Method and device for controlling a stator of an electrical machine and electrical drive system

Abstract

The invention relates to a method for controlling a stator of an electrical machine (330) having a rotor and the stator, comprising controlling (401) the stator of the electrical machine (30) using an asynchronous switching method with a carrier frequency having a predetermined switching frequency (308); determining (402) an actual switching frequency (307); and controlling (403) the actual switching frequency (307) to the predetermined switching frequency (308) by changing the carrier frequency (311). The invention further relates to a corresponding control device and an electric drive system.

Description

[0001] The present invention relates to a method and a device for controlling a stator of an electrical machine having a rotor and the stator, a computing unit and a computer program for carrying out the method, and an electrical drive system. Background of the invention

[0002] The task of a power converter (so-called inverter) in an electric drive, e.g. in a vehicle (traction drive or range extender), is primarily to provide the electric machine with a multi-phase alternating voltage generated from the direct voltage provided by a direct voltage source, such as a battery. This alternating voltage is generated by switching power switches on and off (so-called commutation), for which different modulation methods can be used. A distinction is made between time-synchronous methods, also known as asynchronous switching methods or asynchronous PWM or carrier frequency methods, and angle-synchronous methods (FFC - Fundamental Frequency Clocking).

[0003] In an asynchronous switching process, for example, a voltage signal can be modulated using pulse width modulation (PWM). The switching pattern is based on a fixed switching or calculation grid, i.e. the carrier frequency. Each power switch of the converter is switched on and off a maximum of once per PWM period. If, in a specific control situation, switching occurs too quickly or too frequently, or if the system-specific minimum pulse duration is not met, it can be planned not to switch at all during such a period. Thus, in some processes, periods can occur in which the actual switching frequency / switching frequency is lower than the carrier frequency. The current switching frequency in each case results from the specific sequence of switching operations and the intervals between them.DE 10 2008 040 144 A1 shows an example method with switching frequencies that are lower than the carrier frequency. So-called flat-top methods or discontinuous PWM methods (DPWM) are also methods with a switching frequency that is lower than the carrier frequency. Disclosure of the invention

[0004] According to the invention, a method and a device for controlling a stator of an electrical machine having a rotor and the stator, a computing unit and a computer program for implementing the method, and an electric drive system with the features of the independent patent claims are proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

[0005] The invention utilizes the measure of determining or ascertaining how many switching operations actually take place in the power converter or inverter to control the stator of the electrical machine. If this actual switching frequency is lower than the setpoint, the carrier frequency is increased until the setpoint is reached.

[0006] In detail, the method comprises controlling the stator of the electric machine using an asynchronous switching method with a carrier frequency and a predetermined switching frequency, determining an actual switching frequency and regulating the actual switching frequency to the predetermined switching frequency by changing the carrier frequency.

[0007] The invention thus achieves a reduction in DC-link voltage ripple and harmonic losses in the electric machine. Controllability is improved by the larger number of control loops (measuring, processing, and adjusting). Voltage adjustment, especially at high speeds, is achieved with higher resolution, making the modulated voltage more precise.

[0008] In embodiments of the invention, limit values ​​or limitations can also be included in the control, for example to ensure that the limits of the power converter (in particular load) are adhered to and the power loss is not increased compared to continuous voltage modulation methods, such as SVPWM (space vector PWM), and that the computing capacities of the executing computing unit (e.g. microcontroller) are not exceeded.

[0009] The actual switching frequency is highly dependent on the operating point and, to some extent, time-dependent. Therefore, an executing processing unit is not able to precisely account for every operating point. The switching processes are discrete in time and, in the field weakening range, also strongly dependent on the control level. A slightly different operating point can lead to a different actual switching frequency.

[0010] By means of the control system according to the invention, which changes the carrier frequency, these fluctuating conditions can be taken into account automatically, depending on the situation, with limited effort, optionally taking into account predefined limits.

[0011] Actual value acquisition in the control loop allows for precise detection of when the actual switching frequency drops and by how much. Within the limits of measurement accuracy, there is no uncertainty caused by characteristic maps and their interpolation points.

[0012] The higher carrier frequency reduces the DC link voltage ripple, which is always an optimization goal for inverters. This is because the DC link voltage ripple interferes with other devices on the DC link and causes losses, for example, in the DC link capacitor. Depending on the definition of the corner point, the invention can even reduce the maximum DC link voltage ripple at the corner point. This is the case if the first switching operations are already suspended at the corner point by the modulation method. The corner point and its DC link voltage ripple are relevant to the design of the capacitor selection. A smaller DC link voltage ripple at the corner point enables a smaller and thus more cost-effective capacitor or lower power levels.

[0013] The higher carrier frequency reduces the harmonic losses of the machine, as these are inversely proportional to the carrier frequency or actual switching frequency. Lower losses improve efficiency and increase continuous power. Continuous power is particularly relevant for design at the highest speeds.

[0014] The higher carrier frequency improves the control cycles and thus the controllability of the operating point. More control cycles lead to more stable control in dynamic, noisy, or unfavorable operating points. Noisy or unfavorable operating points are often found in cases of severe overmodulation and small integer switching frequencies (e.g., less than 25) in the field weakening. A higher carrier frequency would increase the switching frequency. More stable control increases efficiency and torque quality.

[0015] The higher carrier frequency makes the modulated voltage more precise. A smaller switching frequency makes the voltage less precise. A higher carrier frequency increases the switching frequency, thus modulating the set voltage more effectively.

[0016] The invention can be advantageously applied to a permanent magnet synchronous machine (PSM) or electrically excited synchronous machine (ESM) as an electrical machine, but also to other types of machines that require commutation of the stator current, such as asynchronous machines (ASM), etc.

[0017] A computing unit according to the invention, e.g. a control unit of a device for controlling a stator of an electrical machine, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0018] The implementation of a method according to the invention in the form of a computer program or computer program product with program code for carrying out all method steps is also advantageous, since this entails particularly low costs, in particular if an executing control unit is also used for other tasks and is therefore already present. Finally, a machine-readable storage medium is provided with a computer program stored thereon, as described above. Suitable storage media or data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible. Such a download can be wired or cable-based or wireless (e.g. via a WLAN network, a 3G, 4G, 5G or 6G connection, etc.).

[0019] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0020] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows a schematic representation of a block diagram of an electric drive system according to an embodiment; Fig. Figure 2 shows a schematic representation of an asynchronous switching process and a resulting curve A of the fundamental voltage. Fig. 3 shows a schematic representation of a control loop structure as it may form the basis of a method for controlling a stator of an electrical machine according to one embodiment. Fig. 4 shows a schematic representation of a flow chart as may form the basis of a method for controlling a stator of an electrical machine according to one embodiment. Embodiment(s) of the invention

[0021] Fig. 1 shows a schematic representation of a block diagram of an electric drive system 1 with a device 10 for controlling a stator of an electric machine 30. The electric drive system 1 comprises, for example, an electric machine 30 with the stator, which can be fed by a power converter 11, and a rotor. For this purpose, the power converter 11 can be fed, for example, by a DC voltage source such as a battery 20 or the like. The example of a three-phase electric machine 30 shown here serves only to improve understanding and does not represent a limitation of the present invention. Furthermore, any electric machines 30 with a number of electrical phases other than three are of course also possible. For example, it can also be a five- or six-phase electric machine 30 or an electric machine 30 with any other number of phases.

[0022] To control the stator of the electric machine 30, the power converter 11 can convert the DC voltage provided by the battery 20 into a suitable AC voltage. In the case of a three-phase electric machine 30, the power converter 11 can, for example, convert the DC voltage into a three-phase AC voltage. In particular, the amplitude of the AC voltage and / or the value of the output current from the power converter 11 to the stator windings (phases) of the electric machine 30 can be adjusted based on a predetermined setpoint S.

[0023] For example, the power converter 11 can be a power converter with multiple half-bridges. In particular, the power converter 11 can comprise at least one half-bridge with two switching elements for each phase of the electrical machine 30. For example, the power converter 11 for a three-phase electrical machine 30 can have a B6 topology. The switching elements of the power converter 11 can be controlled by the control device 12 using suitable control signals using the setpoint S. In this case, the control device 12 can, for example, provide a control signal for each switching element of the power converter 11 in order to open or close the corresponding switching element. The following description describes, in particular, the control signal for one switching element of the switching elements of a power converter 11. The control signals of the remaining switching elements are formed in the same way.The control of an upper switching element of a half-bridge is complementary to the control of the corresponding lower switching element. In addition, dead times or similar factors may also need to be taken into account.

[0024] Fig. Figure 2 shows a schematic representation of a control signal of an asynchronous switching method for controlling a switching element in a power converter 11 for controlling the electrical machine 30 and a resulting output signal A of the fundamental voltage. For better understanding, only a few pulses for one period of the output signal are shown. As in Fig. As can be seen in Figure 2, the switching element in the power converter 11 is controlled based on a fixed time frame (time-synchronous) with the period T, which is the inverse of the carrier frequency. Within each time frame, the corresponding switching element is switched on and off at most once. By varying the ratio between the on-time and off-time, the voltage level of the output signal A can be adjusted accordingly. For example, the period T of a clock pulse can be 100 µs, so that the clock frequency or carrier frequency of the signal is 10 kHz. Furthermore, any other period T or clock frequencies are of course also possible. As shown in Fig. As can be further seen in Figure 2, a corresponding voltage level of the output signal A results depending on the duty cycle of a pulse.

[0025] In Fig. 3 schematically shows a control loop structure 300 according to an embodiment of the invention, which is described below together with Fig. 4, in which a block diagram of an associated flow chart is shown.

[0026] The control loop structure 300 can be implemented in one embodiment of a method according to the invention for controlling a stator of an electrical machine with a rotor and the stator, in particular in an executing control device 12.

[0027] In the embodiment shown, the control loop structure 300 comprises a control element 310, which is designed, for example, as a PI controller (proportional-integral controller). The minimum dynamics of the controller should be sufficiently fast in relation to the power electronics heating. A control deviation 309 is supplied to the control element 310, which results from the difference between a predetermined first switching frequency 308 (setpoint) and an actual switching frequency 307 (actual value). A carrier frequency 311 is received at the output side of the control element 310 and fed to a controlled system 330. The controlled system can be a voltage modulation function that outputs a switching frequency directly or indirectly.

[0028] The control loop is closed by determining or ascertaining 340 the actual switching frequency 307. In the simplest case, this can be a purely software method, but alternatively it can also be a determination that counts and determines the exact number of switching edges per unit of time via the hardware, for example, in a control signal to the semiconductor switching elements of the power converter or in the switched phase current. The software method can evaluate the signals of the switching commands behind the voltage modulation function and count the number of switching operations and display them per unit of time. The effort is lower here compared to a hardware solution, but requires computing capacity. It can advantageously be provided to filter or average the determined value over a certain period of time in order to avoid imposing disadvantageous periodic changes on the control loop. In particular, a moving average can be formed over a sufficiently large number of carrier frequency clock pulses.

[0029] The control loop structure 300 also has a number of optional elements.

[0030] For example, the predetermined switching frequency 308 can be determined by limiting a target switching frequency 321 in a limiting element 320 to a maximum switching frequency value 322 and / or a minimum switching frequency value 323. The maximum switching frequency value 322 and / or the minimum switching frequency value 323 can result, for example, from hardware limits. The target switching frequency 321 can be constant and / or defined as a function of system objectives, e.g., as efficiently as possible while adhering to DC link voltage ripple limits when the switching frequency is equal to the carrier frequency.

[0031] Limit 320 is designed to prevent excessively high or low switching frequencies. In particular, it is intended to prevent overloading the power electronics due to excessive switching operations.

[0032] Furthermore, the carrier frequency 311 can be limited in a limiting element 350 to a maximum carrier frequency value 352 and / or to a minimum carrier frequency value 353. The limiting element 350 serves to prevent carrier frequencies that are too high or too low. This is advantageous, for example, if the carrier frequency is coupled with the calculation of the current / torque control loop or other limits in order to prevent overloading the computing time of the executing computing unit. The limiting can also be implemented more strictly than technically or hardware-necessary. The limit values ​​can, for example, be variable depending on the situation. This allows the controller to automatically correct small deviations, but larger ones are prevented.

[0033] A carrier frequency 354 limited in this way is then fed to the control system 330.

[0034] It can optionally be provided that the minimum carrier frequency value 353 is specified by the predetermined switching frequency 308, since the carrier frequency cannot be lower than the switching frequency. By limiting the carrier frequency, it is advantageous but optional to implement an anti-wind-up function in the control loop 300.

[0035] Furthermore, a feedforward control 360 can optionally be implemented, in which a feedforward control value 361 is calculated from the predetermined switching frequency 308 in accordance with a calculation rule, for example a characteristic map, which feedforward control value 361 is combined with a controller output variable 362 to form the carrier frequency 311 in a conventional manner, for example additively.

[0036] In one embodiment of the invention as in Fig.4, in a step 411, the stator of the electric machine 30 is controlled using an asynchronous switching method with a carrier frequency at a predetermined switching frequency 308.

[0037] In a step 402, an actual switching frequency 307 is determined.

[0038] In a block 403, the actual switching frequency 307 is controlled to the predetermined switching frequency 308 by changing the carrier frequency 311.

[0039] In block 403, in particular, a comparison 404 of the predetermined switching frequency 308 with the actual switching frequency 307 can be performed. If these are equal, the carrier frequency 311 is not changed.

[0040] If, however, the actual switching frequency 307 is less than the predetermined switching frequency 308, the carrier frequency 311 is increased in a block 405. If, however, the actual switching frequency 307 is greater than the predetermined switching frequency 308, the carrier frequency 311 is reduced in a block 406.

[0041] It is advantageous if the modulation method is one that does not reduce the drive by increasing the carrier frequency, since otherwise an increase in the carrier frequency would also increase the field weakening current due to the lower voltage. A method without drive reduction is No-Zero-Pointer (NZP), as described, for example, in WO 2023 / 198336 A1.

[0042] However, the invention can also be operated with control methods that influence the modulation depending on the carrier frequency, as is known, for example, from DE 10 2008 040 144 A1 cited above. However, a possible change in the field weakening current must then be taken into account. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2008 040 144 A1 [0003, 0042] WO 2023 / 198336 A1

[0041]

Claims

[1] Method for controlling a stator of an electrical machine (30) having a rotor and the stator, comprising: Controlling (401) the stator of the electrical machine (30) using an asynchronous switching method with a carrier frequency with a predetermined switching frequency (308); Determining (402) an actual switching frequency (307); Regulating (403) the actual switching frequency (307) to the predetermined switching frequency (308) by changing the carrier frequency (311). [2] The method according to claim 1, wherein the controlling (403) of the actual switching frequency (307) to the predetermined switching frequency (308) by changing the carrier frequency (311) comprises: Increasing (405) the carrier frequency (311) if the actual switching frequency (307) is less than the predetermined switching frequency (308). [3] Method according to claim 1 or 2, wherein the control (403) of the actual switching frequency (307) to the predetermined switching frequency (308) by changing the carrier frequency (311) comprises: Reducing (406) the carrier frequency (311) if the actual switching frequency (307) is greater than the predetermined switching frequency (308). [4] Method according to one of the preceding claims, further comprising: Limiting (350) the carrier frequency (311) to a maximum (352) and / or minimum (353) carrier frequency value. [5] Method according to one of the preceding claims, further comprising: Determining the predetermined switching frequency (308) by limiting (320) a target switching frequency (321) to a maximum (322) and / or minimum (323) switching frequency value. [6] Method according to one of the preceding claims, further comprising: Determining (360) a pre-control value (361) as a function of the predetermined switching frequency (308) and changing the carrier frequency (311) as a function of the pre-control value (361). [7] Computing unit (12) which is designed to carry out all method steps of a method according to one of the preceding claims. [8] Device (10) for controlling a stator of an electrical machine (30), comprising: a power converter (11) designed to be coupled to an electrical machine (30) comprising the stator and a rotor, and to provide an electrical voltage for controlling the stator of the electrical machine (30); and a computing unit (12) according to claim 7, which is electrically coupled to the power converter (11) and provides the control signals for the power converter (11). [9] Electric drive system (1), with: a device (10) for controlling a stator of an electrical machine (30) according to claim 8, and an electrical machine (30) having the stator and a rotor, which is electrically coupled to the power converter (11) of the device (10) for controlling the stator of the electrical machine (30). [10] Computer program which causes a computing unit (12) to carry out all method steps of a method according to one of claims 1 to 8 when it is executed on the computing unit. [11] A machine-readable storage medium having stored thereon a computer program according to claim 10.

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