Control for power dissipation in dielectric subject to HF field - by automatic resonance tuning and a reference voltage cycle

摘要

Reproducible energy input to a succession of items subjected to an HF heater is obtd. by using an electromechanical circuit as a slave to the prim. HF circuit to compensate for variations in the dimensions or electrical properties of the item being heated. The applicator is mounted in a slave circuit having mutual induction with the circuit incorporating the prim. HF source such as a quartz generator. In parallel with the applicator is a variable condenser pref. of the rotary butterfly pattern set up so that the resonant frequency of the slave circuit matches that of the primary circuit when the plans of the variable condenser are half meshed. The energy is applied as a pulsed input maintained at optimum resonance. Useful for HF induction heating for e.g. welding plastics components together, curing adhesives, esp. for treating materials with low dielectric loss characteristics. Pref. the rotor of the butterfly condenser is driven at e.g. 50 Hz by a synchronous motor. The primary circuit incorporates both series and parallel resonance feedback loops. Pref. the cathode voltage of valve in the primary circuit is monitored and compared with a reference level to control the power output in the primary circuit. By programmed variation of the reference voltage, the power input can be similarly programmed, e.g. to reduce risk of arcing during preliminary heating in a welding cycle.