Method for determining the rotor temperature of a permanent magnet electric machine

Abstract

The invention relates to a method for determining the rotor temperature of a rotor (5) equipped with permanent magnets (6) of a permanent magnet electric machine (2) for a motor vehicle, comprising the steps: - measuring at least one winding voltage (u1, u2, u3) on at least one stator winding (W1, W2, W3) of a stator (4) of the permanent magnet electric machine (2) during rotation of the rotor (5), - determining a rotor temperature-dependent induced voltage induced in the stator windings (W1, W2, W3) by the rotation of the rotor (5) based on the measured winding voltage (u1, u2, u3), - comparing the induced voltage with a reference induced voltage predetermined at at least one reference rotor temperature, - determining the rotor temperature based on the comparison.

Description

The invention relates to a method for determining a rotor temperature of a rotor, equipped with permanent magnets, of a permanently excited electric machine for a motor vehicle. The invention also relates to an electric drive unit and to a motor vehicle.In the present case, interest is directed to a permanently excited electric machine, which is used, for example, as drive machines for electrified motor vehicles, that is to say electric or hybrid vehicles. Such permanently excited electric machines usually have a stator with energizable stator windings for exciting a stator magnetic field and a rotor with permanent magnets for exciting a rotor magnetic field. During operation of the electric machine, the latter can heat up as a result of power loss, as a result of which a rotor temperature of the rotor and therefore also a temperature of the permanent magnets increase. In order to protect the components of the electric machine, in particular in order to prevent demagnetization of the permanent magnets, monitoring of the rotor temperature is desirable. In addition, the rotor temperature influences the current rotor flux and thus a regulation of the setpoint torque of the permanently excited electric machine. In this case, the use of temperature sensors on the rotor for temperature monitoring is technically difficult to implement on account of the rotation of the rotor. It is also known from the prior art, for example from EP 2 318 818 B1, to determine the rotor temperature by means of a thermal model based on estimated values of the rotor temperature and a stator temperature. However, such a method is very complicated and limited with regard to its accuracy.It is an object of the present invention to be able to determine a temperature of a rotor of a permanently excited electric machine for a motor vehicle in a simple manner and with high accuracy.This object is achieved according to the invention by a method, an electric drive unit and a motor vehicle having the features according to the respective independent patent claims. Advantageous embodiments of the invention are the subject matter of the dependent claims, the description and the figure.A method according to the invention serves for determining a rotor temperature of a rotor, equipped with permanent magnets, of a permanently excited electric machine for a motor vehicle. In the method, at least one winding voltage on at least one stator winding of a stator of the permanently excited electric machine is measured during the rotation of the rotor and a rotor temperature-dependent induction voltage induced in the stator windings by the rotation of the rotor is determined on the basis of the measured winding voltage. The induction voltage is compared with a reference induction voltage predetermined at at least one reference rotor temperature, and the rotor temperature is determined on the basis of the comparison.The invention also includes an electric drive unit for a motor vehicle. The electric drive unit has at least one permanently excited electric machine with a stator equipped with energizable stator windings and a rotor equipped with permanent magnets. In addition, the electric drive unit has a measuring device for measuring at least one winding voltage on at least one of the stator windings and a control device which is designed to receive the at least one winding voltage measured during the rotation of the rotor, to determine a rotor temperature-dependent induced voltage induced in the stator windings on the basis of the at least one measured winding voltage, to compare the at least one induced voltage with at least one rotor-specific reference induced voltage predetermined at at least one reference temperature and to determine the rotor temperature on the basis of the comparison. A motor vehicle according to the invention comprises an electric drive unit according to the invention. The motor vehicle is an electrified motor vehicle and has the at least one permanently excited electric machine as a drive machine.By means of the method, the rotor temperature, for example for monitoring the permanent magnets of the rotor and / or for controlling the torque of the permanently excited electric machine, can be determined and monitored during operation of the electric machine during the travel of the motor vehicle. Due to the mutual induction between stator and rotor, the permanently excited rotor magnetic field of the rotating rotor induces a voltage in the stator windings. The stator-side winding voltage or phase voltage present at the stator windings can be measured with the aid of voltage sensors at the stator windings. From these measured voltage signals transmitted to the control device, the control device can extract the portion caused by the mutual induction and determine it as the at least one induction voltage.In the case that the winding voltage across the energized stator winding is measured during normal driving operation, in which the electric drive machine provides a drive torque, at least one winding current flowing through the at least one stator winding is measured for this purpose, in particular. A winding current-excited component of the winding voltage can be determined on the basis of the at least one measured winding current, and the mutual inductance-induced component can be determined on the basis of the at least one measured winding voltage and the winding current-excited component. For this purpose, the measuring device can have current sensors which can measure the winding currents or phase currents. On the basis of the measured winding currents, that portion of the winding voltage which is generated by energizing the stator windings can be determined and calculated out.In certain operating ranges of the electric machine, for example in coasting operation, in which the electric machine is towed through the motor vehicle and the rotor is thereby rotated when stator windings are not energized, the rotor temperature can, on the other hand, be determined only on the basis of the at least one measured winding voltage without the use of measured winding currents. In this case, the measured winding voltage corresponds to the induction voltage, so that the determination of the rotor temperature is particularly simple and inexpensive here and is thus particularly advantageous.The determined induction voltage is then compared with at least one predetermined reference induction voltage stored in a storage device of the drive unit. The reference induction voltage corresponds to the induction voltage at a defined, determined reference temperature of the rotor, wherein the at least one reference induction voltage is determined specifically for the rotor. For example, the at least one reference induction voltage can be determined rotor-specifically on the basis of at least one predetermined characteristic variable of the rotor. For example, a number and / or a position and / or a magnetization and / or a geometry of the permanent magnets of the rotor can be used as the at least one characteristic variable of the rotor. The data of the permanent magnets can be taken, for example, from a manufacturer's data sheet and used for determining the reference induction voltage. Alternatively or additionally, each rotor can be supplied with at least one reference temperature at the production line end and rotated. Subsequently, a voltage across at least one stator winding is measured and stored as the at least one reference induction voltage. The induction voltage is thus generated at the end of the production line under defined conditions and is used as a reference induction voltage.In this case, it can be provided that, in the event of a deviation of the determined rotor temperature from the reference rotor temperature, a correction factor for the rotor temperature is determined and is used for torque regulation of the permanently excited electric machine. This embodiment is based on the finding that the rotor temperature influences the magnetic rotor flux or the rotor magnetic field and thus the torque regulation. In order to be able to set the torque as exactly as possible, the correction factor is provided to the torque control.The embodiments presented with reference to the method according to the invention and the advantages thereof correspondingly apply to the drive unit according to the invention and to the motor vehicle according to the invention.Further features of the invention are evident from the claims, the figure and the description of the figures. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figure can be used not only in the respectively specified combination but also in other combinations or alone.The invention will now be explained in more detail on the basis of a preferred exemplary embodiment and with reference to the drawing.The single FIG. 1 shows a schematic illustration of an electric drive unit 1 for an electrically drivable motor vehicle. The electric drive unit 1 has a permanently excited electric machine 2, which is electrically connected to an inverter 3 of the electric drive unit 1 via phase lines L 1, L 2, L 3. The permanently excited electric machine 2 has a stator 4 with stator windings W 1, W 2, W 3, which are electrically connected to the inverter 3 via the phase lines L 1, L 2, L 3 and to which a polyphase alternating current i 1, i 2, i 3 for exciting a magnetic stator flux can be supplied via the phase lines L 1, L 2, L 3. The permanently excited electric machine 2 also has a rotor 5, which is equipped with permanent magnets 6 for exciting a magnetic rotor flux.During operation of the electric machine 2, losses can occur in the components of the electric machine 2, as a result of which the components heat up. As a result of the heating of the rotor 5, magnetic properties of the permanent magnets 6 also change up to demagnetization of the permanent magnets 6. The rotor temperature is not measured directly by means of a temperature sensor on the rotor 5, since attachment and signal transmission of a temperature sensor on the rotor 5 are technically difficult to implement because of rotation.Rather, the rotor temperature is determined with the aid of a measuring device 7, which here has voltage sensors V 1, V 2, V 3 on the stator windings W 1, W 2, W 3 and via which the winding voltages u 1, u 2, u 3 or phase voltages can be measured. In addition, the measuring device 7 here has a current sensor I 1, I 2, I 3 per phase line L 1, L 2, L 3, which current sensor measures the respective winding current i 1, i 2, i 3. Alternatively, one of the winding currents, for example the winding current i3, could also be determined mathematically via the measured winding currents i1, i2, so that the phase line L3does not require its own current sensor I3.The measured winding voltages u 1, u 2, u 3 and winding currents i 1, i 2, i 3 can be transmitted to a control device 8 of the drive unit 1. This control device 8 uses the measured winding voltages u 1, u 2, u 3 to determine the rotor-induced induced voltages which occur in the windings W 1, W 2, W 3 as a result of the magnetic coupling between the stator 4 and the rotor 5 and are dependent on the rotor temperature. In the case where no current i 1, i 2, i 3 is supplied to the stator windings W 1, W 2, W 3 due to operation, the measured winding voltages u 1, u 2, u 3 correspond to the respective induction voltages. In the event that a current i1, i2, i3is supplied to the stator windings W1, W2, W3in an operational-induced manner, the current-induced portion and the counter-induced portion, i.e. the induction voltage, can be determined by the control device 8 with the aid of the measured currents i1, i2, i3.The determined induction voltages are compared with reference induction voltages which correspond to the induction voltages at at least one predetermined reference rotor temperature. A deviation between the determined induction voltages and the predetermined reference induction voltages is an indication that the current rotor temperature deviates from the reference rotor temperature. The actual rotor temperature can be determined on the basis of a magnitude of the deviation. The reference induction voltages can be determined and stored, for example, from known, rotor-specific data or via end-of-line measurements.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedEP 2 318 818 B1

[0002]

Claims

Method for determining a rotor temperature of a rotor (5) of a permanently excited electric machine (2) for a motor vehicle, which rotor temperature is equipped with permanent magnets (6), having the steps: - measuring at least one winding voltage (u1, u2, u3) on at least one stator winding (W1, W2, W3) of a stator (4) of the permanently excited electric machine (2) during the rotation of the rotor (5), - determining a rotor-temperature-dependent induction voltage induced into the stator windings (W1, W2, W3) by the rotation of the rotor (5) on the basis of the measured winding voltage (u1, u2, u3), - comparing the induction voltage with a reference induction voltage predetermined at at least one reference rotor temperature, - determining the rotor temperature on the basis of the comparison.Method according to Claim 1, characterized in that the at least one winding voltage (u1, u2, u3) is measured at the unenergized stator windings (W1, W2, W3), and the at least one measured winding voltage (U1, u2, u3) is determined as the induction voltage.Method according to Claim 2, characterized in that the at least one winding voltage (u1, u2, u3) is measured during coasting of the motor vehicle, in which the rotor (5) is rotated by the moving motor vehicle.Method according to Claim 1, characterized in that - the at least one winding voltage (u1, u2, u3) is measured on at least one energized stator winding (W1, W2, W3), - at least one winding current (i1, i2, i3) flowing through the at least one stator winding (W1, W2, W3) is measured, - a winding-current-excited portion of the winding voltage (u1, u2, u3) is determined on the basis of the at least one measured winding current (i1, i2, i3), and - the at least one induction voltage is determined on the basis of the at least one measured winding voltage (u1, u2, u3) and the winding-current-excited portion.Method according to one of the preceding claims, characterized in that the at least one reference induction voltage is determined on a rotor-specific basis on the basis of at least one predetermined characteristic variable of the rotor (5).Method according to Claim 5, characterized in that a number and / or a position and / or a magnetization and / or a geometry of the permanent magnets (6) of the rotor (5) is used as the at least one characteristic variable of the rotor (5).Method according to one of the preceding claims, characterized in that at least one reference temperature is applied to the rotor (5) at the end of the production line and is rotated, a voltage on at least one stator winding (W1, W2, W3) is measured and stored as the at least one reference induction voltage.Method according to one of the preceding claims, characterized in that, in the event of a deviation of the determined rotor temperature from the reference rotor temperature, a correction factor for the rotor temperature is determined and is used for the torque control of the permanently excited electric machine (2).Electric drive unit (1) for a motor vehicle, having: - at least one permanently excited electric machine (2) having a stator (4) equipped with stator windings (W1, W2, W3) which can be supplied with current and a rotor (5) equipped with permanent magnets (6), - a measuring device (7) for measuring at least one winding voltage (u1, u2, u3) on at least one of the stator windings (W1, W2, W3), - a control device (8) which is designed to receive the at least one winding voltage (u1, u2, u3) measured during the rotation of the rotor (5), to determine a rotor-temperature-dependent induction voltage induced in the stator winding (W1, W2, W3) on the basis of the at least one measured winding voltage (u1, u2, u3), comparing the at least one induction voltage with a reference induction voltage predetermined at at least one reference temperature and determining the rotor temperature on the basis of the comparison.Motor vehicle having an electric drive unit (1) according to Claim 9.

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