High Frequency induction welding accounts for the majority of welded tubing produced worldwide, yet it is still a largely misunderstood process. Part of the reason is that the process is very forgiving, however a thorough understanding of it can lead to higher product yields and quality.
Principals of operation
Induction welding is a form of Electrical Resistance Welding (ERW) in which the large rotary transformer common in low frequency ERW is replaced by a “virtual transformer” consisting of the work coil (primary winding) and the tube itself (secondary winding). A ferromagnetic core inside the tube has a similar role to the laminated iron core in a conventional transformer.
Current flowing in the coil causes a magnetic field to develop surrounding the coil, part of which intersects with the open tube. This causes an electric field on the outer surface of the tube which in turn creates a voltage difference across the edges of the strip. At the frequencies used for induction welding, the interaction between electric and magnetic fields can cause currents to flow in unexpected ways.
The “skin effect” confines current to within a few thousandths of an inch of the surface, so the voltage across the strip edges tends to cause current to flow circumstantially around the inside surface of the tube in the opposite direction to the induced current on the outside surface.
Because the faying edges of the strip are in close proximity to one another from the coil to the apex of the vee, they have a very low value of inductance, and it is inductance rather than resistance that governs current flow at high frequencies. This is sometimes refered to as “proximity effect”.
It can be seen from this that there are two main paths along which current can flow when a voltage is applied or induced across the edges of the strip. The key to operating a high frequency welder efficiently is to direct the majority of the current along the faying edges where it does useful work in heating them, and minimize the wasteful parasitic current that flows around the inside surface of the tube.This is done by making the impedance of the vee low relative to that of the I.D. surface.
