DE102022112596A1 - Method for controlling a multi-phase electric motor with coupled inductors, control circuit and an electric motor having the same - Google Patents

Abstract

In various embodiments, a method for controlling a multi-phase electric motor with coupled inductors by means of a control circuit is provided, the control circuit having a first clocked converter and at least a second clocked converter for generating pulse width modulated voltage signals, an inductance being coupled to each of the output connections of the converters and the inductors coupled to output terminals of the same name of the converters are inductively coupled to one another in pairs, and has a control circuit for controlling the converters, the method comprising: providing a first carrier signal in order to provide the first switching signal on its basis by means of a carrier-based modulation method; Providing a second carrier signal in order to provide the second switching signal based on it using a carrier-based modulation method; and setting a phase shift between the first carrier signal and the second carrier signal or between a reference signal used as part of the carrier-based modulation method and the carrier signals depending on an operating point of the electric motor.

Description

The present invention relates to a method for controlling a multi-phase electric motor with coupled inductors by means of a suitably configured control circuit. The present invention further relates to a control circuit for a multi-phase electric motor with coupled inductors and to such an electric motor in combination with the control circuit.

Nowadays, the topic of electromobility is becoming increasingly important and vehicles equipped with an electric motor are gaining market share. Electric motors of traction drives are currently usually designed as three-phase with a concentrated winding, diameter winding or a stretched winding. The electric motor is powered by the energy of the traction battery, whereby voltage-based inverters (voltage source inverters, VSIs) are usually used as converters to convert the voltage appropriately and to control or regulate the electric motor accordingly. For this purpose, a clocked voltage signal is provided at the outputs of the converter, which results in an approximately sinusoidal current flow due to the large inductances of the electric motor. The clocked converters used work according to the principle of pulse width modulation (PWM) or pulse amplitude modulation (PAM), the former being more common in electric drives.

This converter typically includes two power semiconductor switches per phase (half bridge), for example IGBTs, Si-Mosfets, SiC-Mosfets, or GaN transistors, and a filter capacitor in the intermediate circuit. The inverter converts the battery voltage through n-half bridges into n-phase alternating voltages according to a selected pulse width modulation (PWM) scheme, e.g. using space vector modulation. The alternating voltages in turn generate the necessary motor current per phase in order to obtain the desired motor torque M and the target rotational speed ω.

The motor current is induced by a fundamental voltage, which generates the rotating field and is responsible for torque generation. However, by providing the voltage using clocked converters, undesirable harmonic voltages arise, which superimpose the fundamental voltage. The harmonic voltages in the conductor voltages arise, for example, in the sidebands with the clock frequency and multiples of the clock frequency of the converter, which provides the voltage pulses. The clock frequency usually corresponds to the frequency of the triangular carrier signal used in carrier-based modulation. By Fourier transforming the pulsed voltage, these unwanted components can be determined in terms of their amplitude and phase. The amplitudes of the harmonic voltages depend in particular on the intermediate circuit voltage and the degree of modulation (modulation level) of the converter.

In general, voltage components at harmonic frequencies in the sidebands are undesirable because they usually lead to losses in the machine. The harmonic voltages cause harmonic currents, which are only limited by the effective resistances and the effective harmonic reactances and therefore depend on the design of the electric motor. The harmonic currents lead to additional losses both in the winding (current heat losses) and in the laminated core (iron losses), which in turn cause an undesirable increase in the operating temperature of the electric motor. There are also losses in the permanent magnets in the rotor (eddy current losses). These additional losses can significantly reduce the continuous power in the rotor-critical system, i.e. a system operating at the limit temperature, and also lead to a reduction in the range in the battery-powered vehicle. If necessary, disruptive noises can also be excited by the harmonic currents. In summary, the undesirable effects of harmonic voltages lead, on the one hand, to a reduction in the life expectancy of electric motors and, on the other hand, to a reduction in power during operation.

The conventional three-phase supply of the electric motor offers hardly any opportunity to effectively suppress the harmonic fields caused by the converter supply without significantly influencing the machine behavior. For example, you can try to reduce the unwanted harmonic component of the voltage by using suitable filters (e.g. low-pass filters). However, these filters usually have to be large in size and are therefore only suitable for mobile applications to a limited extent - which is why attempts to reduce these signals have so far been avoided. The adverse consequences of harmonic currents that then occur, such as losses in the permanent magnets, can be reduced by secondary measures such as magnet segmentation. However, these measures are complex and expensive.

Despite the challenges described above, three-phase electric motors are widely used. The use of electric motors with more than three phases can offer numerous advantages over them, such as a lower susceptibility to the failure of individual phases and the possibility of reducing unwanted vibrations in the machine. For example, if an electrical machine is operated with six phases, it is possible to operate it with two conventional converters for three-phase machines.

Against this background, the object of the present invention can be seen as providing a method for controlling or operating a multi-phase electric motor, the operation of which can be optimized with regard to the disadvantageous consequences caused by harmonic voltages.

This object is achieved by means of a method for controlling a multi-phase electric motor with coupled inductances by means of a suitably configured control circuit, further by means of the control circuit for a multi-phase electric motor with coupled inductances and such an electric motor with the control circuit. Further preferred embodiments of the invention can be found in the dependent claims.

According to the invention, a method for driving a multi-phase electric motor with coupled inductors is provided, the driving being carried out by means of a driving circuit. Here, the control circuit has a first clocked inverter and at least one second clocked inverter, the first inverter and the at least second inverter having a predetermined number of output connections and being set up to send a first pulse width modulated voltage signal or to provide a second pulse width modulated voltage signal, an inductance being coupled to each of the output connections and the inductances coupled to output connections of the same name of the converter being inductively coupled to one another in pairs. As is well known from the prior art, the fundamental frequency components of the voltage signals provided at the output connections of a clocked converter are out of phase with one another, by 120° for a three-phase winding of an electric motor.

The control circuit further has a control circuit which is coupled to the first converter and to the at least second converter and is set up to provide a first switching signal for the first converter and a second switching signal for the second converter.

The method according to the invention, which is provided on the basis of the control circuit, comprises providing a first carrier signal in order to provide the first switching signal on its basis using a carrier-based modulation method, and, analogously, providing a second carrier signal in order to provide the second on its basis using a carrier-based modulation method To provide switching signal. Furthermore, the method according to the invention includes setting a phase shift between the first carrier signal and the second carrier signal or between a reference signal used as part of the carrier-based modulation method and the carrier signals - depending on an operating point of the electric motor. For the following description, reference will only be made to the phase shift between the carrier signals.

The operating point of the electric motor can be determined by at least one operating parameter, but is usually specified by an n-tuple, which is characterized by n operating parameters. For example, the operating parameters can be the speed of the electric motor and the torque generated by it.

The method according to the invention can be used, for example, in a six-phase electric motor. Each clocked inverter can have three output connections and a phase can be provided at each, with the fundamental oscillations of the phases being phase-shifted in pairs by 120° relative to one another. The control circuit can therefore be designed in particular for a six-phase electrical machine which has two identical windings. Two standard 3-phase inverters can be used for such a 2x3-phase electrical machine. The two inverters can be designed for half the maximum current (compared to a conventional 3-phase system). At the same time, the number of turns per winding system can be adjusted so that the flow and the fundamental oscillation flux density remain unaffected, i.e. correspond to the flow or the fundamental oscillation flux density of a conventional 3-phase system. However, it should be noted that the control circuit according to the invention can have, in addition to six phases, multiples thereof in order to be designed for operation with an electric motor, which correspondingly has six phases or multiples thereof.

In the method according to the invention, the phase shift between the carrier signals is set with respect to each group (each pair, if two multi-phase subsystems are used, for example a 2x3-phase system) of corresponding pulse width modulated voltage signals of the converters, ie with respect to the carrier signals for generating those pulse width modulated voltage signals, which would be in phase or antiphase without the phase shift added or set as part of the method according to the invention - depending on whether the corresponding switches in the converters are operated in phase or antiphase. The carrier signal can be, for example, a triangle signal.

The converters can be converters known from the prior art, which are usually connected between the traction battery and the electric motor and essentially correspond to an inverter. The at least two inverters used can be essentially the same in terms of the components used and their specifications, so that the same input parameters lead to the same output variables (within the manufacturing tolerance). To convert the voltage, the converters can be operated according to a modulation method, for example according to pulse width modulation (PWM). The undershoot method can be used here, in which a reference signal is compared with a carrier signal (e.g. triangular signal) and if the reference signal falls below or exceeds the bridge switches in the converter are opened or closed.

To implement the method according to the invention, the converters can be suitably controlled by means of the control circuit, whereby the operation of an electric motor connected to the converter is regulated. For this purpose, the control circuit can be coupled to control inputs of the first and second converters using appropriate control lines. The phase shift between the carrier signals of the first and at least the second converter can be adjusted depending on the operating point of the electric motor in such a way that their attenuation is effected by a mutually compensating superimposition of harmonics. The compensating superposition takes place with respect to the same harmonics, for example with regard to harmonics of the same name in the even or odd carrier bands (sidebands) in the pulse width modulated voltage signals.

The prerequisite for implementing the method according to the invention is that the phase-corresponding current paths (output paths) of the first and at least second converters are each magnetically coupled in pairs by two inductors (coils) and additionally preferably by a highly magnetizable material. The inductors magnetically coupled in pairs at the output connections of the phases each form a mutually coupled inductor (MCI).

A fundamentally undesirable harmonic voltage results in a harmonic current, which in turn causes a magnetic flux in the corresponding winding of the electric motor. The harmonic currents or the magnetic fluxes caused by them do not contribute to the average torque formation in the electric motor. In the method according to the invention, which harmonics should be dampened can be selected by specifically adjusting the phase shift between the carrier signals depending on the operating point of the electric motor. This is advantageous because the amplitudes of the harmonic voltages vary depending on the operating point of the electric motor. The attenuation is achieved by a destructive superposition of the corresponding magnetic fluxes in the inductors that are inductively coupled to one another. This dampens the corresponding harmonics and reduces or, ideally, minimizes their influence on the operation of the electric motor. The phase shift between the carrier signals can, for example, be adjusted so that the magnetic fluxes in the electrical machine, which are caused by harmonics, are reduced or completely minimized.

For example, at an operating point of the electric motor, the phase shift between the carrier signals can be adjusted in such a way that the magnetic fluxes generated by the pulse width modulated voltage signals are superimposed in the paired inductors in such a way that the harmonics in even-numbered sidebands are strongly or maximally damped, while the harmonics experience little attenuation in odd-numbered sidebands or vice versa.

For example, in a further operating point of the electric motor, the phase shift between the carrier signals can be adjusted such that the magnetic fluxes generated by the pulse width modulated voltage signals are superimposed in the paired inductors in such a way that harmonics are dampened. It should be emphasized here that the introduction tion of a phase shift between the carrier signals essentially has no influence on the fundamental oscillation.

According to further embodiments of the method, in the underlying control circuit, the inductors coupled to one another in pairs can be arranged relative to one another in such a way that their magnetic fluxes are superimposed in the same direction or in opposite directions. For example, the inductors coupled to one another in pairs can be coupled to one another by means of an element made of a highly magnetizable material, i.e. a material with high magnetic permeability (ferromagnetic material), for example iron or ferrite. Here, the inductors can be magnetically positively or magnetically negatively coupled to one another, depending on the sense of orientation or depending on the direction of the current flowing through them. The type of magnetic coupling (i.e. positive or negative) can be adapted to the overall system and depends on whether the converters are operated in the same or anti-phase and whether the corresponding coils within the multi-phase windings are connected in parallel or anti-parallel relative to one another, i.e. whether the winding systems of the electrical machine are rotated electrically or mechanically by 0° or 180°.

According to further embodiments of the method, in the underlying control circuit, the inductors coupled to one another in pairs can each be wound around a common core, for example a toroidal core, made of the highly magnetizable material, so that the magnetic fluxes of the inductors in the core overlap in the same direction or in opposite directions.

According to further embodiments of the method, the phase shift can be adjusted such that at least the dominant harmonic in the magnetic fluxes in the air gap of the machine is dampened and, in the optimal case, extinguished. A dominant harmonic refers to a harmonic that delivers a dominant voltage and current contribution or the largest contribution among all harmonics. If several dominant harmonics are dampened or extinguished, these may be harmonics which, in descending order, have the greatest impact on the operating behavior of the electrical machine. The method according to the invention pursues a dynamic adjustment of the phase shift according to the current operating point of the electric motor, since the contributions of the harmonics are also dependent on this and improved or optimal operation of the electric motor can be achieved individually at each operating point.

According to further embodiments of the method, this further comprises comparing the current operating point of the electric motor with an operating point map of the electric motor that is divided into discrete areas, a phase shift being assigned to each discrete area, and adjusting the phase shift in accordance with the phase shift assigned to the discrete area, to which the current operating point has been assigned. In other words, the adjustment of the phase shift can be done in discrete values, which can reduce the requirements for the computing hardware used in the control circuit. If a finer tuning of the electric motor is desired, the adjustment can of course be carried out continuously, i.e. without discretization of the operating point map.

In general, a previous simulation, e.g. a previous FEM simulation, of the associated electric motor including power electronics can be used to determine which harmonics should be dampened depending on the operating point in order to optimize the operation of the entire system. It can then be stored in the control circuit of the control circuit how the control signals of the inverters should be shifted depending on the operating point in order to achieve optimal operation of the electric drive.

According to the invention, a control circuit for a multi-phase electric motor with coupled inductors is also provided, to which reference has already been made in the context of the method according to the invention. The control circuit has a first clocked inverter and at least one second clocked inverter, wherein the first inverter and the at least second inverter have a predetermined number of output connections and are set up to receive a first pulse width modulated voltage signal or a first pulse width modulated voltage signal at each of their output connections when controlled by means of a switching signal to provide a second pulse width modulated voltage signal, wherein an inductance is coupled to each of the output connections and the inductances coupled to output connections of the same name of the converters are inductively coupled to one another in pairs, the voltage signals at the output connections of a respective converter being phase-shifted with respect to one another. The control circuit further has a control circuit which is coupled to the first converter and to the at least second converter and is set up to have a first switching signal for the first converter and a second switching signal for the second converter, the control circuit being set up to carry out the method according to the invention previously described herein. The paired magnetic coupling always refers to a phase of the first converter and a corresponding phase of the at least second converter.

According to the invention, an electric motor with coupled inductors (as previously described) and a previously described control circuit is further provided. Each output connection of the first converter is coupled to a phase of a first multi-phase winding, preferably a three-phase winding, of the electric motor and each output connection of the second converter is coupled to a phase of a second multi-phase winding, also a preferably three-phase winding, of the electric motor.

According to further embodiments of the electric motor, the paired inductors can be magnetically positively coupled to one another if the winding systems of the electric motor are wound in opposite directions or if the control circuit is set up to switch corresponding switches of the clocked converters in anti-phase to generate the pulse width modulated voltage signals. In other words, the inductors are positively coupled (superimposition of the magnetic fluxes in the same direction) if the winding systems have an angular offset of 180°. It should be noted here that the angular offset between the winding systems is generally adjusted both structurally and mechanically - through appropriate placement of the inputs and outputs of the winding coils - and electrically - through in-phase or anti-phase operation of the converters.

In a symmetrical modification to the previous case, according to further embodiments of the electric motor, the paired inductors can be magnetically negatively coupled to one another if the winding systems of the electric motor are wound in phase or if the control circuit is set up to have corresponding switches of the clocked converters in phase to generate the pulse width modulated voltage signals switch. In other words, the inductors are negatively coupled (superposition of the magnetic fluxes in opposite directions) if the winding systems have no angular offset (or an angular offset of 0°).

The present invention is based on the idea of optimizing the operation of an electric drive with regard to its losses and/or noise. Consequently, in the method according to the invention, the phase shift between the first carrier signal and the second carrier signal can be adjusted with a view to minimizing the losses occurring in the electric drive, i.e. in order to keep the losses of the electric drive as low as possible at each operating point. Since the amplitudes of the harmonic voltages (and thus the amplitudes of the interfering harmonic currents) depend in particular on the intermediate circuit voltage and on the degree of modulation or control level of the respective converter, the phase shift between the carrier signals can be adjusted at each operating point so that the losses are minimal or at least compared to normal operation not optimized according to the invention. Due to the impedance, which increases linearly with frequency and is caused by the inductance of the windings, no significant harmonic current will be induced by harmonic voltages in higher-order sidebands and will therefore not make any significant contribution to interference. Therefore, in practice attempts are made to dampen or minimize the harmonics in the low sidebands.

It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or alone, without departing from the scope of the present invention.

• 1A zeigt eine Ausführungsform des erfindungsgemäßen Ansteuerschaltkreises, an den ein Elektromotor angeschlossen ist.
• 1B zeigt eine weitere Ausführungsform des erfindungsgemäßen Ansteuerschaltkreises, an den ein Elektromotor angeschlossen ist.
• 2 veranschaulicht die Kopplung zwischen jeweils zwei Phasen innerhalb des erfindungsgemäßen Ansteuerschaltkreises.
• 3 zeigt ein Diagramm, in dem ein Frequenzspektrum eines beispielhaften pulsweitenmodulierten Spannungssignals veranschaulicht ist.
• 4 zeigt ein Diagramm, welches die Abhängigkeit der Amplituden der Oberschwingungen vom Aussteuergrad (PWM) veranschaulicht.
• 5 zeigt ein Diagramm, in dem die Abhängigkeit des Phasenunterschiedes der Seitenbänder in Abhängigkeit von der Phasenverschiebung des Trägersignals veranschaulicht ist.
• 6 zeigt ein Diagramm, in welchem die erfindungsgemäße betriebspunktabhängige Phasenverschiebung implementiert werden kann.

Further advantages and refinements of the invention result from the description and the accompanying drawings.

• 1A shows an embodiment of the control circuit according to the invention, to which an electric motor is connected.
• 1B shows a further embodiment of the control circuit according to the invention, to which an electric motor is connected.
• 2 illustrates the coupling between two phases within the control circuit according to the invention.
• 3 shows a diagram in which a frequency spectrum of an exemplary pulse width modulated voltage signal is illustrated.
• 4 shows a diagram that illustrates the dependence of the amplitudes of the harmonics on the control level (PWM).
• 5 shows a diagram in which the dependence of the phase difference of the sidebands as a function of the phase shift of the carrier signal is illustrated.
• 6 shows a diagram in which the operating point-dependent phase shift according to the invention can be implemented.

In order to explain the invention, the operation of a 2x3-phase electric machine is considered below as an exemplary multi-phase electric motor. By coupling inductors coupled together in pairs, unwanted signals can be attenuated while the desired signal - the fundamental oscillation - remains unchanged. For such a connection it is necessary that the machine has windings (coils) wound in pairs (anti-)parallel. The inductors are positively coupled for anti-parallel wound windings and negatively coupled for parallel wound windings. In this way, the situation can be avoided in which the fundamental vibration leads to saturation of the iron core.

In 1A An overall system is illustrated, comprising an exemplary embodiment of a 2x3-phase control circuit 1 and an electric motor 7 coupled to it, the operation of which can be controlled using the method according to the invention. The control circuit 1 is set up to operate an electrical machine with two three-phase winding systems with angular offset (anti-parallel connection, which is equivalent to an offset of the phase angle by 180 °). The control circuit 1 has a first clocked converter 2 and a second clocked converter 3. The two converters 2, 3 are controlled by a control circuit 9. Each of the converters 2, 3 has corresponding switching elements and is set up to provide clocked voltage signals in the form of three phases 4, 5 at each of the output connections by means of a modulation method, preferably pulse width modulation. The control circuit 9 is set up to provide corresponding switching signals for the switching elements arranged in the converters 2, 3 by means of carrier-based modulation based on a comparison of the reference signal 10 with the carrier signal 11. Although in the 1A only one carrier signal 11 is symbolized, each converter 2, 3 is assigned a respective carrier signal, between which a phase difference is set depending on the operating point of the electric motor 7. Each output connection of the first converter 2 is coupled to an associated inductance 6 (for the sake of simplicity, all in 1A shown inductors are provided with a reference number). Each of the inductors 6 is in turn coupled to a corresponding connection of the electric motor 7. The situation is analogous with the output connections of the second converter 3. In the example shown, the electric motor 7 is a 6-phase electric motor, which is fed by two separate three-phase winding systems. The first converter 2 energizes a first winding of the electric motor 7, whereby a first three-phase subsystem is formed. The first winding can, for example, have three distributed coil windings. Analogously, the second converter 3 is coupled to a second winding of the electric motor 7, which can also have three distributed coil windings. This corresponds to the in 1A electric motor 7 shown is a (2x3)-phase electric motor. In 1A an intermediate circuit capacitor 10 and a traction battery 8 are also shown upstream of the control circuit 1 and used jointly by both converters 2, 3, a DC-DC converter can also be connected between these, which converts the DC voltage provided by a traction battery 8.

In the box marked with reference number 12, an exemplary winding scheme is shown in the form of an original winding, i.e. the smallest repeatable winding unit. The stator of the electric motor 7 can, for example, have 12 slots into which the original winding shown is inserted twice.

As can be seen from the winding diagram 12, the two winding systems are arranged anti-parallel to one another, since in each slot the connections of the coil belonging to the first subsystem are arranged opposite to the connections of the coil belonging to the second subsystem. Therefore, corresponding phases of the first and second converters 2, 3 are magnetically positively coupled to one another in pairs by means of the coupling inductances 6. Since the coils of the two separate winding systems in the electric motor 7 are connected in anti-parallel, the two converters 2, 3 are energized in anti-phase or opposite directions, that is, corresponding switches within the converters 2, 3 are closed and opened in anti-phase. This makes it possible to ensure that, during operation of the electric motor 7, the current flow caused by the fundamental oscillation flows in the same direction through the wires of the two separate windings, which are arranged in a groove, and in particular that the magnetic fluxes generated by them do not cancel each other out. However, since corresponding phases of the two converters 2, 3 are magnetically positively coupled to one another in pairs and the windings of the two subsystems are connected in anti-parallel, the fundamental magnetic oscillation fluxes cancel each other out in the coupling inductances 6, each of which is inductively coupled in pairs. As has already been explained several times, by adjusting the phase shift between the carrier signal of the first converter 2 and the second converter 3, harmonics in the pulse width modulated Voltage signal can be dampened or, ideally, eliminated.

In 1 B is a opposite the in 1A The overall system shown illustrates a slightly modified overall system, in which the 2x3-phase machine 7 has two winding systems connected in parallel (equivalent to an offset of the phase angle by 0 °). Accordingly, corresponding phases of the first and second converters 2, 3 are magnetically negatively coupled to one another in pairs by means of the coupling inductances 6. Since they are in the 1A and 1B The overall systems shown basically have the same structure, the same components are provided with the same reference numbers and are not described again in detail.

At the in 1B In the exemplary embodiment of the control circuit 1 shown, the two winding systems are different, as can be seen from the associated winding diagram 12, in which the original winding is illustrated 1A arranged parallel to each other, since in each groove the connections of the coil belonging to the first subsystem and connections of the coil belonging to the second subsystem are aligned the same. Since the coils of the two separate winding systems in the electric motor 7 are connected in parallel, the two converters 2, 3 are energized in the same phase or in common mode, that is, corresponding switches within the converters 2, 3 are closed and opened in the same phase. Like the one in 1A The control circuit 1 shown can also be ensured here that during operation of the electric motor 7, the current flow caused by the fundamental oscillation flows in the same direction through the wires of the two separate windings, which are arranged in a groove. Because of the in-phase operation of the converters 2, 3, corresponding phases of the first and second converters 2, 3 are magnetically negatively coupled to one another in pairs by means of the coupling inductances 6.

In comparison to using a single 3-phase winding with only one inverter, i.e. a (1x3)-phase electric motor, in the 1A and 1B sketched (2x3)-phase electric motor 7, each of the two converters 2, 3 can be designed for half the maximum current.

2 illustrates a magnetic coupling between two phases within the control circuit 1 according to the invention, which is realized by means of coils 22, 24 as coupling inductors 6 and by means of a core 20. The conductors 21, 23 belonging to phases of the same name (eg u, v or w) are shown, with a coupling inductance 6 being connected to both conductors 21, 23. Without limiting the generality, the first conductor 21 represents a phase of the first converter 2 and the second conductor 23 represents a corresponding phase of the second converter 3. The coil 22 coupled to the first conductor 21 and the coil 24 coupled to the second conductor 23 are each from one in 2 indicated current i _N or i _N * flows through, which is directed in opposite directions. Consequently, the two coils 22, 24, which are arranged opposite one another, are magnetically negatively coupled, since the magnetic fluxes generated by the coils 22, 24 in the core 20 are oriented in opposite directions, as indicated by the arrows shown on both sides of the core 20. This illustrates 2 a magnetic negative coupling between corresponding phases of the two converters 2, 3, as in the in 1B control circuit 1 shown is realized.

In the in 3 Diagram 30 shown shows a frequency spectrum of an exemplary pulse width modulated voltage signal, which can be determined by Fourier transformation of the pulse width modulated voltage signal. The pulse width modulated voltage signal output by each of the converters 2, 3 can, as already mentioned, generally be generated in a manner well known from the prior art, for example by applying the undershoot method in which a reference signal 10 is compared with the carrier signal 11. The correspondingly formed pulse-modulated voltage signal also represents the switch position of the respective half-bridge in the associated converter.

In the diagram 30, a frequency is plotted on the x-axis 31, normalized to the frequency of the reference signal (and therefore unitless), and the effective voltage in volts is plotted on the y-axis 32, i.e. that at the corresponding output terminal of the converter output voltage. Near the origin of the diagram 30 you can see the largest voltage contribution 33, which represents the fundamental vibration component. In the area of the simple carrier frequency, which corresponds to the clock frequency of the converter, a first group 34 of voltage contributions can be seen, which corresponds to harmonic voltages in the first sideband. Analogously, in the area of twice the carrier frequency, a second group 35 of voltage contributions can be seen, which corresponds to harmonic voltages in the second sideband. Furthermore, further groups of voltage contributions can be seen, which, however, have relatively small amplitudes and are also at high frequencies. In diagram 30 only the amplitudes of the different harmonics are shown shown sluggishly. However, their phase position cannot be read from diagram 30.

As already mentioned, through a suitable choice of the phase shift between the carrier signal 11 of the first converter 2 and the second converter 3, specific harmonic components in the pulsed voltage signals can be essentially eliminated or maximally attenuated by destructive superimposition of their magnetic fluxes in the coupled inductors. Alternatively, several harmonic components can be dampened, but then to a lesser extent.

The basic idea of the invention can be seen in the fact that the harmonics damped or eliminated by means of the paired inductors do not even reach the electrical machine and cannot form corresponding current flows therein. This means that an unnecessarily higher current and therefore thermal load on the electrical machine can be prevented in advance. In addition, the iron cores used for coupling the inductors can be relatively delicate, especially in comparison to sine filters, the use of which for smoothing the phase voltages in the control circuit of an electrical machine is known from the prior art, since the coupling in opposite directions according to the invention for connecting the windings Coupling inductors prevent saturation of the iron cores due to the fundamental vibrations.

This in 4 Diagram 40 shown illustrates the dependence of the amplitudes of the harmonics on the degree of modulation of the PWM voltage conversion by the converters 2, 3, whereby the degree of modulation can also be used to specify the operating point of the electric motor 7. The level of control or the degree of modulation of the PWM-based voltage conversion in the converter 2, 3 is plotted on the x-axis 41 (normalized to a corresponding space vector modulation) and a standardized voltage amplitude of the harmonic voltages is plotted on the y-axis 42. A first function graph 43 represents the total voltage, which is composed of the fundamental voltage, which is represented by a second function graph 44, and the contributions of the harmonic voltages, which are represented by the other function graphs. A third function graph 45 represents the contribution of all harmonics, with a fourth function graph 46 specifically representing voltage contributions in the second sideband in the frequency spectrum of the clocked voltage signal provided by the converter 2, 3. This contribution corresponds to the root mean square of the harmonic voltages in the first sideband. A fifth function graph 47 represents voltage contributions from harmonics in the first sideband in the frequency spectrum of the clocked voltage signal provided by the converter 2, 3 and corresponds to the root mean square of the harmonic voltages in the second sideband.

Based on the in 4 The diagram shown shows that, depending on the set degree of modulation of the converters 2, 3, the contributions of all harmonic voltages, which are represented by the third function graph 45, have their maximum at a degree of modulation of approximately 0.6. These contributions become smaller towards smaller and higher degrees of modulation. Furthermore, it can be seen that only from degrees of modulation of approximately 0.9 do the contributions of the harmonic voltages around the first dominant sideband 47 become larger than the contributions of the harmonic voltages around the second dominant sideband 46. At degrees of modulation below 0.9, voltage contributions around the second dominant sideband 46 represents the largest contribution to the harmonic voltage. The in 4 The illustrated relationships between the degree of modulation of the converter 2, 3 and the voltage contributions from the fundamental and harmonics can be obtained through theoretical or experimental analysis. Since the degree of modulation varies over different operating points of the electric motor 7, the dominant harmonics or sidebands also vary accordingly.

By appropriately adjusting the phase shift between the carrier signals 11 of the two converters 2, 3, disruptive magnetic fluxes caused by harmonic voltages in the phases of the converters 2, 3, which in turn induce harmonic currents, can be reduced. The choice of phase shift depends on the sidebands in which the harmonics are to be reduced, and thus significantly on the operating point of the electric motor 7.

In the in 5 Diagram 50 shown shows the correlation between the phase shift between the carrier signals of the two converters 2, 3 (plotted in degrees on the x-axis 51) and the resulting phase difference between corresponding sidebands (plotted in degrees on the y-axis 52). The first function graph 53 shows the phase shift for harmonic contributions in the first sideband and the second function graph 54 shows the phase shift for harmonic contributions in the second sideband. From the diagram 50 it can be seen that by appropriately selecting the phase shift between the carrier signals of the converters 2, 3, the phase difference between corresponding sidebands can in principle be adjusted as desired. To minimize a harmonic contribution to the magnetic flux in the electric motor, a phase difference of +/-180° is required in a system with two subsystems. It can be seen from diagram 50 that this can be achieved for harmonics in the first sideband considered here or in the second sideband, but not for both at the same time. As part of the method according to the invention, a phase shift between the carrier signals 11 is set depending on the operating point of the electric motor 7 in such a way that optimal operating efficiency of the electric motor 7 is achieved.

The operating point-dependent phase shift between the carrier signals is in 6 illustrated. In the diagram 60 shown there, the speed of the electric motor 7 is plotted on the x-axis and the torque of the electric motor 7 on the y-axis 62. The diagram shows the typical torque-speed characteristic curve 63 of an electric motor. The operating point-dependent setting of the phase shift between the carrier signals of the converters can be done continuously, so that in principle every point or every set of points on the torque-speed characteristic curve 63 is assigned a phase shift, which is set. Alternatively, the characteristic curve 63 can also be discretized, which is illustrated by the areas 64 in the diagram 60, whereby the computing effort can be reduced during operation of the electric motor. An exemplary operating strategy can include setting a corresponding phase shift of the carrier signals depending on the speed (surfaces 64 arranged vertically one above the other are combined into a strip) or the torque (surfaces 64 arranged horizontally next to one another are combined into strips) or both becomes. In the latter case, this means that each operating point is assigned to an area 64 and each area 64 is in turn assigned an optimal phase shift. For example, the carrier signals can be shifted in such a way that, as a result, exactly the dominant harmonics in the paired inductors are at least partially, and in the optimal case, completely attenuated. Due to the complementary connection of the inductors coupled in pairs and the associated winding phases, the effects of those harmonics which do not experience any attenuation in two inductors coupled in pairs and thus reach the electrical machine are superimposed in the same direction in the air gap of the electrical machine, so that they are there Experience damping. Although their effects are superimposed, in particular the magnetic fluxes caused in the air gap of the electrical machine, these “passed” harmonics initially lead to corresponding current flows in the windings of the electrical machine and put a thermal load on them without contributing to the formation of torque. However, since these harmonics belong to higher orders, they experience higher impedance (which grows linearly with inductance), so their adverse influence is limited.

Claims (10)

Method for controlling a multi-phase electric motor with coupled inductors by means of a control circuit, the control circuit having: a first clocked converter and at least one second clocked converter, wherein the first converter and the at least second converter have a predetermined number of output connections and are set up to provide a first pulse width modulated voltage signal or a second pulse width modulated voltage signal at each of their output connections when controlled by means of a switching signal , wherein an inductance is coupled to each of the output connections and the inductances coupled to output connections of the same name of the converters are inductively coupled to one another in pairs; and a control circuit which is coupled to the first converter and to the at least second converter and is configured to provide a first switching signal for the first converter and a second switching signal for the second converter; wherein the method comprises: Providing a first carrier signal in order to provide the first switching signal based on it using a carrier-based modulation method; Providing a second carrier signal in order to provide the second switching signal based on it using a carrier-based modulation method; Setting a phase shift between the first carrier signal and the second carrier signal or between a reference signal used as part of the carrier-based modulation method and the carrier signals depending on an operating point of the electric motor. Procedure according to Claim 1 , wherein the inductors coupled to one another in pairs are coupled to one another by means of an element made of a ferromagnetic material. Procedure according to Claim 1 or 2 , whereby the inductors coupled to one another in pairs are arranged relative to one another in such a way that their magnetic fluxes overlap in the same direction or in opposite directions. Procedure according to one of the Claims 1 until 3 , whereby the phase shift is adjusted such that at least one harmonic is attenuated in the pulse width modulated voltage signals. Procedure according to one of the Claims 1 until 4 , whereby the phase shift is adjusted in such a way that at least the dominant harmonic in terms of voltage is attenuated in the pulse width modulated voltage signals. Procedure according to one of the Claims 1 until 5 , further comprising: comparing the current operating point of the electric motor with an operating point map of the electric motor divided into discrete areas, a phase shift being assigned to each discrete area; Setting the phase shift according to the phase shift assigned to the discrete area to which the current operating point has been assigned. Control circuit for a multi-phase electric motor with coupled inductors, comprising: a first clocked converter and at least a second clocked converter, the first converter and the at least second converter having a predetermined number of output connections and being set up to be controlled by means of a switching signal at each of their output connections to provide a first pulse width modulated voltage signal or a second pulse width modulated voltage signal, wherein an inductance is coupled to each of the output connections and the inductances coupled to output connections of the same name of the converters are inductively coupled to one another in pairs, the voltage signals at the output connections of a respective converter being out of phase with one another; and a control circuit which is coupled to the first converter and to the at least second converter and is set up to provide a first switching signal for the first converter and a second switching signal for the second converter, the control circuit being set up to carry out the method according to one of Claims 1 until 6 to carry out. Electric motor with coupled inductors and a control circuit according to Claim 7 , wherein each output terminal of the first converter is coupled to a phase of a first multi-phase winding of the electric motor and each output terminal of the second converter is coupled to a phase of a second multi-phase winding of the electric motor. Electric motor according to Claim 8 , wherein the paired inductors are magnetically positively coupled to one another when the winding systems of the electric motor are wound in opposite directions or when the control circuit is set up to switch corresponding switches of the clocked converters in anti-phase to generate the pulse width modulated voltage signals. Electric motor according to Claim 8 , wherein the paired inductors are magnetically negatively coupled to one another when the winding systems of the electric motor are wound in phase or when the control circuit is set up to switch corresponding switches of the clocked converters in phase to generate the pulse width modulated voltage signals.

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