Rotor for an electric machine and electric machine

Abstract

Rotor for an electric machine, wherein: - the rotor has a rotor shaft (2) and a rotor core (1), - the rotor has a sensor with at least one sensor element (6) configured to detect a first measurement quantity representative of a torque of the rotor, and / or configured to detect a second measurement quantity representative of a temperature of the rotor, wherein the respective sensor element (6) is arranged on the rotor core, - the rotor core (1) has at least two first recesses (4) that serve to reduce a rotational mass of the rotor and are separated by a web (5), - and the respective sensor element is arranged on one of the webs (5) that are formed between two adjacent first recesses (4).

Description

The invention relates to a rotor of an electric machine and an electric machine comprising the rotor. Furthermore, the invention relates to a method and a corresponding device for operating an electric machine. Electric machines are increasingly used as drive motors in motor vehicles. For efficient operation of the electric machine, precise knowledge of the current torque output is advantageous. Furthermore, determining or estimating an instantaneous temperature is necessary for the safe operation of the electric machine in many applications. Depending on the type and application of the electric machine, it is typically necessary to monitor the temperature of multiple components of that machine. US 2011 / 0273121 A1 relates to an electric machine that has a temperature sensor coupled to a rotor assembly of the electric machine in order to detect the temperature of the rotor assembly. DE 102004004687 A1 relates to an electric motor that includes a sensor element for detecting the torque of the electric machine. DE 102017011969 A1 relates to the design of a rotor unit for an electric motor. The rotor unit has a rotor segment body with a plurality of receiving openings in which magnetic elements are arranged. The behavior of strain gauges, which are suitable, among other things, for detecting torque, is described, for example, in “Wikipedia: Strain gauges; URL:https: / / de.wikipedia.org / w / index.php?title=Dehnungsmessstreifen&oldid=180676272”.WO 2008 / 025681 A1 describes that sensors are provided for monitoring an electrical machine, which may include temperature sensors, rotary encoders or the like. The object underlying the invention is to create a rotor for an electric machine and an electric machine as well as methods and corresponding devices that enable precise control of the electric machine and / or more efficient operation of the electric machine. The problem is solved by the features of the independent patent claims. Advantageous embodiments of the invention are characterized in the dependent claims. According to a first aspect, the invention is characterized by a rotor for an electric machine. The rotor comprises a rotor shaft and a rotor core. The rotor has a sensor with at least one sensor element. The sensor element is configured to detect a first measured quantity that is representative of a torque of the rotor, and / or is configured to detect a second measured quantity that is representative of a temperature of the rotor. The respective sensor element is arranged on the rotor core. The rotor core has at least two first recesses, which serve to reduce the rotor's rotational mass and are separated by a bridge. The respective sensor element is arranged on one of the bridges, which are formed between two adjacent first recesses. This enables simple integration and reliable force measurement. Advantageously, the torque delivered by the electric machine and / or the rotor temperature can be measured directly. It is not necessary to determine the rotor's torque or temperature using a model. Integrating at least one sensor element for torque measurement increases the machine's torque accuracy. Direct torque measurement allows for the optimization of the electric machine's stator currents, particularly with regard to maximizing efficiency. Depending on the measured torque, the stator currents can be adjusted to account for tolerances in the electric machine, such as manufacturing tolerances and component aging over its lifetime. This can at least partially eliminate the need for expensive materials, such as permanent magnets, and tight manufacturing tolerances. The torque determined by direct measurement can be significantly more accurate than a rotor torque calculated using a model based on parameters such as current, voltage, and rotor temperature. This allows deviations between the actual torque and the torque determined by direct measurement to be kept considerably smaller than the 10% that can occur with an indirect torque determination. Due to the geometrically close mounting position of the sensor element on the rotor and the good thermal coupling of the sensor element to the rotor, the rotor temperature can be determined very accurately. This more precise temperature determination enables a more accurate design of the electric machine, as temperature tolerances can be defined more tightly. With more precise knowledge of the rotor temperature, the temperatures of other machine elements can also be determined more accurately. This allows for a more dynamically precise determination of the electric machine's torque and a smaller safety margin regarding the maximum operating temperature. Machine design can thus be improved or optimized. The sensor element can be attached to the rotor, for example, by clamping, gluing, or screwing it on. The sensor element can be attached to the rotor using a detachable or a permanent connection. In a further advantageous embodiment according to the first aspect, the at least two first recesses extend along a longitudinal axis of the rotor core. In a further advantageous embodiment according to the first aspect, the at least two first recesses are arranged symmetrically around the rotor shaft in the circumferential direction of the rotor core. Advantageously, this avoids or at least minimizes imbalance in the electric machine. By directly attaching the sensor element to the rotor, the rotor can be balanced after the sensor element is applied, thus minimizing any imbalances caused by the sensor element. In a further advantageous embodiment according to the first aspect, the rotor core has second recesses for arranging permanent magnets or windings, and the first recesses are arranged radially between the second recesses and the rotor shaft. Integrating the magnets has the advantage that the rotor diameter can be reduced. Furthermore, by concentrating the magnetic field, the magnetic flux density of the air gap can be increased, which allows for lower magnetic control of the electric machine. In a further advantageous embodiment according to the first aspect, the sensor element comprises at least one piezoresistive strain gauge. This has the advantage that the sensor can be provided in a compact and cost-effective manner. In a further advantageous embodiment according to the first aspect, at least one strain gauge exhibits a resistance with a predetermined temperature dependence. This has the advantage that, in addition to the rotor torque, the rotor temperature can be very accurately measured and / or determined using the same sensor element. Here, too, measurement close to the magnets, for example in a permanent magnet synchronous machine or on a squirrel-cage motor, is particularly advantageous. In a further advantageous embodiment according to the first aspect, the sensor comprises a first evaluation electronics unit that is electrically coupled to the at least one sensor element for receiving sensor signals from the at least one sensor element, wherein the first evaluation electronics unit is arranged in a bearing shell of the rotor. This enables a space-saving arrangement of the sensor. In a further advantageous embodiment according to the first aspect, the first evaluation electronics are electrically coupled to a second sensor element for receiving second sensor signals, wherein the second sensor element is configured to detect a third measured quantity that is representative of an angular position or change in angular position of the rotor. This enables a space-saving and cost-effective arrangement of the various sensors used for controlling the electric machine. According to a second aspect, the invention is characterized by an electric machine having a rotor according to the first aspect. Advantageous embodiments of the first aspect also apply to the second aspect. According to a third and fourth aspect, the invention is characterized by a method and a corresponding device for operating an electric machine. The electric machine has a rotor according to the first aspect. Depending on the first measured quantity, a torque of the rotor and / or, depending on the second measured quantity, a temperature of the rotor are determined. Preferably, the electric machine is controlled and / or regulated depending on the determined torque and / or temperature. Advantageous designs according to the first aspect also apply to the third and fourth aspects. Exemplary embodiments of the invention are explained below with reference to the schematic drawings. Figure 1a shows a perspective view of an embodiment of a rotor of an electric machine, Figure 1b shows a front view of another embodiment of a rotor of an electric machine, Figure 2 shows a model for a field-oriented control of an electric machine, and Figure 3 shows an exemplary electrical equivalent circuit for a sensor element. Elements of the same construction or function are provided with the same reference symbols across all figures. Fig. 1a shows a perspective view of an embodiment of a rotor of an electric machine. The electric machine is, for example, a permanent magnet synchronous machine. As shown in Fig. 1, the rotor comprises a rotor core 1 and a rotor shaft 2, the rotor shaft 2 being arranged within the rotor core 1; in particular, the rotor shaft 2 is fixedly arranged in a shaft hole in the rotor core 1 to enable the rotor core 1 to rotate the rotor shaft 2 together with the rotor core 1. The material of the rotor core 1 comprises, for example, silicon steel. As shown in Fig. 1a, at least two first recesses 4 are arranged in the rotor core 1, separated by a web 5. In the embodiment shown in Fig. 1, the rotor core 1 has four first recesses 4. The first recesses 4 extend along an axial longitudinal direction of the rotor shaft 2. The first recesses 4 serve in particular to reduce the rotational mass of the rotor. The first recesses 4, for example, are arranged symmetrically around the rotor shaft 2 in the circumferential direction of the rotor core 1. Preferably, the respective first recesses 4 are identical or at least very similar in their design. However, the first recesses 4 can be designed differently from rotor to rotor. The rotor core 1, for example, has second recesses 3 for arranging permanent magnets or windings. The first recesses 4 are preferably arranged radially between the second recesses 3 and the rotor shaft 2. The rotor comprises a sensor with at least one sensor element 6 configured to detect or provide for evaluation a first measured quantity that is representative of a rotor torque, and / or configured to detect or provide for evaluation a second measured quantity that is representative of a rotor temperature. The respective sensor element 6 is arranged on the rotor core. For example, the respective sensor element 6 is arranged on one of the webs 5, each of which is formed between two adjacent first recesses 4. Fig. 1b shows a front view of another embodiment of an electric machine rotor. The rotor comprises a rotor core 1, a rotor shaft 2, first recesses 4, and second recesses 3. The rotor shaft 2 is, for example, designed as a hollow shaft. In contrast to the embodiment shown in Fig. 1a, the second recesses 3 in the embodiment shown in Fig. 1b are shaped differently. Furthermore, the rotor in Fig. 1b has four sensor elements 6, which are, for example, arranged symmetrically around the rotor shaft 2 to minimize possible imbalances. The sensor elements 6 shown in Fig. 1a and Fig. 1b include, for example, at least one piezoresistive strain gauge. At least one strain gauge is, for example, arranged on a silicon chip. The strain gauges are, for example, glazed onto the silicon chip. The strain gauges are, for example, designed as a microsystem chip, or microelectromechanical system (MEMS) chip. For example, the sensor element 6 has four strain gauges that form a measuring bridge. Fig. 2 shows an exemplary electrical equivalent circuit diagram of such a measuring bridge of the sensor element 6 with four strain gauges. The sensor is designed to determine the torque of the electric machine by measuring the voltage across the measuring bridge. The torque transmitted via the shaft causes a torsion of the rotor, which manifests as compressive and tensile stresses at a 45° angle along a longitudinal axis of the rotor. The resistance value of each strain gauge exhibits a predetermined temperature dependence. Fig. 3 shows an exemplary electrical equivalent circuit diagram of the sensor element 6. The strain gauges are electrically coupled to form a measuring bridge. As shown in Fig. 3, each strain gauge is assigned a temperature-dependent resistance R1T, R2T, R3T, R4T and a pressure- or tension-dependent resistance R1P, R2P, R3P, R4P. In this case, the strain gauges are connected together to form a closed ring or a square. The measuring bridge has four terminals, each with an associated terminal resistor RA1, RA2, RA3, RA4. Two voltage terminals V+, V-, which are arranged opposite each other, are connected to a voltage source, for example, and two measuring terminals S+, S-, which are arranged opposite each other, are connected to a voltmeter. For example, the temperature of the rotor is determined based on a determined change in the resistance of at least one strain gauge of the sensor element 6 and a data set representing the specified temperature dependence of the at least one strain gauge. A torque transmitted via the rotor causes rotor torsion, which manifests as compressive and tensile stresses at a 45° angle to the rotor's axial direction. This rotor torsion leads to a change in the resistance of the strain gauges, which can be evaluated, for example, by measuring the stress across the strain gauges. The sensor includes a first evaluation electronics unit (not shown in Figures 1a and 1b) which is electrically coupled to the at least one sensor element 6 for receiving sensor signals from the at least one sensor element 6. The first evaluation electronics unit is, for example, arranged in a bearing shell of the rotor. The first evaluation electronics unit is, for example, wirelessly coupled to a control device, which can also be referred to as a device for operating the electric machine. The control device is designed to determine a torque of the rotor depending on the first measured quantity and / or a temperature of the rotor depending on the second measured quantity and, for example, to control and / or regulate the electric machine depending on the determined torque and / or the determined temperature. Optionally, the first evaluation electronics are electrically coupled to a second sensor element for receiving secondary sensor signals. This second sensor element is configured to acquire or provide for evaluation a third measured variable that is representative of the rotor's angular position or change in angular position. The second sensor element may, for example, include a resolver. Fig. 3 shows a model for the control of an electric machine. The control system is, for example, designed as a field-oriented control system. Field-oriented control (also known as vector control) is a control method for three-phase machines. The goal of this control method for asynchronous or synchronous machines is to achieve decoupled control of magnetic flux and torque in order to replicate the behavior of a DC shunt-wound machine. The model shown in Fig. 3 applies to a permanent magnet synchronous machine. In the forward direction, the model comprises a PI current controller 20, a first unit 22 configured to perform an inverse Clarke transform and a space vector pulse-width modulation, and, for example, a B6 inverter 24. A second unit 26, configured to perform a Clarke-Parks transform, is arranged in a feedback loop. Furthermore, a third unit is connected upstream of the control loop, configured to determine, based on input variables such as rotational speed, magnetic flux, and requested torque Treq, and a predefined characteristic curve, target components Id_s and Iq_s of the stator current that are supplied to the control loop. The torque of a permanent magnet synchronous machine can be described by Eq. 1: Here, Tq denotes internal torque (electromagnetic torque without mechanical torque losses), Iq, Id components of the stator current in the field-oriented d / q coordinate system, Ψ rotor flux in the field-oriented d / q coordinate system, Ld, Lq components of the stator inductances in the field-oriented d / q coordinate system, and Zp number of pole pairs. The values ​​Ψ, Ld, and Lq are not precisely known due to manufacturing tolerances and the aging of the electric machine. Furthermore, the determination of the actual components Iq_i and Iq_i of the stator current is also subject to tolerances due to measurement inaccuracies of shunts or Hall sensors, as well as tolerances in the rotor position. The delivered torque is therefore subject to tolerances that negatively affect control accuracy, especially in hybrid drives, and the efficiency of the electric machine. The target components Id_s and Iq_s of the stator current are determined, for example, using a characteristic map. This map represents the inverse of equation (1) below and uses a requested torque, rotational speed, magnetic flux, and temperature as input variables. Since tolerances for the actual machine are not taken into account in the characteristic map, efficient machine operation is often not guaranteed. Furthermore, it is not certain that the requested torque will actually be achieved. Deviations of approximately 10% between the requested and actual torque are possible. Direct measurement of the torque delivered to the rotor enables improved torque pre-control. The sensitivity of the sensor is increased by the targeted positioning of the strain gauges on the webs 5 of the recesses on the rotor. By specifically positioning the sensor on the ribs 5 of the recesses, the torque and temperature of the rotor can be determined very accurately. Integrating a sensor element 6 increases the torque accuracy of the machine. The torque sensor is also suitable for adapting the torque feedforward control. Furthermore, temperature measurement close to the magnets is particularly advantageous for permanent magnet synchronous machines and close to a squirrel-cage resistor in asynchronous machines.

Claims

Rotor for an electric machine, wherein: - the rotor has a rotor shaft (2) and a rotor core (1), - the rotor has a sensor with at least one sensor element (6) configured to detect a first measurement quantity representative of a torque of the rotor, and / or configured to detect a second measurement quantity representative of a temperature of the rotor, wherein the respective sensor element (6) is arranged on the rotor core, - the rotor core (1) has at least two first recesses (4) that serve to reduce a rotational mass of the rotor and are separated by a web (5), - and the respective sensor element is arranged on one of the webs (5) that are formed between two adjacent first recesses (4). Rotor according to claim 1, wherein the at least two first recesses (4) extend along a longitudinal axis of the rotor core (1). Rotor according to claim 1 or 2, wherein the at least two first recesses (4) are arranged symmetrically around the rotor shaft (2) in the circumferential direction of the rotor core (1). Rotor according to any one of the preceding claims 1 to 3, wherein the rotor core (1) has second recesses (3) for arranging permanent magnets or windings and the first recesses (4) are arranged in a radial direction between the second recesses (3) and the rotor shaft (2). Rotor according to one of the preceding claims, wherein the sensor element (6) has at least one piezoresistive strain gauge. Rotor according to claim 5, wherein the at least one strain gauge has a resistance with a predetermined temperature dependence. Rotor according to one of the preceding claims, wherein the sensor has a first evaluation electronics which is electrically coupled to the at least one sensor element (6) for receiving sensor signals from the at least one sensor element (6) and which is arranged in a bearing shell of the rotor. Rotor according to claim 7, wherein the first evaluation electronics are electrically coupled to a second sensor element for receiving second sensor signals, the second sensor element being configured to detect a third measured quantity that is representative of an angular position or change in angular position of the rotor. Electric machine comprising a rotor according to any one of claims 1 to 8. Method for operating an electric machine, wherein the electric machine has a rotor according to one of claims 1 to 8 and depending on the first measured quantity a torque of the rotor and / or depending on the second measured quantity a temperature of the rotor is determined. Device for operating an electric machine configured to perform the method according to claim 10.

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