Method for operating an electric motor

Abstract

A method for operating an electric motor (12), wherein the electric motor (12) is in a rotor-stator-rotor configuration and has a first gap (22) with a first thickness d1 between a first rotor (18) and a stator (20) and a second gap (26) with a second thickness d2 between the stator (20) and a second rotor (24), and wherein the first rotor (18) and the second rotor (24) can be electrically excited separately, characterized in that the method comprises the following steps: a) detecting a first open-circuit voltage of the first rotor (18) and detecting a second open-circuit voltage of the second rotor (24); b) classifying the first open-circuit voltage and the second open-circuit voltage; and c) setting the electrical excitation of at least one of the first rotor (18) and the second rotor (24) based on at least one of the classified open-circuit voltages.

Description

The present invention relates to a method for operating an electric motor. The present invention further relates to an electric drive system and a computer program product, which are configured to carry out the method for operating the electric motor. Electric motors, especially as drive systems for vehicles, are becoming increasingly widespread. Electric motors often feature an arrangement of multiple stators and / or rotors positioned close together on a single shaft. Due to its design and manufacturing processes, the assembly is subject to tolerances. These tolerances necessitate large air gaps between the rotor and stator. Furthermore, deviations from the nominally identical air gaps can lead to uneven forces acting on the assembly components. This, in turn, can result in undesirable effects such as uneven bearing loading, which negatively impacts service life and therefore costs, or negatively affects NVH (noise, vibration, and harshness). The generic patent DE 10 2020 126 068 A1 relates to an axial flux machine with a rotor and two stators spaced axially apart from the rotor. A first stator comprises a first stator winding. A second stator comprises a second stator winding. To reduce axial vibrations, a device is provided for generating an axially directed force on the rotor by differentially energizing opposing phase coils of the first and second stator windings. US 9 157 416 B2 shows an axial flux machine with a stator-rotor-stator arrangement, wherein the rotor is a permanent magnet rotor. The state-of-the-art solutions can be further improved, particularly with regard to compensating for tolerances of the assembly and reducing the associated disadvantages. The object of the present invention is to overcome at least one disadvantage of the prior art, at least partially. In particular, the object of the present invention is to provide a solution that can advantageously compensate for component tolerances and / or reduce their disadvantages. The problem is solved by a method having the features of claim 1. The problem is further solved by a computer program product having the features of claim 8 and by an electric drive system having the features of claim 9. Preferred embodiments of the invention are described in the dependent claims, in the description or the figures, wherein further features described or shown in the dependent claims or in the description or the figures, individually or in any combination, may constitute subject matter of the invention unless the context clearly indicates otherwise. A method for operating an electric motor is described, wherein the electric motor is in a rotor-stator-rotor configuration and has a first gap with a first thickness d1 between a first rotor and a stator, and a second gap with a second thickness d2 between the stator and a second rotor, and wherein the first rotor and the second rotor can be electrically excited separately, the method comprising the following steps: a) detecting a first open-circuit voltage of the first rotor and detecting a second open-circuit voltage of the second rotor; b) classifying the first open-circuit voltage and the second open-circuit voltage; and c) setting the electrical excitation of at least one of the first rotor and the second rotor based on at least one of the classified open-circuit voltages. Such a method can compensate for design-related tolerances, especially of the rotor-stator-rotor assembly, or reduce or completely prevent their disadvantages. The method described here is thus used to operate an electric motor. The electric motor is not fundamentally limited in its design; however, it is essential that the electric motor be in a rotor-stator-rotor configuration. This configuration is specifically defined as having a first gap of thickness d1 between a first rotor and a stator, and a second gap of thickness d2 between the stator and a second rotor. The stator and the rotors are advantageously arranged on the same axis, with the first and second rotors positioned axially opposite each other and adjacent to the stator. The first and second rotors can be electrically excited independently. This can be achieved, in particular, by energizing the first and / or the second rotor, with each being possible to be energized independently.For this purpose, the rotors have a coil which, in particular, can run continuously through the rotors in corresponding windings. The stator can also be actively excited. However, it is not fundamentally excluded that the stator has one or more passive magnets. For example, the electric motor can be an axial flux machine (AFM) with two rotors, also known as a double rotor, which is configured in a rotor-stator-rotor configuration. Due to the technology, axial flux machines have two rotor disks located outside the stator, which are considered two rotors within the meaning of the invention. However, radial flux machines (RFM) with electrical excitation in the rotor are also conceivable and covered by the scope of the invention. For example, due to the design, the thickness d1 of the first gap may differ from the thickness d2 of the second gap. To compensate for the resulting disadvantages, the method for operating the aforementioned electric motor comprises the following steps. According to process step a), a first open-circuit voltage of the first rotor and a second open-circuit voltage of the second rotor are detected. The open-circuit voltage of both rotors is thus determined from the rotor-stator-rotor configuration. This is easily accomplished using the existing measuring systems and electronics used to control the current. According to procedure step b), the first and second open-circuit voltages are then classified. This classification can be performed in various ways, as explained in detail below. It is essential that the open-circuit voltage is indicative of the distance between the respective rotor and the stator. In other words, the first open-circuit voltage is indicative of the distance d1, and the second open-circuit voltage is indicative of the distance d2. According to process step c), the electrical excitation of at least one rotor is adjusted based on at least one of the classified open-circuit voltages. This means that the excitation is not simply based on a predetermined value, but rather that the classification of the first and / or second open-circuit voltage is taken into account when adjusting the excitation of the first and / or second rotor. This step can be achieved, for example, via appropriate control in the inverter, such as by reducing or increasing the voltage. Alternatively, a hardware implementation using a suitable series resistor in the excitation circuit is also possible. The method described here thus exploits the fact that the open-circuit voltage of the respective rotors can provide an indication of the respective distance between the rotors and the stator. It may be sufficient to infer, for example, a difference in the open-circuit voltages from a difference in the thicknesses d1 and d2, also referred to as gap thicknesses. It has been shown that an asymmetrical structure, particularly one characterized by differing gap thicknesses d1 and d2, can have disadvantages. Such asymmetries can occur in electric motors due to design and manufacturing processes, as the stator and rotor assembly is subject to certain tolerances. These tolerances necessitate large air gaps between the rotor and stator. Furthermore, deviations from the nominally identical air gaps can lead to uneven forces acting on the assembly components. This, in turn, can result in undesirable effects, such as uneven bearing loading, which negatively impacts service life and thus costs, or negatively affects NVH (noise, vibration, and harshness) behavior. If, according to the invention, at least one rotor is energized and thus excited based on at least one of the classified open-circuit voltages, these effects can be effectively counteracted. It has been shown that exciting at least one rotor, preferably both rotors, based on the classification makes it possible to effectively counteract the disadvantages that arise in particular from an asymmetry. In particular, the present invention works as follows. With uniform excitation of the first and second rotors and an asymmetrical gap thickness, a different force acts from the rotors on the stator. Since this respective force depends, in addition to the gap thickness d1 and d2, on the magnetic flux of the first and second rotors respectively, the magnetic flux and thus ultimately the acting force can be adjusted by asymmetrically supplying current to the rotors. This allows the forces acting on the bearings and also the NVH behavior to be improved. This makes it possible to reduce the bearing load on the rotors, which can significantly increase the service life of the corresponding assembly. This also results in improved NVH (Noise, Vibration, Harshness) behavior. Consequently, the operation of a vehicle with a suitably powered electric motor can be significantly improved and made particularly comfortable for passengers. Furthermore, advantages arise in the manufacture of such systems. By counteracting differing gap thicknesses, a potentially coarser tolerance of gap thicknesses can be achieved, as this effectively counteracts the disadvantages that arise from such variations. Likewise, the components can be designed to be smaller. Furthermore, a reduction in air gap thickness can potentially improve performance. Additionally, the forces acting on the rotors can be reduced. A further advantage is that the method is particularly simple. This is especially true when compared to a stator-rotor-stator configuration, where excitation control would be significantly more complex. Therefore, the method according to the present invention is characterized by its simplicity and, consequently, by a greatly reduced susceptibility to errors. It may be preferred that a classification according to process step b) is carried out based on a difference in the open-circuit voltages, i.e., the first open-circuit voltage and the second open-circuit voltage. In this embodiment, it can thus be exploited that it is not absolutely necessary to determine the absolute gap thicknesses d1 and d2, but that to achieve the advantages of the invention, it is preferably sufficient to determine a difference in the thicknesses d1 and d2. Accordingly, this embodiment may be preferred and may be particularly easy to implement. In principle, it may be preferable for classification according to procedure step b) to be based on predefined limit values. This design can also make the procedure particularly simple, as no complex calculations are necessary; instead, a comparatively simple and error-free comparison with predefined data can be performed. This can also be particularly easy in the previously described design, where classification according to procedure step b) is based on a difference in open-circuit voltages, since it is easy to check whether the difference lies within a predefined range of values ​​or not. Accordingly, it can be advantageous for the predefined limit values ​​to address an asymmetry, and thus a difference, between the first thickness d1 and the second thickness d2. As described above, the limit values ​​can, in this case, define a range in which the thicknesses are equal or exhibit a tolerable difference. In particular, counteracting an asymmetry can effectively mitigate the aforementioned disadvantages. Similarly, a difference in open-circuit voltages can be attributed to an asymmetry, i.e., a difference in gap thickness. This can be achieved by determining the limit values ​​through appropriate experiments or by providing them through simulations. Particularly when an asymmetry exists, it can be advantageous to excite the first and second rotors asymmetrically, i.e., with different intensities, in process step c). Different or asymmetrical excitation allows for the effective and precise control of any disadvantages arising from differing gap thicknesses d1 and d2. For example, the excitation of one rotor can be reduced and / or increased in the second, starting from a planned or intended excitation. The excitation level is fundamentally dependent on the respective open-circuit voltage or its classification, and thus, for instance, on the degree of asymmetry. It may be further preferred that the method is a calibration procedure which is carried out at predefined operating points during the operation of an electric motor. In this embodiment, the method can be carried out periodically at given operating points, such as when starting or commissioning the electric motor, during servicing, or even during operation, for example, by specialized real-time control during operation or when faults occur. This allows for the detection and correction of varying gap thicknesses. However, it is also possible that the excitation of the rotors is carried out for a certain period of time based on the classified data without further control, i.e., without carrying out the method according to the invention, since the risk of a changing gap thickness may be negligible after short operating times. Regarding further technical features or advantages of the method, reference is made to the description of the computer program product, the drive system, the figures and the description of the figures. Furthermore, a computer program product is described, comprising instructions which, when the program is executed by a computer, cause it to perform the procedure as described above. Such a computer program product can, for example, be loaded onto a control unit of the electric motor or a control unit for the electric motor and include control commands that initialize the steps of the procedure described here. Accordingly, the control unit can, for instance, initialize corresponding measurements and control the excitation of the rotors, as described above with reference to the procedure. The advantage of such a computer program product can be seen in the fact that, as is also relevant to the method, by detecting in particular different gap thicknesses between the stator and the rotors, the resulting disadvantages, such as increased bearing load and a negatively affected NVH behavior, can be counteracted. Regarding further technical features or advantages of the computer program product, reference is made to the description of the method, the drive system, the figures and the description of the figures. Furthermore, an electric drive system is described, comprising an electric motor, wherein the electric motor is in a rotor-stator-rotor configuration and has a first gap with a first thickness d1 between a first rotor and a stator and a second gap with a second thickness d2 between the stator and a second rotor, and wherein the first rotor and the second rotor can be electrically excited separately, wherein the drive system has a control unit configured to execute a method as described. Regarding the electric motor, reference is made to the preceding description. Preferably, the electric motor can be an axial flux machine, but this is not the only possible configuration. The drive system includes a control unit, which can be part of the electric motor or connected to it, and which is configured to execute the described method. For this purpose, a computer program product described above can be loaded onto the control unit or a data processing device included therein. The advantage of such a drive system can be seen in the fact that, as is also relevant to the method, by detecting in particular different gap thicknesses between the stator and the rotors, the resulting disadvantages, such as increased bearing load and a negatively affected NVH behavior, can be counteracted. Regarding further technical features or advantages of the drive system, reference is made to the description of the method, the computer program product, the figures and the description of the figures. In particular, the following features can be combined individually or in any combination with the aforementioned items: For example, the first thickness can differ from the second thickness. In other words, the distances of the two rotors to the stator can differ. Furthermore, this difference can arise due to manufacturing deviations or tolerances. In other words, it may be preferable if this is a static or constant thickness difference between the two thicknesses. Furthermore, it may be preferred that the first rotor has a first rotor winding, particularly for exciting the first rotor. Alternatively or additionally, it may be preferred that the second rotor has a second rotor winding, particularly for exciting the second rotor. It may also be advantageous if the stator has a stator winding and / or is designed without permanent magnets. The first rotor, in particular the first rotor winding, can be excited by means of a first voltage, a first DC voltage or a first DC voltage level, preferably by applying the first voltage, the second DC voltage or the second DC voltage level to the first rotor winding. The second rotor, in particular the second rotor winding, can be excited by means of a second voltage, a second DC voltage or a second DC voltage level, preferably by applying the second voltage, the second DC voltage or the second DC voltage level to the second rotor winding. Preferably, the first voltage, the first DC voltage or the first DC voltage level can differ from the second voltage, the second DC voltage or the second DC voltage level, particularly in its value or magnitude. The first voltage, first DC voltage or first DC voltage level may preferably be higher than the second voltage, second DC voltage or second DC voltage level if the first thickness is greater than the second thickness. The second voltage, second DC voltage, or second DC voltage level can be higher than the first voltage, first DC voltage, or first DC voltage level if the second thickness is greater than the first. The first voltage, first DC voltage, or first DC voltage level can be proportional to the first thickness, while the second voltage, second DC voltage, or second DC voltage level can be proportional to the second thickness. The first voltage, first DC voltage or first DC voltage level can be provided by a first DC voltage source or a first voltage regulator and / or the second voltage, second DC voltage or second DC voltage level by a second DC voltage source or a second voltage regulator. The DC voltage sources or DC voltage regulators can be operated, controlled and / or regulated independently of each other, in particular to provide the appropriate and / or different voltage, DC voltage or DC voltage level for the rotor windings. The DC voltage sources or DC voltage regulators can preferably be integrated into an inverter. The inverter can preferably serve as a voltage source for the stator, in particular the stator winding, preferably for three-phase current or three-phase voltage. The open-circuit voltage can preferably be measured using the rotor windings, in particular by measuring the voltages induced in the rotor windings. For this purpose, voltage or current sensors can be provided that measure the induced voltage, especially the open-circuit voltage. A significant advantage of the invention, particularly in comparison to so-called stator-rotor-stator arrangements, is that the invention enables independent excitation of the two rotors, in particular rotor windings, of the rotor-stator-rotor arrangement, which allows for individual adjustment to compensate for different gap thicknesses during operation. The invention is further explained below with reference to the figures, whereby one or more features of the figures, individually or in combination, can constitute a feature of the invention. Furthermore, the figures are to be considered merely exemplary and in no way limiting. Fig. 1 shows an electric drive system according to an embodiment of the present invention; and Fig. 2 shows a detailed view of an electric motor for a drive system from Fig. 1. Figure 1 shows a schematic representation of an electric drive system 10, which can be used, for example, in an electrically powered vehicle. The drive system 10 basically comprises an electric motor 12 and a control unit 14, which is connected to the electric motor 12 via a data connection 16. Through the data connection 16, the control unit 14 can influence the operation of the electric motor 12 and retrieve data on the function of the electric motor 12. The electric motor 12 is shown in detail in Fig. 2. The electric motor 12 is configured as a rotor-stator-rotor and is specifically designed as an axial flux machine. Accordingly, the electric motor 12, or rather the rotor-stator-rotor configuration, has a first gap 22 with a first thickness d1 between a first rotor 18 and a stator 20, and a second gap 26 with a thickness d2 between the stator 20 and a second rotor 24. The first rotor 18 and the second rotor 24 each comprise a correspondingly wound coil 28, 30 to form different poles 32, 34. Accordingly, the first rotor 18 and the second rotor 24 can be electrically excited separately by applying a corresponding current. It is further shown that the stator 20 also has a coil 36 in order to be electrically excited, forming different poles 38, 40. In principle, however, the stator 20 can also comprise a permanent magnet. Often, the thicknesses d1 and d2 differ due to the design, resulting in different forces, indicated by arrows 42 and 44, acting on the stator 20 from the first rotor 18 and the second rotor 24 when the first rotor 18 and the second rotor 24 are excited or energized in the same way. Since this force depends not only on the gap thicknesses d1 and d2 but also on the magnetic flux of the first rotor 18 and the second rotor 24, the magnetic flux, and thus ultimately the acting force, can be adjusted by asymmetrically energizing the rotors 18 and 24. This allows for improvements in the forces acting on the bearings and also in the NVH (noise, vibration, and harshness) behavior. Any asymmetry between the gap thicknesses d1 and d2 can be inferred from the open-circuit voltages. A corresponding method, which may preferably be a calibration method, comprises the following process steps: a) detecting a first open-circuit voltage of the first rotor 18 and detecting a second open-circuit voltage of the second rotor 24; b) classifying the first open-circuit voltage and the second open-circuit voltage, in particular based on a difference between the open-circuit voltages and based on predefinable limit values; and c) setting the electrical excitation of at least one of the first rotor 18 and the second rotor 24 based on at least one of the classified open-circuit voltages, wherein in process step c) the first rotor 18 and the second rotor 24 are excited asymmetrically. The limit values ​​preferably relate to an asymmetry of the first distance d1 and the second distance d2.

Claims

A method for operating an electric motor (12), wherein the electric motor (12) is in a rotor-stator-rotor configuration and has a first gap (22) with a first thickness d1 between a first rotor (18) and a stator (20) and a second gap (26) with a second thickness d2 between the stator (20) and a second rotor (24), and wherein the first rotor (18) and the second rotor (24) can be electrically excited separately, characterized in that the method comprises the following steps: a) detecting a first open-circuit voltage of the first rotor (18) and detecting a second open-circuit voltage of the second rotor (24); b) classifying the first open-circuit voltage and the second open-circuit voltage; and c) setting the electrical excitation of at least one of the first rotor (18) and the second rotor (24) based on at least one of the classified open-circuit voltages. Method according to claim 1, characterized in that a classification according to method step b) is carried out based on a difference in the open-circuit voltages. Method according to one of claims 1 or 2, characterized in that a classification according to method step b) is carried out based on predefinable limit values. Method according to claim 3, characterized in that the predefinable limit values ​​relate to an asymmetry of the first thickness d1 and the second thickness d2. Method according to one of claims 1 to 4, characterized in that in process step c) the first rotor (18) and the second rotor (24) are excited asymmetrically. Method according to one of claims 1 to 5, characterized in that the method is a calibration method which is carried out at predefinable operating points during the operation of an electric motor (12). Method according to one of claims 1 to 6, characterized in that the electric motor (12) is an axial flux machine. Computer program product comprising instructions which, when the program is executed by a computer, cause it to execute a method according to any one of claims 1 to 7. An electric drive system (10) comprising an electric motor (12), wherein the electric motor is in a rotor-stator-rotor configuration and has a first gap (22) with a first thickness d1 between a first rotor (18) and a stator (20) and has a second gap (26) with a second thickness d2 between the stator (20) and a second rotor (24), and wherein the first rotor (18) and the second rotor (24) can be electrically excited separately, characterized in that the drive system (10) has a control unit (14) which is configured to carry out a method according to one of claims 1 to 7. Electric drive system (10) according to claim 9, characterized in that the electric motor (12) is an axial flux machine.

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