Method for operating an electrical circuit, electrical circuit and motor vehicle

Abstract

A method for operating an electrical circuit, wherein the electrical circuit includes a DC converter, an inverter and an electric machine, wherein the inverter is connected on the direct current side to the output of the DC converter and on the alternating current side to the electric machine, wherein the electric machine is operated using a torque specification and/or a rotational speed specification, wherein the level of the output voltage of the DC converter is set as a function of a current torque specification and/or a current rotational speed specification.

Claims

1. A method for operating an electrical circuit, wherein the electrical circuit comprises: a DC converter, an inverter and an electric machine, wherein the inverter is connected on the direct current side to the output of the DC converter and on the alternating current side to the electric machine, wherein the electric machine is operated using a torque specification and/or a rotational speed specification, wherein the level of the output voltage of the DC converter is set as a function of a current torque specification and/or a current rotational speed specification wherein a torque of the electric machine is compensated and maintained when a rotational speed of the electric machine is above a predetermined value.

2. The method according to claim 1, wherein, by the set output voltage of the DC converter, the torque of the electric machine is changed, in particular increased.

3. The method according to claim 2, wherein the torque of the electric machine is changed in such a manner that a torque decrease of the electric machine above a rated rotational speed is at least partially compensated and in that a mechanical power of the electric machine above the rated rotational speed is at least sectionally constant and increases above the rated rotational speed with increasing rotational speed, in particular continuously.

4. The method according to claim 1, wherein the output voltage of the DC converter is set in such a manner that an efficiency of the electric machine for an operating point of the electric machine, comprising the current torque specification and/or the current rotational speed, is increased.

5. The method according to claim 1, wherein a DC converter is used, the input of which is connected to a direct current source, in particular to a fuel cell or a battery, wherein the output voltage of the DC converter is set as a function of a voltage of the direct current source and/or a maximum acceptable discharge current of the direct current source.

6. The method according to claim 1, wherein the output voltage of the DC converter is set as a function of a maximum acceptable operating voltage of the electric machine.

7. The method according to claim 1, wherein an inverter, a pulse inverter is used and/or in that, as electric machine, an asynchronous machine, a permanently excited synchronous machine and/or a separately excited synchronous machine is used.

8. An electrical circuit comprising: a control device, a DC converter, an inverter and an electric machine, wherein the inverter is connected to the output of the DC converter on the direct current side and to the electric machine on the alternating current side, wherein the electric machine can be operated using a torque specification and/or a rotational speed specification, wherein in that the control device is configured for carrying out a method for operating an electrical circuit, wherein the electrical circuit further comprises the DC converter, an inverter and an electric machine, wherein the inverter is connected on the direct current side to the output of the DC converter and on the alternating current side to the electric machine, wherein the electric machine is operated using a torque specification and/or a rotational speed specification, wherein the level of the output voltage of the DC converter is set as a function of a current torque specification and/or a current rotational speed specification, wherein a torque of the electric machine is compensated and maintained when a rotational speed of the electric machine is above a predetermined value.

9. A vehicle comprising an electrical circuit according to claim 8.

10. A motor vehicle according to claim 9, wherein the electric machine is a traction electric motor of the motor vehicle and/or in that the DC converter is connected at its input to a traction energy storage, implemented in particular as high-voltage battery or as fuel cell, of the motor vehicle.

11. The method according to claim 2, wherein the output voltage of the DC converter is set in such a manner that an efficiency of the electric machine for an operating point of the electric machine, comprising the current torque specification and/or the current rotational speed, is increased.

12. The method according to claim 3, wherein the output voltage of the DC converter is set in such a manner that an efficiency of the electric machine for an operating point of the electric machine, comprising the current torque specification and/or the current rotational speed, is increased.

13. The method according to claim 2, wherein a DC converter is used, the input of which is connected to a direct current source, in particular to a fuel cell or a battery, wherein the output voltage of the DC converter is set as a function of a voltage of the direct current source and/or a maximum acceptable discharge current of the direct current source.

14. The method according to claim 3, wherein a DC converter is used, the input of which is connected to a direct current source, in particular to a fuel cell or a battery, wherein the output voltage of the DC converter is set as a function of a voltage of the direct current source and/or a maximum acceptable discharge current of the direct current source.

15. The method according to claim 4, wherein a DC converter is used, the input of which is connected to a direct current source, in particular to a fuel cell or a battery, wherein the output voltage of the DC converter is set as a function of a voltage of the direct current source and/or a maximum acceptable discharge current of the direct current source.

16. The method according to claim 2, wherein the output voltage of the DC converter is set as a function of a maximum acceptable operating voltage of the electric machine.

17. The method according to claim 3, wherein the output voltage of the DC converter is set as a function of a maximum acceptable operating voltage of the electric machine.

18. The method according to claim 4, wherein the output voltage of the DC converter is set as a function of a maximum acceptable operating voltage of the electric machine.

19. The method according to claim 5, wherein the output voltage of the DC converter is set as a function of a maximum acceptable operating voltage of the electric machine.

20. The method according to claim 2, wherein an inverter, a pulse inverter is used and/or in that, as electric machine, an asynchronous machine, a permanently excited synchronous machine and/or a separately excited synchronous machine is used.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Additional advantages and details of the invention result from the embodiment examples described below as well as in reference to the drawings. Said drawings are diagrammatic representations and show:

(2) FIG. 1 a side view of a motor vehicle according to the invention,

(3) FIG. 2 a first diagram for explaining embodiment examples of a method according to the invention,

(4) FIG. 3 a second diagram for explaining embodiment examples of a method according to the invention, and

(5) FIG. 4 a third diagram for explaining embodiment examples of a method according to the invention.

DETAILED DESCRIPTION

(6) In FIG. 1, a side view of a motor vehicle 1 according to the invention is represented. The motor vehicle 1 comprises an electrical circuit 2 according to the invention. The electrical circuit 2 comprises an electric machine 3, an inverter 4, a DC converter 5 as well as a direct current source 6. Here, the inverter 4 is connected to the electric machine 3 on its alternating current side. The direct current side of the inverter 4 is connected to the DC converter 5. The DC converter 5 is moreover connected to the direct current source 6. The inverter 4 is implemented as a pulse inverter, and the electric machine 3 is designed as an asynchronous machine, a permanently excited synchronous machine or a separately excited synchronous machine.

(7) A direct current, which is converted via the inverter 4 into an alternating current for operating the electric machine (3), can be drawn from the direct current source 6. The electric machine 3 here represents a traction electric motor of the motor vehicle 1, by which the motor vehicle 1 can be moved in an electric driving operation. The direct current source 6 represents a traction energy storage of the motor vehicle 1 and can be implemented, for example, as a high-voltage energy storage such as a high-voltage battery or a fuel cell. The direct current source can here have, in particular, a nominal voltage between 400 V and 840 V, in particular of 400 V, 800 V or 840 V. The voltage provided by the direct current source 6 represents the input voltage of the DC converter 5. This input voltage can be converted by the DC converter 5 into an output voltage of the DC converter 5, wherein the output voltage can be higher or lower than the input voltage of the DC converter 5. The output voltage generated by the DC converter 5 represents the input voltage on the direct current side of the inverter 4 or an intermediate circuit voltage of the electrical circuit 2.

(8) The motor vehicle 1 moreover comprises a control device 7, by means of which the level of the output voltage of the DC converter 5 can be set as a function of a current torque specification and/or as a function of a current rotational speed specification and/or of an operating point of the electric machine 3, which comprises the current torque specification and/or the current rotation speed specification. The current torque specification and/or the current rotational speed specification and/or the operating point can be transmitted, for example, from a motor control device (not represented here) of the motor vehicle to the control device 7.

(9) The current torque specification and/or rotational speed specification, specified, for example, by the motor control device, is used to operate the electric machine at an operating point determined by the motor control device. By an adjustment of the output voltage of the DC converter 5 as a function of the current torque specification and/or of the current rotational speed specification by the control device 7, an operation of the electric machine 3 with a changed torque can occur. In particular, an operation of the electric machine 3 with an increased torque, a constant and/or increased mechanical power above the rated rotational speed and/or an increased efficiency is possible, as is explained below.

(10) In FIG. 2, a first diagram is represented, in which a rotational speed n of the electric machine 3 is plotted on the abscissa, and a torque M and a mechanical power P of the electric machine 3 are plotted on the ordinate. Here, the torque M of the electric machine 3 is depicted by the respective curves represented with solid lines, and the mechanical power P of the electric machine 3 is depicted by the respective curves represented with dashed lines.

(11) As can be seen, in the rotational speed range between 0 and a rated rotational speed n.sub.rated, the electric machine 3 has a constant torque M.sub.a. Due to the constant torque M.sub.a in the range between 0 and n.sub.rated, the mechanical power P of the electric machine 3 increases correspondingly in a constant manner in the range between 0 and n.sub.rated.

(12) Two torque curves 8, 9 are represented, each representing the course of the torque M versus the rotational speed n. Here, the curve 8 shows, as an example, the course of the torque of a permanently excited synchronous machine, and the curve 9 shows the course of the torque of an asynchronous machine, in each case without adjustment of the level of the output voltage of the DC converter 5. The mechanical power P resulting in each case from the torque M and the rotational speed n is plotted for the permanently excited synchronous machine on curve 10 and correspondingly for the asynchronous machine on curve 11 versus the rotational speed n. Due to the design, above the rated rotational speed n.sub.rated, the torque of the permanently excited synchronous machine as well as the torque of the asynchronous machine decrease. Correspondingly, a mechanical power P of the electric machine 3 which decreases for higher rotational speeds also results from the power curves 10 and 11.

(13) This decrease of the torque or the decrease of the mechanical power can be at least partially compensated by an adjustment of the output voltage of the DC converter 5. This is represented in FIG. 3.

(14) In FIG. 3, a second diagram is represented, wherein the axes and the form of representation correspond to the first diagram from FIG. 2. In FIG. 3, two power curves 12, 13 are depicted, which represent the mechanical power P of the electric machine 3 versus the rotational speed n. Here, the first power curve 12 has a course in which the mechanical power of the electric machine 3 remains constant above the rated rotational speed n.sub.rated. The constant mechanical power is generated in that, as a function of the current torque specification and/or of the current rotational speed specification of the electric machine 3, the output voltage of the DC converter 5 is adjusted in such a manner that the torque of the electric machine 3 is increased as a function of the rotational speed, so that, for all the acceptable rotational speeds above the rated rotational speed n.sub.rated, a constant mechanical power of the electric machine 3 results. The torque curve associated with the first power curve 12 is represented as first torque curve 14.

(15) It is also possible to adjust the level of the output voltage of the DC converter 5 in such a manner that, starting at the mechanical power P.sub.a, at the rotational speed n.sub.rated, an increased mechanical power of the electric machine 3 is generated. Here, the output voltage of the DC converter 5 is adjusted in such a manner that the mechanical power in accordance with the second power curve 13 for rotational speeds increases continuously with the rotational speed above the rated rotational speed n.sub.rated. The torque curve associated with the second power curve 13 is represented as curve 15.

(16) It can be seen that by the adjustment of the level of the output voltage of the DC converter 5, a decrease of the torque of the electric machine 3 above the rated rotational speed can be at least partially compensated, or a mechanical power of the electric machine can be kept constant above the rated rotational speed, or, in particular for all acceptable torque specifications and/or rotational speed specifications, it can be increased with respect to the mechanical power at the rated rotational speed n.sub.rated.

(17) At operating points of the motor vehicle which are below a maximum possible mechanical power of the electric machine 3, the level of the output voltage of the DC converter 5 can be adjusted in such a manner that, for the respective operating point, an improved efficiency results. As an example, an operating point 16 is represented, which comprises a current rotational speed specification n.sub.i as well as a current torque specification M.sub.i. By the control device 7, here, as a function of the current rotational speed specification n.sub.i and the current torque specification M.sub.i, an adjustment of the level of the output voltage of the DC converter 5 is carried out in such a manner that the electric machine 3 is operated at the operating point 16 with an improved efficiency. The efficiency of the electric machine 3 is here increased with respect to an operation of the electric machine 3 at the operating point 16 without adjustment of the output voltage.

(18) In addition to a power curve with a constant course starting at the rated rotational speed, a curved course of the curve corresponding to a traction force hyperbola is also possible, in which, at least for some of the acceptable rotational speeds above the rated rotational speed n.sub.rated, a constant mechanical power of the electric machine 3 results. Such an embodiment example is represented in FIG. 4 as a third power curve 17, wherein the axes and the form of representation correspond to the preceding diagrams. For the mechanical power as well, in addition to a constant increase starting at the rated rotational speed, other curve courses, for example, non-linear curve courses are also possible, in which the mechanical power increases with the rotational speed with a constant slope only starting at a rotational speed above the rated rotational speed. Such an embodiment example is represented as fourth power curve 18. A curve course corresponding to a traction force hyperbola, in which the mechanical power increases continuously with variable slope, is also possible. The torque curve associated with the third power curve 17 is represented as curve 19, and the torque curve associated with the fourth power curve 18 is represented as curve 20.

(19) The respective course of the power curves 12, 13, 17, 18 and/or the associated values for the level of the output voltage 5 can, for example, be stored as characteristic curves in a storage device of the control device 7 of the motor vehicle 1. An operation of the electric machine 3 according to one of the power curves 12, 13, 17, 18 can be specified, for example, by a motor control device of the motor vehicle 1 and/or switched over as a function of a user input.

(20) The adjustment of the output voltage of the DC converter 5 here always occurs taking into consideration a maximum discharge current of the direct current source 6 as well as a maximum acceptable operating voltage of the electric machine 3. Thereby, it can be achieved that the electric machine 3 is operated always within the acceptable physical limits. In addition, it is possible that the adjustment of the output voltage of the DC converter 5 also occurs as a function of a voltage of the direct current source 6, so that, in addition, a voltage of the direct current source 6 which decreases due to a decrease of the charge state can also be compensated during the operation of the electric machine 3.

(21) By the adjustment of the level of the output voltage of the DC converter 5 as a function of the current torque specification and/or of the current rotational speed specification, an improved operation of the electric machine 3 is possible. On the one hand, the efficiency of the operation of the electric machine 3 can be improved, and, on the other hand the electric machine 3 can be operated with in particular a higher torque compared to what would be possible if an unchanged voltage of the direct current source 6 were used as intermediate circuit voltage of the inverter 4. Moreover, by the adaptation of the level of the output voltage of the DC converter 5, an operation of the electric machine 3 according to adjustable characteristic curves with improved efficiency and/or performance of the electric machine 3 (performance shaping) is enabled. This makes it possible, for example, to generate a power curve which is independent of machine type in the case of an already established battery design and/or to increase a performance of the electric drive of a motor vehicle 1 in the case of an already established battery design.