Abstract: A circuit arrangement is disclosed comprising a converter for converting a three-phase system of sinusoidal voltages into three trapezoidal control voltages for controlling control units serving three direct cycloconverters. More particularly, in accord with the invention, the converter acts on the sinusoidal control voltages such that during each region from .pi./3 to 2.pi./3 of a half-wave of each particular voltage that voltage is reduced to .sqroot.3/2 times its peak value, while the other two voltages are amplified or increased by an amount equal to the difference between the aforesaid peak value and .sqroot.3/2 times such peak value.

Abstract: The invention concerns a circuit arrangement for use with an electric melting furnace of a type which is fed by a converter, the latter converter being equipped with thyristors and controlled by a control unit and a series-connected current regulator. More specifically, in accord with the invention, the circuit arrangement is provided with monitoring circuits for monitoring the operation of the thyristors. These monitoring circuits, in turn, generate signals which control switches for selectively short-circuiting the feedback resistors of the regulator.

Abstract: A circuit arrangement is disclosed comprising three direct converters which are fed from an a.c. voltage network. The converters are arranged in a Y configuration and are connected to a three-phase load. To increase the transmitted power and to vary the reactive power requirement, the circuit arrangement is provided with means for generating a control voltage for the control unit of at least one of the three converters such that the output voltage of this converter has a periodic waveform which includes essentially no fifth and seventh order harmonics. Additionally, the load-side conductor currents of the three converters are controlled with control signals having sinusoidal waveforms. A particular form used for the control voltage is a trapezoidal waveform.

Abstract: The invention concerns a converter arrangement for electrode melting furnaces, particularly for electric slag remelting of metals. A current regulator in the form of a PI controller is associated with the converter arrangement. The former is influenced by an actual current value, a reference current value, and additionally, by a control quantity. The control quantity replaces the current regulation of the regulator by a voltage control in dependence on periodic or aperiodic conditions which may be due to operational causes or to trouble and interfere with the melting process in the furnace. More particularly, for the duration of such conditions, the control quantity causes the output voltage of the regulator, which voltage acts on the control unit feeding the converter, to be limited upward to an adjustable value.