Method and device for detecting the operating state of an electric motor

Abstract

Method for detecting a functional state of an electrical machine (1) with a rotor (2) and a stator (3), comprising: - Driving the electric machine (1) until it runs at a certain speed, and - Short-circuiting a stator coil (4) of the stator (3) of the electric machine (1), characterized by - Detecting an induced voltage and / or an induced current in the stator coil (4) that is induced when the rotor (2) of the electric machine (1) comes to a stop, and - Comparing the measured induced voltage and / or the measured induced current with a predefined reference value to infer the operating state of the electric machine (1), wherein, when comparing the induced voltage with the predefined reference value, a comparison is made between a course of the induced voltage during the coasting of the rotor (2) and a reference curve, and / or, when comparing the induced current with the predefined reference value, a comparison is made between a course of the induced current during the coasting of the rotor (2) and a reference curve, wherein a magnetic field of the rotor (2) is generated by an excitation current which flows through at least one rotor coil (5) of the rotor (2) and a value of the excitation current flowing in the rotor coil (5) is measured before or during the coasting of the rotor (2) of the electric machine (1).and the predefined comparison value is selected depending on the value of the rotor current recorded.

Description

State of the art

[0001] The present invention relates to a method and a device for detecting a functional state of an electric motor.

[0002] Permanent magnets are also used in alternating current electric machines to increase the efficiency and power output of the electric machine. Alternatively or additionally, energized coils are used to generate a magnetic field for the electric machine's rotor. If the permanent magnets become demagnetized due to external influences, such as overheating, or if the magnetic field in the electric machine is only provided with reduced strength, for example, due to a short circuit in a coil, the efficiency and power of the electric machine decrease.

[0003] Currently, such demagnetization of electrical machines is not diagnosed, making it very difficult to diagnose the cause of the reduced performance. Finding such a fault in an electrical machine, for example, in a workshop, during final inspection at the factory, or during normal operation, requires increased effort, which in turn leads to additional costs.

[0004] DE 102 43 219 A1 and US 2 650 706 A disclose methods that enable detection of magnetization in an electric motor.

[0005] Furthermore, DE 10 2014 205 254 A1 discloses a method and a device for determining a demagnetization of a permanent magnet of an electric motor.

[0006] DE 10 2016 207 375 A1 discloses a method for detecting a degree of demagnetization of an electric machine.

[0007] DE 10 2011 006 516 A1 discloses a method for operating an electrical machine in a short-circuit mode, in particular for use in a motor vehicle. Disclosure of the invention

[0008] The method according to the invention for detecting a functional state of an electrical machine having a rotor and a stator comprises driving the electrical machine until it runs at a predetermined speed, short-circuiting a stator coil of the stator of the electrical machine, detecting an induction voltage and / or an induction current in the stator coil that is induced when the rotor of the electrical machine stops, and comparing the detected induction voltage and / or the detected induction current with a predefined comparison value in order to conclude the functional state of the electrical machine.

[0009] The device according to the invention for detecting a functional state of an electrical machine with a rotor and a stator comprises control electronics which are designed to drive the electrical machine until it runs at a certain speed, to short-circuit a stator coil of the stator of the electrical machine, to detect an induction voltage and / or an induction current in the stator coil which is induced when the rotor of the electrical machine stops, and to compare the detected induction voltage and / or the detected induction current with a predefined comparison value in order to conclude the functional state of the electrical machine.When comparing the induction voltage with the predefined comparison value, a comparison is made between a course of the induction voltage when the rotor runs out and a comparison curve and / or when comparing the induction current with the predefined comparison value, a comparison is made between a course of the induction current when the rotor runs out and a comparison curve.

[0010] An electrical machine is, in particular, a motor or a generator. The functional state is, in particular, a state that describes a state of a desired available magnetic field in the electrical machine. The electrical machine is preferably driven by appropriately supplying current to the rotor and / or the stator. This current is provided, in particular, by means of an inverter. Alternatively, the electrical machine can be driven by external means. The stator coil is preferably short-circuited by appropriately switching the inverter. The induced current is a current that flows in the windings of the stator coil. The induced voltage is preferably detected at the ends of the stator coil. The induced voltage and the induced current are therefore quantities that are measured.

[0011] A rotor's magnetic field is generated by an excitation current flowing through at least one rotor coil. This is particularly the case when the electrical machine is an induction machine (IM). In such induction machines, interturn short circuits can occur. These are difficult to detect by resistance measurement, but can be detected using the present invention.

[0012] A value of the excitation current flowing in the rotor coil is recorded before or during the rotor of the electric machine's deceleration, and the predefined comparison value is selected based on the value of the rotor current recorded during this time. The induction voltage and the induction current induced in the stator coil during the rotor deceleration depend on the excitation current flowing in the rotor coil. By selecting the predefined comparison value based on the value of the rotor current recorded during this time, errors in the comparison that could be caused by a change in the magnitude of the rotor current can be excluded. This ensures particularly reliable detection of the functional status.

[0013] This makes it possible to determine the degree of magnetization of the electrical machine. This is done in particular via control software, since the necessary sensors and control elements are usually present anyway when the electrical machine is used. This applies in particular to the use of the electrical machine in a vehicle, particularly when the electrical machine is used to drive the vehicle. The method according to the invention is preferably carried out in a workshop, during final testing in a factory or while the electrical machine is in operation. The detected functional state primarily describes the degree of magnetization of the rotor and / or the stator. It can therefore be clearly detected whether the electrical machine has a fault that affects the magnetic field of the rotor and / or the stator.In particular, a short circuit between turns in the electrical machine and the demagnetization of permanent magnets can be detected. According to the invention, coasting is understood to mean that the rotor continuously reduces its speed because it is no longer being driven. This does not mean that the rotor must come to a standstill during coasting. Coasting also occurs if the speed has merely been reduced.

[0014] The electric machine is preferably installed in a vehicle. The method can be a component of on-board software and can be initiated during a final factory inspection or in a service workshop using a diagnostic tester. Diagnostic results using the method according to the invention can preferably be read from an error memory of the electric machine.

[0015] The procedure is particularly suitable for a factory final test of the electrical machine, for execution during a workshop service and as on-board diagnostics during regular operation.

[0016] The subclaims show preferred developments of the invention.

[0017] Furthermore, it is advantageous if, when comparing the induction voltage with the predefined comparison value, a comparison is made between a profile of the induction voltage as the rotor runs down and a comparison curve and / or if, when comparing the induction current with the predefined comparison value, a comparison is made between a profile of the induction current as the rotor runs down and a comparison curve. The profile is preferably a temporal profile. In other words, a comparison is made with several predefined comparison values, each of the comparison values ​​being assigned to a rotational speed or a point in time since the start of the rotor running down. The comparison curve is preferably a curve which indicates a profile of the induction voltage or the induction current over a rotational speed of the rotor.The comparison curve is further preferably a curve which indicates a course of the induction voltage or the induction current over time. Thus, an expected course of the induction voltage is compared with the actually detected induction voltage. Alternatively or additionally, an expected course of the induction current is compared with the actually detected induction current. In particular, a difference is formed between the comparison curve and the course of the detected induction voltage and / or the course of the detected induction current. This difference is preferably integrated over the course, i.e. over the course over time or over the speed. By comparing the detected induction voltage and / or the detected induction current over the duration of the rotor run-down, further errors can be detected.For example, individual peaks in the curve of the recorded induction voltage and / or the curve of the recorded induction current can be used to indicate contact problems in the stator.

[0018] Furthermore, it is advantageous if the method is executed repeatedly, wherein, upon repeated execution of the method, the predefined comparison value corresponds to the detected induction voltage and / or the detected induction current that was detected during a previous execution of the method. Thus, it can be concluded whether a functional state of the electrical machine deteriorates over a period of operation.

[0019] It is also advantageous if the electric machine runs at a specific speed for a specified time interval before short-circuiting the stator coil. This ensures that the currents supplying the rotor and / or stator are constantly regulated. Thus, any falsification of a measurement result due to control loop intervention can be avoided.

[0020] Furthermore, it is advantageous if the rotor's magnetic field is generated at least partially by a permanent magnet. More preferably, the rotor's magnetic field is generated exclusively by a permanent magnet. This is particularly the case with synchronous machines with permanent magnets.

[0021] Demagnetization of permanent magnets is particularly difficult to detect, as it can only be measured directly with great effort. However, the method according to the invention makes this possible in a simple manner.

[0022] A motor unit comprising the device according to the invention and the electrical machine, wherein the electrical machine is a motor or a generator, is particularly advantageous.

[0023] The electric motor is preferably a synchronous motor with permanent magnets or an induction motor. Such electric motors are particularly advantageous for use in electric vehicles. Especially in this automotive environment, reliable detection of functional states is essential to enable vehicle servicing. Short description of the drawings

[0024] Embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawing: Fig. 1 a representation of a motor unit which comprises the device according to the invention for detecting a functional state of an electrical machine, and Fig. 2 a flowchart of a method according to the invention for detecting a functional state of an electrical machine. Embodiments of the invention

[0025] Fig. Figure 1 shows a schematic representation of a motor unit 10 according to a first embodiment of the invention. This comprises control electronics 6 and an electric machine 1. The motor unit 10 is a drive unit of a vehicle. The electric machine 1 is an electric motor. The electric motor and thus the electric machine 1 can also be operated as a generator, which is the case, for example, during recuperation. The control electronics 6 is an engine control unit of the vehicle.

[0026] The electric machine 1 comprises a rotor 2 and a stator 3. The stator 3 comprises several stator coils 4. The rotor 2 comprises a rotor coil 5. The rotor coil 5 serves to generate a magnetic field of the rotor 2. The magnetic field of the rotor 2 is generated when an excitation current flows through the rotor coil 5. The rotor coil 5 of the rotor 2 can be contacted via a first sliding contact 7 and a second sliding contact 8 in order to transmit the excitation current to the rotor 2. The Fig. The electric motor shown in Figure 1, i.e., the electric machine 1, is an induction motor. The stator coils 4 and the rotor coil 5 are coupled to the control electronics 6.

[0027] The method according to the invention is controlled by the control electronics 6.

[0028] The functional status of the electric machine 1 is detected when requested by a service call. Alternatively, this occurs routinely when the vehicle in which the motor unit is installed is started. Alternatively or additionally, the control electronics 6 detects situations in which the rotor 2 will run out anyway, and the method is initiated at this time to detect the functional status of the electric machine during operation in the vehicle.

[0029] First, the electric machine 1 is driven until it runs at a specific speed. The specific speed can be freely selected. For example, it can be set to 1000 revolutions per minute. However, it is possible to use any specific speed as a starting value for the method. The specific speed is preferably a value that the electric machine 1 has at a time when it is detected that the electric machine 1 is coasting down. A comparison value, described in more detail below, is selected according to the specific speed.

[0030] In this exemplary first embodiment, the electric machine 1 is accelerated to a speed of 1000 revolutions / minute. Once this has occurred, the electric machine 1 is held constant at this speed for a predetermined time interval. For example, the electric machine 1 rotates constantly at a speed of 1000 revolutions / minute over a time interval of, for example, 100 ms.

[0031] Once this time interval, in this case 100 ms, has elapsed, the stator coil 4 of the stator 3 is short-circuited. In this first embodiment, all three stator coils 4 of the stator 3 are short-circuited. This state is also referred to as an active short circuit, or AKS for short. This state is already present in current vehicles powered by electric motors 1 and can therefore be triggered by the control electronics 6. To achieve this state, the transistors of an inverter are switched accordingly to short-circuit the ends of the individual stator coils 4.

[0032] Because the stator coils 4 of the stator 3 are short-circuited, the electric machine 1 is no longer driven and begins to coast down. This means that the speed of the electric machine 1 continuously decreases. At the time the stator coils 4 are short-circuited, the control electronics 6 detects a value of the excitation current flowing in the rotor coil 5. The control electronics 6 regulates the excitation current during the coasting of the rotor so that it remains constant at this predetermined value. A value of the excitation current flowing in the rotor coil 5 is thus detected before the rotor 2 of the electric machine 1 coasts down.

[0033] During the coasting of the rotor 2 of the electric machine 1, the control electronics 6 detects an induced voltage and an induced current flowing through a stator coil 4 of the stator 3. As previously mentioned, the stator 3 has multiple coils 4. The method is carried out identically for each of the coils. The method is therefore described below using a single stator coil 4 of the stator 3 as an example.

[0034] The induced voltage and the induced current flowing through the stator coil 4 of the stator 3 are measured. The induced voltage and the induced current are thus recorded. Because the rotor 2 is moving and simultaneously generates a magnetic field, a current is induced in the stator coil 4 of the stator 3. This current is measured. Because the speed of the rotor 2 continuously decreases, the measured induced voltage and the measured induced current also continuously decrease. This occurs according to a curve characteristic of the electrical machine 1. This characteristic curve depends on the speed of the electrical machine and the excitation current flowing through the rotor coil 5 of the rotor 2.

[0035] In the following, the recorded induction voltage is first examined. The recorded characteristic curve, which results for the induction voltage when the rotor 2 of the electric machine 1 runs down, is compared with a reference curve. The reference curve has a curve shape that corresponds to that of the characteristic curve when the electric machine 1 is undamaged or fully functional.

[0036] The control electronics 6 compare the detected induction voltage with a predefined comparison value in order to determine the functional state of the electrical machine 1. In the simplest case, the predefined comparison value is a single value that describes the induction voltage and / or the detected induction current at a specific point in time since the start of the rotor rundown or at a specific speed of the electrical machine 1. A continuous comparison can also take place as a function of the rotor speed via a characteristic curve that represents the speed and the corresponding induction voltage, as is exemplified by a query step 29 in Fig. 2. The comparison value corresponds to a single point on the comparison curve. In this first embodiment, the induction voltage is compared with several comparison values ​​assigned to different speeds of the electric machine 1. For this purpose, the characteristic curve, i.e., the course of the induction voltage over the rotor's rundown, is compared with the comparison curve.

[0037] To provide the comparison curve, a plurality of possible comparison curves are stored in the control electronics 6. These represent the induced voltage over the speed of the rotor 2 or, alternatively, over a period of time since the start of coasting. Comparison curves are stored for several different excitation currents. The control electronics 6 selects the appropriate curves from the plurality of possible comparison curves based on the previously measured excitation current and the determined speed of the rotor 2.

[0038] A comparison is made between the selected comparison curve and the recorded values ​​of the induction voltage and / or the induction current, depending on the excitation current and rotor speed, whereby, for example, the selected comparison curve is compared with the characteristic curve. It is assumed that the comparison curve and the measured value describe a temporal progression of the induction voltage since the start of the rundown of the rotor 2. In this case, an induction voltage and / or an induction current was recorded for a plurality of points in time during the rundown of the rotor 2. At the same time, a predefined comparison value is available for each of the points in time via the associated comparison curve. When the first induction voltage is compared with the respectively associated predefined comparison value, a difference is formed between the recorded induction voltage and the predefined comparison value.If a value of this difference is greater than a predefined threshold value, an onboard diagnosis of the control electronics 6 stores an entry in an error memory, which indicates that the functional state of the electrical machine 1 is faulty.

[0039] Alternatively or additionally, an induction current is considered instead of the induction voltage. The induction current is evaluated according to the induction voltage, with the comparison curves describing the course of the induction current when the electrical machine 1 is running down.

[0040] Thus, when comparing the induction voltage with the predefined comparison value, a comparison is made between the course of the induction voltage as rotor 2 runs down and a comparison curve. Similarly, when comparing the induction current with the predefined comparison value, a comparison is made between the course of the induction current as rotor 2 runs down and a comparison curve.

[0041] It is advantageous if, during the comparison, not only individual values ​​from the characteristic curve are considered separately with individual values ​​from the comparison curves, but the curves are considered as a whole. For example, a difference curve is formed which results from the difference values ​​between values ​​of the recorded induction voltage and the respective associated values ​​of the comparison curve used. The difference curve is thus a temporal progression of the difference between a recorded characteristic curve and a comparison curve over the speed or over time. The excitation current for generating the magnetic field in the rotor 2 can also remain constant. In this case, the comparison curve and thus the induction voltage and / or the induction current only depend on the rotor speed. The control electronics 6 is designed to detect periodic components in the difference curve.In this way, for example, it can be concluded that there is a faulty contact of the rotor 2 in the area of ​​the sliding contacts 7, 8 if periodic peaks occur in the difference curve.

[0042] The comparison curves are provided, for example, by the method being carried out for the first time at the factory for different excitation currents and / or specific speeds. Since it can be assumed at the factory that a functional state of the electrical machine 1 is fully present, i.e. that the electrical machine 1 is functioning within the specified parameters, such values ​​can be used as reference values. The method is thus repeatedly carried out, with the method being carried out for the first time at the factory and later carried out again when the vehicle is in operation. When the method is repeated, i.e. during vehicle operation, the predefined comparison value of the first induction voltage and the recorded induction current, which was measured at the factory, are used as the predefined comparison value.For example, when the process is repeated, the predefined comparison value corresponds to the induction voltage recorded at the factory and thus to a value recorded during a previous run of the process.

[0043] In an alternative embodiment of the invention, the electric machine 1 is a synchronous machine with permanent magnets. In this case, it is not necessary to provide an excitation current in the rotor 2. Otherwise, the motor unit 1 corresponds to the motor unit 1 according to the first embodiment of the invention, although the dependence of the comparison values ​​on the excitation current can be omitted, since such a current does not exist.

[0044] Fig. Figure 2 shows a flowchart 20, on which software executed by the control electronics 6 is preferably based. Three initial parameters 21, 22, 23 are initially recorded.

[0045] The first initial parameter 21 determines whether the electric machine 1 is rotating at the specified speed, also referred to as the target speed. If this is the case, a logical "1" is provided for an AND operation 24.

[0046] The second initial parameter 22 detects whether the stator coils 4 of the stator 3 are short-circuited. This checks whether an active short circuit exists. If this is the case, a logical "1" is provided for the AND gate 24.

[0047] The third initial parameter 23 determines whether detection of the functional state of the electrical machine 1 is desired. For this purpose, a service interface is queried, for example. If detection of the functional state of the electrical machine is desired, a logical "1" is provided for the AND gate 24.

[0048] If the necessary logical values, i.e., the logical "1" in each case, are provided based on the initial parameters 21, 22, 23, the condition represented by the AND operation 24 is fulfilled. This is detected by an IF query 25, which checks whether the AND operation 24 outputs a logical "1." If this is the case, the method according to the invention is started in this case, assuming that the electric machine 1 has already been driven up to the specified speed.

[0049] During IF query 25, a time query is optionally executed to check whether the specified time interval has elapsed, over which the electric machine is to maintain the specified speed. This checks whether the initial conditions 21, 22, 23 are met throughout the time interval. If this is the case, the stator coil 3 of the stator 3 is short-circuited. If this is not the case, the IF query 25 is executed again until all of the initial conditions 21, 22, 23 have been met throughout the time interval. This is Fig. 2 is represented by a reset step 26.

[0050] As soon as the stator coil 4 of the stator 3 is short-circuited, the electric machine 1 begins to coast down. In a time recording step 27, the time that has elapsed since the stator coil 4 was short-circuited, i.e., since the rotor began coasting down, is recorded. At the same time, in a recording step 28, measured values ​​for the induction voltage and the induction current are continuously recorded and provided to the control electronics 6. The measured values, which describe the induction voltage and the induction current at different times, can thus be queried as values ​​by the software executed by the control electronics 6.

[0051] In a query step 29, a comparison curve stored in the control electronics 6 is retrieved. According to a period of time that has elapsed since the short-circuiting of the stator coil 4, a comparison value is selected from the retrieved comparison curve. This value is compared with a currently measured value of the induction voltage or induction current associated with the corresponding time. This occurs in a comparison step 30.

[0052] In comparison step 30, a difference is calculated between the measured induction voltage, or alternatively the measured induction current, and the comparison value. This difference is checked as a difference value to determine whether it is greater than a possible tolerance. The tolerance is provided as a parameter of the software of the control electronics 6 and is provided by a memory call 31. If the difference value is less than the tolerance, the functional state of the electrical machine 1 is assessed as completely satisfied. This is determined in a test step 32. If the difference value is greater than the tolerance, i.e., if the measured or measured induction voltage or the measured induction current deviates from the corresponding value of the comparison curve by more than a predetermined tolerance, the functional state of the electrical machine 1 is assessed as not satisfied.In this case, an onboard diagnostic management system creates an entry in the error log. This occurs in error evaluation step 33.

[0053] This diagnostic can clearly determine whether electrical machine 1 has a short circuit between turns or whether permanent magnets are demagnetized. This type of fault detection was previously impossible.

[0054] In summary, the electric machine 1 is started and accelerated to a certain speed. It is then allowed to coast down, an active short circuit is requested in the control software, and subsequently, an amplitude of the induced voltage or induced current is measured and compared with a reference value stored in the software. If the electric machine 1 is demagnetized or partially demagnetized, the induced current or induced voltage when the electric machine 1 coasts down is smaller by a defined amount than a reference value defined by an applicable parameter.

[0055] The reference value of the induction voltage and the induction current depend on the speed of the electric machine 1 and are stored in the software as a characteristic curve or characteristic map, which depends on the speed of the electric machine 1. This is also referred to as a comparison curve. If a measured value differs by a defined amount from the reference value, which can be applied as a parameter, then the electric machine is demagnetized or defective.

[0056] The method according to the invention can be carried out in a service workshop and in certain selected driving modes. For example, the method is executed when the vehicle is started or stopped at a traffic light. The method takes between 0.5 and a few seconds.

[0057] In addition to the above written revelation, explicit reference is made to the revelation of Fig. 1 and Fig. 2.

Claims

[1] Method for detecting a functional state of an electrical machine (1) with a rotor (2) and a stator (3), comprising: - Driving the electrical machine (1) until it runs at a certain speed, and - short-circuiting a stator coil (4) of the stator (3) of the electrical machine (1), characterized by - detecting an induction voltage and / or an induction current in the stator coil (4) which is induced when the rotor (2) of the electrical machine (1) stops, and - comparing the detected induction voltage and / or the detected induction current with a predefined comparison value in order to determine the functional state of the electrical machine (1), wherein, when comparing the induction voltage with the predefined comparison value, a comparison is made between a profile of the induction voltage when the rotor (2) is running down and a comparison curve, and / or when comparing the induction current with the predefined comparison value, a comparison is made between a profile of the induction current when the rotor (2) is running down and a comparison curve, wherein a magnetic field of the rotor (2) is generated by an excitation current flowing through at least one rotor coil (5) of the rotor (2), and a value of the excitation current flowing in the rotor coil (5) is detected before or during the rotor (2) of the electrical machine (1) running down,and the predefined comparison value is selected depending on the value of the rotor current detected. [2] Method according to claim 1, characterized by that the method is carried out repeatedly, wherein in a repeated run of the method the predefined comparison value corresponds to the detected induction voltage and / or the detected induction current which was detected in a previous run of the method. [3] Method according to one of the preceding claims, characterized by that the electrical machine (1) runs at the specific speed for a predetermined time interval before the stator coil (4) of the stator (3) is short-circuited. [4] Method according to one of the preceding claims, characterized by that a magnetic field of the rotor (2) is generated partly by a permanent magnet. [5] Device for detecting a functional state of an electrical machine (1) with a rotor (2) and a stator (3), comprising control electronics (6) which are designed to: - to drive the electrical machine (1) until it runs at a certain speed, and - to short-circuit a stator coil (4) of the stator (3) of the electrical machine (1), characterized by that the control electronics (6) are designed to: - to detect an induction voltage and / or an induction current in the stator coil (4) which is induced when the rotor (2) of the electrical machine (1) runs down, and - to compare the detected induction voltage and / or the detected induction current with a predefined comparison value in order to determine the functional state of the electrical machine (1), wherein, when comparing the induction voltage with the predefined comparison value, a comparison is made between a profile of the induction voltage when the rotor (2) is running down and a comparison curve, and / or when comparing the induction current with the predefined comparison value, a comparison is made between a profile of the induction current when the rotor (2) is running down and a comparison curve, wherein a magnetic field of the rotor (2) is generated by an excitation current flowing through at least one rotor coil (5) of the rotor (2), and a value of the excitation current flowing in the rotor coil (5) is detected before or during the rotor (2) of the electrical machine (1) is running down,and the predefined comparison value is selected depending on the value of the rotor current detected. [6] Motor unit (10) comprising a device for detecting a functional state of an electrical machine (1) according to claim 5, further comprising the electrical machine (1), wherein the electrical machine (1) is a motor or a generator. [7] Motor unit according to claim 6, characterized by that the electrical machine (1) is a synchronous machine with permanent magnets or an induction machine.

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