Spot welding: improve quality and reduce cost with inverters

Successful manufacturers constantly look for ways to improve quality while reducing costs. That’s why many are looking beyond conventional spot welding technology for something to help them achieve their cost reduction goals. Resistance welding traditionally utilizes alternating current (AC); however, this technology has limitations in the control of the output energy. Resistance welding with direct current (DC) using inverter technology, however, dramatically improves weld process control through closed loop feedback. This provides a consistent output, in turn lowering scrap and increasing production yield.

The specific advantages of resistance spot welding with inverter technology vary from customer to customer, but one interesting use is for cascade welding. This is when a control (using just one weld schedule), initiates a single air valve to close multiple electrodes, and sequentially fires two or more silicon controlled rectifiers (SCRs). Cascade welding is being used successfully for industrial applications including sheet metal, cross wire, bracket to sheet metal, electronic cabinets, furniture, and lead acid batteries.

Moving from AC to DC with inverter technology

Over the past few decades, power switching technology has gained greater acceptance as a solution to get the precise process control that many users are seeking. There is now a long history of using insulated gate bipolar transistors (IGBTs) as power switching devices in resistance welding, as well as in servo drives and personal computers. The move to electronic switching technology has led to very precise control over the DC output. In addition to this controllability, resistance welding benefits from the use of stable and durable electronic components. As a result, the inverter technology used today to generate DC power now challenges AC welding in almost every welding application.

In fact, AMADA MIYACHI AMERICA has been an active participant in the movement towards inverters. We developed our first inverter for micro welding more than 30 years ago! Shortly thereafter, we developed an inverter for large scale welding and continue to be the industry leader in this technology.

With inverter technology, current feedback is configured with either primary or secondary current feedback. With primary feedback, the current is sensed at the input to the transformer. When secondary feedback is used, the current is sensed after the transformer in what is called the “secondary loop.” In both cases the control uses the sensed current to dynamically adjust the output. Inverter technology also allows for other feedback modes, including automatic voltage compensation (AVC), constant current, constant voltage, and constant power.

Let’s see how it works in the real world

Inverters are great for power distribution cascading, which allows for even power distribution such that each phase of the line can be fired in a sequence that minimizes power requirements of the welding equipment. This is particularly important in systems that are already at their available power limits. In addition, not all buss systems are designed to handle such high power all at once, so sequencing to fire to multiple channels helps reduce the requirements of this design.

Cascading the weld process reduces the need to purchase multiple controls; one cascade control with multiple SCRs can operate multiple electrodes. It also reduces operating costs, allowing for more even power consumption. Power costs can be considerable when operating multiple controls. Especially when all controls are fired at the same time; peak power costs rise quickly.

We recently worked on an interesting tombstone (lead casting) welding application that really highlights the benefits of cascading. The customer was resistance welding lead (Pb) tombstones between internal cells (cell to cell) and terminal posts (cell to post). There were a total of 7 welds per assembly.

Previously, the manufacturer had used one power supply and one head, indexing between each site. This was a time-consuming process where the time between welds is a significant portion of the production cycle.

In addition, each weld affected the position of the tombstones for the next weld. The weld process pulls the material together in one location, but this can cause separation at other locations. The variable gap affects the success rate of the weld.

By switching to the inverter technology (in this case, using an IS‐800CR‐X7 inverter power supply with 7-way cascade), they were able to use 1 power supply and control with 7 heads. This increased productivity; the electrodes squeeze and hold simultaneously, and energy cascades through each of the 7 locations. In addition, as one tombstone was welded in one location, the other 6 heads held the other tombstones in place. This helped decrease defects and variability between the welds.

There are many other recent examples I could cite, but the important takeaway is to consider using resistance welding with DC power using inverter technology. It can be a good option for increasing production yields.