Inverter circuit and method for operating an electric motor

Abstract

The invention relates to an inverter circuit (100), in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operable vehicle, at least comprising a first input terminal (10) for a positive input direct voltage (DC+) and a second input terminal (12) for a negative input direct voltage (DC-), a center terminal (14) for an electrical center potential (S), wherein the center terminal (14) is electrically coupled to an electrical ground potential (M) via a capacitor (36), or wherein the center terminal (14) is electrically coupled to the first input terminal (10) via a capacitor (44) and to the second input terminal (12) via a further capacitor (46), an output terminal (16) for outputting an output alternating voltage (AC), a main current path (20) and a secondary current path (30).An inductance (38) is electrically coupled on the one hand to the central terminal (14) and on the other hand via an additional switch (40, 42) to the first input terminal (10) and the second input terminal (12). The invention further relates to a method for operating an electric motor.

Description

[0001] The invention relates to an inverter circuit, in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operable vehicle, and to a method for operating an electric motor, in particular an electric drive train of an electrically operable vehicle, by means of an inverter circuit.

[0002] Two-stage inverters are typically used in battery-electric vehicles. Three-stage inverters are well-known and offer significant efficiency potential in electric drive systems. To increase the efficiency of the electric motor in the cycle-relevant range, a two-stage inverter can be extended to a three-stage ground-point-coupled inverter using a so-called T-branch.

[0003] EP 2 975 756 A1 discloses an inverter for converting direct current (DC) from a power generator into three-phase alternating current (AC) from a three-phase network. The inverter comprises a primary side with a first DC input line and a second DC input line, a secondary side with a first AC output phase, a second AC output phase, and a third AC output phase, and a filter component for connecting the secondary side to the primary side via a return line. The filter component comprises a first capacitive element and a first switching element connected to the return line. The first AC output phase is connected to the first capacitive element and the first switching element. The filter component allows the size of the inverter's output filter inductances to be reduced.

[0004] CN105680712 B describes a control circuit for a double-T three-level parallel inverter system. The system comprises two T-shaped three-level inverters, with the DC sides of the inverters electrically connected in parallel. Each switching element of the inverters is controlled by the control circuit. The AC sides of the inverters are filtered by a filter component and electrically connected in parallel to achieve grid connection.

[0005] An object of the invention is to provide an inverter circuit, in particular a three-stage inverter circuit for efficiently operating an electric motor of a drive train of an electrically operable vehicle.

[0006] A further object is to provide a method for efficiently operating an electric motor, in particular an electric drive train of an electrically operable vehicle, by means of an inverter circuit.

[0007] The above-mentioned objects are solved by the features of the independent claims.

[0008] Advantageous embodiments and advantages of the invention emerge from the further claims, the description and the drawing.

[0009] According to one aspect of the invention, an inverter circuit, in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operated vehicle, is proposed, at least comprising a first input terminal for a positive input DC voltage and a second input terminal for a negative input DC voltage, a center terminal for an electrical center potential, wherein the center terminal is electrically coupled to an electrical ground potential via a capacitor, or wherein the center terminal is electrically coupled to the first input terminal via a capacitor and to the second input terminal via a further capacitor, an output terminal for outputting an output AC voltage, a main current path with two main switches, each of which is connected to a terminal with the first input terminal orthe second input terminal and the other terminal are electrically connected to the output terminal, and a secondary current path with two secondary switches arranged in a T-shape relative to the main switches, the secondary current path being electrically connected to the center terminal on the one hand and to the output terminal on the other. An inductance is electrically coupled to the center terminal on the one hand and, via an additional switch, to the first input terminal and the second input terminal on the other hand.

[0010] The electrical center potential represents a voltage level that can lie between the positive input DC voltage and the negative input DC voltage. For example, the center potential can correspond to half of the total input DC voltage, the sum of the positive and negative input DC voltages.

[0011] The proposed inverter circuit represents an advantageous further development of a known three-stage inverter with a so-called T-architecture. With such an inverter circuit, it is possible to provide a current or voltage curve for electric motors that is as sinusoidal as possible and to increase the efficiency of the electric drive train of an electrically operated vehicle.

[0012] Preferably, the first input terminal is provided and / or designed for electrical connection to a positive supply voltage provided by a vehicle battery of the electrically operable vehicle and the second input terminal is provided and / or designed for electrical connection to a negative supply voltage provided by the vehicle battery.

[0013] The center terminal is not necessarily intended and / or designed as an input terminal for an electrical voltage (external to the inverter circuit). In particular, the input terminal can be an internal terminal of the inverter circuit.

[0014] In one embodiment, a capacitor of the inverter circuit is integrated via the center terminal into the branch at the center potential, the so-called T-branch of the secondary current path.

[0015] The center terminal of the inverter circuit can be electrically connected via the capacitor, for example, to a ground potential or intermediate potential, or to a midpoint voltage of the vehicle battery. In this case, the capacitor is electrically connected, for example, to the mentioned potential or voltage on the one hand and to the center terminal on the other.

[0016] In an alternative or additional embodiment, two capacitors can also be connected in series between the positive input DC voltage and the negative input DC voltage, wherein the common terminal of the two series-connected capacitors can be electrically coupled to the center terminal. In this way, the voltage of the potential at the center terminal can be adjusted.

[0017] One or the common terminal of the two capacitors can be coupled to the positive and negative input DC voltages via an inductor, with the inductor itself being connected to the positive and negative input DC voltages via two transistors acting as switches. This special circuit arrangement makes it possible to set any voltage across the capacitor, thus achieving the desired sinusoidal current waveform.

[0018] This can advantageously increase the efficiency of the vehicle’s electric drive train.

[0019] According to an advantageous embodiment of the inverter circuit, a voltage on the auxiliary current path can be variably adjusted using the inductance and the two additional switches. A sinusoidal output current can be adjusted at the output terminal using the variable voltage on the auxiliary current path. Thus, an approximately sinusoidal current can be provided at the output terminal for the AC voltage and flow through a connected motor winding. In this way, electrical losses caused by harmonics of the current flow and by the charge reversal of the winding capacitances of the electric motor can be advantageously reduced, thereby increasing the efficiency of the drive train.

[0020] According to an advantageous embodiment of the inverter circuit, two series-connected diodes can be electrically connected in reverse direction to the first input terminal and the second input terminal, with the center terminal being electrically coupled to the common terminal of the series-connected diodes. This protective circuit can advantageously prevent the voltage at the center terminal from rising above the positive input DC voltage or falling below the negative input DC voltage. This can protect the semiconductor switches of the auxiliary current path as well as the additional switches on the inductor.

[0021] According to an advantageous embodiment of the inverter circuit, the main current path can be switched to non-conductive during operation of the secondary current path. This allows the inverter circuit to enable particularly efficient operation of the electric motor at low currents, since the system losses of the electric drive train are lower.

[0022] According to an advantageous embodiment, the inverter circuit can be converted from three-stage operation to two-stage operation and vice versa, depending on at least one electrical load requested at the output terminal. Three-stage operation can advantageously enable particularly efficient operation of the inverter circuit in a low load range of the electric motor, while two-stage operation is usually preferred in a high load range.

[0023] According to a further aspect of the invention, a method for operating an electric motor, in particular an electric drive train of an electrically operable vehicle, by means of an inverter circuit is proposed, at least comprising applying an electrically positive input DC voltage to a first input terminal and an electrically negative input DC voltage to a second input terminal of the inverter circuit; and coupling the electric motor to the at least one output terminal of the inverter circuit. In this case, an inductance electrically coupled to a center terminal is controlled via additional switches that are electrically coupled to the first input terminal and the second input terminal.

[0024] The electrical center potential represents a voltage level which can be between the positive input DC voltage and the negative input DC voltage, but preferably corresponds to half of the total input DC voltage from the sum of the amounts of positive and negative input DC voltage.

[0025] The proposed method uses a modification of a prior-art three-stage inverter with a so-called T-architecture. Such an inverter circuit makes it possible to provide a current or voltage waveform that is as sinusoidal as possible for electric motors and increase the efficiency of the electric drive train of an electrically powered vehicle.

[0026] For example, a capacitor can be integrated via the center terminal into the branch at the center potential, the so-called T-branch of the secondary current path. The center potential can be an electrical ground potential.

[0027] In an alternative or additional embodiment, two capacitors can also be connected in series between the positive input DC voltage and the negative input DC voltage, with the common terminal of the two series-connected capacitors being electrically coupled to the center terminal. In this way, the potential voltage at the center terminal can be adjusted.

[0028] One or the common terminal of the two capacitors can be coupled to the positive and negative input DC voltages via an inductor, with the inductor itself being connected to the positive and negative input DC voltages via two transistors acting as switches. This special circuit arrangement makes it possible to set any voltage across the capacitor, thus achieving the desired sinusoidal current waveform.

[0029] According to an advantageous embodiment of the method, a voltage at the auxiliary current path of the inverter circuit can be variably adjusted using the inductance and the two additional switches. A sinusoidal output current at the output terminal can be adjusted using the variable voltage at the auxiliary current path. This allows an approximately sinusoidal current to be provided at the output terminal for the AC voltage and flow through a connected motor winding. In this way, electrical losses caused by harmonics of the current flow and by the charge reversal of the winding capacitances of the electric motor can be advantageously reduced, thereby increasing the efficiency of the drive train.

[0030] According to an advantageous embodiment of the method, a voltage at the center terminal less than or equal to the positive input DC voltage and / or greater than or equal to the negative input DC voltage can be set by two series-connected diodes, which are electrically connected in the reverse direction to the first input terminal and the second input terminal, wherein the center terminal is electrically coupled to the common terminal of the series-connected diodes. In this way, it can advantageously be prevented that the voltage at the center terminal rises above the positive input DC voltage or falls below the negative input DC voltage. This can protect the semiconductor switches of the secondary current path as well as the additional switches on the inductor.

[0031] According to an advantageous embodiment of the method, the main current path can be switched off during operation of the secondary current path. This allows the inverter circuit to enable particularly efficient operation of the electric motor at low currents, since the system losses of the electric drive train are lower.

[0032] According to an advantageous embodiment of the method, the inverter circuit can be converted from three-stage operation to two-stage operation and vice versa depending on at least one electrical load requested at the output terminal. Three-stage operation enables particularly efficient operation of the inverter circuit in a low load range of the electric motor, while two-stage operation is preferably switched in a high load range.

[0033] Further advantages will become apparent from the following description of the drawings. The drawings illustrate an exemplary embodiment of the invention. The drawings, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0034] It shows: Fig. 1 shows an inverter circuit, in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operable vehicle, according to an embodiment of the invention; Fig. 2 shows an inverter circuit according to a further embodiment of the invention with an alternative wiring of the center terminal with two capacitors; and Fig. 3 an inverter circuit according to a further embodiment of the invention with a protective circuit as overvoltage protection of the switches in the secondary current path.

[0035] The figure shows only an example and is not to be understood as limiting.

[0036] Fig. Figure 1 shows an inverter circuit 100, in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operable vehicle, according to an embodiment of the invention. Inverter circuit 100 can represent one phase of an inverter for operating a multi-phase electric motor.

[0037] The Fig. The inverter circuit 100 shown in Figure 1 comprises a first input terminal 10 for a positive DC input voltage DC+ and a second input terminal 12 for a negative DC input voltage DC-, a center terminal 14 for an electrical center potential S, and an output terminal 16 for outputting an AC output voltage AC to a phase of an electric motor, not shown.

[0038] The electrical center potential S represents a voltage level that can be between the positive input DC voltage HV+ and the negative input DC voltage HV-, but preferably corresponds to half of the total input DC voltage from the sum of the positive and negative input DC voltages HV+, HV-. The center potential S represents an internal potential of the inverter circuit 100.

[0039] In the Fig. In the embodiment shown in Figure 1, the center terminal 14 is electrically coupled to an electrical ground potential M via a capacitor 36.

[0040] Since an electric motor used for a drive train of an electrically operated vehicle is usually a three-phase motor, three phases of the inverter circuit 100 are required to control the electric motor. The entire inverter therefore has three or more, for example six, such inverter circuits 100, such as those shown in Fig. 1. In the case of a three-phase machine, the electric motor can be coupled to the output terminals 16 of at least three such inverter circuits 100.

[0041] The inverter circuit 100 comprises a main current path 20 with two main switches 22, 24, each of which is electrically connected with one terminal to the first input terminal 10 and the second input terminal 12, respectively, and with the other terminal together to the output terminal 16. Furthermore, the inverter circuit 100 comprises a secondary current path 30 with two secondary switches 32, 34 arranged in a T-shape to the main switches 22, 24. The secondary current path 30 is electrically connected on the one hand to the central terminal 14 and on the other hand to the output terminal 16. The central terminal 14 is in the Fig. 1, the input terminal 10 is electrically connected to the electrical ground potential M via a capacitor 36. An inductance 38 in the form of a coil is electrically coupled, on the one hand, to the center terminal 14 and, on the other hand, via an additional switch 40, 42, to one input terminal 10 and the other input terminal 12.

[0042] In a typical three-stage inverter, for example, the main switches 22, 24 of the main current path 20 can each be designed for a current carrying capacity of 800 A rms (rms = root mean square), while the sub-switches 32, 34 of the sub-current path 30 are each designed for a current carrying capacity of 120 A rms.

[0043] The three-stage inverter circuit 100 can thus be converted from three-stage operation to two-stage operation and vice versa, depending on at least one electrical load requested at the output terminal 16. Three-stage operation can increase the efficiency of the electric motor in the low-load range.

[0044] The inverter circuit 100 represents an extension of a conventional three-stage inverter with asymmetrical design of the current carrying capacity of the switching transistors 22, 24, 32, 34.

[0045] By increasing the number of switchable voltage levels, the electrical losses caused by harmonics of the output current can be reduced. Ideally, a purely sinusoidal current can be generated through the motor winding.

[0046] For this purpose, the proposed three-stage inverter circuit 100 is expanded with the components capacitor 36, inductor 38, and the two switching transistors 40, 42. The inductor 38, which is electrically coupled between the capacitor 36 and the auxiliary switches 32, 34 of the auxiliary current path 30 of the inverter circuit 100, is controlled via the additional switches 40, 42, which are electrically coupled to one input terminal 10 and the other input terminal 12.

[0047] These four components, capacitor 36, inductor 38, and the two switching transistors 40, 42, allow any desired voltage to be set across capacitor 36. If the two transistors 32, 34 are conductive and a motor winding of an electric motor is connected to output terminal 16, a current flows through the motor winding. If the voltage across capacitor 36 is varied, a corresponding current flows through the motor winding. The current is approximately sinusoidal. While this part of the circuit is active, the main current path 20 is switched off.

[0048] The circuit for adjusting the voltage across capacitor 36 is active only for low currents. For high motor currents, switches 32, 34 of secondary current path 30 are non-conductive, and switches 22, 24 of primary current path 20 are active. The inverter circuit 100 is then controlled again in conventional two-stage operation.

[0049] By means of the inductance 38 and the two additional switches 40, 42, a voltage across the capacitor 36 of the inverter circuit 100 can be variably adjusted. Using the variable voltage, a sinusoidal output current can be set at the output terminal 16 via the capacitor 36. The main current path 20 can be switched off during operation of the secondary current path 30.

[0050] This can advantageously increase the efficiency of the electric motor in the vehicle’s electric drive train.

[0051] In Fig. 2 shows an inverter circuit 100 according to a further embodiment of the invention with an alternative wiring of the center terminal 14 with two capacitors 44, 46.

[0052] In this embodiment, the center terminal 14 is electrically coupled to the first input terminal 10 via the capacitor 44 and to the second input terminal 12 via the additional capacitor 46. Thus, the two capacitors 44, 46 can be connected between the positive input DC voltage HV+ and the negative input DC voltage HV, with the common terminal of the series-connected capacitors 44, 46 being electrically coupled to the center terminal 14. In this way, the voltage of the potential at the center terminal 14 can be adjusted.

[0053] Similar to the one in Fig. 1 illustrated embodiment can be used in the Fig. In the embodiment shown in Figure 2, a voltage across the capacitors 44, 46 of the inverter circuit 100 can be variably adjusted using the inductor 38 and the two additional switches 40, 42. Using the variable voltage, a sinusoidal output current can be set at the output terminal 16 via the capacitors 44, 46.

[0054] This can advantageously increase the efficiency of the electric motor in the vehicle’s electric drive train.

[0055] Fig. 3 shows an inverter circuit 100 according to a further embodiment of the invention with a protective circuit as overvoltage protection of the switches 32, 34 in the secondary current path 30.

[0056] Here, two series-connected diodes 44, 46 are electrically connected in the reverse direction to the first input terminal 10 and the second input terminal 12, wherein the center terminal 14 is electrically coupled to a common terminal 15 of the series-connected diodes 44, 46.

[0057] The protective circuit advantageously prevents the voltage at the center terminal 14 from rising above the positive input DC voltage HV+ or falling below the negative input DC voltage HV-. This protects the semiconductor switches 32, 34 of the secondary current path 30 as well as the additional switches 40, 42 on the inductor 38.

[0058] Since the rest of the circuit of the Fig. 3 illustrated embodiment to the one in Fig. 2, the mode of operation when generating the sinusoidal output current at the output terminal 16 is the same as in Fig. 2. List of reference symbols 10 first input connection 12 second input connection 14 Central connection 15 common connection 16 Output connector 20 Main current path 22 Main switch 24 main switches 30 bypass path 32 auxiliary switches 34 auxiliary switches 36 Capacitor 38 Inductance / Coil 40 switches 42 switches 44 Capacitor 46 Capacitor 48 diodes 50 diodes 100 inverter circuit DC+ positive input DC voltage DC- negative input DC voltage AC output voltage M electrical mass S mean potential QUOTES CONTAINED IN THE DESCRIPTION

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

[0000] EP 2 975 756 A1

[0003] CN 105680712 B

[0004]

Claims

[1] Inverter circuit (100), in particular a three-stage inverter circuit for operating an electric motor of a drive train of an electrically operable vehicle, at least comprising - a first input terminal (10) for a positive input DC voltage (DC+) and a second input terminal (12) for a negative input DC voltage (DC-), - a center terminal (14) for an electrical center potential (S), wherein the center terminal (14) is electrically coupled to an electrical ground potential (M) via a capacitor (36), or wherein the center terminal (14) is electrically coupled to the first input terminal (10) via a capacitor (44) and to the second input terminal (12) via a further capacitor (46), - an output terminal (16) for outputting an alternating voltage (AC), - a main current path (20) with two main switches (22, 24), each of which is electrically connected with one terminal to the first input terminal (10) or the second input terminal (12) and with the other terminal together to the output terminal (16), - a secondary current path (30) with two secondary switches (32, 34) arranged in a T-shape to the main switches (22, 24), wherein the secondary current path (30) is electrically connected on the one hand to the central terminal (14) and on the other hand to the output terminal (16), wherein an inductance (38) is electrically coupled on the one hand to the central terminal (14) and on the other hand via an additional switch (40, 42) to the first input terminal (10) and the second input terminal (12). [2] Inverter circuit according to claim 1, wherein a voltage at the secondary current path (30) can be variably adjusted by means of the inductance (38) and the two additional switches (40, 42), wherein a sinusoidal output current at the output terminal (16) can be adjusted by means of the variable voltage at the secondary current path (30). [3] Inverter circuit according to one of the preceding claims, wherein two series-connected diodes (44, 46) are electrically connected in the reverse direction to the first input terminal (10) and the second input terminal (12), wherein the center terminal (14) is electrically coupled to the common terminal (15) of the series-connected diodes (44, 46). [4] Inverter circuit according to one of the preceding claims, wherein the main current path (20) is switched non-conductive during operation of the secondary current path (30). [5] Inverter circuit according to one of the preceding claims, wherein the inverter circuit (100) can be converted from a three-stage operation to a two-stage operation and vice versa depending on at least one electrical load requested at the output terminal (16). [6] Method for operating an electric motor, in particular an electric drive train of an electrically operable vehicle, by means of an inverter circuit (100) according to one of the preceding claims, at least comprising Applying an electrically positive input DC voltage (DC+) to a first input terminal (10) and an electrically negative input DC voltage (DC-) to a second input terminal (12) of the inverter circuit (100); Coupling the electric motor to at least one output terminal (16) of the inverter circuit (100), wherein an inductance (38) electrically coupled to a center terminal (14) is controlled via additional switches (40, 42) which are electrically coupled to the first input terminal (10) and the second input terminal (12). [7] Method according to claim 6, wherein a voltage at the secondary current path (30) of the inverter circuit (100) is variably adjusted by means of the inductance (38) and the two additional switches (40, 42), wherein a sinusoidal output current at the output terminal (16) is adjusted by means of the variable voltage at the secondary current path (30). [8] Method according to claim 6 or 7, wherein a voltage at the center terminal (14) is set to be less than or equal to the positive input DC voltage (HV+) and / or greater than or equal to the negative input DC voltage (HV-) by means of two series-connected diodes (44, 46) which are electrically connected in the reverse direction to the first input terminal (10) and the second input terminal (12), the center terminal (14) being electrically coupled to a common terminal (15) of the series-connected diodes (44, 46). [9] Method according to one of claims 6 to 8, wherein the main current path (20) is switched non-conductive during operation of the secondary current path (30). [10] Method according to one of claims 6 to 9, wherein the inverter circuit (100) is converted from a three-stage operation to a two-stage operation and vice versa depending on at least one electrical load requested at the output terminal (16).

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