Central oscillator for induction range using triac burner controls

DETAILED DESCRIPTION The present invention is directed to an induction cooking unit having multiple independently duty cycle controlled induction cooking coils excited by a common oscillator providing power at an ultrasonic frequency.
The invention is especially adapted for but not confined to domestic cooking appliances such as kitchen ranges having a smooth ceramic top with multiple burner-like heating locations.
In accordance with the preferred embodiment of the invention, an ultrasonic square wave inverter or oscillator utilizing fast turn-off thyristors operates from a DC supply which is obtained by fully rectifying, but not filtering, a 60 hertz standard AC line voltage.
The inverter chops the DC supply to provide an alternating square wave output at approximately 25 kilohertz which is fully modulated at 120 hertz.
The square wave output is filtered by a series resonant LC network to provide a fully modulated alternating sinusoidal power output.
Four induction cooking coils arranged as kitchen range burners adapted to be inductively coupled to cooking utensils are excited by the modulated alternating sinusoidal power.
A triac is inserted in series between the alternating sinusoidal power output and each induction cooking coil.
Power to each induction cooking coil is duty cycle controlled by switching its respective triac only at the zero crossing of the AC line voltage which corresponds to the zero point of the modulation envelope of the modulated sinusoidal power output so that the triac does not have to interrupt voltage or current at ultrasonic frequencies.
The triacs are independently switched to provide the duty cycles necessary to achieve the desired average cooking power for each induction cooking coil.
To avoid potential light flicker problems due to line power fluctuations caused by synchronously switching all four of the triacs, the gating signals triggering the triacs are phase displaced relative to each other to cause sequential switching of the triacs, thus increasing effective switching frequency