METHOD FOR DETERMINING A DESIRED VOLUME FLOW FOR A COOLANT

Description

[0001] The present invention relates to a method for determining a desired volume flow for a coolant which is passed through a coolant path for cooling a power converter, as well as a computing unit and a computer program for carrying out the method. State of the art

[0002] Electrical machines can be operated by means of a power converter, particularly when used in a vehicle, especially a hybrid or electric vehicle. In particular, motor and generator operation are possible. Due to the heat generated by power loss in the power converter, particularly in the semiconductor switching elements or power semiconductors, it is expedient to cool such power converters using a coolant, for example, water. Methods for determining a target volume flow for a coolant are known from the publications DE 10 2016 212192 A1, DE 11 2013 000188 T5, and EP 2 907 688 A1. Disclosure of the invention

[0003] According to the invention, a method for determining a target volume flow, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0004] The invention relates to a method for determining a target volume flow for a coolant that is passed through a coolant path to cool a power converter. Such a power converter can in particular be an inverter. As already mentioned, such a power converter typically has semiconductor switching elements, in particular power semiconductors such as MOSFETs or IGBTs, in order to convert a voltage, for example from a direct voltage into an alternating voltage and vice versa. Depending on the type of electrical machine to be controlled by the power converter and also the type of power converter itself, the semiconductor switching elements and thus the power converter can heat up considerably due to corresponding power losses, particularly at high overall power levels, i.e. high currents.

[0005] Such an electric machine can be used in particular in a vehicle, in particular a hybrid or (pure) electric vehicle.

[0006] Typically, such a power converter, or power electronics in general, therefore includes a cooling device or cooling option. For example, a heat sink can be used for this purpose. The semiconductor switching elements and, if applicable, other relevant components are arranged on it, and a coolant is guided along it in a coolant path to dissipate the generated heat. The coolant can, in particular, be a liquid such as water or a water-glycol mixture. The heat sink can have cooling fins or the like located in the coolant path to dissipate the generated heat as efficiently as possible.

[0007] For the targeted and most effective control of a corresponding coolant pump, by means of which the coolant is pumped or guided along the coolant path, knowledge of the required coolant volume flow, i.e., a target volume flow, is required. On the one hand, the converter should not exceed a certain temperature, i.e., it should be protected from overheating, for which a sufficiently high volume flow is necessary. On the other hand, it is also undesirable to provide an unnecessarily high coolant volume flow, which would require unnecessary energy to operate the coolant pump.

[0008] To determine a suitable target flow rate, it is possible to determine the temperature of the semiconductor switching elements or the heat generated and dissipated by them. From this, a suitable target flow rate can then be determined. However, it has now been shown that a particularly accurate target flow rate for the coolant can also be determined by taking into account the converter's intermediate circuit capacitor and its temperature.

[0009] In the proposed method, a temperature of the converter's intermediate circuit capacitor and a coolant temperature are determined. The temperature of the intermediate circuit capacitor can, for example, be measured (or modeled) directly at the intermediate circuit capacitor using a corresponding temperature sensor. The temperature of the coolant can, for example, be measured (or modeled) using a temperature sensor at an inlet of the coolant path. However, it is also conceivable to measure the coolant temperature at another suitable location in the coolant circuit, preferably upstream and in particular near the converter's coolant path.

[0010] Based on a temperature difference between the temperature of the intermediate circuit capacitor and the temperature of the coolant, a value for the target volume flow is then determined. The latter is preferably achieved by first determining a temperature difference between the temperature of the intermediate circuit capacitor and the temperature of the coolant and then determining the value for the target volume flow based on the temperature difference and the temperature of the coolant. A coolant pump for the coolant is then expediently controlled or regulated based on the determined target volume flow. The target volume flow for the semiconductors, for example, can be a separate function and thus independent of the capacitor. Nevertheless, the target volume flow for the power converter or inverter is a max function of the capacitor, semiconductor and other temperature models.

[0011] Based on the temperature difference between the intermediate circuit capacitor – this can be a film capacitor in particular – and the coolant, it is particularly easy to determine how high the coolant's volume flow rate must or should be in order to dissipate the heat that needs to be dissipated. The temperature difference indicates how much heat needs to be dissipated, while the coolant temperature indicates how much heat the coolant can still absorb. For example, a very warm coolant can generally absorb – and dissipate – less heat (per volume or mass) than a colder coolant. Accordingly, the volume flow rate should be higher for a warmer coolant than for a colder coolant.

[0012] The target flow rate can be determined based on the temperature difference between the temperature of the intermediate circuit capacitor and the temperature of the coolant, on the one hand, and the temperature of the coolant, on the other, using a characteristic map. This map can, for example, contain suitable values ​​for the target flow rate for different values ​​of the two aforementioned variables. These values ​​can have been previously determined during test measurements and / or simulations.

[0013] It is now particularly preferred if at least one current value in the power converter is determined. From the temperature of the intermediate circuit capacitor and the temperature of the coolant (or the temperature difference and the temperature of the coolant), an intermediate value for the target volume flow can then be determined, in particular based on the aforementioned characteristic map. This intermediate value is then adjusted, in particular scaled, based on the at least one current value and then used as the value for the target volume flow.

[0014] In particular, a value of a current in an intermediate circuit of the power converter and / or a value of a phase current can be considered as the at least one value of the current in the power converter. Both the current in the intermediate circuit and the phase current are quantities that characterize a current operating status of the power converter, in particular with regard to the current power loss and thus the heat generated and dissipated. This also applies in particular to a future development of the heat generated and dissipated. If the currents are only low, it can be assumed that the heat generated and dissipated will decrease. Therefore, a lower target flow rate can also be selected. Accordingly, for high currents, a higher or lower target flow rate can be selected.This allows the cooling efficiency to be further increased, to the point of being able to switch off the coolant pump completely, at least temporarily, for example in the case of a non-active power converter, thus saving energy and reducing emissions.

[0015] The determination of the target volume flow or a current value thereof is expediently carried out repeatedly, in particular regularly, at specific time intervals. In this case, it is preferred if a time interval between two consecutive determinations of the value for the target volume flow is specified as a function of a change in a current in the power converter. In particular, one of the two currents already mentioned above, namely the current in the intermediate circuit or the phase current, can be used as the current in the power converter. It is also conceivable to use both currents, in which case both can be considered and one of them selected depending on the situation.

[0016] It is useful to specify a longer time interval for a current increase (i.e., a positive gradient) in the power converter than for a current decrease (i.e., a negative gradient). This can ensure that any adverse thermal effects occur, for example, reheating can be avoided. Heating is generally somewhat faster than cooling, which is why a distinction should be made between the two. To prevent an excessive reheating effect, the flow rate is advantageously changed slowly during cooling, while a high flow rate should be requested relatively quickly during heating.

[0017] A computing unit according to the invention, e.g. a control unit of a motor vehicle or a control unit or a power electronics unit of an electrical machine, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0018] Implementing a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for additional tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0019] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0020] The invention is illustrated schematically in the drawing using an embodiment and is described below with reference to the drawing.

[0021] Short description of the drawings Figure 1 shows a schematic diagram of a power converter in which a method according to the invention can be carried out. Figure 2 shows the power converter from Figure 1 in another representation. Figure 3 schematically shows a sequence of a method according to the invention in a preferred embodiment. Embodiment(s) of the invention

[0022] In Figure 1 A power converter 110, designed as a B6 bridge by way of example, is shown schematically, in which a method according to the invention can be carried out and which serves to control an electrical machine 100.

[0023] The power converter 110 has two DC voltage terminals 131, 132, which are connected in the usual way, in addition to an intermediate circuit capacitor 135, embodied, for example, as a film capacitor, to, for example, six semiconductor switching elements 120, for example, MOSFETs or IGBTs. A phase (stator winding) of the electric machine 100 is connected between each two of the semiconductor switching elements 120.

[0024] At this point it should be mentioned that the power converter can be operated not only as an inverter but also as a rectifier, so that the electrical machine as a whole can be operated both as a motor and as a generator.

[0025] Furthermore, the power converter 110 is connected to an on-board power supply 170, for example, in a vehicle, via its DC voltage terminals 131, 132. In turn, other components or consumers are typically connected to the on-board power supply 170, which, however, are not shown here for the sake of clarity.

[0026] During operation of the power converter 110, the individual semiconductor switching elements 120 are now controlled to open or close in a suitable manner by means of a control circuit or a control unit 150. This occurs, for example, at a specific clock frequency. In a conventional control system, for example, one switch is always closed and the other is open in each branch. In this process, a direct voltage U dc is converted into an alternating voltage, so that a phase current I flows in the phases.

[0027] The power converter 110 and optionally the control unit 150 can together form a power electronics system 140 for the electric machine 100 or be part of such a power electronics system. In particular, a current or current flow as well as a voltage can also be measured in the power converter.

[0028] In Figure 2 the converter 110 is off Figure 1in another illustration, in a sectional view. In particular, a heat sink 160 with a plurality of cooling fins 161 is additionally shown here, on the surface of which the semiconductor switching elements 120 are arranged or applied (in this view, only one semiconductor switching element per phase is visible). By appropriately connecting the semiconductor switching elements 120 to the heat sink 160, good heat transfer from the semiconductor switches 120 to the heat sink 160 can be achieved. In addition, the intermediate circuit capacitor 135 is shown, which can also be arranged on the heat sink 160 to enable effective cooling.

[0029] Furthermore, a coolant path 164 is shown, which is defined, for example, by a suitable housing on the side of the heat sink 160 opposite the semiconductor switching elements 120, so that, in particular, the cooling fins 161 are also located therein. The coolant path 164 has an inlet 162 and an outlet 163, so that coolant, for example water, indicated by arrows 180, can enter through the inlet 162 and exit again through the outlet 163. In this way, the heat from the heat sink 160 can be transferred to the coolant 180. A coolant pump, indicated by reference numeral 190 for example, can be used to pump the coolant 180.

[0030] Furthermore, a temperature sensor 181 and 182 are respectively attached to the inlet 162 of the coolant path and to the intermediate circuit capacitor 135, whereby a temperature of the coolant 180 at the inlet 162 of the coolant path and a temperature of the intermediate circuit capacitor 135 can be measured. Additional temperature sensors 183 can be provided on the semiconductor switching elements 120.

[0031] In Figure 3 A schematic representation of a process according to the invention in a preferred embodiment is shown. First, the temperature TC of the intermediate circuit capacitor and the temperature TK of the coolant are determined or measured, for example by means of the Figure 2 mentioned temperature sensors.

[0032] A temperature difference ΔT can then be calculated from the temperature TC and the temperature TK according to the formula ΔT=TC -TK. Based on the temperature difference ΔT and the temperature TK of the coolant, an intermediate value V̇ Z of the target volume flow for the coolant can then be determined using a characteristic map K 1. For this purpose, the values ​​stored in the characteristic map K 1 can be determined, for example, based on test measurements and / or simulations. It is also conceivable that the temperature difference is not explicitly determined or calculated, but that the values ​​stored in the characteristic map apply directly to the temperatures TC and TK.

[0033] Furthermore, a current IZ in the intermediate circuit and / or a phase current IP are determined or measured using appropriate sensors or measuring devices. Based on a respective characteristic curve K 1 or K 2, a factor can then be determined, which can be used to adjust, in particular scale down, the intermediate value V̇ Z. It is advisable to use only one of the two factors, specifically the one that leads to a higher target flow rate, to offset any uncertainties. This then provides the target flow rate V, which can be used to control the coolant pump.

[0034] As already mentioned, this target volume flow can be determined regularly in order to always obtain the currently required value for the target volume flow in order to operate the coolant pump efficiently, and if possible to be able to switch it off temporarily. In this context, a change or gradient ΔI Z or ΔI P can also be determined for the current IZ in the intermediate circuit and / or the phase current IP. If only one gradient is determined and this gradient is positive, i.e. the current is increasing, a time interval Δt between two consecutive determinations of the value for the target volume flow V can be set to a small value, otherwise to a large value.

[0035] A time interval is conveniently specified using a time grid of the model used; for example, the function can be called every 100 ms. For example, if a fixed torque and a fixed speed are used, but with two different DC link voltages, the gradient of the DC link current changes, so that either less or more heat must be dissipated. If the signs differ, the max function applies, meaning the larger value is used.

Claims

1. Method for determining a target volumetric flow rate (V) for a coolant (180), which is conducted through a coolant path (164) to cool a power converter (110), wherein a temperature (TC) of a DC-link capacitor (135) of the power converter (110) and a temperature (TK) of the coolant are determined, and wherein a value for the target volumetric flow rate (V) is determined based on a temperature difference (ΔT) between the temperature (Tc) of the DC-link capacitor and the temperature (TK) of the coolant, characterized in that at least one value of a current in the power converter is determined, and wherein, based on the at least one value of the current, an intermediate value (V̇Z) for the target volumetric flow rate determined from the temperature (TC) of the DC-link capacitor and the temperature (TK) of the coolant, in particular on the basis of a characteristic map (K1), is adapted, in particular scaled, and used as the value for the target volumetric flow rate (V).

2. Method according to Claim 1, wherein a value of a current (IZ) in a DC link of the power converter and / or a value of a phase current (IP) is / are used as the at least one value of the current in the power converter.

3. Method according to any one of the preceding claims, wherein a time interval (Δt) between two successive determinations of the value for the target volumetric flow rate (V) is predefined as a function of a change of a current in the power converter.

4. Method according to Claim 3, wherein, upon an increase of the current in the power converter, a longer time interval is predefined than upon a decrease of the current in the power converter.

5. Method according to any one of the preceding claims, wherein a coolant pump (190) for the coolant (180) is activated based on the determined target volumetric flow rate (V̇).

6. Processing unit (140), which is configured to carry out all method steps of a method according to any one of the preceding claims.

7. Computer program, which causes a processing unit (140) to carry out all method steps of a method according to any one of Claims 1 to 5 when it is executed on the processing unit (140).

8. Machine-readable storage medium having a computer program according to Claim 7 stored thereon.