Method for determining the irreversible demagnetization of the magnets of a rotor of an electric machine

Abstract

Method (1) for determining the health status of the permanent magnets (10) in the rotor of an electric machine based on a voltage parameter of the electric machine which is based on a control effect of a control signal as a measure of the induced back electromotive force.

Description

[0001] The invention relates to a method for determining the irreversible demagnetization of the magnets of a rotor of an electrical machine based on a voltage parameter of the electrical machine. State of the art

[0002] The central approach of the forecast and health management system is to use data on usage and condition from the operation of a single system to detect disruptions and failures at an early stage and define appropriate countermeasures. This allows decisions to be made while taking overall economic interests into account. For example, this allows costs to be reduced, risks to be minimized, usable operating hours to be maximized, and availability to be improved.

[0003] Electric vehicles use a purely electric drive system, known as an e-drive. The drive system of an electric vehicle includes an electric motor, a mechanical reduction gear, an inverter, and a motor controller.

[0004] The inverter inverts DC power from batteries into AC power, which is used to drive the motor. The motor delivers torque to the vehicle's drivetrain. The controller controls the power supplied to the motor by the inverter.

[0005] Compared to combustion engines, EM electric motors generally require less but more careful maintenance during their lifetime.

[0006] Therefore, it is crucial to have a clear understanding of the historical, current and projected health of the electric motor.

[0007] Irreversible demagnetization of a magnet refers to the permanent loss of its magnetic properties. This phenomenon occurs when a magnet is exposed to external factors such as extreme temperatures, high mechanical stress, or opposing magnetic fields that exceed its performance capability. Once a magnet has been irreversibly demagnetized, it cannot regain its original magnetic strength without remagnetization.

[0008] Permanent magnet synchronous motors (PMSM) rely on the permanent magnetic flux of the rotor to couple with the rotating electromagnetic field of the stator, resulting in the conversion of electrical power into mechanical power as rotary motion. The magnetic flux of the rotor changes depending on the operating point and temperature conditions of the machine.

[0009] Different methods are used to detect and quantify irreversible demagnetization.

[0010] Model-based solutions consist of a demagnetization model of the magnet, which is given by the BH characteristic and the dependence of the magnetic properties on temperature, see "Temperature-Dependent Demagnetization Model of Permanent Magnets for Finite Element Analysis", P. Zhou et al., February 2012, IEEE Transactions on Magnetics 48(2):1031-1034. Where B represents the magnetic flux density in Tesla and H the magnetic field strength in amperes per meter (A / m), respectively. Continuous monitoring of the operating point is performed, observing its position relative to the knee point on the demagnetization curve. The knee point is the separation point between the reversible and irreversible demagnetization of the magnet. The result is the relative change in the magnetic remanence.

[0011] A data-driven approach involves generating synthetic data based either on simulations or experiments. Synthetic datasets contain measurements of current, voltage, torque, and speed for various demagnetization levels. The dataset is then used for supervised learning. The approach also includes fitting regression models or training artificial neural networks that represent the statistical relationship between the target of determining demagnetization and the inputs, the measured electrical and mechanical parameters.

[0012] Another approach uses signal injection. This involves feeding a series of signals through the inverter to the motor under standstill conditions. The current flowing through the motor windings is analyzed. First, a baseline is established before the motor is deployed in the field. Once the user puts the motor into operation, the same indicator is monitored over time and compared with the initial values ​​before deployment, which corresponds to the magnet with the optimal remanence.

[0013] DE 102 43 219 A1 discloses a device for detecting faults due to weakening permanent magnets in a motor in a vehicle. The device includes a voltage monitor that detects a permanent magnet-induced voltage in the motor at a predetermined speed and in a no-load state. The voltage monitor is coupled to a processor that records the permanent magnet-induced voltage as measured at the predetermined speed and compares it to a reference voltage that reflects the permanent magnet-induced voltage for the motor with a fully magnetized permanent magnet. The processor determines a difference between the detected permanent magnet-induced voltage and the reference voltage. The difference is analyzed to determine whether a component is defective.In particular, the voltage induced by the permanent magnet is a function of the relative positions and locations of the permanent magnets in the motor. This relationship is used to detect a faulty magnet. Specifically, the permanent magnets are designed so that a change in magnetic resistance or magnet strength is used to identify the faulty magnet. A diagnostic code is set to alert others of the location of the faulty magnet for replacement or other corrective action.

[0014] It is an object of the invention to propose a method for determining the health status of an electrical machine by an irreversible degree of demagnetization, which method uses control signals, temperature and speed measurements and / or estimations of speed and temperature to enable data-driven condition monitoring for the rotor of a permanent magnet synchronous motor. Description of the invention

[0015] The problem is solved by a method for determining the health status of the permanent magnets in the rotor of an electrical machine based on a voltage parameter of the electrical machine, which is based on a control effect of a control signal as a measure of the induced counter-electromotive force.

[0016] The proposed method relies on the control effect of the control signal and is not an actual measurement of the induced counter electromotive force at the terminals of the machine.

[0017] The voltage parameter depends on the rotor temperature and the rotor speed.

[0018] A model and data-driven approach is used.

[0019] The method collects baseline data in a baselining step, which is terminated by a major demagnetization event.

[0020] After the baselining step, the continuous monitoring step begins.

[0021] The deviation of the voltage parameter indicates the degree of irreversible demagnetization of the permanent magnets.

[0022] The information is also used to evaluate the effects of driving after an ASC event, i.e. in the so-called “limp-home mode”.

[0023] By quantifying the irreversible demagnetization after an active short circuit, it becomes possible to reduce heavy rare earths in the electrical machine.

[0024] The procedure represents part of an overall procedure for monitoring the health of an electrical machine.

[0025] The collected data and the degree of irreversible demagnetization of the permanent magnets are transmitted to the driver and / or a remote monitoring system. Description of the characters Fig. 1 shows a history of the health status of an electrical machine, Fig. 2 shows a process diagram.

[0026] The proposed solution targets on-board condition monitoring of the rotor of a permanent magnet synchronous motor. Descriptive analysis methods are used to quantify the condition. Assuming that each component of the machine exhibits either an optimistic or conservative deterioration characteristic under real operating conditions, the end of life for each component is expected to deviate from the design specifications.

[0027] Fig. Figure 1 schematically shows the health status of the electrical machine, starting from its commissioning at time 0 until the end of its service life (EoL). The decreasing health status is visible in the middle line of the diagram. The diagram also shows two additional curves. The EoL curve c follows a very conservative estimate so that the lifetime ends earlier. In an optimistic estimate EoL o the end of life is delayed.

[0028] The diagram shows very schematically the influence that various parameters have on estimating the health status of the electrical machine.

[0029] This application proposes a model- and data-driven approach to monitoring the condition of permanent magnets. This method involves quantifying the extent of irreversible demagnetization in the event of a significant demagnetization event, such as an active short circuit, and is described in the block diagram of the Fig. 2 shown.

[0030] The idea is based on monitoring the back electromotive force (BEMF) at appropriate driving intervals, which correspond to the operating points where the motor is almost not loaded and the rotor is rotating.

[0031] At appropriate driving intervals i d ~0 with i d - direct component of the current, i q ~0 with i q - quadratic component of the current, ω > 0 with ω - rotor speed.

[0032] The concept consists of two stages. Stage 1: Collect basic data

[0033] In Fig. 2, the process begins with step 2 with the collection of measurement data and control signals. The rotor temperature T R , the rotor speed ω and the current data i d , i q , and q An appropriate driving interval 3 is determined using the parameters mentioned above.

[0034] The first stage is the determination of the baseline data, a so-called baselining 4, in which a considerable number of BEMF, Back Electromotive Force data points 6 are collected for different operating points.

[0035] The following equations are used in the rotor-fixed reference system and describe the dependencies of voltage u, current i and flux ψ for a motor with a specific parameter set. ud=Rsid+dψd / dt−ωψq uq=Rsiq+dψq / dt+ωψd ψd=Ldid+ψPM ψq=Lqiq with ud - DC component of the voltage, u q - quadratic component of the voltage i d - direct component of the current i q - quadratic component of the current ψ d - direct component of the flow ψ q - quadratic component of the flow ψ PM - Flux of the permanent magnet R s - Stator resistance L d - Stator inductance in the d-axis L q - Stator inductance in the q-axis ω - rotor speed.

[0036] In an idle scenario (i d ~0 and i q ~0) with a rotor speed greater than 0 (ω > 0), equations 2 and 3 represent the following relationship between u q , ω and ψ PM here: uq=ωψPM(Tr), where T r is the estimated / measured rotor temperature.

[0037] Since uq as a quadratic component of the voltage in direct proportion to the flux of the permanent magnet ψ PM it is used as an indicator of the remanent flux density at a certain rotor temperature T R and speed ω. It should be noted that u q ' in step 7 is a control function, a control signal, and not an actual measurement of the induced BEMF at the terminals of the machine windings. This control signal comes from a proportional-integral controller. The integral component is essential for the function, as it ensures that the desired setpoints are reached even in the event of parameter uncertainties or demagnetization. The baselining phase ends when a major demagnetization event occurs, for example, an active short circuit (ASC).

[0038] Alternatively, a statistical approach can be used to determine the actual termination of the baseline simulation when a sufficiently representative range of operating points is covered. Stage 2: Continuous monitoring phase

[0039] The end of baselining phase 4 marks the beginning of continuous monitoring 5. Baselining phase 4 contains data points from u q ' with respect to the measured rotor speed ω and temperature T R and thus represents one or more completely healthy magnets that have not been subjected to irreversible demagnetization.

[0040] In the monitoring phase, the current and q -Data points with the corresponding u q ' data from baselining phase 4. One of the challenges is to identify the identical operating points in both phases, baselining and the continuous monitoring phase.

[0041] To solve this problem, a regression model is proposed that takes the values ​​of u q with the rotor speed ω and the rotor temperature T R during the baseline process. In this way, it is possible to determine the baseline value of the u q for each data point of interest in the continuous monitoring phase in step 9.

[0042] The relative difference of the u q -Values dr=(u′q−uq) / u′q Indicates the degree of irreversible demagnetization of the permanent magnets. In step 10, the data is displayed or further used as the health status of permanent magnets.

[0043] The permanent magnet condition indicator can be used as part of a comprehensive concept for monitoring the health of the electric motor or the entire system, which takes into account the principle of multifactorial aging.

[0044] The permanent magnet health indicator can be used by original equipment manufacturers to gain insights into fleet management and to reduce the occurrence of early failures and detect over-engineering.

Claims

[1] Method (1) for determining the state of health of the permanent magnets (10) in the rotor of an electrical machine based on a voltage parameter (u q ) of the electrical machine, which is based on a control effect of a control signal as a measure of the induced counter-electromotive force. [2] Method according to claim 1, characterized by that the voltage parameter (u q ) on the rotor temperature (T r ) and the rotor speed (w). [3] Method according to claim 1 or 2, characterized by that a model and data-driven approach is used. [4] Method according to one of the preceding claims, characterized by that the method collects baseline data in a baselining step, which is terminated by a major demagnetization event. [5] Method according to claim 4, characterized by that after the baselining step, the continuous monitoring step begins. [6] Method according to one of the preceding claims, characterized by that the deviation of the voltage parameter (u q ) indicates the degree of irreversible demagnetization of the permanent magnets. [7] Method according to one of the preceding claims, wherein the method is part of an overall method for monitoring the health of an electrical machine. [8] Method according to one of the preceding claims, wherein the collected data and the degree of irreversible demagnetization of the permanent magnets are transmitted to the driver and / or a remote monitoring device.

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