Method for open-loop/closed-loop control of at least two parallel oscillating circuit inverters feeding induction furnaces

In a method Of open-loop/closed-loop control of current converter valves of at least two load-guided parallel oscillating circuit inverters being supplied from a common source which is controlled by an open-loop/closed-loop control and being connected in series on the DC side, each inverter having one induction furnace with a compensating capacitor,

• which includes igniting two diagonally opposed current converter valves at a time in one inverter to establish a flow of current from the source with a series-connected choke through the parallel oscillating circuit connected to that inverter, the improvement which comprises:
• adjusting load phase angles of the parallel oscillating circuit inverters, resulting in specified individual powers of the parallel oscillating circuits wherein a maximum power that can be output by the source is taken into account, and attaining a specified ratio between the individual powers of the parallel oscillating circuits, and
• intervening into the open-loop/closed-loop control of the source only after one of the load phase angles to be established reaches a minimum value defined with valve hold-off intervals, while operating the source in fully driven fashion at load phase angles to be established above the minimum value,
• supplying the inverters from a direct voltage source with a series-connected choke, and reducing the direct voltage when intervening into the open-loop/closed-loop control,
• determining a varied current as an auxiliary variable while taking both the varied powers to be established and the voltage into account;
• ascertaining a varied impedance of each parallel oscillating circuit from the varied oscillating circuit current and the varied oscillating circuit power to be established;
• and determining the load phase angles from the ascertained varied impedances.