Method and device for operating an electric motor

1. Method for operating an electric motor, comprising the steps of:

(a) detecting zero-crossings and phase angles of applied phase-shifted sinusoidal voltage phases of a multi-phase power supply network; and

(b) connecting half-waves of the applied sinusoidal voltage phases by means of semiconductor switches in accordance with switching patterns applied to the semiconductor switches, said patterns being synchronised with the detected phase angles and zero-crossings, to generate phase-shifted quasi-sinusoidal voltage phases which are applied directly to the electric motor, wherein to start up the electric motor the frequency of the quasi-sinusoidal voltage phases is stepped up incrementally by changing the switching patterns applied to the semiconductor circuits until the network frequency of the voltage supply network is reached, wherein after the network frequency of the power supply network is reached the electric motor is supplied directly by the power supply network, via mechanical contacts connected in parallel with the semiconductor switches.

2. Method according to claim 1 , wherein the generated phase-shifted quasi-sinusoidal voltage phases have the same frequency as one another, this being a fraction of the network frequency of the multi-phase power supply network.

3. Method according to claim 2 , wherein the semiconductor switches are formed by galvanically separated power semiconductors, which are actuated in accordance with a switching pattern read out from a data store.

4. Method according to claim 2 , wherein the quasi-sinusoidal voltage phases are at a phase angle of approximately 120° to one another.

5. Method according to claim 2 , wherein when the switching pattern applied to the semiconductor switches is changed the direction of rotation of a rotary field for driving the electric motor is maintained.

6. Method according to claim 1 , wherein the semiconductor switches are formed by galvanically separated power semiconductors, which are actuated in accordance with a switching pattern read out from a data store.

7. Method according to claim 6 , wherein the first half-wave and/or the last half-wave of a sinusoidal voltage phase, which is connected through by means of the semiconductor switches in accordance with the applied switching pattern, are truncated to generate the quasi-sinusoidal voltage phases.

8. Method according to claim 6 , wherein the quasi-sinusoidal voltage phases are at a phase angle of approximately 120° to one another.

9. Method according to claim 6 , wherein when the switching pattern applied to the semiconductor switches is changed the direction of rotation of a rotary field for driving the electric motor is maintained.

10. Method according to claim 1 , wherein the quasi-sinusoidal voltage phases are at a phase angle of approximately 120° to one another.

11. Method according to claim 10 , wherein when the switching pattern applied to the semiconductor switches is changed the direction of rotation of a rotary field for driving the electric motor is maintained.

12. Method according to claim 1 , wherein when the switching pattern applied to the semiconductor switches is changed the direction of rotation of a rotary field for driving the electric motor is maintained.

13. Method according to claim 1 , wherein the method is a method for operating a three-phase asynchronous motor.

14. Device for operating an electric motor, comprising:

a detection unit for detecting zero-crossings and phase angles of applied phase-shifted sinusoidal voltage phases of a multi-phase power supply network; and

a switching unit for connecting half-waves of the applied sinusoidal voltage phases by means of semiconductor switches in accordance with switching patterns applied to the semiconductor switches, said patterns being synchronised with the detected phase angles and zero-crossings, to generate phase-shifted quasi-sinusoidal voltage phases which are applied directly to the electric motor, wherein the switching unit is a hybrid switching unit, which has mechanical contacts which are connected in parallel with the semiconductor switches of the switching unit,

wherein a control unit is provided which is configured and operable to actuate semiconductor switches of the switching unit in accordance with the switching patterns read out from a data store,

wherein to start up the electric motor the frequency of the quasi-sinusoidal voltage phases is stepped up incrementally by the switching unit by operation of the control unit changing the switching patterns applied to the semiconductor circuits until the network frequency of the voltage supply network is reached, wherein after the network frequency of the power supply network is reached the electric motor is supplied directly by the power supply network, via mechanical contacts connected in parallel with the semiconductor switches.

15. Device according to claim 14 , wherein the mechanical contacts bridge the semiconductor switches of the switching unit, which are connected in parallel therewith, in continuous operation once the frequency of the quasi-sinusoidal voltage phases reaches the network frequency of the power supply network when the electric motor is started up.

16. Device according to claim 14 , wherein a reversing stage for setting a direction of rotation of a rotary field for driving the electric motor is provided.

17. Device according to claim 14 , wherein an overload protection stage is provided.

18. Motor start-up means for starting up an electric motor, comprising a device according to claim 14 .

19. Device according to claim 15 , wherein a reversing stage for setting a direction of rotation of a rotary field for driving the electric motor is provided.

20. Device according to claim 14 , wherein the device is a device for operating a three-phase asynchronous motor.

21. Motor start-up means according to claim 18 , wherein the motor start-up means is a motor start-up means for starting up a three-phase asynchronous motor.