The invention relates to a device for controlling the voltage at the output of a polyphase cycloconverter to which a load, for example a rotating-field motor (3), is connected. For power factor improvement, the reference point (A) of the cycloconverter is electrically isolated from the reference point (B) of the load (3). A function generator (6a) is used to generate a d.c. voltage signal, proportional to the output voltage, of a predetermined magnitude if the output voltage is less than a predetermined value. The d.c. voltage signal is superimposed on the sinusoidal control signals of each phase in adding stages (6), the said signals being present at the control inputs of the control sets (51-53) for the rectifiers of the cycloconverter.

Device for the control of the output voltage of a polyphase cycloconverter

In a method for driving a direct converter which is connected to a multi-phase network, is constructed from partial converters and supplies a three-phase load without any circulating current, with the partial converter being modulated taking into account the zero voltage between the star points of the load and of the partial converters, which are not connected to one another, it is intended to be achieved that only three 3-pulse partial converters (that is to say only 18 converter valves) can be used and that nevertheless the second harmonic in the network current is prevented. This is achieved in that the signal ( alpha 1, alpha 2, alpha 3) for modulating each partial converter (21, 22, 23) is initially modified via a characteristic transmitter (2) in such a manner that the modified signal corresponds to the deviation in the amplitude of the second harmonics from that amplitude when the modulation is zero. Each modified signal is multiplied, in a multiplier (31, 32, 33), by the absolute value of the partial converter current (i21, i22, i23). The sum of the product is used, via a regulator (4), as the regulation signal for modulating the individual partial converters.

Method for driving a direct converter

The method involves using a bridge consisting of six semiconductor switches with antiparallel diodes. Six switches are driven so that a three-phase system exists at RST, displaced w.r.t. a virtual star point, and the respective phase in the block is driven intermittently at any operating point at fully controllable three-phase current. The voltage (U) may be formed at any time point from the difference between the value of the highest voltage in the three-phase system RST and the maximum producible voltage.

Drive method for pulse inverter by driving six switches so that three=phase system is displaced w.r.t. virtual star point

The three phase converter with a return section for each phase, for operating a machine at high loads and low speeds, with their output sides star connected, reduces the blind loading. Within the control region, each section is fully controlled by a variable additional voltage. The resulting combined output voltage is sinusoidal. Each section is cyclically reversed at the output frequency of the direct converter. The frequency of the additional control voltage corresponds to the output frequency multiplied by the number of phases. The control voltage is obtained from the differences of maximum and minimum sinusoidal values.

Grumbrecht Peter Dipl Ing

Papers

Control system for three=phase converter - applies voltage varying at output frequency times number of phases to three converter sections