Medium Frequency Transformers

In numerous applications of induction heating (surface hardening, small-pieces heating), single-coil windings are small inductors with a low impedance are used, which makes it impossible to connect them directly to a standard mid-frequency converter with a rated output voltage of 400V to 800V.

The impedance created is typically solved by adding a transformer between the converter output and the inductor.  Characterisitic parameters of MF transformers are:

• variable frequency range (i.e. 3-10 kHz)
• power output (i.e. 800 kVA during minimal frequency)
• transmission range (i.e. 12 possible winding configurations)
• maximum voltage (i.e. 800V on the primary).

Other desired parameters are good efficiency, low voltage, nízké napětí nakrátko and a low mass.

The various designs of transformers all have one common principle: water-cooled copper windings, a core made from plates no thicker than 0.2mm (or for higher frequencies, made from ferrite semi-product).  The core normally has water cooling as well.

An example of a rather complex design solution is alternatively placed primary and secondary disk coils on a twin-column or jacketed core. The secondary windings are single-coil and are connected in parallel.  Primary coils have one ro more coils and their varied count allowed them to be connected into series, and with this, the transformer transmission can be gradually changed.