Good coil design is essential in realizing the improved efficiency possible with today’s solid state welders. Unlike earlier vacuum tube welders, transistor inverters operate at high currents& low voltages. Coil currents can exceed 3000 amps, which is ten times higher than a typical vacuum tube welder. The power loss in a circuit due to resistance is given by (current)2x resistance, so it is essential to keep the coil resistance very low. With 3000 amps of current flowing in a coil with a resistance of 1/100 ohm, the power lost in the coil is 90 kilowatts! This can very easily eliminate any gains due to the higher efficiency of solid state welders.
Because the electrical resistance of copper rises with temperature, proper coil cooling is extremely important. In some installations, coil coolant may require a refrigerative chiller. In all cases, coils must be designed for adequate transfer of heat away from the surfaces where it is generated. EHE coils are fabricated from 99.9% pure, oxygen free high conductivity copper, with silver brazed joints & careful consideration is given to cooling system design.
Many larger coils use spray cooling. This has the advantage that coolant is in contact with the entire surface of the coil, eliminating the need for lateral thermal conduction which may result in “hot spots”. Spray cooled coils are frequently much less expensive to manufacture.
Most solid state welders use either single or two turn coils. Two turn coils are available with either left hand or right hand helix direction. The choice of helix direction depends on the weld roll configuration and mill direction, and should be selected to provide the shortest vee length. In general left hand helix coils are used when mill direction is left to right and right hand helix coils are used for right to left mills.
Single turn coils are often supplied in two sections that are bolted or clamped together, to enable a coil to be replaced without cutting & re-threading the mill.
Coil design for vacuum tube welders is less critical, however a well designed coil can still result in a considerable improvement in welding efficiency. With wholesale electricity costs in the United States hitting $0.50 per kilowatt/hour in some locations, improving welding efficiency by just 10% can reduce operating costs by $250,000.00 a year!
The price list on the facing page covers our most common coil types. In addition, we can offer a wide range of specialized coils & coil holders to suit all types of HF welding
equipment.
