METHOD FOR OPERATING AN ELECTRIC MACHINE, DEVICE, ELECTRIC MACHINE, MOTOR VEHICLE

Description

[0001] The invention relates to a method for operating an electrical machine having a rotatably mounted rotor and at least one motor winding, wherein a temperature load of the machine is determined, and wherein the motor winding is supplied with an electrical motor current to generate a predetermined target torque as a function of the temperature load.

[0002] Furthermore, the invention relates to a device for operating an electrical machine, comprising a control unit.

[0003] Furthermore, the invention relates to an electrical machine with such a device.

[0004] Furthermore, the invention relates to a motor vehicle with such an electric machine. State of the art

[0005] The publications DE 10 2014 109677 A1, JP 2010 226914 A and GAGAS BRENT S ET AL: "Magnet Temperature Effects on the Useful Properties of Variable Flux PM Synchronous Machines and a Mitigating Method for Magnetization Changes", IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, Vol. 53, No. 3, May 1, 2017 (2017-05-01), pages 2189-2199, XP011650333, ISSN: 0093-9994, disclose methods and devices for operating electrical machines. An electrical machine typically has a rotatably mounted rotor and at least one motor winding. During operation of an electrical machine, the motor winding is usually supplied with an electrical motor current to generate a predetermined target torque.The motor winding is thus energized or supplied with the motor current in such a way that a drive magnetic field is generated and the electric machine generates or provides the specified target torque.

[0006] During operation, an electrical machine heats up. Temperatures exceeding 230°C can cause permanent magnets in a permanent magnet arrangement of the electrical machine to become demagnetized. An electrical machine with at least partially demagnetized permanent magnets will no longer deliver the specified power or will fail completely if the permanent magnets are completely demagnetized. To protect the permanent magnets from demagnetization, it is known from the prior art to determine the thermal load of the electrical machine and to take this thermal load into account when energizing the motor winding.For example, the published patent application WO 2019 / 096497 A1 discloses a method for operating an electrical machine, according to which the motor winding of the machine is energized at high temperatures such that the magnetic stator flux of the machine is reduced compared to lower temperatures. Disclosure of the invention

[0007] The method according to the invention with the features of claim 1 has the advantage of increasing the durability of the electrical machine. According to the invention, when a temperature load exceeding a predetermined load threshold is detected, a field strengthening mode of the machine is set, wherein the motor winding is energized in the field strengthening mode such that the motor current has a positive field-forming current component (Id). The electrical motor current flowing through the motor winding is generally formed by a torque-forming current component (Iq) and a field-forming current component (Id). Relative to a rotor-fixed coordinate system, the field-forming current component flows parallel to the orientation of the magnetic field of the permanent magnet arrangement.The torque-generating current component flows perpendicular to the orientation of the magnetic field of the permanent magnet arrangement and corresponds to the torque generated by the machine. Depending on the operating point of the machine, the torque-generating current component is positive, negative, or zero. The field-generating current component can also be positive, negative, or zero. According to the invention, when the temperature load exceeding the load threshold is detected, the motor winding is energized such that the motor current has a positive field-generating current component. Due to the positive field-generating current component, a field strengthening of the magnetic field of the permanent magnet arrangement is achieved, from which the term field strengthening mode is derived. Thus, due to the positive field-generating current component, a magnetic field is generated that supports the correct alignment of the micropoles of the permanent magnets of the permanent magnet arrangement.In this respect, the positive field-forming current component prevents demagnetization of the machine's permanent magnet arrangement or achieves remagnetization of the permanent magnet arrangement. This prevents high temperature loads on the electric machine from impairing the magnetization of the permanent magnets. This results in the increased durability of the electric machine achieved by the method according to the invention. Preferably, in the field strengthening mode, a target current vector for the motor current is specified, which has a positive field-forming current component, wherein the motor winding is then energized depending on the specified target current vector such that the motor current has the positive field-forming current component. According to the invention, the rotor has the permanent magnet arrangement. The permanent magnets are then connected to the rotor in a rotationally fixed manner.In this case, the motor winding is designed as a stator winding and is arranged distributed around the rotor.

[0008] According to a preferred embodiment, the motor winding is energized in the field-strengthening mode such that the percentage of the positive field-forming current in relation to the motor current is at least 10%. This at least substantially prevents demagnetization of the permanent magnets. Preferably, the percentage of the positive field-forming current is at least 20%, particularly preferably at least 50%.

[0009] According to a preferred embodiment, the motor winding is energized in the field strengthening mode such that the positive field-forming current component is as great as possible. This provides the advantages described above with regard to preventing demagnetization or achieving remagnetization to a particularly high degree. The greatest possible level of the positive field-forming current component is limited in particular by the fact that the torque-forming current component must have a certain level so that the electric machine generates the specified target torque. The motor winding is therefore set to an operating point of the electric machine depending on the specified target torque, at which the specified target torque is generated and the positive field-forming current component is as great as possible.

[0010] According to the invention, when a temperature load below the load threshold is detected, the motor winding is energized in such a way that the torque-generating current component at least substantially corresponds to the motor current. The motor current is thus at least substantially free of a field-generating current component. This enables the electric machine to operate with particularly high efficiency. A positive field-generating current component is not necessary because the load threshold is undershot.

[0011] Preferably, the temperature load on the rotor is determined as the temperature load on the machine. This is advantageous in that the rotor, or rather the permanent magnet arrangement of the rotor, is the part of the electrical machine that needs to be protected from temperature-related damage. By determining the temperature load on the rotor, a particularly precise decision can be made as to whether the positive field-forming current component is advantageous or necessary to prevent demagnetization or achieve remagnetization, or whether the positive field-forming current component currently provides no advantage due to the low temperature load. Preferably, a temperature sensor is used to detect the temperature of a stator of the electrical machine, and the temperature of the rotor is determined using a model based on the detected stator temperature.

[0012] Preferably, a temperature, in particular the temperature of the rotor, is determined as the temperature load. In this case, a threshold temperature is specified as the load threshold. For example, a temperature between 210°C and 250°C is specified as the threshold temperature, particularly preferably a temperature of 230°C. Alternatively, a temperature integral of the temperature, in particular the temperature of the rotor, is preferably determined as the temperature load. In this case, a temperature integral threshold is specified as the load threshold.

[0013] According to a preferred embodiment, it is provided that a threshold speed is specified for the rotor, and that the field strengthening mode is only set when the actual rotor speed falls below the threshold speed. Such a procedure prevents an undesirably high electrical voltage from being induced in the motor winding due to the rotation of the rotor at high actual rotor speeds that exceed the threshold speed. According to a preferred embodiment, it is provided that a threshold torque is specified for the target torque, and that the field strengthening mode is only set when the target torque falls below the threshold torque. If the target torque exceeds the threshold torque, the provision of the positive field-forming current component could impair the ability of the electric machine to generate the target torque at the specified level.This is undesirable.

[0014] The device according to the invention for operating an electrical machine having a rotatably mounted rotor and at least one motor winding is characterized by the features of claim 7, which includes a control unit specifically designed to carry out the method according to the invention when used as intended. This also results in the aforementioned advantages. Further preferred features and combinations of features emerge from the above description and from the claims.

[0015] The electrical machine according to the invention has a rotatably mounted rotor and at least one motor winding. The machine is characterized by the features of claim 8 and the device according to the invention. This also results in the aforementioned advantages. Further preferred features and combinations of features emerge from the above description and from the claims. The electrical machine is preferably designed as a permanent magnet synchronous machine. In this respect, the electrical machine has a permanent magnet arrangement. The permanent magnet arrangement is particularly preferably part of the rotor and is thus connected to the rotor in a rotationally fixed manner.

[0016] The motor vehicle according to the invention is characterized by the features of claim 9, which include the electric machine according to the invention. This also results in the aforementioned advantages. Further preferred features emerge from the above description and from the claims.

[0017] The invention is explained in more detail below with reference to the drawings. Figure 1 shows a motor vehicle in a schematic representation and Figure 2 shows a method for operating an electrical machine of the motor vehicle.

[0018] Figure 1 shows a schematic representation of a motor vehicle 1. The motor vehicle 1 has a front wheel axle 2 with two front wheels 3 and 4 and a rear wheel axle 5 with two rear wheels 6 and 7.

[0019] The motor vehicle 1 also has a drive arrangement 8 with an electric machine 9, which is designed to drive at least one of the wheels 3, 4, 6, and 7. For example, the machine 9 is designed to drive the front wheels 3 and 4, the rear wheels 6 and 7, or both the front wheels 3 and 4 and the rear wheels 6 and 7. Optionally, the drive arrangement 8 has, in addition to the machine 9, at least one further electric machine and / or at least one internal combustion engine.

[0020] The electric machine 9 has a rotor 10, which is rotatably mounted in a housing (not shown) of the machine 9. The rotor 10 has a permanent magnet arrangement 12 with at least one permanent magnet 11. In this case, several permanent magnets 11 are present. The permanent magnets 11 are connected to the rotor 10 in a rotationally fixed manner and are distributed in the circumferential direction of the rotor 10. For example, the permanent magnets 11 are neodymium-iron-boron magnets 11.

[0021] The machine 9 also has a motor winding 13. In this case, the motor winding 13 is a stator winding 13 of a stator 23 of the electric machine 9. In this case, the stator winding 13 has three phases 14, 15, and 16. The phases 14, 15, and 16 are distributed around the rotor 10 such that the rotor 10 can be rotated by appropriately energizing the phases 14, 15, and 16. In addition to the stator winding 13, the stator 23 has, for example, a support (not shown) for the stator winding 13.

[0022] The phases 14, 15 and 16 are electrically connected to an energy storage device 18 of the motor vehicle 1 by a power electronics unit 17 of the machine 9 having a plurality of switching elements.

[0023] The electric machine 9 also has a device 19 for operating the machine 9. The device 19 has a control unit 20, which is designed to control or switch the switching elements of the power electronics 17.

[0024] The motor vehicle 1 also has a speed sensor 21 associated with the rotor 10. The speed sensor 21 is designed to detect the actual speed N Ist of the rotor 10. The speed sensor 21 is communicatively connected to the control unit 20 in order to provide the control unit 20 with its sensor signal, i.e., the detected actual speed N Ist .

[0025] The motor vehicle 1 also has a temperature sensor 22. The temperature sensor 22 is assigned to the stator 23 and is designed to detect the temperature of the stator 23. The temperature sensor 22 is communicatively connected to the control unit 20 in order to provide the control unit 20 with its sensor signal, i.e., the detected temperature.

[0026] In the following, with reference to Figure 2 an advantageous method for operating the electrical machine 9 by means of the control unit 20 is described.

[0027] In a first step S1, the control unit 20 specifies a target torque M Soll to be generated by the engine 9. For example, the control unit 20 determines the level of the specified target torque M Soll depending on an actuation of an accelerator pedal by a driver of the motor vehicle 1.

[0028] In a second step S2, the temperature sensor 21 detects the temperature of the stator 23 and provides the detected temperature to the control unit 20.

[0029] In a third step S3, the control unit 20 determines a temperature load of the electric machine 9 as a function of the temperature detected by the temperature sensor 22. In this case, the control unit 20 determines the temperature of the rotor 10 as the temperature load of the electric machine 9. In this case, the control unit 20 determines the temperature of the rotor 10 preferably in a model-based manner as a function of the detected temperature of the stator 23.

[0030] In a fourth step S4, the speed sensor 21 detects the actual speed N Ist of the rotor 10 and provides the detected actual speed N Ist to the control unit 20.

[0031] The steps S1, S2, S3 and S4 are carried out continuously so that the target torque M target , the temperature load of the rotor 10 and the actual speed N actual of the rotor 10 are continuously determined or recorded.

[0032] In a fifth step S5, the control unit 20 decides whether the electric machine 9 should be operated in a standard mode or in a field-strengthening mode or field-strengthening operation. To this end, in step S5, the control unit 20 takes into account the specified target torque M target , the determined temperature load of the rotor 10, and the detected actual speed N actual of the rotor 10. Preferably, step S5 is also performed continuously.

[0033] In this case, the control unit 20 specifies a threshold load for the temperature load, a threshold speed for the actual speed N Ist , and a threshold torque for the target torque M Soll . In this case, the control unit 20 specifies a threshold temperature as the threshold load, particularly preferably a threshold temperature of 230°C. In step S5, the control unit 20 then compares the target torque M Soll with the specified threshold torque, the determined temperature load with the specified threshold load, and the detected actual speed N Ist with the specified threshold speed.

[0034] If the comparison shows that the temperature load exceeds the threshold load, that the actual speed N actual falls below the threshold speed, and that the target torque M target falls below the threshold torque, reference is made to a sixth step S6. In this step S6, the electric machine 9 is then operated in field strengthening mode. The control unit 20 then controls the switching elements of the power electronics 17 such that an electric motor current flowing through the motor winding 13 has a torque-generating current component Iq and a positive field-generating current component Id. The torque-generating current component Iq causes the electric machine 9 to generate the predetermined target torque M target. The positive field-generating current component Id creates a magnetic field that is aligned parallel to the magnetic field of the permanent magnets 11.This magnetic field prevents the permanent magnets 11 from becoming demagnetized when the temperature load on the rotor 10 exceeds the threshold load. In particular, this ensures that already demagnetized parts of the permanent magnets 11 are remagnetized. Operating the electric machine 9 in the field-strengthening mode thus increases the durability of the permanent magnet arrangement 12 and thus the durability of the electric machine 9.

[0035] However, if the comparison in step S5 shows that the target torque M target exceeds the threshold torque, that the temperature load falls below the threshold load, and / or that the actual speed N actual exceeds the threshold speed, reference is made to a seventh step S7. In the seventh step S7, the electric machine 9 is then operated in the standard mode. The control unit 20 then controls the switching elements of the power electronics 17 such that the electric motor current flowing through the motor winding 13 is at least substantially formed by the torque-forming current component Iq. The motor current is therefore at least substantially free of a positive or negative field-forming current component Id when the machine 9 is operated in the standard mode.In step S7, the motor winding is also energized in such a way that the torque-generating current component Iq ensures that the electric machine 9 generates the desired or predefined target torque M target.

Claims

1. Method for operating an electric machine, which has a rotatably mounted rotor (10) with a permanent magnet arrangement and has at least one motor winding (13), wherein a temperature load of the machine (9) is determined, and wherein an electric motor current is applied to the motor winding (13) to generate a specified desired torque (Mdesired) in dependence on the temperature load, wherein the permanent magnet arrangement comprises permanent magnets, characterized in that when a temperature load below a specified load threshold value is determined, the motor winding (13) is energized in such a way that a torque-forming current component (Iq) of the motor current corresponds at least substantially to the motor current and when a temperature load above the load threshold value is determined, a field-strengthening mode of the machine (9) is set, so that the electric machine is operated in the field-strengthening mode, wherein the motor winding (13) in the field-strengthening mode is energized in such a way that the motor current has a positive field-forming current component (Id).

2. Method according to Claim 1, characterized in that the motor winding (13) in the field-strengthening mode is energized in such a way that the percentage share of the positive field-forming current component (Id) in the motor current is at least 10%, preferably at least 20%, particularly preferably at least 50%.

3. Method according to one of the preceding claims, characterized in that the motor winding (13) in the field-strengthening mode is energized in such a way that the positive field-forming current component (Id) is as large as possible.

4. Method according to one of the preceding claims, characterized in that the temperature load of the rotor (10) is determined as the temperature load of the machine (9).

5. Method according to one of the preceding claims, characterized in that a threshold speed is specified for the rotor (10), and in that the field-strengthening mode is only set in the presence of an actual speed (Nactual) of the rotor (10) below the threshold speed.

6. Method according to one of the preceding claims, characterized in that a threshold torque is specified for the desired torque (Mdesired), and in that the field-strengthening mode is only set in the presence of a desired torque (Mdesired) below the threshold torque.

7. Device for operating an electric machine, wherein the machine (9) has a rotatably mounted rotor (10) and at least one motor winding (13), characterized in that the device (19) has a control unit (20) specifically configured to carry out the method according to one of the preceding claims when it is used as intended.

8. Electric machine, having a rotatably mounted rotor (10), and having at least one motor winding (13), characterized by a device (19) according to the preceding claim.

9. Motor vehicle characterized by an electric machine (9) according to the preceding claim.