Method for operating an electric motor

Abstract

The invention relates to a method for operating an electric motor (4) by means of pulse width modulation, in which during a period (T Periode ) at least one pulse-width modulated signal pulse (70) at a pulse position (T0) with a pulse duration (T on ), wherein a resulting amplitude spectrum (64) has an amplitude value (A) at a critical frequency (ω kritisch ), wherein at least one time parameter (T Periode , T0, T on ) of the pulse width modulation by a sum of the time parameter (T Periode , T0, T on ) and a correction factor (T korr ), where the correction factor (T korr ) is changed in such a way that a calculated amplitude spectrum (64') at the critical frequency (ω kritisch ) a reduced amplitude value (I Amp ), and wherein the pulse width modulation is carried out with the time parameter (T Periode , T0, T on) and the changed correction factor (T korr ) is carried out.

Description

The invention relates to a method for operating an electric motor by means of pulse width modulation, in which at least one pulse width modulated signal pulse is generated during a period duration. The invention further relates to an electric machine with an electric motor and software on a data carrier.Adjusting systems operated or actuated by electric motor as motor vehicle components, such as, for example, window lifters, seat adjusters, door and sliding roof drives or radiator fan drives, and pumps and interior fans typically have an electric drive with a controlled electric motor. For such electric motor drives, so-called brushless electric motors (brushless direct current motor, BLDC motor) are increasingly frequently used, in which the wear-prone brush elements of a rigid (mechanical) commutator are replaced by an electronic commutation of the motor current.Electromotive drives in motor vehicles are typically supplied by a (high-voltage) battery which serves as an internal energy store of the vehicle. This energy store supplies the electric motor with electrical energy in the form of direct current (or direct voltage) via the on-board power supply system. For the conversion of this direct current into an alternating current suitable for the motor, a power converter (inverter, inverter) is connected between the energy store and the electric motor.The power converter has a bridge circuit which is connected via an electrical intermediate circuit to the direct current or direct voltage supply of the energy store. The motor current is generated by pulse width modulation (PWM) of the semiconductor switches in the bridge circuit, which supplies a polyphase output current (three-phase current).In PWM, the width (duration) of the voltage pulses is varied to control the average voltage and thus the power delivered to the motor. This is done by a predetermined timing scheme in which the semiconductor switches change between conducting and blocking states in a rhythmic pattern, enabling efficient and accurate control of the motor power.As a result of the switching processes of the semiconductor switches during the course of the PWM, pulsed currents are generated in the intermediate circuit or in the on-board power supply system. These currents or current ripple may result in electromagnetic compatibility (EMC) disturbances. These EMC interference manifests itself in undesired electromagnetic interference, which can impair the normal function of other electronic devices or systems in the on-board power supply system.In order to meet EMC requirements imposed on the electric motor or electromotive drive, the current ripple is reduced, for example, by filtering, in particular by means of capacitors and / or inductances. Such filtering thus requires additional components, as a result of which increased costs and an increased installation space requirement arise. Furthermore, it may occur that individual frequencies of the current ripple are not sufficiently attenuated by the filtering. In particular, if the filter has a resonance with the on-board power supply system, the EMC requirements can be exceeded at certain points.The invention is based on the object of specifying a particularly suitable method for operating an electric motor by means of pulse width modulation. In particular, the aim is to reduce effort and costs for complying with EMC requirements by means of a targeted control of the pulse width modulation. The invention is furthermore based on the object of specifying a particularly suitable electric machine for carrying out the method and a particularly suitable software on a data carrier.The method according to the invention is provided for operating an electric motor by means of a PWM and is suitable and configured for this purpose. The electric motor is preferably part of a motor vehicle, and is connected to an intermediate circuit of a (vehicle) motor vehicle.During the course of PWM, at least one pulse-width-modulated signal pulse (PWM pulse) is generated during a period duration at a pulse position having a pulse duration.A "period duration" is to be understood here and in the following in particular as the time period of a complete PWM cycle repetition.A "pulse duration" (pulse width, pulse length, pulse width) is to be understood here and below in particular as a time period in which the PWM pulse is kept at a high (or low) level within the period duration.A "pulse position" is to be understood here and in the following in particular as the specific placement of the PWM pulse within the period duration, i.e. a temporal position of the PWM pulse with respect to the beginning of the period duration. The pulse position relates here, for example, to the center of the PWM pulse. Alternatively, the pulse position can also relate to a start or an end of the PWM pulse.An amplitude spectrum resulting from the PWM, in particular the amplitude spectrum of the resulting EMC interference or current ripple in an intermediate circuit or on-board electrical system connected to the electric motor, has an amplitude value at at least one critical frequency which does not meet predefined EMC requirements. This means that, for example, the amplitude spectrum of the EMC disturbances in the intermediate circuit or on-board power supply system caused by the PWM is monitored and detected, and that the method is started, for example, if the amplitude value does not meet the predefined EMC requirements.An "amplitude spectrum" is to be understood here and in the following in particular as the magnitude of a frequency spectrum.A "critical frequency" is to be understood here and below in particular as a specific frequency or frequency range in the amplitude spectrum of the current ripple, at which or within the amplitude values of the EMC interference occur, which exceed predefined EMC requirements or at least are to be regarded as critical in this respect. The critical frequency depends on various factors, such as the structure of the PWM signals, the type and nature of the electronic components involved and the specific application conditions.According to the method, at least one time parameter of the pulse width modulation is replaced by a sum of the time parameter and a correction factor. The time parameter is thus supplemented or modified with a correction factor.A "time parameter" is to be understood here and in the following in particular as a quantifiable time measure which characterizes the essential aspects of the PWM process. These time parameters include, in particular, the period duration, the pulse duration and the pulse position within the period duration.A "correction factor" is understood to mean a time variable which can be adjusted or changed in the course of the method and is added to the time parameter, and with which the pulse width modulation or the amplitude spectrum resulting therefrom is influenced.With regard to the method according to the invention, the time parameter is a fixed time variable, which is supplemented by the correction factor as a variable time variable. In this case, the correction factor is changed in such a way that a calculated amplitude spectrum at the critical frequency has a reduced amplitude value. Subsequently, the PWM is performed with the time parameter and the correction factor. Thus, a particularly suitable method for operating an electric motor by means of a PWM is realized. In particular, the EMC behavior of the electric motor is thus improved without the need for additional electrical components that are cost-intensive and space-intensive.The invention is based on the finding that the current ripple of the EMC interference has an associated amplitude spectrum which is due to the PWM or which can be changed or influenced by the PWM. According to the invention, no energy is taken from the amplitude spectrum by the correction factor, the spectrum or the peak characterized by the critical frequency and the amplitude value is only (at least partially) frequency-shifted by the correction factor. In particular, the peak is shifted towards frequencies which are not critical with regard to the EMC requirements. According to the invention, the energy of the current ripple is therefore shifted, wherein the energy of the EMC can also change in sum as a result together with filtering.Preferably, an estimated amplitude spectrum is calculated for a number of correction factors and it is checked whether the amplitude value at the critical frequency is reduced in comparison to the current amplitude spectrum and / or whether the amplitude value reaches or falls below a predefined or stored threshold value (for example an EMC requirement). The correction factor is preferably set such that the amplitude value becomes minimum at the critical frequency or at least falls below the threshold value.The correction factor is, for example, one or two orders of magnitude smaller than the associated time parameter.A single (rectangular) PWM pulse can be described, for example, by the following frequency spectrum:Here, F Pulse is the frequency spectrum, ω is the (circular) frequency, I Amp is the current level or current amplitude during one pulse (PWM amplitude) and corresponds to the phase current, T on is the pulse duration, T 0 is the pulse position, and j is the imaginary unit (√-1).A pulse sequence is the sum of individual pulses. This applies both to the frequency range and in the time range, so that the following results as the frequency spectrum F(ω) for n individual pulses:Here, i is the running index via the sum of the n individual pulses. In the case of equidistant pulses, T 0( i)=i×T Pulsabstand. applies here, for example. If only one PWM pulse is generated in one pulse period, T Pulsabstand is equal to the period duration (T Peri-ode). Each period can have a different period duration. Preferably, all phases or two phases have a PWM with different pulse width and pulse height per period.The amplitude spectrum is the magnitude of the frequency spectrum |F(ω)|. The concept of the invention is to replace one of the time parameters T by T+T korr wherein T korr is the correction factor, and to vary the correction factor T korr in such a way that the magnitude of the frequency spectrum at the critical frequency (|F(ω kritisch)|), i.e. the amplitude value, is minimal or at least reduced.In one conceivable embodiment, the pulse position (T 0), the pulse duration (T on) or the period duration (T Periode) is used as the time parameter.For example, the pulse duration T on(i) is replaced in the course of the method by T on(i)+ T on-korr(i) wherein T onkorr(i) is the correction factor for the pulse duration at the running index i. If the pulse duration (T on(i)) is used as a time parameter, it is accepted that an average phase or motor voltage for the electric motor is briefly disturbed.A further possibility is to vary the period duration (T Periode) of the PWM in a suitable manner. If T Periode is replaced by T Periode+ T Periodekorr where T Periodekorr is the correction factor for the period duration, then T Periodekorr can be selected such that |F(ω kritisch)| becomes minimum. In this case, the period duration T on is preferably varied in such a way that an average phase voltage before and after the change in the correction factor is substantially identical. In other words, the period duration T on is adjusted accordingly such that a desired average phase voltage for the electric motor is maintained.In the following, without limiting generality, the variant in which the pulse position T 0(i) is varied is explained in more detail. For this purpose, T 0(i) is replaced by T 0(i)+ T 0korr(i) where T 0korr(i) is the correction factor for the pulse position for the running index i. The corresponding frequency spectrum is thusAccording to the method, T 0korr(i) is selected or set in such a way that |F(ω kritisch)| is minimal or at least reduced.In order to reduce the computing effort, when performing the method, preferably all past and next signal pulses, i.e. the signal pulse to be generated next, are taken into account. This means that the sum of the frequency spectrum of the last signal pulse (n-1) and the frequency spectrum for the arriving signal pulse (n) is calculated, so that the following applies as an approximation. The correction factor is preferably always calculated only for the next period consisting of up to three individual pulses.In an advantageous development, the correction factor is limited by an upper limit (T korrMax) during the change. In other words, a maximum value for the correction factor is limited so that |T korr(i)| < T korrMax. Such a limitation ensures that a PWM control or PWM regulation for the motor operation (for example by means of field-oriented regulation) is disturbed or influenced as little as possible. A quasi-continuous value range is therefore specified for the correction factor, and that calculated amplitude spectrum with the lowest amplitude value at the critical frequency is sought for correction factors within this value range. For example, the upper limit or the maximum value is dimensioned to be approximately 1 μs (microsecond). Alternatively, the upper limit can be dimensioned to approximately 1% of the time parameter to be corrected, so that the correction factor is varied, for example, in a range ±1% of the time parameter.In a particularly simple and computation-reduced configuration, a value range with a number of discrete correction values is stored for the correction factor. Instead of a continuous value range, a set of stored correction values or correction times is therefore predefined. Preferably, at least two, for example three, correction values are stored as a value range, e.g. Tcorr(n) ∈{-1 μs, 0 μs, 1 μs}. The amplitude spectrum is calculated for each correction value, wherein that correction value is selected as the value for the correction factor for carrying out the PWM at which the calculated amplitude spectrum has the lowest or minimum amplitude value at the critical frequency. This ensures a particularly simple implementation of the method.In one conceivable embodiment, the resulting amplitude spectrum is measured. In particular, the amplitude (I Amp) of the amplitude spectrum is measured or determined at the critical frequency. With regard to the above-described approximation for determining the frequency or amplitude spectrum, the amplitude for the frequency spectrum of the preceding signal pulse (I Amp(n-1)) is measurable. In particular, the frequency or amplitude spectrum of the preceding signal pulse can be measured, and the value for the amplitude can be derived therefrom. For this purpose, the current ripple or EMC interference is measured, for example, by an averaging measurement (average measurement) over several milliseconds (ms) or by a peak measurement (peak measurement) in the range of a few milliseconds and is then converted into the frequency or amplitude spectrum by a Fourier transformation. During active deployment, preferably only the value for the critical frequency(s) is calculated.In an expedient development, the measured amplitude (I Amp(n-1)) is used in the calculation of the frequency or amplitude spectrum, so that the most accurate possible prediction of the amplitude spectrum is made possible.The amplitude of the future current ripple (I Amp(n)) is unknown at the time of calculation. As an approximation, in the simplest case, for example, the amplitude for the past signal pulse can be estimated as a value for the arriving amplitude (I Amp(n)= I Amp(n-1)). It is likewise possible to estimate it on the basis of the current components Id, Iq of a field-oriented motor control with corresponding rotation. Such estimates are subject to a certain error, but this may already be a sufficiently good approximation to sufficiently reduce the amplitude at the critical frequency to meet a predetermined EMC requirement.Preferably, the amplitude of the future current ripple (I Amp(n)) is estimated in the calculation of the amplitude spectrum. Here and in the following, "estimation" or "estimation" is to be understood as meaning an approximate determination of the future amplitude, in particular by evaluating the measured frequency spectrum for the past signal pulse, for example by eye gaze, precharacterized measurements, stored tables or characteristic curves, or by means of statistical-mathematical methods. In the case of static motor operation, the phase voltages are approximately sinusoidal, so that a comparatively simple estimation is possible. Alternatively, a motor model for the electric motor may be used to estimate or predict the amplitude.In a preferred embodiment, the method is carried out for more than one critical frequency. This means that the amplitude values are reduced at a plurality of critical frequencies. For example, the sum of the amplitude spectra for the critical frequencies is minimized in this case, i.e. min(|F(ωkri-table1)|+|F(ω kritisch2)|). Alternatively, a weighted sum can also be minimized, i.e. min(g1×|F(ω kritisch1)| + g2×|F(ω kritisch2)|), wherein g1and g2are weighting factors. The weighting can be effected, for example, on the basis of the respective amplitude values at the critical frequencies, that is to say that the frequency spectrum with a higher amplitude value is weighted more strongly at the critical frequency than the frequency spectrum with a lower amplitude value. Other cost functions are likewise conceivable.The electric machine according to the invention is provided in particular as an electric motor drive in a motor vehicle and is suitable and configured for this purpose. The electric machine has an electric motor which is preferably designed without brushes with a stator and with a rotor rotatably mounted therein. The stator has a number of phase windings which are led on the one hand to the power converter and on the other hand are connected in a star connection, for example, in a common connection point (star point).The electric machine further comprises a bridge circuit connected or coupled to the electric motor and a controller, i.e. a control device. The bridge circuit is preferably part of a power converter, in particular an inverter. The controller is part of the power converter, for example. The controller is generally suitable and configured for carrying out the method described above using programming and / or circuitry. The controller is thus specifically configured to add a correction factor to a time parameter of the PWM during operation and to set it in such a way that an amplitude value of an amplitude spectrum at a critical frequency is reduced.In a preferred embodiment, the controller is formed at least in the core by a microcontroller having a processor and a data memory, in which the functionality for carrying out the method according to the invention is implemented by programming in the form of operating software (firmware), with the result that the method is automatically carried out-if appropriate in interaction with a user-when the operating software is executed in the microcontroller.The controller may alternatively also be formed, within the scope of the invention, 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 circuit-related means.The electric machine operated with the method thus has improved behavior with respect to EMC radiation and with respect to noise generation arising as a result of the switching processes of the semiconductor switches. The method according to the invention is suitable and set up in particular for use in speed-controlled systems. In principle, however, the application is not restricted to the automobile sector.An additional or further aspect of the invention provides software on a medium or data carrier for carrying out or executing the method described above. This means that the software is stored on a data carrier and is provided for carrying out the method described above, and is suitable and configured for this purpose. This realizes particularly suitable software for the operation of an electric motor, with which the functionality for carrying out the method according to the invention is implemented by program technology. The software is thus in particular operating software (firmware), wherein the data carrier is, for example, a data memory of the controller.An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. In the drawings, schematic and simplified representations show: FIG. 1 shows an electric machine with a power source and with an electric motor and with a power converter connected therebetween, FIG. 2 shows three phase windings of a three-phase electric motor of the machine in a star connection, FIG. 3 shows a bridge module of a bridge circuit of the power converter for driving a phase winding of the electric motor, FIG. 4 is an equivalent circuit diagram for the current source, FIG. 5 shows a block diagram for pulse width modulation, and FIG. 6 shows a flow chart for a method for operating the electric motor.The invention is explained below by way of example with reference to a drive with a B6 circuit. However, the invention can be applied to other arrangements.Parts and sizes corresponding to one another are always provided with the same reference numerals in all figures.FIG. 1 shows an electric machine 2 for an electric motor drive of a vehicle, not shown in detail, for example of a motor vehicle or an electrically driven or drivable bicycle (e-bike). The machine 2 comprises a three-phase brushless electric motor 4, which is connected to a current source (voltage supply) 8 by means of a power converter (converter, inverter) 6. In this exemplary embodiment, the power source 8 comprises an energy store in the vehicle in the form of a (motor vehicle) battery 10, and a (DC voltage) power supply 12 connected thereto as part of an on-board power supply system, which extends at least partially into the power converter 6.The intermediate circuit 12 is substantially formed by a forward line 12 aand a return line 12 b, by means of which the power converter 6 is connected to the battery 10. The lines 12 aand 12 bare at least partially led into the power converter 6, in which an intermediate circuit capacitor 14 and a bridge circuit 16 are connected between them.During operation of the machine 2, an input current I E supplied to the bridge circuit 16 is converted into a three-phase output current (motor current, three-phase current) I U, Iv, Iw for the three phases U, V, W of the electric motor 4. The output currents lu, Iv, Iw, which are also referred to below as phase currents, are conducted to the corresponding phase (windings) U, V, W (FIG. 2 ) of a stator, which is not shown in more detail.FIG. 2 shows a star circuit 18 of the three phase windings U, V, W. The phase windings U, V and W are each led by one (phase) end 22, 24, 26 to a respective bridge module 20 (FIG. 3 ) of the bridge circuit 16, and are connected to one another by the respective opposite end in a star point 28 as a common connection terminal. In the illustration of FIG. 2, the phase windings U, V and W are each shown by means of an equivalent circuit diagram in the form of an inductance 30 and an ohmic resistor 32 and a respective voltage drop 34, 36, 38.The voltage 34, 36, 38 dropping across the phase winding U, V, W in each case is schematically represented by arrows and results from the sum of the voltage drops across the inductance 30 and the ohmic resistor 32 and the induced voltage 40.The star circuit 18 is controlled by means of the bridge circuit 16. In this embodiment, during operation, switching is carried out at a high switching frequency to each of the phase windings U, V, W between a high (DC) voltage level of the feed line 12 aand a low voltage level of the return line 12 b.The high voltage level is in particular an intermediate circuit voltage U ZK of the intermediate circuit 12, wherein the low voltage level is preferably a ground potential U G. This clocked actuation is implemented as a PWM actuation--shown by arrows in FIG. 1--by a controller 42, with which a control and / or regulation of the rotational speed, the power and the direction of rotation of the electric motor 4 is possible.The bridge modules 20 each comprise two semiconductor switches 44 and 46, which are illustrated in FIG. 2 merely schematically and by way of example for the phase W. The bridge module 20 is connected on the one hand with a potential connection 48 to the supply line 12 aand thus to the intermediate circuit voltage U ZK. On the other hand, the bridge module 20 is connected in G with a second potential connection 50 to the return line 12 band thus to the ground potential U. The respective phase end 22, 24, 26 of the phase U, V, W can be connected either to the intermediate circuit voltage U ZK or to the ground potential U G via the semiconductor switches 44, 46.If the semiconductor switch 44 is closed (conductive) and the semiconductor switch 46 is opened (non-conductive, blocking), the phase end 22, 24, 26 is connected to the potential of the intermediate circuit voltage U ZK. Accordingly, upon opening of the semiconductor switch 44 and closing of the semiconductor switch 46, the phase U, V, W is in G contact with the ground potential U. As a result, it is possible by means of the PWM control to apply two different voltage levels to each phase winding U, V, W.In FIG. 3, a single bridge module 20 is shown in simplified form. In this exemplary embodiment, the semiconductor switches 44 and 46 are realized as MOSFETs (metal oxide semiconductor field-effect transistors), which switch in each case in a clocked manner between a switched-on state and a blocking state by means of the PWM control. For this purpose, the respective gate connections are led to corresponding control voltage inputs 52, 54, by means of which the signals of the PWM control of the controller 42 are transmitted.FIG. 4 shows an equivalent circuit diagram for the current source 8, During operation, the battery 10 generates a battery voltage U Bat and a corresponding battery current I Bat for operating the converter 6, In FIG. 4, the internal resistance of the battery 10 is represented as an ohmic resistor 56 and a self-inductance of the battery 10 as an inductance 58. A shunt resistor 60 is connected in the return line 12 b.Depending on the switching states of the (power) semiconductor switches 44, 46, the phase current I U, I V, I W flows via the shunt resistor 60. With measurements and the knowledge of the switching states of the semiconductor switches 44, 46, the phase currents lu, Iv, Iw are reconstructed by the controller 42. Other measurement methods can also be used to determine the motor currents (e.g. direct phase current measurement). Together with the measured and / or calculated phase voltages (U U, U V, U W) the controller 42 has the phase voltages (U U, U V, U W) and the phase currents I U, I V, I W available to it.In the exemplary embodiment of FIG. 1, the motor current is detected by means of a current meter 62, for example by means of the shunt resistor 60, and is fed to the controller 42. The controller 42 controls and / or regulates the motor operation on the basis of motor variables, in particular on the basis of the detected phase currents I U, I V, I W and the calculated phase voltages U U, U V, U W, and on the basis of other variables (e.g. motor resistance, motor inductance, duty cycle of the PWM voltage). For example, a field-oriented regulation for the electric motor 4 is realized in this case.Owing to the switching processes of the semiconductor switches 44, 46 during the PWM, pulsed currents are generated in the intermediate circuit 12 or in the on-board power supply system.These currents or current ripple may result in electromagnetic compatibility (EMC) disturbances.In the following, a method for operating the electric motor 4 is described by way of example with reference to FIG. 5 and FIG. 6, which method is provided and configured to reduce such EMC disturbances.FIG. 5 shows, in a schematic and simplified illustration, two sections 66 and 68 arranged vertically one above the other, which each show a time diagram of the PWM (left) and an amplitude spectrum 64 (right) horizontally next to one another.The time diagrams show, for example, two periods of the PWM, one period being, for example, 50 μs long. The time t is plotted horizontally, that is to say on the X or abscissa axis, wherein a PWM amplitude, for example in volts, is plotted vertically, that is to say along a Y or ordinate axis not shown in more detail.In the case of the amplitude spectra 64, a (circular) frequency ω is plotted along the X or abscissa axis, the amplitude being plotted along a Y or ordinate axis, not shown in more detail.In the course of the pulse width modulation PWM, for example, at least one signal pulse (PWM pulse) 70 per period is generated for each phase U, V, W. The PWM is characterized here by time parameters, in particular by the period duration T Periode, the pulse duration T on and the pulse position T 0. The pulse duration T on is substantially the switch-on time of the semiconductor switches 44, 46, i.e. how long the respective semiconductor switches 44, 46 are switched on. The pulse duration T on is centered here about the pulse position T 0 within the period duration T Periode.According to the method, at least one of the time parameters is supplemented by a correction factor (correction time) T korr in order thus to bring about a frequency shift of the amplitude spectrum 64 to EMC non-critical frequencies by means of the changed PWM. In the following, without limiting generality, an execution of the method is described, in which the pulse position T 0 is replaced by a sum of the pulse position T 0 and the correction factor T korr.In a threshold value comparison 72, the controller 42 first checks whether a current amplitude spectrum 64 of the EMC disturbances or of the current ripple meets a predefined or stored EMC requirement. The EMC requirement is characterized, for example, in that an amplitude threshold value S must not be exceeded in a specific frequency range in the amplitude spectrum 64.The controller 42 thus checks in the threshold value comparison 72 whether the amplitude spectrum 64 reaches or exceeds the amplitude threshold value S. For this purpose, for example, an average or peak measurement is carried out by means of the ammeter 62, and the amplitude spectrum 64 is determined from the current ripple on the basis of a Fourier transformation, in particular by means of a frequency spectrum, for example by means of FFT (Fast Fourier Transform). When the amplitude threshold value S is reached or exceeded, the method is started. The corresponding frequency value of the amplitude spectrum 64 for the amplitude value A at which the amplitude threshold value S is reached or exceeded is referred to below as critical frequency ω kritisch. Alternatively, the method can be carried out or started during the entire engine operation, independently of the amplitude value A.The critical frequency ω kritisch can be determined here by means of special devices independently of the method according to the invention. For example, a resonant frequency of the electric machine 2 is determined.Section 66 of FIG. 5 shows an operating situation, for example, in which amplitude spectrum 64 has a frequency peak whose amplitude value A is greater than amplitude threshold value S. In such an operating situation, the controller 42 starts the method described below.In a first method step 74, the pulse position T 0 is replaced by the sum of the pulse position T 0 and the correction factor T korr. Section 68 of FIG. 5 shows a corresponding PWM pulse sequence in which signal pulse 70 generated next is shifted by T korr in period duration T Periode from pulse position T 0.In a subsequent second method step 76, an amplitude spectrum 64' is calculated, which is presumably obtained on the basis of the changed PWM time parameter (T 0+ T korr) in the case of the EMC disturbance.The amplitude spectra 64' is calculated as the magnitude of the frequency spectrum |F(ω)|. For the PWM pulse sequence, the frequency spectrum F(ω) is approximately described by the following formulaDuring the calculation, the frequency spectrum F(ω) is essentially composed of the frequency spectrum F n-1( ω) of the preceding pulse and the frequency spectrum F n( ω) for the coming (future) pulse, the frequency spectrum F n( ω) being provided with the correction factor T korr. Preferably, the frequency component or the frequency spectrum F(ω) is calculated only for the critical frequency ω kritisch.The frequency spectrum F n-1( ω) is in particular the measured amplitude spectrum 64 from the threshold value comparison 72. For the frequency spectrum F n( ω), the amplitude I Amp is not known during the calculation, and is estimated for the calculation, for example, by means of a motor model for the electric motor 4 and / or the intermediate circuit 12 (or on-board power supply system). Alternatively, the amplitude I Amp determined from the measured frequency spectrum F n-1( ω) is used approximately for the calculation of the frequency spectrum F n( ω).Method steps 74 and 76 are preferably repeated for a number of different correction factors T korr. Thus, a plurality of amplitude spectra 64' are calculated for different correction factors T korr. For this purpose, the value of the correction factor Tcorr is changed and the corresponding amplitude spectrum 64' is then calculated.The correction factor T korr can be limited by an upper limit (T korr-Max) during the change. In other words, it is possible that a maximum value limits the correction factor T korr so that |T korr| < T korrMax. Such a limitation ensures that a PWM control or PWM regulation for the motor operation (for example by means of field-oriented regulation) is disturbed or influenced as little as possible. For example, the upper limit T korrMax is dimensioned to be approximately 1 μs (microsecond). Alternatively, the upper limit can be dimensioned to approximately 1% of the pulse position T 0 so that the correction factor T korr is varied, for example, in a range ±1% around the pulse position T 0.Alternatively, a set or number of correction values T korr can also be stored. Preferably, at least two, for example three, correction values are stored as a value range, e.g. T korr ∈{-1 μs, 0 μs, 1 μs}.In a subsequent method step 78, the calculated amplitude spectra 64' are compared with one another, and that correction factor T korr is selected at which the magnitude of the respective amplitude spectrum 64' at the critical frequency is the smallest at the critical frequency ω kritisch. For a set MT korr of different correction factors T korr is thus determined.In method step 80, during the PWM, the next signal pulse 70 is generated with the determined correction factor T korr. As is shown, for example, in the amplitude spectrum 64' in section 68 of FIG. 5, no energy is taken from the amplitude spectrum 64 by the correction factor T korr and the spectrum or the peak characterized by the critical frequency ω kritisch and the amplitude value A is frequency-shifted by the correction factor T korr( at least partially) to a frequency ω neu.The method described above can also be carried out for a plurality of critical frequencies. For example, the sum of the amplitude spectra for the critical frequencies is minimized in this case. At two critical frequencies ω kritisch1 and ω kritisch2 it is thus determined in method step 78, for example.The claimed invention is not limited to the above-described embodiments. Rather, other variants of the invention can also be derived herefrom by the person skilled in the art within the scope of the disclosed claims without departing from the subject matter of the claimed invention. In particular, all individual features described in connection with the various exemplary embodiments can also be combined in another manner within the scope of the disclosed claims without departing from the subject matter of the claimed invention.List of reference characters2 Electrical machine 4 Electric motor 6 Power converter 8 Current source 10 Battery 12 Intermediate circuit 12 a Hin line 12 b Rückleitung line 14 Intermediate circuit capacitor 16 Bridge circuit 18 Star circuit 20 Bridge module 22, 24, 26 Phase end 28 Star point 30 Inductance 32 Resistance 34, 36, 38 Voltage drop 40 Voltage 42 Controller 44, 46 Semiconductor switch 48, 50 Potential terminal 52, 54 Control voltage inputs 56 Resistance 58 Inductance 60 Shunt resistor 62 Current meter 64, 64' Amplitude spectrum 66, 68 Section 70 Signal pulse 72 Threshold value comparison 74, 76, 78, 80 Method step I E Input current U, V, W Phase lu, Iv, Iw phase current U ZK intermediate circuit voltage U G ground potential U Bat battery voltage I Bat battery current S amplitude threshold value A amplitude value I Amp amplitude ω kritisch frequency T periode period duration T on pulse duration T 0 pulse position T korr correction factor t time ω frequency ωneufrequency

Claims

Method for operating an electric motor (4) by means of pulse width modulation, in which during a period duration (T Periode) at least one pulse width modulated signal pulse (70) is generated at a pulse position (T 0) having a pulse duration (T on) wherein a resulting amplitude spectrum (64) has an amplitude value (A) at a critical frequency (ω kritisch) - wherein at least one time parameter (T Periode, T 0, T on) of the pulse width modulation is determined by a sum of the time parameter (T Periode, T 0, T on) and a correction factor (T korr) - wherein the correction factor (T korr) is changed in such a way that a calculated amplitude spectrum (64') at the critical frequency (ω kritisch) has a reduced amplitude value (I Amp) and - wherein the pulse width modulation is carried out with the time parameter (T Periode, T 0, T on) and the changed correction factor (T korr).Method according to Claim 1, characterized in that the pulse position (T 0), the pulse duration (T on) or the period duration (T Periode) is used as the time parameter.Method according to Claim 2, characterized in that, if the period duration (T Periode) is used as a time parameter, the pulse duration (T on) is varied in such a way that an average phase voltage for the signal pulse (70) is substantially identical before and after the change in the correction factor (T korr).Method according to one of Claims 1 to 3, characterized in that the correction factor (T korr) is limited by an upper limit and / or a lower limit during the change.Method according to one of Claims 1 to 4, characterized in that a value range with a number of discrete correction values is stored for the correction factor (T korr) wherein the amplitude spectrum (64') is calculated for each correction value, and that correction value is selected as the value for the correction factor (T korr) at which the calculated amplitude spectrum (64') has the minimum amplitude value (A) at the critical frequency (ω kritisch).Method according to one of Claims 1 to 5, characterized in that the amplitude spectrum (64) is measured.Method according to Claim 6, characterized in that an amplitude (I Amp) for the calculation of a future amplitude spectrum (64') is determined from the measured amplitude spectrum (64).Method according to one of Claims 1 to 7, characterized in that an amplitude (I Amp) is estimated during the calculation of the amplitude spectrum (64').Method according to one of claims 1 to 8, characterised in that the method is carried out for more than one critical frequency (ω kritisch).Electric machine (2), having an electric motor (4) and a bridge circuit (16), and also a controller (42) which actuates the bridge circuit (16), for carrying out a method according to one of Claims 1 to 9.Software on a data carrier for carrying out a method according to one of Claims 1 to 9, when the software runs on a computer.

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