In MIG welding, what should be constant? Is it welding voltage or current? This is a question that bewilders most of the welders. The answer is that the MIG welding actually requires constant voltage-type power supplies. This is exactly in contrast to TIG and stick welding which use power supplies that comes from the constant current. Constant voltage is actually crucial in MIG welding since it ensures a stable arc gap size, resulting in stable arc operation and smooth metal transfer from the wire tip.
The main difference noted on voltage and current is on the way they respond to varying electrical resistance in the arc. The greater the resistance would be if the arc is longer. Keep in mind that increasing the arc gap size causes a drop in current in constant voltage. Therefore it causes a decrease in the power and heat input of the arc. This can be substantiated by the Joule’s law. If it is a constant current machine, the voltage increases as the arc gap is increased thereby causing the heat input to increase. So altogether, the longer arc gap would be colder in a Constant Voltage machine. And on the other hand in constant current machine, the longer arc gap would be hotter. Here the heat is a lot less focused as a bigger arc has greater surface area to wade off the heat.
On the whole, the constant voltage is important in MIG welder in order to balance the melting off rate and heat input as well as the arc length.