An operating circuit for coupling a synchronous machine with a voltage network, comprising a converter circuit which can be controlled by a control unit and has a converter input connectable to the synchronous machine and a converter output connectable to the voltage network, wherein the converter circuit comprises a plurality of converter switches (20, 21, 22, 23) controllable by the control unit and a switching arrangement controllable by the control unit, wherein the converter circuit can be switched between a direct converter configuration and a start converter configuration by means of the switching arrangement, wherein the converter circuit in the start converter configuration forms a network-commutated and a load-commutated converter and the control unit is arranged for a frequency start-up of the synchronous machine in motor operation, wherein the control unit is configured to control the converter circuit in the direct converter configuration depending on the voltage profile at the converter input in order to set an alternating voltage having a predetermined frequency at the converter output, wherein the synchronous machine works in generator mode and provides electrical energy for the voltage network, wherein the converter circuit comprises at least two converter branches, each having a first output terminal and a second output terminal and each having a first input terminal, a second input terminal and a third input terminal, and wherein each converter branch comprises a bridge circuit with four controllable converter switches (20, 21, 22, 23) for each input terminal (39, 40, 41), wherein a first bridge point is connected via a first converter switch to the first output terminal and via a second converter switch to the second output terminal, and wherein a second bridge point is connected via a third converter switch to the first output terminal and is connected to the second output terminal via a fourth converter switch. a converter circuit which can be controlled by a control unit and has a converter input connectable to the synchronous machine and a converter output connectable to the voltage network, wherein the converter circuit comprises a plurality of converter switches (20, 21, 22, 23) controllable by the control unit and a switching arrangement controllable by the control unit, wherein the converter circuit can be switched between a direct converter configuration and a start converter configuration by means of the switching arrangement, wherein the converter circuit in the start converter configuration forms a network-commutated and a load-commutated converter and the control unit is arranged for a frequency start-up of the synchronous machine in motor operation, wherein the control unit is configured to control the converter circuit in the direct converter configuration depending on the voltage profile at the converter input in order to set an alternating voltage having a predetermined frequency at the converter output, wherein the synchronous machine works in generator mode and provides electrical energy for the voltage network, wherein the converter circuit comprises at least two converter branches, each having a first output terminal and a second output terminal and each having a first input terminal, a second input terminal and a third input terminal, and wherein each converter branch comprises a bridge circuit with four controllable converter switches (20, 21, 22, 23) for each input terminal (39, 40, 41), wherein a first bridge point is connected via a first converter switch to the first output terminal and via a second converter switch to the second output terminal, and wherein a second bridge point is connected via a third converter switch to the first output terminal and is connected to the second output terminal via a fourth converter switch.