Induction Heater - History

History

The basic principle involved in induction heating was discovered by Michael Faraday as early as 1831. Faraday's work involved the use of a switched DC supply provided by a battery and two windings of copper wire wrapped around an iron core. It was noted that when the switch was closed a momentary current flowed in the secondary winding, which could be measured by means of a galvanometer. If the circuit remained energized then the current ceased to flow. On opening the switch a current again flowed in the secondary winding, but in the opposite direction. Faraday concluded that since no physical link existed between the two windings, the current in the secondary coil must be caused by a voltage that was induced from the first coil, and that the current produced was directly proportional to the rate of change of the magnetic flux.

Initially the principles were put to use in the design of transformers, motors and generators where undesirable heating effects were controlled by the use of a laminated core.

Early in the 20th century engineers started to look for ways to harness the heat-generating properties of induction for the purpose of melting steel. This early work used motor generators to create the medium frequency (MF) current, but the lack of suitable alternators and capacitors of the correct size held back early attempts. However, by 1927 the first MF induction melting system had been installed by EFCO in Sheffield, England.

At around the same time engineers at Midvale Steel and The Ohio Crankshaft Company in America were attempting to use the surface-heating effect of the MF current to produce localized surface case hardening in crankshafts. Much of this work took place at the frequencies of 1920 and 3000 Hz as these were the easiest frequencies to produce with the equipment available. As with many technology-based fields it was the advent of World War II which led to huge developments in the utilization of induction heating in the production of vehicle parts and munitions.

Over time, the technology advanced and units in the 3 to 10 kHz frequency range with powers outputs to 600 kW became common place in induction forging and large induction hardening applications. The motor generator would remain the mainstay of MF power generation until the advent of high voltage semiconductors in the late 1960s and early 1970s.

Early in the evolutionary process it became obvious to engineers that the ability to produce a higher radio frequency range of equipment would result in greater flexibility and open up a whole range of alternative applications. Methods were sought to produce these higher RF power supplies to operate in the 200 to 400 kHz range.

Development in this particular frequency range has always mirrored that of the radio transmitter and television broadcasting industry and indeed has often used component parts developed for this purpose. Early units utilised spark gap technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode (valve) based oscillators. Indeed, many of the pioneers in the industry were also very involved in the radio and telecommunications industry and companies such as Phillips, English Electric and Redifon were all involved in manufacturing induction heating equipment in the 1950s and 1960s.

The use of this technology survived until the early 1990s at which point the technology was all but replaced by power MOSFET and IGBT solid state equipment. However there are still many valve oscillators still in existence, and at extreme frequencies of 5 MHz and above they are often the only viable approach and are still produced.

Mains frequency induction heaters are still widely used throughout manufacturing industry due to their relatively low cost and thermal efficiency compared to radiant heating where piece parts or steel containers need to be heated as part of a batch process line.