The primary function of an impeder is to increase the impedance of the parasitic current path around the inside circumference of the tube, thus diverting more of the available energy into the weld “vee”. Impeders also concentrate the magnetic flux created by current in the work coil, so that a greater amount of energy is induced into the tube.
The design & placement of the impeder are among the most important areas in which efficiency can be improved. Impeders are critical components in the induction welding process, however since they are low cost consumable items, many buyers base their choice on price alone. Welder manufacturers have always used electrical efficiency as a major selling point for their particular brand of equipment, however in reality, there is only a difference of a few percentage points between the best and the worst on the market, and this can usually be seen only under carefully controlled laboratory conditions. In contrast, the selection of an impeder can affect weld speeds for a given power level by as much as fifty percent. This would increase the speed of a mill from 200 to 400 feet per minute without any increase in weld power, or if speed were kept constant, would cut power costs in half. Since the average annual cost of impeders for a single 2" tube mill is less than $8,000.00, “cheap” impeders are frequently a very costly item.
The life expectancy of an impeder is another important factor to consider. Most cost less than $25.00 each, but it takes an average of twenty minutes to change one. If downtime is only rated at a conservative $500.00 per hour, the cost of downtime is seven times the cost of the impeder itself!
The most important component in an impeder is the ferrite. Some manufacturers use a grade that is inexpensive, but is designed for use as antenna rods in portable radios, not for the high power density and high temperatures which occur in tube welding. Impeder ferrite should have the highest possible saturation flux density and amplitude permeability, and at the same time should have electrical & magnetic losses low enough to keep cooling requirements reasonable. Sometimes these parameters are mutually exclusive, so picking the right grade of ferrite requires a thorough knowledge of high frequency welder operation, as well as magnetic circuit design.