Determining a winding temperature of a winding of an electric motor

Abstract

The invention relates to a method for determining a winding temperature of a winding (6) of an electric motor (5) and a power converter (1), which is configured to control an electric motor (5). In the method, at least one measured operating state of the electric motor (5) is ascertained for a measurement of an electrical winding resistance of the winding (6). In a measured operating state of the electric motor (5), the winding resistance is measured and the winding temperature is calculated from the measured winding resistance. In an operating state of the electric motor (5) that differs from a measured operating state, the winding temperature is calculated from at least one operating parameter of the electric motor (5) using a thermal model.

Description

[0001] Description

[0002] Determining a winding temperature of an electric motor winding

[0003] The invention relates to a method for determining a winding temperature of a winding of an electric motor. Furthermore, the invention relates to a power converter configured to control an electric motor. The electric motor can generally be a rotating electrical machine designed as a synchronous motor or asynchronous motor, a linear motor, or a reluctance motor. Furthermore, the term "electric motor" also includes correspondingly designed generators.

[0004] Many electric motors are operated in overload mode using an inverter. It is important to protect the electric motor windings from overheating. Overheating can also occur in water-cooled or forced-ventilated electric motors if the cooling system fails. To protect against overheating, a temperature sensor is often installed on the winding of an electric motor and its measured values ​​are monitored. The temperature sensor must be electrically isolated from the winding. This means that the thermal connection between the temperature sensor and the winding is not ideal and can lead to a measurement delay of, for example, 10 seconds or more. Furthermore, individual winding phases of the winding can heat up more than others, for example when the electric motor is under load and at a standstill.If the temperature sensor is not located on the most heavily loaded winding phase, the protection implemented with the temperature sensor may not be sufficient. Furthermore, the temperature sensor or its supply cable may fail. As a sensitive component in an otherwise massive electric motor, a temperature sensor is therefore not only at risk of failure, but its measurements are also not particularly accurate due to delay times and a suboptimal installation position.

[0005] DE 10 2014 005 706 A1 discloses a method for operating an electric motor controlled by power electronics. In the method, a motor temperature is determined based on an initial temperature at an initial time using a motor temperature model. The initial temperature at the initial time is calculated from at least one measured motor parameter using a predetermined formula, and the predetermined formula for calculating the initial temperature is automatically recalibrated.

[0006] US 2021 / 0305929 A1 discloses a system for controlling a synchronous motor drive that receives a command voltage signal and identifies a resistance imbalance signature in the synchronous motor drive based on the command voltage signal. Based on the resistance imbalance signature, the system determines respective phase resistances corresponding to phases of a synchronous motor of the synchronous motor drive. Based on the phase resistances and an estimated average resistance, one or more phases of the synchronous motor are identified between the phases of the synchronous motor, corresponding to one or more phase resistances representing a resistance imbalance.

[0007] Ching-Yin Lee, "Effects of unbalanced voltage on the operation performance of a three-phase induction motor, " in IEEE Transactions on Energy Conversion, Vol. 14, No. 2, pages 202-208, June 1999, doi: 10.1109 / 60.766984 discloses a real load test to investigate the effects of an unbalanced voltage supply on the performance of an induction motor.

[0008] The invention is based on the object of providing an improved method and an improved device for determining a winding temperature of a winding of an electric motor.

[0009] The object is achieved according to the invention by a method having the features of claim 1 and a power converter having the features of claim 14.

[0010] Advantageous embodiments of the invention are the subject of the dependent claims.

[0011] In the method according to the invention for determining a winding temperature of a winding of an electric motor, at least one measurement operating state of the electric motor is defined for measuring an electrical winding resistance of the winding. In a measurement operating state of the electric motor, the winding resistance is measured, and the winding temperature is calculated from the measured winding resistance. In an operating state of the electric motor that differs from a measurement operating state, the winding temperature is calculated using a thermal model from at least one operating parameter of the electric motor.

[0012] The invention therefore combines measurements of a winding resistance of the winding of an electric motor with a thermal model for determining a winding temperature of the winding. This exploits the fact that the winding resistance is temperature-dependent and therefore the winding temperature can be calculated from the winding resistance. However, a measurement of the winding resistance is only possible in certain operating states of the electric motor. In particular, the motor current must be large enough for the measurement so that a sufficiently high voltage drop across the winding resistance. In addition, for example, a speed of a rotor of the electric motor relative to a stator of the electric motor - in the case of a rotating electrical machine this is an angular speed of the rotor and in the case of a linear motor a translational speed of the rotor - must not be too large to be able to carry out the measurement.Therefore, according to the invention, measuring operating states of the electric motor are defined in which a measurement of the winding resistance is possible and is carried out. In other operating states of the electric motor, i.e., in operating states in which a measurement of the winding resistance is not possible, a thermal model is used to calculate the winding temperature from at least one operating parameter of the electric motor.

[0013] The method according to the invention thus enables the winding temperature to be determined without a temperature sensor. This eliminates the aforementioned disadvantages of using a temperature sensor, namely the risk of temperature sensor failure and inaccuracies and delays in measurements with the temperature sensor. Furthermore, the costs for the temperature sensor, as well as its installation and cabling, are also eliminated.

[0014] In particular, the invention enables a very precise determination of the winding temperature in the measured operating states by measuring the winding resistance. This is particularly advantageous because the measured operating states are often critical operating states in which individual winding phases are subjected to particularly high loads, for example, operating states with negligible or very low rotor speed at high motor current. The winding resistance values ​​determined in the measured operating states can also be advantageously used as initial values ​​for the thermal model and / or for tracking the thermal model.

[0015] In one embodiment of the invention, in the case of a multi-phase electric motor, the temperature of the winding phase of the electric motor winding that has the highest temperature in the operating state is determined as the winding temperature in an operating state of the electric motor. The temperature of the winding phase that has the highest temperature in the operating state is preferably determined using the thermal model.

[0016] The above-mentioned embodiment of the invention takes into account that it is generally not possible to measure the phase resistances of the winding phases of the individual phases of a multi-phase electric motor. Rather, the winding resistance can generally only be measured as an averaged resistance which is made up of components of the individual phase resistances, whereby these components depend on the instantaneous position of a current space vector of the current in the winding. However, with the aid of the thermal model and taking into account the respective position of the current space vector, it is possible to determine with good accuracy from the averaged resistance the temperature of the winding phase of the electric motor winding which has the highest temperature in the operating state (see the exemplary embodiments described below). This temperature is crucial for protecting the motor.Therefore, according to the invention, this temperature is determined as the winding temperature of the winding. The calculation of this temperature is preferably implemented in the thermal model, so that the thermal model is also configured, in particular, to calculate from the measured winding resistances the temperature of the winding phase that has the highest temperature in the respective measuring operating state.

[0017] In a further embodiment of the invention, a motor current of the electric motor is used as an operating parameter for calculating the winding temperature in an operating state of the electric motor that is different from a measured operating state. Alternatively or additionally, for example, a speed of a rotor of the electric motor relative to a stator of the electric motor is used as an operating parameter for calculating the

[0018] Winding temperature in an operating state of the electric motor different from a measuring operating state is used.

[0019] The above-mentioned embodiments of the invention take into account that the motor current and the rotor speed of the electric motor are the operating parameters of the electric motor which are essential for the winding temperature.

[0020] In a further embodiment of the invention, the winding resistance of the electric motor winding is measured by detecting at least one electric current flowing in the winding and a corresponding electric voltage, and calculating the winding resistance of the electric motor winding from the measured values. Instead of measuring the voltage, a nominal voltage could in principle also be used to determine the winding resistance. However, deviations between the actual voltage and the nominal voltage must be taken into account with sufficient accuracy. Such deviations arise, for example, due to locking times of semiconductor switches used to generate the voltage.

[0021] In a further embodiment of the invention, an operating state in which a magnitude of a current space vector of an electric current flowing in the winding is greater than a predetermined magnitude threshold value is defined as a measuring operating state of the electric motor. This embodiment of the invention takes into account the above-mentioned criterion for selecting a measuring operating state of the electric motor as an operating state in which the motor current is large enough for the measurement to cause a sufficiently high voltage drop across the winding resistance.

[0022] In a further embodiment of the invention, an operating state in which a rotor of the electric motor does not move relative to a stator of the electric motor or moves only slowly, for example, slower than a predetermined speed threshold, is defined as a measuring operating state of the electric motor. This embodiment of the invention takes into account the above-mentioned criterion for selecting a measuring operating state of the electric motor as an operating state with a vanishing or low rotor speed.In addition, the measuring operating state is determined, for example, by the fact that the magnitude of a current space vector of an electric current flowing in the winding at the end of a measuring period does not differ significantly from the magnitude at the beginning of the measuring period, for example by less than a predetermined deviation threshold, and the winding resistance is determined as an average value over several measurements carried out during the measuring period, for example over approximately 200 measurements. By determining the winding resistance as an average value over several measurements under the conditions mentioned, the measurement error is reduced and the accuracy of the measurement is increased.

[0023] According to the invention, an operating state is defined as a measuring operating state during which an electric current flowing in the winding is modulated with a modulation frequency, and the winding resistance is calculated from the modulations of the electric current and an associated electric voltage applied to the winding. In the case of a three-phase electric motor, for example, a current along a d-axis and / or a current along a q-axis of a d / q coordinate system is modulated with the modulation frequency.

[0024] The aforementioned embodiment of the invention is a measuring method for measuring the winding resistance, which can be used alternatively or in addition to the measuring method described above (measurements at low rotor speeds under the above-mentioned conditions). The winding resistance and the winding temperature are determined from the modulated current and an associated modulated voltage. This measuring method has the advantage that it can also be used when the electric motor is operating at higher rotor speeds. However, it requires a modulation of the current in the winding, which does not impair the operation of the electric motor and still enables the winding resistance to be measured. With this measuring method, too, the thermal model is preferably used to determine the temperature of the winding phase with the highest temperature from the measured values.

[0025] In a further embodiment of the invention, the winding temperature for a winding of a stator of the electric motor is determined.

[0026] In a further embodiment of the invention, when calculating the winding temperature in an operating state different from a measured operating state, the thermal model uses at least one winding temperature determined in a measured operating state of the electric motor as an initial value. This embodiment of the invention takes into account that the thermal model generally calculates the winding temperature iteratively, starting from initial values. Since a winding temperature determined in a measured operating state of the electric motor is a measured value or is calculated from measured values, it is suitable as a realistic initial value for calculating the winding temperature with the thermal model.

[0027] In a further embodiment of the invention, the thermal model uses at least one winding temperature determined in a measured operating state of the electric motor to track the thermal model when calculating the winding temperature in an operating state different from a measured operating state. This embodiment of the invention enables the thermal model to be tracked, for example by means of a so-called observer, with realistic values ​​of the winding temperature determined in measured operating states, thus improving the accuracy of the thermal model.

[0028] A power converter according to the invention is designed to control an electric motor and comprises

[0029] - a current measuring device arranged to measure at least one electric current in a winding of the electric motor,

[0030] - a voltage measuring device which is arranged to measure at least one electrical voltage corresponding to a measured current, and

[0031] - a computing unit which is configured to calculate a winding temperature of the winding of the electric motor from measured values ​​recorded by the current measuring device and the voltage measuring device in a defined measuring operating state of the electric motor and to calculate the winding temperature in an operating state of the electric motor which is different from a measuring operating state using a thermal model from at least one operating parameter of the electric motor.

[0032] A power converter according to the invention enables the implementation of the method according to the invention. Therefore, the advantages of a power converter according to the invention correspond to the above-mentioned advantages of the method according to the invention.

[0033] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0034] FIG 1 is a block diagram of an embodiment of a power converter for controlling an electric motor, FIG 2 is a schematic diagram of a winding of an electric motor,

[0035] FIG 3 is a flow diagram of an embodiment of the method according to the invention,

[0036] FIG 4 a PTL element for calculating a winding temperature of an electric motor.

[0037] Figure 1 (FIG. 1) shows a block diagram of an exemplary embodiment of a power converter 1 for controlling an electric motor 5. In this exemplary embodiment, the electric motor 5 is three-phase and has a stator and a rotor. The stator has a winding 6, for which winding temperatures are determined in various operating states of the electric motor 5, as described below.

[0038] Figure 2 (FIG. 2) schematically shows the winding 6 of the stator of the electric motor 5. The winding 6 has a winding phase 7, 8, 9 for each phase of the electric motor 5. The winding phases 7, 8, 9 are electrically connected to one another in a star connection and, for example, are each guided through grooves of a laminated core of the stator.

[0039] A winding phase 7 of a first phase, referred to as the U phase, has an electrical phase resistance RU. A winding phase 8 of a second phase, referred to as the V phase, has an electrical phase resistance RV. A winding phase 9 of the third phase, referred to as the W phase, has an electrical phase resistance RW. The phase resistances RU, RW, RV are temperature-dependent, i.e. they depend on a temperature of the respective winding phase 7, 8, 9.

[0040] During operation of the electric motor 5, an electrical phase current IU flows in the winding phase 7 of the U phase, an electrical phase current IV flows in the winding phase 8 of the V phase and an electrical phase current IW flows in the winding phase 9 of the W phase.

[0041] The power converter 1 has, for example, in a known manner, for each phase of the electric motor 5 a half-bridge (not shown here) with two semiconductor switches that are controlled in a pulse-width modulated manner. The power converter 1 also comprises a current measuring device 2 that is set up to measure each phase current IU, IV, IW, and a voltage measuring device 3. The voltage measuring device 3 is set up to measure a phase voltage UU, UV, UW for each winding phase 7, 8, 9 between the respective winding phase 7, 8, 9 and an electrical reference line 10 that is at an electrical reference potential. The reference potential is, for example, a potential of an electrical direct voltage of an electrical intermediate circuit of the power converter 1, from which its semiconductor switches are supplied with electrical voltage.

[0042] The power converter 1 further comprises a computing unit 4 configured to determine the winding temperature of the winding 6 of the electric motor 5 according to the method described below. The computing unit 4 can, in particular, be integrated into a control unit of the power converter 1, which controls the semiconductor switches of the power converter 1. For example, the computing unit 4 can be realized as a computer program implemented in the control unit and executable by the control unit, which is configured to determine the winding temperature of the winding 6 of the electric motor 5.

[0043] Figure 3 (FIG. 3) shows a flow diagram of an exemplary embodiment of the method according to the invention with method steps 11, 12, 13 for determining the winding temperature of the winding 6. In a first method step 11, at least one measuring operating state of the electric motor 5 is defined for measuring the winding resistance during a measuring period. As a measuring operating state, for example, an operating state is defined in which the rotor of the electric motor 5 does not move or moves only slowly, for example slower than a predetermined speed threshold value, relative to the stator of the electric motor 5, and an absolute value of a current space vector which is formed from the phase currents IU, IV, IW does not differ significantly at the end of the measuring period from the absolute value at the beginning of the measuring period, for example by less than a predetermined deviation threshold value.

[0044] In a second method step 12, the winding resistance is measured in a measuring operating state of the electric motor 5 and the winding temperature is calculated from the measured winding resistance.

[0045] To measure the winding resistance, the phase currents IU, IV, IW and phase voltages UU, UV, UW are measured multiple times during the measuring period, for example about 200 times each, and an average value is calculated from the measured values ​​for each phase current IU, IV, IW and each phase voltage UU, UV, UW. From the average values ​​of the phase currents IU, IV, IW, for example, a current space vector is calculated which, in a rotor-fixed d / q coordinate system, has a component id along a d-axis and a component iq along a q-axis of the coordinate system. Furthermore, a voltage space vector is calculated from the average values ​​of the phase voltages UU, UV, UW, which voltage space vector has a component ud along the d-axis and a component uq along the q-axis in the d / q coordinate system. From the current space vector and the voltage space vector, the winding resistance is calculated as a space vector resistance R RZScalculated, for example, according to RRZS= [ud -iq+ (uq-uL) -iq) ] / (id 2 + IQ 2 ), where uL denotes a voltage induced by the rotor in the q-direction, which depends on a rotor speed of the rotor and is treated, for example, as proportional to the rotor speed.

[0046] The winding temperature is determined using a thermal model of the electric motor 5, the temperature of the winding phase 7, 8, 9 that has the highest temperature in the measured operating state. For example, the winding temperature is determined from the winding resistance according to

[0047] T=T0+ (R RZS / RO-1) • (1 / k) -K calculated with the following notations:

[0048] To: Reference temperature , Ro : Value of the space vector resistance R RZS at the reference temperature , k: coefficient for the temperature dependence of the winding resistance depending on the material of the winding, K: correction factor .

[0049] The correction factor K is used to calculate the space vector resistance R RZS The temperature of the winding phase 7, 8, 9 which has the highest temperature in the measuring operating state is to be calculated as the winding temperature T. For example, the correction factor K is calculated according to

[0050] K= (3 / 2) -I 2 -maXi{PTl (Ii 2 )} / sumi{PTl (Ii 2 ) -Ii 2} with the following notations: I: magnitude of the current space vector, i: index that runs over the three phases U, V, W, PTl (Ii 2 ) : Output value of a PTl element 14 for phase i (see Figure 4 and its description), maxi{PTl (Ii 2 )} : Maximum of PT1 (IU 2 ), PT1 (IV 2 ) and PT1 (IW 2 ) , sum ± { PT1 ( I ± 2 ) -Ii 2}=PT1 (IU 2 ) • IU 2 + PT1 (IV 2 ) • IV 2 + PT1 ( IW 2 ) -IW 2. The correction actuator K takes into account that, as a rule, the measurements of the phase currents IU, IV, IW and phase voltages UU, UV, UW do not enable the phase resistances RU, RV, RW of the winding phases of the individual phases to be determined, because the voltages of the phase windings 7, 8, 9 relative to the star point of the star point connection of the phase windings 7, 8, 9 (see Figure 2) cannot be determined from the measurements of the phase voltages UU, UV, UW, each measured relative to the reference potential of the reference line 10. The mean value of the phase voltages UU, UV, UW only results in the star point voltage if all phase resistances RU, RV, RW are the same, which, however, cannot be assumed here due to the different heating of the individual phase windings 7, 8, 9.

[0051] Figure 4 ( FIG 4 ) shows schematically the PTI element 14 for the U-phase of the winding 6 . The input value for the PTI element 14 is essentially the square IU 2 of the phase current IU, multiplied by the ratio AT / I o 2 , where AT is a temperature increase AT of the winding phase 7 of the U-phase at a reference current I o , for example at a standstill current of the electric motor 5 . Initial value PTl ( IU 2 ) =ATu of the PTl element 14 is a temperature increase ATu of the winding phase 7 of the U-phase as a function of the input value compared to the reference temperature. The PTl elements for the phases V and W correspond to the PTl element for the U-phase, whereby the input values ​​of these PTl elements are the squares IV 2 or IW 2 instead of IU 2 contain .

[0052] The determination of the winding temperature described with reference to Figures 2 and 3 can be modified in various ways, and in particular refined. For example, the calculation of the winding temperature can also take into account the influence of a laminated core of the stator and / or the rotor of the electric motor 5. In a third method step 13, in an operating state of the electric motor 5 that is different from a measured operating state, the winding temperature is calculated using the thermal model from at least one operating parameter of the electric motor 5. A motor current of the electric motor 5, for example, is used as an operating parameter for calculating the winding temperature. Alternatively or additionally, the speed of the rotor of the electric motor 5 relative to the stator of the electric motor 5, for example, is used as an operating parameter for calculating the winding temperature.For example, when calculating the winding temperature, the thermal model uses a winding temperature determined in a measured operating state of the electric motor 5 as an initial value. Furthermore, winding temperatures determined in each measured operating state of the electric motor 5 can be used to track the thermal model.

[0053] Although the invention has been illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Patent claims 1. Method for determining a winding temperature of a winding (6) of an electric motor (5), wherein - at least one measuring operating state of the electric motor (5) is defined for measuring an electrical winding resistance of the winding (6), - in a measuring operating state of the electric motor (5), the winding resistance is measured and the winding temperature is calculated from the measured winding resistance and - in an operating state of the electric motor (5) different from a measuring operating state, the winding temperature is calculated using a thermal model from at least one operating parameter of the electric motor (5), - wherein an operating state is defined as a measuring operating state during which an electric current flowing in the winding (6) is modulated with a modulation frequency, and the winding resistance is calculated from the modulations of the electric current and an associated electric voltage applied to the winding (6).

2. Method according to claim 1, wherein in the case of a multi-phase electric motor (5), the temperature of that winding phase (7, 8, 9) of the winding (6) of the electric motor (5) which has the highest temperature in the operating state is determined as the winding temperature in an operating state of the electric motor (5).

3. Method according to claim 2, wherein the temperature of the winding phase (7, 8, 9) which has the highest temperature in the operating state is determined using the thermal model.

4. Method according to one of the preceding claims, wherein a motor current of the electric motor (5) is used as an operating parameter for calculating the winding temperature in a different operating state from a measuring operating state operating state of the electric motor (5).

5. Method according to one of the preceding claims, wherein a speed of a rotor of the electric motor (5) relative to a stator of the electric motor (5) is used as an operating parameter for calculating the winding temperature in an operating state of the electric motor (5) different from a measuring operating state.

6. Method according to one of the preceding claims, wherein the winding resistance of the winding (6) of the electric motor (5) is measured by detecting at least one electrical current (IU, IV, IW) flowing in the winding (6) and a corresponding electrical voltage (UU, UV, UW) and calculating the winding resistance of the winding (6) of the electric motor (5) from the measured values.

7. Method according to one of the preceding claims, wherein an operating state in which an amount of a current space vector of an electric current flowing in the winding (6) is greater than a predetermined amount threshold value is defined as a measuring operating state of the electric motor (5).

8. Method according to one of the preceding claims, wherein an operating state is defined as a measuring operating state of the electric motor (5) in which a rotor of the electric motor (5) does not move or moves only slowly, for example slower than a predetermined speed threshold value, relative to a stator of the electric motor (5).

9. Method according to claim 7 or 8, wherein the measuring operating state is additionally determined in that an amount of a current space vector of an electric current flowing in the winding (6) does not differ significantly at the end of a measuring period from the amount at the beginning of the measurement period, for example by less than a specified deviation threshold, and the winding resistance is determined as an average value over several measurements carried out during the measurement period.

10. Method according to one of the preceding claims, wherein in the case of a three-phase electric motor (5) a current along a d-axis and / or a current along a q-axis of a d / q coordinate system is modulated with the modulation frequency.

11. Method according to one of the preceding claims, wherein the winding temperature for a winding (6) of a stator of the electric motor (5) is determined.

12. Method according to one of the preceding claims, wherein at least one winding temperature determined in a measuring operating state of the electric motor (5) is used as an initial value by the thermal model in the calculation of the winding temperature in an operating state different from a measuring operating state.

13. Method according to one of the preceding claims, wherein at least one winding temperature determined in a measuring operating state of the electric motor (5) is used by the thermal model in the calculation of the winding temperature in an operating state different from a measuring operating state to track the thermal model.

14. Power converter (1) which is designed to control an electric motor (5), the power converter (1) comprising - a current measuring device (2) which is arranged to measure at least one electric current (IU, IV, IW) in a winding (6) of the electric motor (5), - a voltage measuring device (3) which is designed to measure at least one of a measured current (IU, IV, IW) corresponding electrical voltage (UU, UV, UW), and - a computing unit (4) which is configured to calculate a winding temperature of the winding (6) of the electric motor (5) in a defined measuring operating state of the electric motor (5) from measured values ​​acquired by the current measuring device (2) and the voltage measuring device (3) and to calculate the winding temperature with a thermal model from at least one operating parameter of the electric motor (5) in an operating state of the electric motor (5) different from a measuring operating state, - wherein an operating state is defined as a measuring operating state during which an electric current flowing in the winding (6) is modulated with a modulation frequency, and the winding resistance is calculated from the modulations of the electric current and an associated electric voltage applied to the winding (6).

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