I recently posted a blog about closed loop welders and how you can get the most out of using them, and it occurred to me that some of you may not be familiar with the different resistance spot welding power supply technologies, how they work, and what they can be used for. So here is a short description of the four different types, including both closed loop and open loop designs.
First of all, let's talk about the difference between closed loop and open loop welders. Simply put, closed loop welders use sensors to measure the current and voltage during the weld so you can adjust for part and process variation as you go; open loop welders don't - you just get what you get.
CLOSED LOOP TECHNOLOGIES: Linear DC and HF Inverter
In Linear DC power supplies, a capacitor bank is charged up and the welding energy is released through a bank of transistors. Linear DC power supplies deliver an ultra stable output with a very fast rise time. Most DC power supplies can be programmed in constant current, constant voltage, or constant power. Time control can be programmed in increments as small as 0.01 milliseconds. Because DC power supplies offer the best low energy control, it is the best choice for welding fine wires and thin foils.
High frequency inverter technology utilizes pulse width modulation circuitry to control the weld energy. 3-phase input current is full wave rectified to DC, which is then switched to produce an AC current at the primary of the welding transformer. The resulting secondary current, when rectified, is in the form of DC with an imposed, low-level AC ripple. Like Linear DC welders, High Frequency Inverters can be programmed for constant current, voltage, or power operation. Time control can be programmed in 1 millisecond or 0.01 millisecond increments. High Frequency Inverters have very high repetition rates, so they are frequently used for automated applications.
OPEN LOOP TECHNOLOGIES: Capacitor Discharge (CD) and Direct Energy (AC):
Capacitor Discharge (CD) power supplies store energy in a capacitor bank prior to the weld. The energy is discharged through a pulse transformer to the weld head. The resulting high peak current and very fast rise time is useful for welding very conductive parts. The level of charge on the capacitor bank is usually programmed in watt-seconds or % energy. Time control is achieved by changing the transformer tap settings, which changes the pulse duration, or pulse width. Unfortunately, since a capacitor discharge power supply is open loop (no feedback), changes in the secondary circuit, such as loose cables or corroded connections can result in inconsistent energy delivery to the parts.
Direct Energy (AC) power supplies take energy directly from the power line as the weld is being made. Coarse current control is achieved by changing the tap settings on the welding transformer, which changes the voltage of the output. Fine adjustment of weld current is achieved by controlling the amount, in percent, of the AC power that is applied to the primary of the welding transformer. The weld time is controlled in line cycles (1 cycle = 16.67 milliseconds @ 60 Hz), the minimum usually being one half cycle. Line voltage fluctuations can affect the weld current delivered by open loop AC power supplies. For this reason, the input line must be well regulated. AC power supplies are general purpose welders with high energy output (not suitable for critical, fine welding applications). The longer welding times are useful for resistance brazing applications.
For more information read Fundamentals of Resistance Welding and The Benefits of Closed-loop Control for the Resistance Welding Process.