A frequency converter is connected to the machine slip-ring rotor. It consists of a rectifier with diodes in a three-phase bridge circuit, a d.c. intermediate circuit with a smoothing inductor, and at least one invertor with controllable diodes in a three-phase bridge circuit. The inverter is connected to a frequency converter transformer primary winding, whose secondary winding is connected to an a.c. network. The slip-ring rotor (5) of the asynchronous machine (4) is connected to the primary winding (8) of a transformer (10) with two secondary windings (11, 12) shifted by 30 deg. relative each other. The first secondary winding (11) is connected to a first rectifier (13), and the second winding (12) to a second rectifier (14) with diodes (15, 17, 16, 18) in a three-phase bridge circuit.
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