DE102020123731A1 - Method for operating a drive device for a motor vehicle and corresponding drive device - Google Patents

Abstract

The invention relates to a method for operating a drive device (1) for a motor vehicle, which has an electric traction machine which is electrically connected to a traction battery via an inverter and an intermediate circuit (2), a backup capacitor ( 6) with a backup capacitor capacitance. In order to reduce an intermediate circuit voltage present in the intermediate circuit (2) to a voltage limit value when the traction battery is electrically decoupled from the intermediate circuit (2), the intermediate circuit (2) is provided with a discharge capacitor (8) arranged electrically in parallel with the backup capacitor (6). connected to a discharge capacitor capacitance that is greater than the backup capacitor capacitance. The invention also relates to a drive device (1) for a motor vehicle.

Description

The invention relates to a method for operating a drive device for a motor vehicle that has an electric traction machine that is electrically connected to a traction battery via an inverter and an intermediate circuit, with a backup capacitor having a backup capacitor capacitance being present in the intermediate circuit. The invention also relates to a drive device for a motor vehicle.

From the prior art, for example, the publication DE 103 30 284 A1 famous. This describes a device for limiting an overvoltage in the intermediate circuit of a traction converter. The device has two turn-off semiconductor switches electrically connected in series, an inductance, a double-layer capacitor and a resistor that can be switched on by means of a switch, the inductance and the double-layer capacitor being electrically connected in series and this series connection between a connection point of the two turn-off semiconductor switches and a negative busbar of the intermediate circuit are connected and wherein the switchable resistor is electrically switchable in parallel with the double-layer capacitor. This is intended to reduce overvoltages that occur in the intermediate circuit of the traction converter.

The object of the invention is to propose a method for operating a drive device for a motor vehicle which has advantages over known methods, in particular enabling an extremely rapid reduction of an intermediate circuit voltage present in the intermediate circuit, for example in the event of an accident of the motor vehicle.

According to the invention, this is achieved with a method for operating a drive device for a motor vehicle with the features of claim 1 . It is provided that in order to reduce an intermediate circuit voltage present in the intermediate circuit to a voltage limit value when the traction battery is electrically decoupled from the intermediate circuit, the intermediate circuit is connected to a discharge capacitor which is arranged electrically in parallel with the backup capacitor and has a discharge capacitor capacitance which is greater than the backup capacitor capacitance.

Advantageous configurations with expedient developments of the invention are specified in the dependent claims.

The drive device is used to drive the motor vehicle, ie to provide a drive torque aimed at driving the motor vehicle. To generate the drive torque, the drive device has at least one drive unit, namely the electric traction machine. In order to provide the drive torque, the traction machine is at least intermittently charged with electric current, which is temporarily stored in the traction battery. Like the traction machine, the traction battery is part of the drive system.

The traction battery is preferably in the form of a high-voltage battery for efficient temporary storage of the electrical power. That is, a source voltage of the traction battery is 200V or more, 400V or more, 600V or more, or 800V or more. In any case, the source voltage is greater than 100 V. The electrical current temporarily stored in the traction battery is in the form of direct current. On the other hand, alternating current or three-phase current is required to operate the traction machine. For this reason, the traction machine is electrically connected to the traction battery via the inverter. The inverter is in turn connected to the traction battery via the intermediate circuit, so that conversely the traction machine is connected to the intermediate circuit via the inverter. The intermediate circuit is in the form of a direct current circuit and accordingly has two electrical poles which are connected to electrical connections of the traction battery which have different electrical potentials.

In order to avoid or at least reduce a fluctuation in the intermediate circuit voltage present in the intermediate circuit during operation of the traction machine and, accordingly, of the inverter, the intermediate circuit has the back-up capacitor. The two poles of the intermediate circuit are electrically connected to one another via the backup capacitor. The back-up capacitor is electrically connected in parallel with the inverter. The backup capacitor has the backup capacitor capacitance, which is selected in such a way that the above-mentioned fluctuations in the intermediate circuit voltage are at least reduced or even completely prevented.

However, the presence of the back-up capacitor also means that a not inconsiderable amount of electrical energy is temporarily stored in the intermediate circuit, at least as long as the electric traction machine is being operated. It may be necessary, for example in the event of an accident involving the motor vehicle or in the run-up to such an accident, to increase the intermediate circuit voltage significantly reduce, namely to or even below the voltage limit. For this purpose, the electrical energy can be discharged from the intermediate circuit and in particular from the back-up capacitor. As a result, the danger emanating from the electrical current for the motor vehicle and/or a user of the motor vehicle is significantly reduced.

For example, provision can be made for the electrical energy present in the intermediate circuit to be converted into thermal energy, for example with the aid of at least one discharge resistor and/or by correspondingly operating the inverter. However, the resulting thermal energy makes it necessary to dimension the component used to dissipate the electrical energy to be comparatively large and temperature-resistant. For example, the discharge resistor or the inverter must be designed accordingly. In addition, the electrical energy removed from the intermediate circuit in this way is lost for further use.

For this reason, it is provided according to the invention to use the discharge capacitor to reduce the intermediate circuit voltage. This is electrically connected to the intermediate circuit in order to reduce the intermediate circuit voltage, so that it is electrically connected to the intermediate circuit in parallel with the backup capacitor. After the discharge capacitor has been electrically connected to the intermediate circuit, the discharge capacitor is charged with the electrical energy present in the intermediate circuit. Before dismantling or at the beginning of dismantling, the traction battery is electrically separated from the intermediate circuit. Accordingly, no further electrical energy is supplied to the intermediate circuit from the traction battery, but only the electrical energy or charge present at the beginning of the dissipation is present in the intermediate circuit.

The electrical charge present in the intermediate circuit is then distributed to the backup capacitor and the discharge capacitor. The intermediate circuit voltage present in the intermediate circuit drops accordingly. The discharge capacitor capacitance of the discharge capacitor is greater than the backup capacitor capacitance and is selected in such a way that the intermediate circuit voltage drops below the voltage limit value due to the charging of the discharge capacitor. Accordingly, the discharge capacitor capacitance is selected as a function of the backup capacitor capacitance in order to reduce the intermediate circuit voltage, starting from the intermediate circuit voltage present at the start of dissipation, down to or even below the voltage limit value.

The procedure described enables the intermediate circuit voltage to be reduced extremely quickly without the electrical energy being converted into thermal energy. Rather, the electrical energy or charge present in the intermediate circuit is used to charge the discharge capacitor, as a result of which the voltage drops rapidly without the generation of thermal energy. The use of the discharge capacitor also enables rapid discharging of the support capacitor or rapid reduction of the intermediate circuit voltage several times in succession, without a negative influence due to thermal energy having to be expected. The discharge capacitor is in the form of a super-cap or an ultra-cap, for example. An embodiment of the discharge capacitor as a double-layer capacitor or lithium-ion capacitor is preferred.

The discharge capacitor capacitance can be used to determine in a simple manner how far the intermediate circuit voltage is to be reduced. In addition, an appropriate selection of the discharge capacitor capacity enables a desired time span to be set, within which the intermediate circuit voltage is reduced to or below the voltage limit value, in particular starting from a source voltage of the traction battery. For example, the discharge capacitor capacity is selected in such a way that the intermediate circuit voltage is reduced to below the voltage limit value within a maximum of five seconds or less than five seconds.

A voltage of at most 60 V, at most 45 V or at most 30 V, for example, is used as the voltage limit value. Such a voltage limit value ensures that, after the intermediate circuit voltage has been reduced, the motor vehicle cannot be damaged due to the electrical energy present in the intermediate circuit and/or the user of the motor vehicle cannot be endangered by this electrical energy.

A development of the invention provides that a value which is at most 20%, at most 15% or at most 10% of a source voltage of the traction battery is used as the voltage limit value. A source voltage of the traction battery of at least 150 V, at least 225 V or at least 300 V results for the above-mentioned voltage limit value of at most 30 V. For the voltage limit value of 45 V there results a source voltage of at least 225 V, at least 300 V or at least 450 V. For the preferred voltage limit of 60 V, however, a source voltage of at least 300 V, at least 450 V or at least results 600 V. Despite the high source voltage, the damage or risk of injury explained is significantly reduced or even completely eliminated by the low selection of the voltage limit value.

aufgestellt werden, wobei U₁ die Quellenspannung der Traktionsbatterie, C₁ die Stützkondensatorkapazität, U₂ den Spannungsgrenzwert und C₂ die Entladekondensatorkapazität beschreibt. Hieraus ergibt sich bei gegebener Stützkondensatorkapazität, gegebener Quellenspannung und gegebenem Spannungsgrenzwert die Beziehung $${\text{C}}_2={\text{C}}_1\left({\text{U}}_1/{\text{U}}_2-1\right)$$

für die notwendige Entladekondensatorkapazität, um die Zwischenkreisspannung ausgehend von der Quellenspannung der Traktionsbatterie auf den Spannungsgrenzwert abzusenken. Das Verhältnis aus Entladekondensatorkapazität zu Stützkondensatorkapazität ergibt sich folglich aus der Beziehung $${\text{C}}_2/{\text{C}}_1={\text{U}}_1/{\text{U}}_2-1.$$

A development of the invention provides that a value is used for the backup capacitor capacitance which is greater than the backup capacitor capacitance by a factor of at least 5, at least 7.5 or at least 10. The amount of charge Q present in the backup capacitor results from the backup capacitor capacitance and the intermediate circuit voltage before or at the beginning of the dissipation, ie in particular from the source voltage of the traction battery. The relationship Q = CU is used here, where Q stands for the amount of charge, C for the capacitance and U for the voltage. If one assumes that the amount of charge in the back-up capacitor is distributed loss-free between the back-up capacitor and the discharge capacitor, then the relationship $${\text{u}}_1{\text{C}}_1={\text{u}}_2\left({\text{C}}_1+{\text{C}}_2\right)$$

be set up, where U ₁ describes the source voltage of the traction battery, C ₁ the backup capacitor capacity, U ₂ the voltage limit and C ₂ describes the discharge capacitor capacity. The relationship results from this for a given backup capacitor capacitance, a given source voltage and a given voltage limit value $${\text{C}}_2={\text{C}}_1\left({\text{u}}_1/{\text{u}}_2-1\right)$$

for the necessary discharge capacitor capacity to lower the intermediate circuit voltage from the source voltage of the traction battery to the voltage limit value. The ratio of discharge capacitor capacity to backup capacitor capacity consequently results from the relationship $${\text{C}}_2/{\text{C}}_1={\text{u}}_1/{\text{u}}_2-1.$$

Assuming that the voltage limit corresponds to 10% of the source voltage of the traction battery, the discharge capacitor capacitance must be greater than the backup capacitor capacitance by a factor of nine in order to discharge the backup capacitor to such an extent that the intermediate circuit voltage, starting from the source voltage, reaches the voltage limit is reduced. Other values may result for other voltage ratios. The factors specified as minimum values ensure reliable discharge of the backup capacitor for different voltage ratios and correspondingly reliable reduction of the intermediate circuit voltage to the voltage limit value.

A development of the invention provides that at the beginning of the dissipation of the intermediate circuit voltage, the inverter is set in such a way that the traction machine is electrically isolated from the intermediate circuit. So not only the traction battery should be electrically separated from the intermediate circuit, but also the traction machine. This prevents additional electrical energy or charge from being supplied to the intermediate circuit during generator operation of the traction machine. This reliably prevents an increase in the intermediate circuit voltage.

Of course, it can be provided that in addition to the connection of the discharge capacitor to the intermediate circuit, the inverter is controlled to convert the electrical energy present in the intermediate circuit into thermal energy, so that the electrical energy is discharged in the direction of the inverter on the one hand and in the direction of the discharge capacitor on the other . Such a procedure enables the intermediate circuit voltage to be reduced particularly quickly.

A development of the invention provides that a current strength of the electric current fed to the discharge capacitor is limited by means of a limiting resistor connected in series with the discharge capacitor. The discharge capacitor is therefore not electrically connected directly to the intermediate circuit, but only indirectly via the limiting resistor. The limiting resistor is selected such that on the one hand a sufficiently rapid reduction in the intermediate circuit voltage to the voltage limit value is ensured, but on the other hand the amperage of the electric current flowing when the discharge capacitor is charged corresponds at most to a specific maximum amperage. The maximum current intensity is preferably selected in such a way that the discharge capacitor is not damaged even if the back-up capacitor is repeatedly discharged using the discharge capacitor. This improves the fatigue strength of the drive device.

A further development of the invention provides that after the reduction in the intermediate circuit voltage, the electric current temporarily stored in the discharge capacitor is fed to at least one current consumer. The current consumer preferably has a nominal voltage which corresponds to the voltage limit value. For example, provision is made to electrically separate the discharge capacitor from the intermediate circuit after the reduction of the intermediate circuit voltage to the voltage limit value and to connect it to the current consumer. Those in the discharge capacitor The electrical energy contained is not lost, but is used sensibly.

A development of the invention provides that the discharge capacitor is discharged at least temporarily via a discharge resistor. The discharging capacitor is preferably discharged after the intermediate voltage has been reduced to the voltage limit value. Provision can be made to electrically separate the discharge capacitor from the intermediate circuit after the reduction in the intermediate circuit voltage and instead to electrically connect it to the discharge resistor, so that the electrical energy contained in the discharge capacitor is reduced via the discharge resistor and thereby converted into thermal energy.

However, provision can of course also be made for the discharge capacitor to be electrically connected to the discharge resistor while the intermediate circuit voltage is being reduced, so that the intermediate circuit voltage is reduced partially by means of the discharging capacitor and partially by means of the discharging resistor. The use of the discharge resistor makes it possible, on the one hand, to quickly reuse the discharge capacitor to reduce the intermediate circuit voltage and, on the other hand, to reduce the intermediate circuit voltage more quickly than is possible using the discharge capacitor alone.

A development of the invention provides that the discharge capacitor is electrically isolated from the intermediate circuit while it is being discharged. This has already been pointed out above. The result of this is that normal operation of the drive device is possible parallel to the discharging of the discharging capacitor. For example, after the reduction of the intermediate circuit voltage, provision is made to electrically separate the intermediate circuit from the discharge capacitor and instead to electrically connect it to the traction battery, so that the intermediate circuit voltage is subsequently increased in the direction of the source voltage, in particular up to the source voltage. Such a procedure is provided in particular if the intermediate circuit voltage is reduced as a function of a risk of an accident in the motor vehicle without the accident actually occurring. This ensures that the traction machine can be started up quickly after the intermediate circuit voltage has been reduced.

A development of the invention provides that after the reduction of the intermediate circuit voltage by means of the discharge capacitor, the discharge capacitor is electrically isolated from the intermediate circuit and the discharge capacitor is discharged by means of the discharge resistor if a source voltage of the discharge capacitor is greater than a capacitor voltage limit value. Such a procedure is particularly advantageous if the reduction in the intermediate circuit voltage does not only take place down to the voltage limit value, but down to a lower voltage. With such a design of the discharging capacitor, it is possible to use the discharging capacitor for a number of successive discharging processes of the back-up capacitor, that is to say for multiple reductions in the intermediate circuit voltage to the voltage limit value or below it.

The discharging capacitor is preferably only discharged by means of the discharging resistor if the source voltage of the discharging capacitor is too high or is greater than the capacitor voltage limit value. This is preferably smaller than the voltage limit value and selected in particular in such a way that it is always ensured that the intermediate circuit voltage can be reduced to the voltage limit value by means of the discharge capacitor. If the discharge capacitor has a source voltage that is so close to the voltage limit value that it is to be expected that the intermediate circuit voltage can no longer be reduced to the voltage limit value during a subsequent dissipation, the discharge capacitor should be discharged at least to this extent by means of the discharge resistor that during the next reduction of the intermediate circuit voltage this can take place down to the voltage limit value. This ensures that the intermediate circuit voltage is always reliably reduced down to the voltage limit value.

The invention also relates to a drive device for a motor vehicle, in particular for carrying out the method according to the statements in this description, with an electric traction machine which is electrically connected to a traction battery via an inverter and an intermediate circuit, with a backup capacitor having a Backup capacitor capacity is present. The drive device is provided and configured to reduce an intermediate circuit voltage present in the intermediate circuit to a voltage limit value when the traction battery is electrically decoupled from the intermediate circuit by connecting the intermediate circuit to a discharge capacitor which is arranged electrically in parallel with the backup capacitor and has a discharge capacitor capacitance which is greater than the backup capacitor capacitance .

The advantages of such a configuration of the drive device or such a procedure have already been pointed out senior Both the drive device and the method for operating it can be further developed according to the statements made within the scope of this description, so that reference is made to them in this respect.

The features and feature combinations described in the description, in particular the features and feature combinations described in the following description of the figures and/or shown in the figures, can be used not only in the combination specified in each case, but also in other combinations or on their own, without going beyond the scope of the leave invention. The invention is therefore also to be considered to include embodiments that are not explicitly shown or explained in the description and/or the figures, but result from combinations of features from the explained embodiments or can be derived from them.

• 1 eine schematische Darstellung eines Bereichs einer Antriebseinrichtung für ein Kraftfahrzeug.

The invention is explained in more detail below with reference to the exemplary embodiments illustrated in the drawing, without the invention being restricted. It shows the only

• 1 a schematic representation of a region of a drive device for a motor vehicle.

the 1 shows a schematic representation of a region of a drive device 1 of a motor vehicle, which has an electric traction machine, not shown here. The electric traction machine is electrically connected via an inverter, also not shown, to an intermediate circuit 2, which in turn is electrically connected to a traction battery, not shown, for example via at least one contactor 3, in the exemplary embodiment shown here via several contactors 3.

The intermediate circuit 2 has a first pole 4 and a second pole 5, which are characterized by different electrical potentials. The first pole 4 is electrically connected, for example, to a first electrical connection and the second pole 5 to a second electrical connection of the traction battery. In the intermediate circuit 2 there is a back-up capacitor 6, via which the two poles 4 and 5 are electrically connected to one another. In addition, the two poles 4 and 5 are connected to one another via an electrical circuit 7 which has at least one discharge capacitor 8 .

The discharge capacitor 8 is electrically connected to the intermediate circuit 2 via an electrical switch 9 . In the exemplary embodiment shown here, the discharge capacitor 8 is directly electrically connected to the second pole 5 on the one hand and connected to the first pole 4 via the switch 9 on the other hand. The switch 9 is preferably electrically connected to the first pole 4 of the intermediate circuit 2 via a limiting resistor 10 .

The switch 9 can be controlled by a control device 11 . This means that the control unit 11 can set or sets a switching state on the switch 9 which is selected from a plurality of switching states. In a first switching state, the discharge capacitor 8 is electrically isolated from the intermediate circuit 2, in particular the first pole 4. In a second switching state, however, it is electrically connected to the intermediate circuit 2, namely both poles 4 and 5 of the intermediate circuit 2.

It can be provided that the discharge capacitor 8 is coupled to an electrical connection 12 in the first switching setting of the switch 9 and is decoupled from the connection 12 in the second switching setting. Furthermore, the drive device 1 has a connection 13 which is permanently electrically connected to the second pole 5 . In the charged state, the discharge capacitor 8 provides electrical current at a specific voltage at the connections 12 and 13 . For example, a current consumer is electrically connected to terminals 12 and 13.

In addition or as an alternative, the electrical circuit 7 has a discharge resistor 14 which is arranged electrically between the two terminals 12 and 13 . For example, the discharge resistor 14 is connected directly to the connection 13 on the one hand and to the connection 12 on the other side via an electrical switch 15 . The switch 15 can preferably be set by means of the control unit 11 . In the first switching state of the switch 5 shown here, the discharge resistor 14 is electrically isolated from the connection 12 . In a second switching state, on the other hand, it is connected to the terminal 12, so that electrical energy present in the discharge capacitor 8 can be dissipated via the discharge resistor 14, in particular converted into thermal energy.

The circuit 7 serves to reduce an intermediate circuit voltage present in the intermediate circuit to a voltage limit value. For this purpose, the traction battery is first decoupled from the intermediate circuit 2, for example by adjusting the at least one contactor 3 accordingly electrically connected to the intermediate circuit 2, in particular by appropriate setting of the switch 9. Electrical energy temporarily stored in the backup capacitor 6 is subsequently distributed to the backup capacitor 6 and the discharge capacitor 8, as a result of which the intermediate circuit voltage drops. A capacity of the discharge capacitor 8, which is also referred to as the backup capacitor capacity, is selected such that the intermediate circuit voltage drops to a voltage limit value which is preferably at most 20%, at most 15% or at most 10% of a source voltage of the traction battery.

The configuration of the drive device 1 described and the associated method ensure rapid and reliable reduction of the intermediate circuit voltage without the electrical energy present in the back-up capacitor 6 being converted solely into thermal energy. Rather, it is at least partially supplied to the discharge capacitor 8, from which it can subsequently be supplied to other purposes. This means that the energy present in the discharge capacitor 8 is not lost but can be used in a meaningful way.

Reference List

1

drive device

2

intermediate circuit

3

contactor

4

1 Pole

5

2 Pol

6

backup capacitor

7

circuit

8th

discharge capacitor

9

counter

10

limiting resistor

11

control unit

12

connection

13

connection

14

discharge resistance

15

counter

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 10330284 A1 [0002]

Claims (10)

Method for operating a drive device (1) for a motor vehicle, which has an electric traction machine which is electrically connected to a traction battery via an inverter and an intermediate circuit (2), wherein in the intermediate circuit (2) there is a back-up capacitor (6) with a Support capacitor capacity is present, characterized in that in order to reduce an intermediate circuit voltage present in the intermediate circuit (2) to a voltage limit value when the traction battery is electrically decoupled from the intermediate circuit (2), the intermediate circuit (2) has a discharge capacitor (8 ) is connected to a discharge capacitor capacitance that is greater than the backup capacitor capacitance. procedure after claim 1 , characterized in that a value which is at most 20%, at most 15% or at most 10% of a source voltage of the traction battery is used as the voltage limit value. Method according to one of the preceding claims, characterized in that a value is used for the backup capacitor capacitance which is greater than the backup capacitor capacitance by a factor of at least 5, at least 7.5 or at least 10. Method according to one of the preceding claims, characterized in that at the start of the reduction in the intermediate circuit voltage, the inverter is set in such a way that the traction machine is electrically isolated from the intermediate circuit (2). Method according to one of the preceding claims, characterized in that a current strength of the electric current fed to the discharge capacitor (8) is limited by means of a limiting resistor (10) connected in series with the discharge capacitor (8). Method according to one of the preceding claims, characterized in that after the reduction in the intermediate circuit voltage, the electric current temporarily stored in the discharge capacitor (8) is fed to at least one current consumer. Method according to one of the preceding claims, characterized in that the discharge capacitor (8) is discharged at least temporarily via a discharge resistor (14). Method according to one of the preceding claims, characterized in that the discharge capacitor (8) is electrically isolated from the intermediate circuit (2) while it is being discharged. Method according to one of the preceding claims, characterized in that after the reduction of the intermediate circuit voltage by means of the discharge capacitor (8), the discharge capacitor (8) is electrically isolated from the intermediate circuit (2) and the discharge capacitor (8) is discharged by means of the discharge resistor (14). , if a source voltage of the discharge capacitor (8) is greater than a capacitor voltage limit. Drive device (1) for a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, with an electric traction machine which is electrically connected to a traction battery via an inverter and an intermediate circuit (2), wherein in the intermediate circuit (2) a back-up capacitor (6) with a back-up capacitor capacitance is present, characterized in that the drive device (1) is provided and designed to reduce an intermediate circuit voltage present in the intermediate circuit (2) to a voltage limit value when the traction battery is electrically decoupled from the intermediate circuit (2). To connect the intermediate circuit (2) to a discharge capacitor (8) which is arranged electrically in parallel with the support capacitor (6) and has a discharge capacitor capacitance which is greater than the support capacitor capacitance.

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