Vacuum Tube High Frequency-Induction Welders

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 thousanths of an inch of the surface, so the voltage across the strip edges tends to cause current to flow circumfrentially 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 minimise 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.

Vee Length
Vee length depends on coil position, and to some extent on coil length, since heating starts to occur even before the strip enters the coil.
Coil position is usually determined by the diameter and size of the weld roll box, whereas coil length is generally dictated by the matching capabilities of the welder.
There are two factors involved here:-

  • The high efficiency of induction welding is due to the fact that only a very small mass of metal is heated. Increasing the vee length allows more time for heat to be conducted away from the edges, so more energy is needed & a wider heat affected zone results.
  • The distribution of current between the vee and the inside surface of the tube depends on the relative impedances of the two circuits. A longer vee has a higher impedance, which directs more of the available current around the inside of the tube. This is particularly important when welding small diameter tubing, since the small space available for impeders limits their effectiveness. There are several schools of thought regarding optimum vee length, but all agree that minimum electrical power is used with short vee lengths and short work coils. Power distribution across the edge face of the strip is fairly even, however there is less thermal conduction away from the corners, which may result in their overheating before the center of the edge reaches forging temperature. This tendency can be reduced by increasing the vee length, or by lowering the welder frequency (more on this later), so optimum vee length is more a function of wall thickness than it is of diameter. As a general rule, I recommend using a minimum length vee & the shortest practical work coil unless there is evidence of uneven temperature distribution. If the weld tends to be cold in the center, the coil should be moved back the minimum distance needed to correct the problem. Because the weld rolls are usually made of steel (D2 or H13), they will heat up readily due eddy currents induced in them by the work coil. This sets the minimum acceptable clearance between coil & rolls. Non magnetic (ceramic or Ampco 25 bronze) or weakly magnetic (Tungsten carbide) rolls will reduce roll heating

Work coils
Current flowing through the work coil establishes a magnetic field that causes energy to be induced into the tube. Coil current increases with weld power & decreases with frequency, but at any frequency used for induction welding, the currents are in the order of hundreds, to thousands of amps. This current all travels close to the coil surface due to the high frequency skin effect, so coils must be designed and made to handle these extremely high currents with minimum losses. We have seen solid state welders with quoted efficiencies of 90% or better degraded to less than 60% because of poor work coil design. Vacuum tube welders are more forgiving of poor coil construction because most operate at higher voltages & lower currents, but well designed & manufactured coils will still save thousands of dollars in energy costs each year.