WO2015193003A1

WO2015193003A1 - Step-up/step-down converter as phase converter for electric machine

Info

Publication number: WO2015193003A1
Application number: PCT/EP2015/058651
Authority: WO
Inventors: Hans Geyer
Original assignee: Robert Bosch Gmbh
Priority date: 2014-06-20
Filing date: 2015-04-22
Publication date: 2015-12-23
Also published as: DE102014211853A1

Abstract

The present invention provides a bidirectional voltage converter. The voltage converter comprises at least four modes of operation for converting a DC voltage into a voltage for actuating an electric machine or for converting a generator voltage into a DC voltage. In this case, the respective converted voltage is smoothed and is provided so as to have a much lower noise component than in the case of conventional power controllers. The circuit design particularly allows operation at high switching frequencies. Hence, the switching frequency can be moved to a range outside the range of human hearing. Furthermore, high switching frequencies also allow a reduction in weight and required installation space for the voltage converter.

Description

Description title

HIGH RESILIARY AS A PHASE CONVERTER FOR ELECTRICAL MACHINE

The present invention relates to a voltage converter and a method for converting an electrical voltage.

Electric drive systems, as used for example in electrically powered vehicles, for example, from a

DC power source, such as a high-power traction battery are fed. The DC voltage provided by this battery must be converted by means of a voltage converter (inverter) into a single-phase or multi-phase AC voltage in order to be able to generate the desired torque on the electric machine. Current inverters for electric drive systems in this case use the principle of pulse width modulation (PWM) via thyristor, IGBT or MOSFET power half-bridges with a corresponding number of phases to be realized. By modulating the duty cycle of the half-bridges assigned to the phases, in conjunction with the motor inductances, a current value through the

Machine windings generated, which causes a corresponding moment in the machine rotor. By a suitable control of the half bridges thus driving pattern can be generated with rotating field character with variable fundamental frequencies, phase relationships and modulation degrees. Due to the inclusion of the motor inductance is achieved so that an arrangement with a low number of components with high efficiency.

In such circuit topologies is of the inverter

Pulse width modulated signal led to the electric machine. To avoid electromagnetic interference, the cable connections received must be sufficiently shielded. Due to the physical characteristics and power dissipation limit of the switching elements used in the current inverters, the PWM switching frequencies are typically limited to lower frequencies in the human hearing range (up to 20kHz).

The German patent application DE 10 2013 201 538 AI discloses a

Inverter system to reduce unwanted noise caused by PWM control in an electric vehicle. For this purpose, the electric motor with an optimized, pseudo-random

Periodic grid activated.

The voltage supply of a conventional inverter is usually initially limited to the current battery or source voltage. If higher voltages are required, for example, to expand the speed range, then the DC voltage must be adjusted via a step-up converter circuit.

There is a need for an efficient and cost effective one

Voltage conversion suitable for driving an electric machine.

Disclosure of the invention

To this end, according to one aspect, the present invention provides a

A voltage converter having a DC voltage terminal comprising a first terminal member and a second terminal member; one

Machine connection, which has a third connection element and a fourth

Connection element comprises; a conversion device, which is adapted to a DC voltage between the first connection element and the second connection element in a phase voltage for an electrical

Machine to convert and provide the converted phase voltage between the third terminal and the fourth terminal, and is further configured to convert a provided by the electric machine between the third terminal and the fourth terminal phase voltage in a predetermined DC voltage and the DC voltage converted between to provide the first connection element and the second connection element; the conversion device in a first operating mode, the DC voltage between the first terminal element and the second terminal element is converted into a phase voltage whose maximum value is greater than the DC voltage between the first terminal element and the second terminal element, in a second operating mode the DC voltage between the first terminal element and the second terminal element in FIG converts a phase voltage whose maximum value is smaller than the DC voltage between the first

Terminal element and the second connection element, in a third

Operating mode converts the phase voltage between the third terminal and the fourth terminal into the predetermined DC voltage, wherein the maximum value of the phase voltage is smaller than the predetermined DC voltage, and in a fourth mode of operation

Phase voltage between the third terminal and the fourth terminal element converted into the predetermined DC voltage, wherein the maximum value of the phase voltage is greater than the predetermined

DC.

In another aspect, the present invention provides a method of converting electrical voltage with the steps of

Providing a voltage converter according to the invention; selecting an operating mode; and driving the voltage converter in

Dependence on the selected operating mode.

Advantages of the invention

It is an idea of the present invention to provide a voltage converter between a DC voltage source and an electrical machine

execute that a bidirectional voltage conversion between both sides can take place. At the same time provides the invention

Voltage converters in both the forward and in the reverse direction each Hoch- or Tiefsetzfunktionalitäten ready. This provides a compact and highly efficient voltage converter for a very wide range of applications. Through the integration of up- and down-converter functionality for

Extension of the voltage level of the source voltage can be

Voltage converters can be used in a very wide range of applications. In this way, output voltages are over the entire

Operating voltage range up to a minimum voltage of 0 volts possible. This allows the operation of an electric machine in a very wide speed range due to the high variability of the output voltage of the voltage converter, regardless of the source voltage of

DC voltage source.

The bidirectional voltage conversion in combination with the stepping up or down of the voltage level also allows the kinetic energy to be fed back into a DC battery during a deceleration process by means of a regenerative electric machine. In this case, in particular due to the integrated boost converter functionality, even with low output voltages of the regenerative electric machine, there is still a return of the energy into the battery. Thus, during braking, a particularly efficient recovery of kinetic energy can take place.

According to one embodiment, the conversion device comprises a first switching element, which is arranged between the first connection point and a first node, and a second switching element, which is arranged between the first node point and the second connection element. The conversion device further includes a third switching element disposed between the third terminal and a second node and a fourth switching element disposed between the second node and the fourth terminal. Furthermore, the conversion device comprises an inductance, which is arranged between the first node and the second node. By integrating an inductance in the conversion device, it is not necessary to include the motor inductance for the voltage conversion. Thus, the voltage conversion completely within the

Conversion device done. Therefore, there is a trouble-free, or at least clearly, at the machine connection of the voltage converter poorer output signal on. A shielding of the electrical connection between the machine connection of the voltage converter and electrical machine is therefore generally not required. At the

Machine connection of the voltage converter is rather an already smoothed output voltage with the machine base frequency ready.

Preferably, the conversion device is operated at a switching frequency of at least 20 kHz. Such switching frequencies are above the human audible range. Thus, the acoustic interference by audible vibrations can be avoided or at least significantly reduced. Furthermore, such high switching frequencies allow miniaturization of the circuitry. In particular, the inductances used can be made smaller at higher switching frequencies.

According to one embodiment, the voltage converter further comprises a control device, which is adapted to the first, the second, the third and the fourth switching element in dependence on a selected one

To control operating mode. This allows flexible control of the voltage converter in all operating modes.

According to another embodiment, the conversion device is further configured to close the third switching element and the fourth switching element in a fifth operating mode. This will be the third

Connection element and the fourth connection element electrically connected to each other at the machine connection. Thus, a connected electric machine can be switched into an active short circuit in this operating mode. The voltage converter can also open in a further operating mode, the third switching element and the fourth switching element, in order to switch a connected electric machine in the operating mode of the freewheel.

According to another embodiment, that between the third

Connection element and the fourth connection element provided

Phase voltage an alternating voltage, in particular a sinusoidal voltage between the potential of the third terminal element and a, from the operating mode and a predetermined PWM ratio

resulting maximum potential of the fourth connection element.

According to another embodiment, the invention determines

Method at the DC voltage terminal of the voltage converter, a DC voltage and / or at the motor terminal of the voltage converter, a phase voltage. The step of selecting the operation mode then selects the operation mode using the detected DC voltage and / or the detected phase voltage.

In another aspect, the present invention includes a

electric drive system with a voltage converter according to the invention. The electric drive system may further comprise an electrical machine with a phase connection, wherein the phase connection with the

Machine connection of the voltage converter is electrically coupled. Further, the electric drive system may include an electrical energy storage, which is electrically coupled to the DC voltage terminal of the voltage converter.

According to a further embodiment, the electric machine is a multi-phase electric machine, in particular an at least two-phase machine. In this case, the electric drive system comprises at least one voltage converter for each phase of the electric machine.

According to one embodiment, the electrical drive system may comprise a plurality of voltage converters connected in parallel for each phase of an electrical machine.

In another aspect, the present invention includes a

Motor vehicle, aircraft or ship with an electric drive system according to the invention.

Short description of the drawing Further embodiments and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. 1 shows a schematic representation of a circuit arrangement of a voltage converter according to an exemplary embodiment; a schematic representation of an electric drive system with a voltage converter according to an embodiment; a schematic representation of an electric drive system with a voltage converter according to another embodiment; a schematic representation of an electric drive system with a voltage converter according to yet another embodiment; a schematic representation of a motor vehicle with an electric drive system according to an embodiment; and a schematic representation of a method for converting an electrical voltage, as it is based on an embodiment.

Description of embodiments

Figure 1 shows a schematic representation of a circuit arrangement for a voltage converter 1, as it is based on an embodiment.

The voltage converter 1 comprises a DC voltage connection 11, a machine connection 12, a conversion device 13 and a

Control device 14. The DC voltage terminal 11 may be connected to a DC voltage source 2, for example a battery. In this case, a first connection element AI with a pole of the DC voltage source. 2 connected and a second connection point A2 with the other pole of the

DC voltage source connected. Furthermore, the voltage converter 1 has a machine connection 12 with a third connection element A3 and a fourth connection element A4. In the single-phase embodiment shown here, the two connection elements A3 and A4 of the machine connection 12 can be connected to the phase connections of an electrical machine 3. The voltage converter 1 further comprises a conversion device 13 with the four switching elements Sl to S4, as well as an inductance L. The switching elements S1 to S4 may be

For example, semiconductor switches Tl to T4 act, which is preferably a freewheeling diode Dl to D4 connected in parallel. As a semiconductor switch, for example, thyristors, bipolar transistors with an insulated gate (IGBT) or MOSFET are possible. In particular silicon carbide switches (SiC) or super junction MOSFETs are particularly suitable for high switching frequencies, in which switching frequencies of 20 kHz and more can be realized with only very low switching losses.

The first switching element Sl is arranged between the first connection element AI of the DC voltage terminal 11 and a first node Kl. The second switching element S2 is between this first

Node Kl and the second terminal AI of

DC voltage terminal 11 is arranged. Furthermore, the third one

Switching element S3 between the third connection element A3 of the

Machine connection 12 and a second node K2 arranged. The fourth switching element S4 is arranged between the second node K2 and the fourth connection element A4 of the machine connection 12.

Between the first node Kl and the second node K2, an inductance L is arranged. The second connection point A2 of the

DC terminal 11 and the fourth terminal A4 of the machine terminal 12 can also be electrically connected to each other and are preferably at the reference potential of

Voltage Converter 1. ur control of the switching elements Sl to S4, the control terminals of these switching elements Sl to S4 connected to a control device 14 become. The control device 14 can be designed to

Receive control signals and / or setpoints for the operation of the electric machine 3. Based on these control signals and / or set values, the control device 14 outputs switching signals to the switching elements S1 to S4 in order to open or close the corresponding switching elements S1 to S4. The control signals or desired values can be provided to the control device 14 via analog or digital signals. For example, the corresponding control signals or setpoint values can be transmitted via a bus system and received by the control device 14.

Furthermore, the control device 14 can also receive measured values via the voltage at the DC voltage connection 11 and / or at the machine connection 12. For example, this voltage sensors 15, 16 am

DC voltage connection 11 and / or are arranged on the machine connection 12, which determine the corresponding voltages and transmit the measured values of the determined voltages to the control device 14. Alternatively, existing sensors can transmit their measured values to the

Control device 14 transmitted to provide information about the voltage at the DC voltage port 11 and / or at the machine port 12 of the control device 14.

Depending on the control of the switching elements Sl to S4 in the

Conversion device 13 of the voltage converter 1 thus different operating modes can be realized. In a first operating mode, the voltage converter 1 operates as a combined boost converter inverter. The voltage applied to the DC voltage terminal 11

DC voltage is thereby raised and simultaneously converted into a voltage which is suitable for an electric machine on

Control machine connection 12. In boost converter mode, the maximum value, that is to say the amplitude of the voltage at the machine connection 12, is greater than the DC voltage which is present at the DC voltage connection 11.

For this purpose, the first switching element Sl is controlled by the control device 14 such that it is permanently closed. Furthermore, the second

Switching element S2 permanently open. The third switching element S3 is called activated active rectifier, so that flows through the freewheeling diode D3 and possibly also by the semiconductor switch T3, the current only in one direction. Finally, the fourth switching element S4 is driven with a predetermined switching frequency. In this case, according to the pulse width modulation principle, the voltage on the side of the machine connection 12 is set in accordance with the duty cycle. It exists between

Input voltage Ul at the DC voltage terminal 11 and

Output voltage U2 at the machine terminal 12 the following relationship:

In this case, T is the period of the predetermined switching frequency f with which the control device 14 controls the fourth switching element S4 and t _e m respectively the period of time within a period T, during which the switching element S4 is closed. As can be seen, already at a duty cycle tein: T of 50%, an output voltage U2 twice the magnitude of

Input voltage Ul can be achieved.

In a second operating mode, the voltage converter 1 operates as a combined buck converter inverter. The voltage applied to the DC voltage terminal 11 DC voltage is thereby reduced and simultaneously converted into a voltage which is suitable to drive an electrical machine on the machine connection 12. In the buck converter mode, the maximum value, that is to say the amplitude of the voltage at the machine terminal 12, is smaller than the DC voltage which is applied to the DC voltage terminal 11.

In this case, the third switching element S3 is permanently closed and the fourth switching element S4 permanently open. The second switching element S2 is driven as an active rectifier, which allows only a current flow in one direction in this mode. The first switching element Sl is finally driven with the predetermined switching frequency f = 1 / T such that on

Machine connection 12 sets the desired output voltage U2. The ratio of output voltage U2 to input voltage Ul is as follows:

It can easily be seen from this formula that in buck converter mode the output voltage U2 can be lowered down to 0 volts when t _e m approaches zero.

In addition to the operating modes described above, in which a DC voltage is converted to the DC voltage terminal 11 in a voltage for driving an electric machine 3, is also a converting the voltage applied to the machine terminal 12 voltage of a generator operated electric machine 3 in a DC voltage of predetermined height possible. The voltage U2, which is provided by the electric machine 3 at the machine terminal 12, may vary and be greater or smaller than the DC voltage to be provided at the DC voltage terminal 11, there to feed an electrical energy storage 2 and, for example, to charge a battery.

Is the amplitude or the maximum value of the voltage U2 am

Machine connection 12 of the voltage converter 1 smaller than that for

Charging the electrical energy storage 2 required DC voltage Ul, so the voltage converter 1 operates in a further operating mode as a combined rectifier and boost converter. This is the third

Switching element S3 permanently closed and the fourth switching element S4 permanently open. The first switching element Sl operates as an active rectifier, which transmits the current in one direction only, and the second switching element S2 is clocked at a predetermined switching frequency. Thus, a voltage Ul is established at the DC voltage terminal 11 according to the following formula:

If the maximum value or the amplitudes of the voltage U2 provided by the electric machine 3 at the machine connection 12 are greater than the voltage U1 with which the electrical energy store 2 is to be charged, the voltage converter 1 operates in a further operating mode as a combined rectifier and step-down converter. For this purpose, the first switching element Sl permanently closed and the second switching element permanently open. The fourth switching element S4 operates as an active rectifier, which only transmits the current in one direction, while the third switching element S3 is clocked at the predetermined switching frequency f = 1 / T. This adjusts the stress ratios according to the following formula:

U1 / U2 = W T.

Moreover, with the voltage converter 1 described above, it is also possible to switch the electric machine 3 to a safe operating state such as the active short circuit or a freewheeling mode.

For the active short circuit this will be the first and the second

Switching element Sl and S2 open and the third switching element S3 and the fourth switching element S4 closed. Thus, the third connection element A3 and the fourth connection element A4 are electrically connected to one another at the machine connection 12, and thus the phase connections of the electrical machine 3 are short-circuited.

On the other hand, if the third switching element S3 and the fourth switching element S4 are open at the same time, then the electric machine 3 is in the

Freewheel mode.

The control of the switching elements Sl to S4 of the conversion device 13 of the voltage converter 1 by the control device 14 based on the formula described above can take place by means of a suitable microcontroller. With appropriate existing computing power, the duty cycle of the switching elements S1 to S4 to be switched can take place via suitable mathematical algorithms. Alternatively, it is also possible to calculate the required duty cycles in advance and store them in a memory of the control device 14. In this way, the

Control device 14 are relieved.

The switching frequency with which the switch elements S 1 to S 4 are driven can be selected in a very wide frequency range. Analogous to For example, switching frequencies in the range of up to 10 kHz are also possible with conventional inverters. However, relatively low switching frequencies require a relatively large inductance L between the first node K1 and the second node K2. By increasing the switching frequency to frequencies above 20 kHz and more, the required inductance L can be correspondingly reduced. This leads to a further reduction of the required installation space and the weight of the voltage converter. 1

In addition, the use of switching frequencies above the human hearing range also leads to a lower acoustic

Impairment. For the use of such high switching frequencies of 20 kHz and more, in particular modern silicon carbide (SiC) switches are advantageous. Such SiC switches have relatively low switching losses even at switching frequencies above 20 kHz. Alternatively, it is also possible to use voltage converters with a super-junction MOSFET, which also have only low switching losses at high switching frequencies.

For a better understanding, the exemplary embodiment described above describes an electric drive comprising a voltage converter 1, an electrical energy store 2 and a single-phase electric machine 3. Moreover, the voltage converter 1 can also be used for electric drives with multi-phase electrical machines 3. In the following, therefore, circuit configurations for a three-phase electric machine 3 are executed. The choice of three phases for a multi-phase electric machine 3 serves only for illustrative purposes. In addition, electrical machines with any other number of phases are also possible.

Figure 2 shows a schematic representation of a circuit concept of a three-phase electric machine 3 with mass star point. Each of the three phases LI, L2, L3 is electrically connected to a connection element, for example the third connection element A3 of the machine connection 12 of a previously described conversion device 13. The respective other connection element, that is, for example, the fourth connection element A4 is electrically connected to the reference potential. The star point of the electric machine 3 is also also with the reference potential electrically connected. On the DC side are all

DC voltage terminals 11 of a common electrical

Energy storage 2 powered. The switching elements of all

Conversion devices 13 can from a common

Control device 14 are controlled. Moreover, it is also possible that a separate control device 14 is present for each phase.

FIG. 3 shows a further schematic illustration of an electric drive with a three-phase electrical machine 3. In this exemplary embodiment, a three-phase system structure with a virtual mass star point is shown. That is, the windings of all phases of the electric machine 3 are electrically connected to each other at the neutral point. However, this star point is not electrically connected to a reference potential. Otherwise, the structure of the electric drive with virtual mass star point is analogous to the structure of the electric drive with mass star point, in which the

Mass star point is at reference potential.

Figure 4 shows a schematic representation of an electric drive with a three-phase electric machine in delta connection. The windings of the electric machine 3 are each connected between two phase terminals of the electric machine 3. In addition, the structure of the electrical energy source 2 and the voltage converter 1 used is analogous to the embodiments described above. Figure 5 shows a schematic representation of a motor vehicle with an electric drive. The electric drive from electrical energy storage 2, the voltage converters 1 and the electric machine 3 can be carried out, for example, according to one of the embodiments described above.

FIG. 6 shows a schematic representation of a flowchart on which a method 100 for converting an electrical voltage according to an exemplary embodiment is based. In a step 110, first a previously described voltage converter 1 is provided. Subsequently, an operation mode is selected in step 120. In this mode of operation it can For example, the above-described operating modes boost converter inverter, the buck converter inverter, boost converter rectifier, buck converter rectifier act. In addition, the safe operating mode in the form of an active short circuit or free-running mode is possible.

Subsequently, in step 130, the voltage converter 1 is driven in dependence on the selected operating mode.

The selection of the operating mode can, in accordance with the previously received setpoint specifications for a torque of the electric

Machine 3 are determined. Alternatively, instead of the torque, an electrical current in the phase or phases of the electric machine 3 can be predetermined, which is to be set by providing the output voltage at the motor terminal 12 of the voltage converter. If the electric machine 3 is to be decelerated in order, for example, to decelerate an electrically driven motor vehicle, a corresponding braking torque can also be preset. Furthermore, the operating mode of the active short circuit or of the freewheeling mode can also be specified for setting a safe operating mode. Furthermore, the

Tension conditions at the machine connection 12 or the

DC voltage connection 11 can be determined. In this way it is in particular possible to determine whether the voltage converter 1 in

Hochsetzstellermodus or to operate in buck converter mode. Furthermore, therefore, a precise regulation for setting the

Voltage ratios at the DC voltage terminal 11 and am

Machine connection 12 possible.

For this purpose, for example, in step 115, the size of the DC voltage at the DC voltage terminal 11 of the voltage converter 1 can be determined. Additionally or alternatively, the size of the phase voltage at the motor terminal 12 of the voltage converter 1 can be determined. Based on the determined voltages, the operating mode can then be determined in step 130 using the determined DC voltage and / or the determined

Phase voltage can be selected. In summary, the present invention relates to a bidirectional voltage converter. The voltage converter comprises at least four operating modes for converting a DC voltage into a voltage for controlling an electrical machine or for converting a

Generator voltage in a DC voltage. The respective converted

Voltage is smoothed and provided with significantly reduced noise component than with conventional current regulators. The circuit construction in particular allows operation with high switching frequencies. Thus, the switching frequency can be shifted to an area beyond the human hearing range. In addition, high switching frequencies also allow one

Reduction of weight and required installation space for the

Voltage converter.

Claims

claims

A voltage converter (1) comprising: a DC voltage terminal (11) having a first terminal

(AI) and a second connection element (A2) comprises; a machine terminal (12) comprising a third terminal (A3) and a fourth terminal (A4); and a conversion device (13) configured to apply a predetermined DC voltage (Ul) between the first

Terminal element (AI) and the second connection element (A2) in a phase voltage (U2) for an electric machine (3) to convert and the converted phase voltage (U2) between the third

Terminal element (A3) and the fourth connection element (A4) to provide, and further adapted to a of the electrical machine (3) between the third connection element (A3) and the fourth connection element (A4) provided phase voltage (U2) in a predetermined DC voltage (Ul) to convert and the converted

DC voltage (Ul) between the first connection element (AI) and the second connection element (A2) to provide; wherein the conversion device (13) converts the DC voltage (U1) between the first connection element AI) and the second connection element (A2) into a phase voltage (U2) in a first operating mode whose maximum value is greater than the DC voltage (U1) between the first connection element (AI) and the second

Connection element (A2), in a second operating mode, the DC voltage (Ul) between the first connection element (AI) and the second connection element (A2) converted into a phase voltage (U2) whose maximum value is smaller than the DC voltage (Ul) between the first connection element (AI) and the second connection element (A2), in a third operation mode, the phase voltage (U2) between the third terminal element (A3) and the fourth terminal element (A4) is converted into the predetermined DC voltage (U1), the maximum value of the phase voltage (U2) being smaller than the predetermined DC voltage (U1), and in a fourth mode of operation, the phase voltage (U2) between the third terminal (A3) and the fourth terminal (A4) is converted into the predetermined DC voltage (Ul), the maximum value of the phase voltage (U2) being greater than the predetermined DC voltage (Ul) ,

Voltage converter (1) according to claim 1, wherein the

Conversion device (13) comprising:

a first switching element (Sl) disposed between the first terminal (AI) and a first node (Kl); a second switching element (S2) disposed between the first node (K1) and the second terminal (A2); a third switching element (S3) disposed between the third terminal (A3) and a second node (K2); a fourth switching element (S4) disposed between the second node (K2) and the fourth terminal (A4); an inductance (L) disposed between the first node (K1) and the second node (K2).

The voltage converter (1) according to claim 2, wherein the voltage converter (1) further comprises a control device (14) adapted to configure the first, second, third and fourth switching elements (S1-S4) in accordance with a selected operating mode head for. A voltage converter (1) according to any one of claims 1 to 3, wherein the conversion device (13) is further configured to close the third switching element (S3) and the fourth switching element (S4) in a fifth operating mode to form the third terminal element (A3). and to electrically connect the fourth terminal (A4).

Voltage converter (1) according to one of claims 1 to 4, wherein between the third connection element (A3) and the fourth

Terminal element (A4) provided phase voltage (U2) a

AC voltage is.

An electric drive system, comprising: a voltage converter according to any one of claims 1 to 5; an electrical energy store (2) connected to the

DC terminal (11) of the voltage converter (1) is electrically coupled; and an electric machine (3) comprising a phase terminal electrically coupled to the machine terminal (12) of the voltage converter (1).

Electric drive system according to claim 6, wherein the electric machine (3) comprises a multi-phase electric machine, and the electric drive system for each phase of the electric machine (3) comprises at least one voltage converter (1).

Motor vehicle, in particular, air, water or land vehicle, with an electric drive system according to claim 6 or 7.

Method (100) for converting an electrical voltage, comprising the steps: Providing (110) a voltage converter (1) according to one of claims 1 to 5;

Selecting (120) an operating mode; and

Driving (130) the voltage converter (1) in dependence on the selected operating mode.

The method (100) of claim 9, comprising the step:

Determining (115) a DC voltage at the DC voltage connection (11) of the voltage converter (1) and / or determining a DC voltage

Phase voltage at the motor terminal (12) of the voltage converter (1); wherein the step (120) of selecting the operation mode is

Operating mode using the determined DC voltage and / or the determined phase voltage selects.