Arc Stability

Inverter power sources have been around for decades. As reliability now equals that of the older transformer rectifier (TR) design, they are more popular than ever.  One reason is superior arc quality.  TR DC output is rectified from 60 Hz AC input. A choke stabilizes the resultant bumpy DC, but the output remains somewhat unstable and erratic (Stage 2).  In contrast, Inverter 60Hz AC input is rectified to 60 Hz DC. It inverts to around 30,000 Hz AC, rectified again and then stabilized through a choke. As the illustration below shows, the inverter DC output seen in stage 5 appears as a straight line. This means a more stable arc.  What the graphic doesn’t show is the exponentially faster amp/volt response. The response occurs within the welding arc (in short circuit transfer). 

In the end, the inverter arc for DC stick and wire processes will be less violent. This will lead to higher operator appeal and produce significantly less spatter.

Efficiency & Portability

Inverter welding power sources rectify 60 Hz AC into DC and then, unlike the older transformer rectifier (TR) design, use high speed switches to convert the filtered DC to around 30,000 Hz AC. It then transforms from high volts/low amps to high amps/low volts.  Per Miller Welders, “a basic premise of welding power source design states that a faster operating frequency enables the power source to use fewer copper windings and a smaller core in its transformer
and inductor.”  

In other words, typically heavy internal components, like the transformer and choke, become a fraction of their size and weight.  For this reason, inverters are much smaller, lighter and more portable than their TR counterparts.  Additionally, high frequency AC dramatically raises efficiency. This lowers input current draw, reduces circuit breaker size requirements and lowers your electric bill.  The pictures below are decent visual representations of transformer size and weight difference. However, many inverter transformers are now the size of a credit card. 

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Flexibility & New Technologies

Inverter welding power sources are far more flexible than the old transformer rectifier (TR) design in two respects.  First, with existing welding processes, they expand what you can do.  For example, a TIG inverter allows you to control AC output frequency from 20 to 250 Hz and shuts off the high frequency in AC welding (which is always “on” in TR machines) which reduces annoying computer and phone line interference.  In short circuit GMAW, inductance adjusts more precisely and stabilizes output power to weld very thin gauge plate. Second, inverters couple with software to create an almost infinite number of possible pulsed MIG waveforms.

Newer technologies addressing open root pipe (Miller RMD/Lincoln STT), thin wall aluminum (Miller Profile Pulse/Lincoln Pulse on Pulse), thin wall steel/stainless steel (Lincoln Power Mode), and super high production submerged arc (Lincoln/Miller AC Subarc) are already here and many more powerful technologies will come as pulse waveforms are better understood.  The basic shape of a waveform can be changed as can the peak and background current, ramp up time, and tail-out.  Any change or changes made will affect the mode of droplet transfer, the arc cone shape, heat input, penetration characteristics etc.