the base materials. This feature also reduces spitting.
The AC welder derives its name from the fact that its output is generally a sine wave of the same frequency as the power line. It extracts energy from the power line as the weld is being made. For this reason, the power line must be well regulated and capable of providing the necessary energy. Some AC welders include a line voltage compensation feature to automatically adjust for power line fluctuations. In its simplest form, the AC welder consists of a welding transformer that steps down the line voltage (normally between 480 to 100 volts) to the welding voltage (typically 2 to 20 volts). The welding current that flows through the secondary of the transformer, and its connected load, is very high, ranging from 10 to more than 100,000 amps. The welding current is allowed to flow for very short periods of time, typically .001 to 2 seconds. AC welders can operate at rates up to 5-6 welds per second.
AC Welding Systems are generally composed of the three elements. The Welding Transformer, the Welding Control, and the Mechanical System.
WELDING TRANSFORMERS – are used in AC machines to change alternating current from the power line into a low-voltage, high amperage current in the secondary winding. A combination of primary and/or secondary taps on the welding transformer are commonly used to provide a macro adjustment of the welding current, as well as adjustment of secondary voltage. Transformer ratings for AC machines are expressed in KVA (kilovoltamperes) for a specified duty cycle. This duty cycle rating is a thermal rating, and indicates the amount of energy that the transformer can deliver for a stated percentage of a specific time period, usually one minute, without exceeding its temperature rating. The RMS Short Circuit Secondary Current specification indicates the maximum current that can be obtained from the transformer. Since heating is a function of the welding current, this parameter gives an indication of the thickness of the materials that can be welded.
Medium (till 1000Hz) Frequency Inverter Welders use sub-millisecond pulse width modulation (switching) technology with closed-loop feedback to control the weld energy in sub-millisecond increments. Three phase input current is full wave rectified to DC and switched at 1kHz to produce an AC current at the primary of the welding transformer. The secondary current is then rectified to produce DC welding current with an imposed, low-level, AC ripple. The high-speed feedback circuitry enables the inverter power supply to adapt to changes in the secondary loop resistance and the dynamics of the welding process. For example, a 1 kHz inverter power supply adjusts the output current every mili-second after rectification, which also allows the weld time (duration of current) to be controlled accurately in increments as small as 1 milliseconds. The high frequency closed loop feedback can be used to control (maintain constant) either current, voltage, or power while also monitoring another of the same three parameters.
Additional benefits of high frequency switching technology include reduced power consumption, smaller welding transformers, and the use of a very short pre-weld “check pulse” to test electrode and parts positioning prior to executing a weld. The result of this pre-weld check can be used to inhibit the weld by setting check limits.
- CONSTANT CURRENT can be used for 65% of all welding applications including those that exhibit low contact resistance, small variability in contact resistance, flat parts, and multiple part “sandwiches.”
- CONSTANT VOLTAGE can be used for applications where the workpieces do not have flat surfaces, e.g. crossed wires, and where the resistance varies significantly, and for extremely short welds (less than 1 millisecond).
- CONSTANT POWER can be used for applications with significant variations in electrical resistance from weld to weld, including applications where the plating erodes and builds-up on the face of the welding electrodes.
Due to their extensive programmability, small transformer size, and robustness, high frequency inverter power supplies are generally the best choice for automation applications.