Method for operating an electrical machine and electrical machine

Abstract

Method for operating an electrical machine (2) with a power source (8) and with an electric motor (4) and with a power converter (6) connected therebetween, - at which an input current (I E ) of the current source (8) is converted into a multi-phase output current (lu, Iv, Iw) for the electric motor (4) by means of a pulse-width modulated control of a number of semiconductor switches (44, 46) of the power converter (6), wherein the or each pulse (Pu, Pv, Pw) is generated at a first time (72, 74, 76) and terminated after a pulse duration (Tu, Tv, Tw) at a second time (78, 80, 82), - in which a frequency spectrum (F1(ω), F2(ω)) is determined for each alternating current component (I1, I2) which is generated in an intermediate circuit (12) of the current source (8) during the pulse-width modulated control of the semiconductor switches (44, 46), and - at which the pulse durations (T U ', T V ', T W') of the pulse (P U ', P V ', P W ') of the pulse width modulated control for a specific frequency (ω0) can be set such that the sum of the frequency spectra (F res (ω)) of the alternating current components (I1, I2) becomes minimal at this frequency (ω0).

Description

[0001] The invention relates to a method for operating an electrical machine having a power source and an electric motor, as well as a power converter connected therebetween. In this method, an input current of the power source is converted into a multiphase output current for the electric motor by means of pulse-width modulated control of a number of semiconductor switches of the power converter. The or each pulse is generated at a first time and terminated at a second time after a pulse duration. The invention further relates to an electrical machine operated according to such a method, in particular for a motor vehicle.

[0002] Electric motor-driven adjustment systems as automotive components, such as window regulators, seat adjusters, door and sunroof drives, or radiator fan drives, as well as pumps and interior blowers, typically feature an electric machine with a controlled electric motor. Brushless electric motors are known, for example, in which a rotor mounted rotatably relative to a stator is driven by a rotating magnetic field. For this purpose, the phase windings of the stator are supplied with a corresponding three-phase or motor current, which is controlled and regulated by a controller as part of the (motor) electronics.

[0003] Such electrical machines typically include a (high-voltage) battery as an on-board energy storage device, from which the electric motor is supplied with electrical energy in the form of direct current. To convert the direct current into motor current, a power converter (inverter) is suitably connected between the energy storage device and the electric motor. A (DC voltage) intermediate circuit, to which a bridge circuit of the power converter is connected, is arranged downstream of the energy storage device. The energy storage device and the intermediate circuit act as a current source to provide the input-side direct current (input current) for the power converter. The motor current is generated as a multi-phase output current by pulse-width modulated (PWM) control of semiconductor switches in the bridge circuit.The pulses of the PWM control switch the semiconductor switches between a conducting and a blocking state.

[0004] The switching operations of the semiconductor switches generate alternating currents in the lines of the intermediate circuit or power source. These alternating currents are considered critical with regard to compliance with EMC (electromagnetic compatibility) guidelines.

[0005] In the publication DG Holmes “The significance of zero space vector placement for carrier-based PWM schemes” (IEEE Transactions on Industry Applications, Vol. 32, Is. 5, 1996, P. 1122-1129) it is known that in pulse width modulation the placement of a zero space vector component within the carrier interval determines the harmonic performance of the modulation strategy.

[0006] The publication by P.A. Dahono, Y. Sato, and T. Kataoka, "Analysis and Minimization of Ripple Components of Input Current and Voltage of PWM Inverters" (IEEE Transactions on Industry Applications, Vol. 32, Is. 4, July-Aug., 1996), teaches that a reference signal that produces minimal input voltage ripple varies depending on the load power factor. Furthermore, it is disclosed that the effective value of the inverter's input current ripple does not depend on the shape of the reference signal.

[0007] The invention is based on the object of providing a particularly suitable method for operating an electrical machine. In particular, the EMC behavior of the electrical machine during operation is to be improved.

[0008] The invention is further based on the object of specifying an electrical machine operating according to such a method.

[0009] With regard to the method, the stated object is achieved according to the invention with the features of claim 1 and with regard to the electrical machine with the features of claim 8. Advantageous embodiments and further developments are the subject of the respective subclaims.

[0010] The method according to the invention is suitable and configured for operating an electrical machine. The electrical machine comprises an energy storage device with a downstream (DC voltage) intermediate circuit, which together are designed as a power source (voltage supply) for an electric motor of the machine. A power converter, for example, in the form of an inverter, is connected between the power source and the electric motor.

[0011] During machine operation, the power converter converts an input current from the power source into a multi-phase, particularly three-phase, output current (motor current, three-phase current) for the electric motor. For this purpose, a number of semiconductor switches of the power converter connected in the intermediate circuit are controlled in a clocked manner using a pulse-width modulated (PWM) controller. The PWM controller has at least one pulse-shaped signal (pulse) for each phase to switch the respective semiconductor switches. The or each pulse is generated at a first time and terminated at a second time after a respective pulse duration.

[0012] According to the method, a frequency spectrum is determined for each alternating current component generated during the pulse-width modulated control of the semiconductor switches in the intermediate circuit of the power source. For this purpose, the alternating current components are converted into the frequency domain, for example using Fourier transformation. The pulse durations of the pulses of the pulse-width modulated control are set such that the sum of the frequency spectra of the alternating current components (sum spectrum) is minimal. This reduces or minimizes the alternating current generated by the alternating current components in the power source, which has a beneficial effect on the EMC behavior of the electrical machine. In particular, this reduces the load on the current or voltage source. Furthermore, it is possible to improve the acoustics of the machine, i.e. it enables noise reduction.

[0013] The frequency spectra of the alternating current components (current block) generally have comparable amplitudes, but differ in phase. By adjusting the pulse durations, the phase positions of the frequency spectra are shifted relative to each other without changing the amplitude of the frequency spectra. By selecting the appropriate pulse durations, it is therefore possible to minimize the total spectrum and thus the alternating current in the power source, at least for certain frequencies.

[0014] The sum spectrum is typically not minimizable across the entire frequency range, but for a frequency to be optimized, particularly the clock frequency of the PWM control, it is sufficient to perform the minimization. The amplitude is minimized when the phase positions of the individual frequency spectra of the alternating current components are shifted oppositely or inversely to each other. In a suitable refinement, to minimize the sum, the individual frequency spectra, particularly in the range around the frequency to be optimized, are phase-shifted by 180° from each other.

[0015] In one embodiment of the invention, the pulse durations are set for a specific frequency, in particular the frequency of a maximum of the or each frequency spectrum. The maximum of the or each frequency spectrum is essentially determined by the clock frequency (fundamental wave component) of the PWM control or the pulses. The relevant frequency is preferably determined dynamically during operation of the electric machine. In other words, the pulse durations for the relevant frequency are set in real time during operation.

[0016] In one possible embodiment, the determination is performed using the clock frequency. This means that each period of the PWM control is optimized individually. For example, it is conceivable to calculate the optimal pulse durations only once per millisecond. This reduces the computational load. However, a circuit design using a bandpass filter for the relevant frequency is also conceivable, with the pulse durations being adjusted during a settling time to achieve a minimum.

[0017] In a practical embodiment, the or each pulse is generated a first time period earlier than the first time point and / or terminated a second time period later than the second time point. By adjusting the first and second time periods, the temporal positions of the first and second time points are effectively shifted during the PWM control. Consequently, the pulse duration is varied or adjusted. This allows a (relative) phase shift of the frequency spectra of the alternating current components to be realized in a simple manner.

[0018] In a suitable embodiment, each pulse for generating the multi-phase output current is provided with the same first and second time duration for adjusting the pulse duration. In other words, a common first time duration is used for all switch-on operations, i.e. the switching of the semiconductor switches from the non-conducting (blocking) to the conducting state, so that all pulses are generated at an earlier (first) time. Likewise, a common second time duration is used for all switch-off operations, i.e. the switching of the semiconductor switches from the conducting to the blocking state, so that all pulses end at a later (second) time. It is also conceivable, for example, that all pulses are generated at a later time and / or end at an earlier time.

[0019] The time intervals used to set the pulse durations of the PWM control can be changed for different periods. This means, for example, that new values for the time intervals are determined for each period of the PWM control, or that the same values for the time intervals are used across multiple periods. It is essential that all pulse durations of the different phases are changed by the same time intervals during a period. In other words, the difference between the pulse durations before and after the pulse durations are adjusted within a period is constant.

[0020] In an advantageous embodiment, the first and second time periods are equal in magnitude. In particular, this only slightly influences the generated phase currents of the output current, so that there is no significant impact on motor operation.

[0021] An additional or further aspect of the method according to the invention provides that the first and second time periods are calculated in advance for different specific frequencies and stored in a table. This realizes a particularly low-complexity method that can simultaneously be flexibly adapted to different frequencies, for example, due to a change in the clock frequency of the PWM control.

[0022] The electric machine is particularly suitable and configured for electric motor drive in a motor vehicle, for example, for an adjustment system as a motor vehicle component. The electric motor is preferably brushless, with a stator and a rotor rotatably mounted therein. The stator has a number of phase windings, which are connected to the power converter on the one hand and, on the other hand, are connected in a star connection, for example, at a common node (star point). The power converter has a controller, i.e., a control unit. The controller is generally suitable and configured—in terms of programming and / or circuitry—to carry out the method described above.The controller is thus specifically designed to detect the alternating current components in the power source during operation and, by controlling and / or regulating the PWM control, to adjust the pulse durations of the (PWM) pulses in such a way that their relative (frequency) phase position is shifted against one another, so that the alternating current in the power source is minimized.

[0023] In a preferred embodiment, the controller is formed at least in its core by a microcontroller with a processor and a data memory in which the functionality for carrying out the method according to the invention is implemented in the form of operating software (firmware) so that the method - if necessary in interaction with a user - is carried out automatically when the operating software is executed in the microcontroller.

[0024] Within the scope of the invention, the controller can alternatively also be formed by a non-programmable electronic component, for example an ASIC (application-specific integrated circuit), in which the functionality for carrying out the method is implemented using circuitry.

[0025] The electric machine operated with the method thus exhibits improved performance with regard to EMC emissions and noise generated by the switching processes of the semiconductor switches. The method according to the invention is particularly suitable and configured for use in speed-controlled systems. In principle, its application is not limited to the automotive sector.

[0026] An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. The drawings show, in schematic and simplified representations: Fig. 1 an electrical machine with a power source and an electric motor and with a power converter connected between them, Fig. 2 three phase windings of a three-phase electric motor of the machine in star connection, Fig. 3 a bridge module of a bridge circuit of the power converter for controlling a phase winding of the electric motor, Fig. 4 an equivalent circuit diagram of the current source, Fig. 5 a diagram of a PWM control of the phase windings, and Fig. 6a, Fig. 6b shows a diagram of the PWM control with a phase shift.

[0027] Corresponding parts and sizes are always provided with the same reference symbols in all figures.

[0028] The Fig. 1 shows an electric machine 2 for an electromotive adjustment system of a motor vehicle (not shown in detail), for example, a window lifter or a seat adjuster. The machine 2 comprises a three-phase electric motor 4, which is connected to a power source (voltage supply) 8 via a power converter 6. In this exemplary embodiment, the power source 8 comprises an internal vehicle energy storage device 10, for example in the form of a (motor vehicle) battery, and a (DC voltage) intermediate circuit 12 connected thereto, which extends at least partially into the power converter 6.

[0029] The intermediate circuit 12 is essentially formed by a forward line 12a and a return line 12b, by means of which the power converter 6 is connected to the energy storage device 10. The lines 12a and 12b are at least partially routed into the power converter 6, in which an intermediate circuit capacitor 14 and a bridge circuit 16 are connected between them.

[0030] During operation of the machine 2, an input current I supplied to the bridge circuit 16 is E converted into a three-phase output current (motor current, three-phase current) lu, Iv, Iw for the three phases U, V, W of the electric motor 4. The output currents lu, Iv, Iw, hereinafter also referred to as phase currents, are applied to the corresponding phases (windings) U, V, W ( Fig. 2) of a stator not shown in detail.

[0031] In the Fig. 2 shows a star connection 18 of the three phase windings U, V, W. The phase windings U, V and W are each connected with a (phase) end 22, 24, 26 to a respective bridge module 20 ( Fig. 3) of the bridge circuit 16, and are connected to each other with the opposite end in a star point 28 as a common connection terminal. In the illustration of the Fig. 2, the phase windings U, V and W are each shown by means of an equivalent circuit in the form of an inductance 30 and an ohmic resistance 32 as well as a respective voltage drop 34, 36, 38. The voltage 34, 36, 38 dropping across the phase windings U, V, W is schematically represented by arrows and results from the sum of the voltage drops across the inductance 30 and the ohmic resistance 32 as well as the induced voltage 40. The voltage 40 induced by a movement of a rotor of the electric motor 4 (electromagnetic force, EMF) is shown in the Fig. 2 is represented by a circle.

[0032] The star connection 18 is controlled by means of the bridge circuit 16. The bridge circuit 16 is designed with the bridge modules 20, in particular as a B6 circuit. In this embodiment, during operation, each of the phase windings U, V, W is switched at a high switching frequency between a high (DC) voltage level of the supply line 12a and a low voltage level of the return line 12b. The high voltage level is in particular an intermediate circuit voltage U ZK of the intermediate circuit 12, wherein the low voltage level preferably has an earth potential U G This clocked control is designed as a - in Fig. 1 by means of arrows - PWM control is carried out by a controller 42, with which a control and / or regulation of the speed, the power and the direction of rotation of the electric motor 4 is possible.

[0033] The bridge modules 20 each comprise two semiconductor switches 44 and 46, which are arranged in the Fig. 2 are shown only schematically and as an example for phase W. The bridge module 20 is connected on the one hand with a potential connection 48 to the supply line 12a and thus to the intermediate circuit voltage U ZK On the other hand, the bridge module 20 is connected to the return line 12b and thus to the ground potential U by a second potential connection 50. G The respective phase end 22, 24, 26 of phase U, V, W is connected via the semiconductor switches 44, 46 either to the intermediate circuit voltage U ZK or with the earth potential U G If the semiconductor switch 44 is closed (conductive) and the semiconductor switch 46 is opened (non-conductive), the phase end 22, 24, 26 is connected to the potential of the intermediate circuit voltage U ZKAccordingly, when the semiconductor switch 44 is opened and the semiconductor switch 46 is closed, the phases U, V, W are connected to the ground potential U G This makes it possible to apply two different voltage levels to each phase winding U, V, and W using PWM control.

[0034] In the Fig. Figure 3 shows a simplified representation of a single bridge module 20. In this embodiment, semiconductor switches 44 and 46 are implemented as MOSFETs (metal oxide semiconductor field-effect transistors), each of which switches between a conducting state and a blocking state in a clocked manner using PWM control. For this purpose, the respective gate terminals are connected to corresponding control voltage inputs 52, 54, via which the PWM control signals of controller 42 are transmitted.

[0035] The Fig. 4 shows an equivalent circuit diagram for the current source 8. During operation, the energy storage device 10 generates a battery voltage U Bat and a corresponding battery current I Bat for operation of the power converter 6. In the Fig. 4, the internal resistance of the energy storage device 10 is shown as an ohmic resistance 56 and the self-inductance of the energy storage device 10 as an inductance 58. A shunt resistor 60 is connected in the return line 12b, across which the intermediate circuit voltage U ZK falls.

[0036] Based on the Fig. 5, the signal curve at the individual phase connections 22, 24, 26 is shown below and explained how the voltage or PWM signals at the individual phase windings U, V, W can be advantageously controlled or regulated and what consequences this has regarding the currents lu, Iv, Iw in the phase windings U, V, W and the input current I Ethe external power source 8 resulting from this.

[0037] The diagram of the Fig. 5 comprises four horizontal sections arranged one above the other. Time is plotted horizontally, meaning on the x-axis or abscissa axis. Examples are shown in the Fig. 5 shows three periods of the PWM control, where one period is, for example, 50 µs (microseconds) long.

[0038] The Fig. Figure 5 shows a PWM control system in which the phase terminals 22, 24, 26 of the electric motor 4 are each controlled with a PWM (pulse) signal Pu, Pv, Pw with a different duty cycle. The instantaneous target voltages Uu, Uv, and Uw of the three phases U, V, and W are shown in Fig. 5 with an instantaneous value of 62, 64, and 66, each represented as a horizontal line. The target voltage values vary over time depending on the speed of the electric motor 4, each in the manner of a sine function. This causes the lines of instantaneous values 62, 64, and 66 to periodically move up and down in the vertical direction, i.e., along the Y or ordinate axis.

[0039] The sawtooth-shaped line in the upper section of the diagram represents a periodically linearly increasing and linearly decreasing counter reading 68 of a counter integrated in the controller 42. The intersection points between the thresholds of the individual phases U, V, W, which are fixed for a specific time, i.e., the instantaneous values 62, 64, 66, with the sawtooth-shaped counter reading 68, represent the times for generating and ending the (PWM) pulses Pu, Pv, Pw, which are applied to the phase windings U, V, W. This means that in the case of a high voltage threshold, the instantaneous value 62, 64, 66 is low, so that the duty cycle of the pulse-shaped pulse Pu, Pv, Pw is long, and the corresponding phase U, V, W is thus supplied with the phase current lu, Iv, Iw or is subjected to a voltage for a long time.

[0040] In the second section 70 of the diagram of the Fig. 5 shows the voltage curves at the phase connections 22, 24 and 26 in a time-resolved manner. In this exemplary embodiment, phase W is initially supplied with a voltage at a time 72. With a time delay, phase winding V is then supplied with a voltage at a time 74. The voltage supply to phase U then begins at a time 76. After a first pulse duration Tu, phase U is disconnected from the voltage signal at a time 78, thus ending pulse Pu. Subsequently, pulse Pv of phase V is ended after a pulse duration Tv at a time 80, and pulse Pw of phase W is ended after a pulse duration Tw at a time 82. This voltage curve is repeated periodically as a pulse sequence for the three phases U, V, W.

[0041] In the third section 84 and the fourth section 86 of the Fig. 5 is a time course of the alternating current I resulting from the PWM control resin the current source 8, for example in the intermediate circuit 12.

[0042] In section 84, the alternating current I res shown for an operating situation in which the phase winding U is flowing with a phase current lu of 4A (amperes), the phase winding V with a phase current Iv of 1A, and the phase winding W with a phase current Iw of 3A. With respect to the directions from and to the star point 28, the phase currents Iv and Iw have a current direction inverted relative to the phase current lu. In other words, the phase currents Iv and Iw are directed opposite to the phase current lu.

[0043] In the period between times 72 and 74, a current of 3A flows through the phase winding W, with an additional current of 1A being added through the phase winding V from time 74 onwards. As a result, the resulting alternating current I resIn the period between times 74 and 76, a current level of 4 A is reached. At time 76, the pulse Pu adds a current which has a direction opposite to the other currents while maintaining the same current amplitude. Thus, for the pulse duration Tu between times 76 and 78, the resulting (total) alternating current I res essentially completely reduced.

[0044] In the period between times 78 and 80, the phase current lu is switched off again through the phase U, so that the alternating current I res in this time period, it again has an amplitude of 4A. At time 80, this amplitude is reduced again to 3A and at time 82, after the termination of the pulse Pw, it is essentially completely reduced.

[0045] As in the Fig. As can be clearly seen in Figure 5, the pulse sequence essentially creates two alternating current components (current blocks) I1 and I2, which repeat periodically during the PWM control. The alternating current component I1 begins at time 72 and ends at time 76, while the alternating current component I2 begins at time 78 and ends at time 82.

[0046] Overall, this results in a high number of switching operations per period of the pulses Pu, Pv, Pw and, due to the strongly varying currents in the intermediate circuit 12, a high alternating current I res , which periodically charges and discharges the intermediate circuit capacitor 14.

[0047] In section 86 of the Fig. Figure 5 shows a situation in which the current directions of the phase currents Iv and Iw are opposite to each other and the current direction in phase U has the same orientation as in phase V. In this case, too, a high number of switching operations with similarly high alternating current components I1 and I2 result.

[0048] Depending on the course of the phase currents lu, Iv, Iw, a different alternating current I res The alternating current I res has a frequency distribution whose main components are particularly at n times the PWM (clock) frequency.

[0049] The following is based on the Fig. 6a and Fig. 6b a method for reducing the alternating current I res in the intermediate circuit 12. For this purpose, the controller 42 uses a modulation method for PWM control, in which the frequency spectrum of the alternating current I resis influenced in such a way that the alternating current component at one frequency, in particular at a main component, is minimized.

[0050] For this purpose, the controller 42 determines, for example by means of Fourier transformation, a frequency spectrum F1(ω), F2(ω) for the alternating current component I1 and I2, respectively, where ω is the angular frequency.

[0051] The frequency spectra F1(ω) and F2(ω) generally have comparable amplitudes, but differ in their respective phase positions. The corresponding frequency spectrum F res (ω) of the resulting alternating current I res Due to the linearity of the Fourier transformation, the sum of the individual frequency spectra F1(ω) and F2(ω) is Fres(ω)=F1(ω)+F2(ω).

[0052] The amplitude of the frequency spectrum F1(ω) is minimal for a specific frequency ω0 when the individual frequency spectra F1(ω) and F2(ω) are shifted in phase oppositely, i.e. by 180°, at the frequency ω0. Mathematically, a change in the phase position results from the shift theorem by multiplying it by a complex phase factor according to the relationship F1′(ω)=e−jωτ1 F1(ω), where j is the imaginary unit and τ1 is a first time period or shift time.

[0053] This means that a shift by a time period τ1 only changes the phase position, but not the amplitude of the frequency spectrum F1(ω). Shifting both frequency spectra F1(ω) and F2(ω) yields: Fres′(ω)=e−jωτ1F1(ω)+e−jωτ2F2(ω), where τ2 describes a time period or shift time for the frequency spectrum F2(ω) of the alternating current component I2.

[0054] By appropriate choice of the time periods τ1 and τ2 it is possible that for the frequency ω0 the frequency spectrum F' res (ω0) is minimized. The frequency ω0 is, in particular, the clock frequency of the PWM control, i.e., the fundamental or repetition frequency of the alternating current components I1 and I2.

[0055] As in the Fig. 6a and Fig. As schematically indicated in Figure 6b, this phase shift is achieved by shifting the alternating current components I1 and I2 relative to one another in time. For this purpose, all phases U, V, W are switched on earlier by a time duration -τ1 during the PWM control. This means that the pulses Pu', Pv', Pw' are generated at a time 72', 74', 76' that is advanced by the time duration -τ1. On the other hand, the phases U, V, W are switched off at a time 78', 80', 82' that is delayed by the time duration τ2. In other words, the pulses Pu', Pv', Pw' are ended later by a time duration τ2. This effectively extends the pulse durations Tu', Tv', Tw' of Pu', Pv', Pw' by a time duration τ1 + τ2. This means that the duty cycle of the PWM control is adjusted during the phase shift.

[0056] In the embodiment of the Fig.6b, the time periods τ1, τ2 are equal in magnitude, which means in particular that the time period -τ1 is equal to the time period τ2. This ensures that the mean value of the resulting alternating current I' res is not significantly changed.

[0057] In a suitable embodiment, the time periods τ1 and τ2 are determined in advance for different frequencies ω0 and stored in a table of the controller 42. Additionally or alternatively, the alternating current components I1 and I2 are detected during operation via the shunt resistor 60, and the time periods τ1 and τ2 are calculated in real time. List of reference symbols 2 machines 4 electric motor 6 power converters 8 Power source 10 energy storage 12 intermediate circuit 12a Outward line 12b Return line 14 DC link capacitor 16 bridge circuit 18 star connection 20 bridge modules 22, 24, 26 phase end 28 Star Point 30 Inductance 32 resistance 34, 36, 38 Voltage drop 40 voltage 42 controllers 44, 46 semiconductor switches 48, 50 Potential connection 52, 54 Control voltage input 56 Resistance 58 Inductance 60 shunt resistance 62, 64, 66 instantaneous value 68 Meter reading Section 70 72, 74, 76 Time 72', 74', 76' Time 78, 80, 82 Time 78', 80', 82' Time 84, 86 Section U, V, W phase / phase winding I U , I V , I W Phase current / output current I E Input current U ZK DC link voltage U G Earth potential I Bat Battery power U BatBattery voltage U U , U V , U W Target voltages P U , P V , P W Pulse signal / pulse P U ', P V ', P W ' Pulse signal / Pulse T U , T V , T W Pulse duration T U ', T V ', T W ' Pulse duration I res , I res ' alternating current I1, I2 alternating current component F1 (ω), F2(ω) frequency spectrum F res (ω), F' res (ω) Frequency spectrum ω, ω0 frequency τ1, τ2 duration / shift time

Claims

[1] Method for operating an electrical machine (2) with a power source (8) and with an electric motor (4) and with a power converter (6) connected therebetween, - at which an input current (I E ) of the current source (8) is converted into a multi-phase output current (lu, Iv, Iw) for the electric motor (4) by means of a pulse-width modulated control of a number of semiconductor switches (44, 46) of the power converter (6), wherein the or each pulse (Pu, Pv, Pw) is generated at a first time (72, 74, 76) and terminated after a pulse duration (Tu, Tv, Tw) at a second time (78, 80, 82), - in which a frequency spectrum (F1(ω), F2(ω)) is determined for each alternating current component (I1, I2) which is generated in an intermediate circuit (12) of the current source (8) during the pulse-width modulated control of the semiconductor switches (44, 46), and - at which the pulse durations (T U ', T V ', T W') of the pulse (P U ', P V ', P W ') of the pulse width modulated control for a specific frequency (ω0) can be set such that the sum of the frequency spectra (F res (ω)) of the alternating current components (I1, I2) becomes minimal at this frequency (ω0). [2] Method according to claim 1, characterized by that to minimize the sum of the frequency spectra (F res (ω))) these are phase-shifted by 180° to each other. [3] Method according to claim 1 or 2, characterized by that the determined frequency (ω0) corresponds to the frequency of a maximum of the or each frequency spectrum (F1(ω), F2(ω)). [4] Method according to claims 1 to 3, characterized by that the or each pulse (P U ', P V ', P W ') is generated earlier than the first time (72, 74, 76) by a first time period (τ1) and / or is terminated later than the second time (78, 80, 82) by a second time period (τ2). [5] Method according to claim 4, characterized by that each pulse (P U ', P V ', P W ') for generating the multiphase output current (I U , I V , I W ) with the same first and second time duration (τ1, τ2) for setting the pulse duration (T U ', T V ', T W '). [6] Method according to claim 4 or 5, characterized by that the first and second time periods (τ1, τ2) are equal in magnitude. [7] Method according to one of claims 4 to 6, characterized by that the first and second time periods (τ1, τ2) are calculated for different specific frequencies (w0) and stored in a table. [8] Electrical machine (2), in particular for a motor vehicle, with a power source (8) and with an electric motor (4) and with an intermediately connected power converter (6) with a controller (42) for carrying out a method according to one of claims 1 to 7.

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