and secondary winding separated by an air gap. Power control was affected by varying the magnetic coupling of the two coils by physically moving them relative to each other. Solid state power supplies In the early days of induction heating, the motor-generator was used extensively for the production of MF power up to 10 kHz. While it is possible to generate multiples of the supply frequency such as 150 Hz using a standard induction motor driving an AC generator, there are limitations. This type of generator featured rotor mounted windings which limited the peripheral speed of the rotor due to the centrifugal forces on these windings. This had the effect of limiting the diameter of the machine and therefore its power and the number of poles which can be physically accommodated, which in turn limits the maximum operating frequency. To overcome these limitations the induction heating industry turned to the inductor-generator. This type of machine features a toothed rotor constructed from a stack of punched iron laminations. The excitation and AC windings are both mounted on the stator, the rotor is therefore a compact solid construction which can be rotated at higher peripheral speeds than the standard AC generator above thus allowing it to be greater in diameter for a given RPM. This larger diameter allows a greater number of poles to be accommodated and when combined with complex slotting arrangements such as the Lorenz gauge condition or Guy slotting which allows the generation of frequencies from 1 to 10 kHz. As with all rotating electrical machines, high rotation speeds and small clearances are utilised to maximise flux variations. This necessitates that close attention is paid to the quality of bearings utilised and the stiffness and accuracy of rotor. Drive for the alternator is normally provided by a standard induction motor for convention and simplicity. Both vertical and horizontal configurations are utilised and in most cases the motor rotor and generator rotor are mounted on a common shaft with no coupling. The whole assembly is then mounted in a frame containing the motor stator and generator stator. 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