Method for activating an electric machine

1. A method for activating an electric machine, which is operable at least as a motor and includes a rotor, a stator winding having a number of phases and a rectifier having a number of half-bridges corresponding to the number of phases, which each have active switching elements situated in a series connection between two DC voltage terminals of the rectifier, between which the phases are connected, the method comprising:

applying alternating current signals, which are phase-offset to one another by switching the switching elements, to the respective phases in a first motor operating mode in which the rotor rotates above a predefined limiting speed, the frequency of the signals being speed-dependently set; and

at least partially applying constant direct-current signals to the respective phases in a second motor operating mode in which the rotor is one of stationary or rotates below the limiting speed, by switching the switching elements as a function of an instantaneous angle position of the rotor, the direct-current signals being selected in such a way that a current flow does not exceed a predefined maximum current absolute value through any of the phases and an absolute value of a space vector in a d/q coordinate system, which is defined on the basis of the current flows through the phases, is greater than the maximum current absolute value;

wherein the electric machine has five phases and the rectifier has five half-bridges, whose switching elements are switched in the second operating mode in such a way that a positive voltage signal having a first absolute value is output via a first of the half-bridges, a negative voltage signal having the first absolute value is output via a second of the half-bridges, a positive voltage signal having half of the first absolute value is output via a third of the half-bridges, a negative voltage signal having half of the first absolute value is output via a fourth of the half-bridges, and no voltage signal is output to the phases via a fifth of the half-bridges.

2. The method as recited in claim 1 , wherein the voltage signals are output at star points of the stator winding having five phases connected to one another in a star circuit.

3. The method as recited in claim 1 , wherein the electric machine has four to six phases and the rectifier has corresponding four to six half-bridges.

4. The method as recited in claim 1 , wherein the absolute value of the space vector in the d/q coordinate system is determined by a Clarke and Park transformation.

5. The method as recited in claim 1 , wherein the maximum current absolute value is ascertained with the aid of a field-oriented regulation.

6. The method as recited in claim 1 , wherein, in the first operating mode, the alternating current signals are applied to the phases in each case by switching the switching elements on the basis of triangle-sine modulation.

7. An electric machine, which is operable at least as a motor, comprising:

a rotor;

a stator winding having multiple phases;

a rectifier having multiple half-bridges corresponding to the number of phases, which each have active switching elements situated in a series connection between two DC voltage terminals of the rectifier, between which the phases are connected; and

a control unit including a processor configured to:

apply alternating current signals, which are phase-offset to one another by switching the switching elements, to the respective phases in a first motor operating mode in which the rotor rotates above a predefined limiting speed, the frequency of the signals being speed-dependently set;

at least partially apply constant direct-current signals to the respective phases in a second motor operating mode in which the rotor is one of stationary or rotates below the limiting speed, by switching the switching elements as a function of an instantaneous angle position of the rotor, the direct-current signals being selected in such a way that a current flow does not exceed a predefined maximum current absolute value through any of the phases and an absolute value of a space vector in a d/q coordinate system, which is defined on the basis of the current flows through the phases, is greater than the maximum current absolute value;

wherein the electric machine has five phases and the rectifier has five half-bridges, whose switching elements are switched in the second operating mode in such a way that a positive voltage signal having a first absolute value is output via a first of the half-bridges, a negative voltage signal having the first absolute value is output via a second of the half-bridges, a positive voltage signal having half of the first absolute value is output via a third of the half-bridges, a negative voltage signal having half of the first absolute value is output via a fourth of the half-bridges, and no voltage signal is output to the phases via a fifth of the half-bridges.

8. The electric machine as recited in claim 7 , wherein the voltage signals are output at star points of the stator winding having five phases connected to one another in a star circuit.

9. The electric machine as recited in claim 7 , wherein the electric machine has four to six phases and the rectifier has corresponding four to six half-bridges.

10. The electric machine as recited in claim 7 , wherein the absolute value of the space vector in the d/q coordinate system is determined by a Clarke and Park transformation.

11. The electric machine as recited in claim 7 , wherein the maximum current absolute value is ascertained with the aid of a field-oriented regulation.

12. The electric machine as recited in claim 7 , wherein, in the first operating mode, the alternating current signals are applied to the phases in each case by switching the switching elements on the basis of triangle-sine modulation.

13. A non-transitory, computer-readable data storage medium storing a computer program, which is executable by a processor, comprising:

a program code having program code for activating an electric machine, which is operable at least as a motor and includes a rotor, a stator winding having a number of phases and a rectifier having a number of half-bridges corresponding to the number of phases, which each have active switching elements situated in a series connection between two DC voltage terminals of the rectifier, between which the phases are connected, by performing the following:

applying alternating current signals, which are phase-offset to one another by switching the switching elements, to the respective phases in a first motor operating mode in which the rotor rotates above a predefined limiting speed, the frequency of the signals being speed-dependently set; and

at least partially applying constant direct-current signals to the respective phases in a second motor operating mode in which the rotor is one of stationary or rotates below the limiting speed, by switching the switching elements as a function of an instantaneous angle position of the rotor, the direct-current signals being selected in such a way that a current flow does not exceed a predefined maximum current absolute value through any of the phases and an absolute value of a space vector in a d/q coordinate system, which is defined on the basis of the current flows through the phases, is greater than the maximum current absolute value;

wherein the electric machine has five phases and the rectifier has five half-bridges, whose switching elements are switched in the second operating mode in such a way that a positive voltage signal having a first absolute value is output via a first of the half-bridges, a negative voltage signal having the first absolute value is output via a second of the half-bridges, a positive voltage signal having half of the first absolute value is output via a third of the half-bridges, a negative voltage signal having half of the first absolute value is output via a fourth of the half-bridges, and no voltage signal is output to the phases via a fifth of the half-bridges.

14. The data storage medium as recited in claim 13 , wherein the voltage signals are output at star points of the stator winding having five phases connected to one another in a star circuit.

15. The data storage medium as recited in claim 13 , wherein the electric machine has four to six phases and the rectifier has corresponding four to six half-bridges.

16. The data storage medium as recited in claim 13 , wherein the absolute value of the space vector in the d/q coordinate system is determined by a Clarke and Park transformation.

17. The data storage medium as recited in claim 13 , wherein the maximum current absolute value is ascertained with the aid of a field-oriented regulation.

18. The data storage medium as recited in claim 13 , wherein, in the first operating mode, the alternating current signals are applied to the phases in each case by switching the switching elements on the basis of triangle-sine modulation.