Resistance Welding Q&A: Transformer vs. Transgun kVA

Q: I have three different welding machines in my plant along with several robotic transguns. A couple of my machines are 50-kVA press machines and my transguns are 100 kVA. Why can my 50-kVA machines produce up to 35-kA secondary current, but my transguns only go as high as 18–20 kA?

Fig. 1 — Picture of resistance welding transformers.

A: This is a great and common question. My first answer is kVA has little to do with how much actual secondary welding current you can get from a given machine or robotic welding gun (transgun).

Since I cannot write a long dissertation on electrical theory here, I am going to do my best to give you the basics and I hope this will help you understand why you are getting more out of your machines with lower kVA ratings than your transguns with higher kVA ratings.

First, let me start with the statement that not all resistance welding (RW) transformers are created equal. There are different model types that were created for different RW applications.

To illustrate this, see Fig. 1, which shows several RW alternating current (AC) welding transformers and medium-frequency direct current (MFDC) power supplies. You can see in the picture that these various models come in all sizes, shapes, and colors, each designed for a specific task/job.

The small silver and green transgun models are designed to be used on transguns and are designed to be small and lightweight. They are rated at 100 kVA at 50% duty cycle.

The large, red AC machine-type transformer is rated at 75 kVA at 50% duty cycle. It is designed to be used in a stand-alone press welding machine and be able to produce high secondary currents for heavy steel and projection welding. The red one weighs more than five times the smaller ones, yet is rated lower in kVA. So once again, I want to stress that kVA rating is not what allows you to get more or less secondary current.

Let’s dig deeper into your specific question of kVA vs. secondary-current output. My first step here is to expose you to Ohm’s Law, which states that E = I × R (E = volts, I = current in amps, R = resistance in Ohm’s). Note: There is Ohm’s Law for AC and Ohm’s Law for DC, but for my basic explanation, we will just use Ohm’s Law for DC — Fig. 2.

Fig. 2 — Ohm’s Law.

The most important thing to understand is that secondary volts and total secondary-circuit resistance or impedance are the most important factors in how much current your transformer can produce. Voltage drives current, not kVA.

The next thing I want to point out is that the welding transformer, size of the secondary circuit, length, material, and cross section of secondary conductors of a transgun or machine, and the resistance of the material you are welding, are all the things that add up to total secondary resistance/impedance. See Fig. 3 for a simple illustration of secondary losses.

Fig. 3 — Illustration of secondary losses.

Let’s try to put all of what we have learned together and answer your question.

The transguns on your robots have smaller transgun-style transformers and lightweight welding guns. Those transformers contain less copper and have higher internal impedance or resistance compared to larger machinetype transformers, which have a lot more copper and much lower internal impedance/resistance. It is also important to note the difference between the actual transgun and a typical RW stand-alone press welding machine. The transgun has smaller cross-section conductors (gun arms), and many are made of aluminum to keep the guns light on the end of the robot. This means higher secondary resistance/ impedance compared to a typical RW press welding machine, which has a large copper bus and copper shunts, which mean a much lower resistance/impedance to current flow.

So now you might ask: What is kVA? kVA is the thermal capacity rating and in the most basic terms, the more kVA you have, the more welds per min you can make at a given secondary voltage, current, and duty cycle.

To sum it all up. The reason your 50-kVA machines will reach higher secondary currents compared to your 100 kVA transguns is due to lower internal resistance/impedance in the physically larger 50-kVA transformers and lower secondary circuit resistance/impedance in the press welding machines. It has nothing to do with the kVA rating, which is only a thermal rating.

This article was written by Don DeCorte, vice president of sales at RoMan Manufacturing Inc., and past chairman of the Resistance Welding Manufacturing Alliance (RWMA). He is also an AWS Life Member, Counselor, past director, and Certified Resistance Welding Technician.

The Resistance Welding Manufacturing Alliance (RWMA), an industry partner of the American Welding Society, is an active network of industry professionals dedicated to the advancement of resistance welding standards and processes.