As the inductor is the area where the heating takes place, coil design is one of the most important elements of the system and is a science in itself.
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.
[6] 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.
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.
Methods were sought to produce these higher RF power supplies to operate in the 200 to 400 kHz range.
Early units utilised spark gap technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode (valve) based oscillators.
[8] 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.
Due to its flexibility and potential frequency range, the valve oscillator based induction heater was until recent years widely used throughout industry.
The unit consists of three basic elements: The DC (direct current) power supply consists of a standard air or water cooled step-up transformer and a high voltage rectifier unit capable of generating voltages typically between 5 and 10 kV to power the oscillator.
Many latter day units feature thyristor power control which works by means of a full wave AC (alternating current) drive varying the primary voltage to the input transformer.
Another very popular method was to use a two part tank coil with a primary and secondary winding separated by an air gap.
[12] In the early days of induction heating, the motor-generator was used extensively for the production of MF power up to 10 kHz.
This type of machine features a toothed rotor constructed from a stack of punched iron laminations.
This necessitates that close attention is paid to the quality of bearings utilised and the stiffness and accuracy of rotor.
The whole construction is mounted in a cubicle which features a heat exchanger and water cooling systems as required.
The motor-generator became the mainstay of medium frequency power generation until the advent of solid state technology in the early 1970s.
In the early 1970s the advent of solid state switching technology saw a shift from the traditional methods of induction heating power generation.
Initially this was limited to the use of thyristors for generating the 'MF range of frequencies using discrete electronic control systems.
State of the art units now employ SCR (silicon-controlled rectifier),[14] IGBT or MOSFET technologies for generating the 'MF' and 'RF' current.
The inverter converts the DC supply to a single phase AC output at the relevant frequency.