Determining winding temperature of winding of electric motor

Abstract

The invention relates to a method for determining a winding temperature of a winding (6) of an electric motor (5) and to a current converter (1) which is provided for controlling the electric motor (5). In the method, in order to measure the winding resistance of the winding (6), at least one measurement operating state of the electric motor (5) is specified. In a measurement operating state of the electric motor (5), a winding resistance is measured and a winding temperature is calculated from the measured winding resistance. The winding temperature is calculated from at least one operating parameter of the electric motor (5) using a thermal model in an operating state of the electric motor (5) that differs from the measured operating state.

Description

Technical Field

[0001] The invention relates to a method for determining the winding temperature of a winding of an electric motor. The invention also relates to a current converter which is provided for controlling an electric motor. The electric motor can generally be a rotating electric machine designed as a synchronous motor or asynchronous motor, a linear motor or a reluctance motor. Furthermore, the term electric motor also includes generators of corresponding design. Background Art

[0002] Many electric motors are operated with overload on the current converter. It is important to protect the windings of the electric motor from overheating. Overheating may also occur in water-cooled or externally ventilated electric motors due to cooling failures. In order to prevent overheating, temperature sensors are usually arranged on the windings of the electric motor, and the measured values ​​of the temperature sensors are monitored. The temperature sensor must be reliably electrically separated from the winding. As a result, the thermal connection between the temperature sensor and the winding is not ideal and may lead to a measurement delay of, for example, 10 seconds or longer. In addition, for example, when the electric motor is loaded in a stationary state, individual winding bundles of the winding can be heated more strongly than other winding bundles. If the temperature sensor is not arranged on the winding bundle that is most strongly loaded, the protection achieved by the temperature sensor may be insufficient. In addition, the temperature sensor or the input line of the temperature sensor may fail. Therefore, as a sensitive component in a large electric motor, the temperature sensor is not only at risk of failure, but also does not measure particularly accurately due to the delay time and the non-optimal installation position.

[0003] DE 10 2014 005 706 A1 discloses a method for operating an electric motor which is controlled by a power electronics system. In the method, starting from a starting temperature at a starting time, the motor temperature is determined by means of a motor temperature model, wherein the starting temperature at the starting time is calculated from at least one measured motor parameter by means of a predetermined formula and the predetermined formula for calculating the starting temperature is automatically recalibrated.

[0004] US2021 / 0305929 A1 discloses a system for controlling a synchronous motor drive, the system receiving a command voltage signal and identifying a resistance imbalance flag in the synchronous motor drive according to the command voltage signal. Based on the resistance imbalance flag, the system determines a corresponding phase resistance corresponding to a synchronous motor of the synchronous motor drive. Based on the phase resistance and the estimated average resistance, one or more phases of the synchronous motor corresponding to one or more phase resistances exhibiting resistance imbalance are identified between the phases of the synchronous motor.

[0005] Ching-Yin Lee, "Effect of Unbalanced Voltage on the Operating Performance of a Three-Phase Induction Motor," IEEE Transactions on Energy Conversion, Vol. 14, No. 2, pp. 202-208, June 1999, doi: 10.1109 / 60.766984 discloses a real load test to examine the effect of asymmetrical voltage supply on the performance of an induction motor. Summary of the invention

[0006] The object of the present invention is to specify an improved method and an improved device for determining the winding temperature of an electric motor.

[0007] According to the invention, this object is achieved by a method having the features of claim 1 and by a current converter having the features of claim 14 .

[0008] Advantageous embodiments of the invention are the subject matter of the dependent claims.

[0009] In the method according to the invention for determining the winding temperature of an electric motor, at least one measuring operating state of the electric motor is specified for measuring the winding resistance of a winding. In the measuring 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 is different from the measuring operating state, the winding temperature is calculated from at least one operating parameter of the electric motor using a thermal model.

[0010] The present invention therefore combines the measurement of the winding resistance of a winding of an electric motor with a thermal model for determining the winding temperature of the winding. In this case, use is made of the fact that the winding resistance is temperature-dependent and the winding temperature can therefore be calculated from the winding resistance. Of course, the winding resistance can only be measured in certain operating states of the electric motor. In particular, the motor current used for the measurement must be large enough to drop a sufficiently high voltage across the winding resistance. In addition, for example, the speed of the rotor of the electric motor relative to the stator of the electric motor, i.e. the angular velocity of the rotor (rotor) in the case of a rotating electric machine and the translational velocity of the rotor in the case of a linear motor, must not be too large in order to be able to perform the measurement. Therefore, according to the present invention, a measuring operating state of the electric motor is determined, in which the measurement of the winding resistance can be realized and performed. In other operating states of the electric motor, that is, in operating states in which the winding resistance cannot be measured, a thermal model is used instead to calculate the winding temperature from at least one operating parameter of the electric motor.

[0011] The method according to the invention can therefore determine the winding temperature without a temperature sensor. This eliminates the above-mentioned disadvantages of using a temperature sensor, namely the risk of failure of the temperature sensor and the inaccuracy and time delay of the measurement with the temperature sensor. In addition, the costs for the temperature sensor and its installation and wiring are also eliminated.

[0012] In particular, the invention enables very precise determination of the winding temperature in the measuring operating state by measuring the winding resistance. This is particularly advantageous since the measuring operating state is usually a critical operating state in which the individual winding bundles are subjected to particularly high loads, for example in operating states in which the rotor speed disappears or is very low at high motor currents. Furthermore, the value of the winding resistance determined in the measuring operating state can advantageously be used as a starting value for a thermal model and / or for updating the thermal model.

[0013] In the embodiment of the present invention, in the case of a multi-phase electric motor, the temperature of the winding bundle of the winding of the electric motor having the highest temperature in the operating state is determined as the winding temperature in the operating state of the electric motor. Here, the temperature of the winding bundle having the highest temperature in the operating state is preferably determined using a thermal model.

[0014] The above-described design of the present invention takes into account that it is usually impossible to measure the phase resistance of the winding bundles of the individual phases of a multi-phase electric motor. More precisely, usually only an average resistance can be measured as the winding resistance, which is composed of components of the individual phase resistances, wherein these components depend on the instantaneous position of the current space vector in the winding. However, it is possible to determine the temperature of the winding bundle of the winding of the electric motor with the highest temperature in the operating state with good accuracy by means of a thermal model and taking into account the corresponding position of the current space vector from the average resistance (see the embodiment described below for this). This temperature is decisive for protecting the motor. Therefore, according to the present 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 provided in particular for calculating the temperature of the winding bundle with the highest temperature in the corresponding measured operating state from the measured winding resistance.

[0015] In a further embodiment of the invention, the 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 the measured operating state. Alternatively or additionally, for example, the speed of the rotor of the electric motor relative to the stator 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 the measured operating state.

[0016] The above-described embodiment of the present invention takes into account that the motor current and the rotor speed of the electric motor are operating parameters of the electric motor that are relevant to the winding temperature.

[0017] In a further embodiment of the invention, the winding resistance of the winding of the electric motor is measured by detecting at least one current flowing in the winding and the voltage corresponding thereto and calculating the winding resistance of the winding of the electric motor from the measured values. Instead of measuring the voltage, in principle, the voltage setpoint value of the voltage can also be used to determine the winding resistance. However, deviations of the actual voltage value from the voltage setpoint value must be taken into account with sufficient accuracy. Such deviations arise, for example, due to the locking times of the semiconductor switches that generate the voltage.

[0018] In a further embodiment of the invention, an operating state is designated as a measuring operating state of the electric motor when the magnitude of the current space vector of the current flowing in the winding is greater than a predefined magnitude threshold value. This embodiment of the invention takes into account the above-mentioned criteria for selecting the measuring operating state of the electric motor as an operating state, in which the motor current for measurement is sufficiently large to drop a sufficiently high voltage across the winding resistance.

[0019] In a further embodiment of the invention, an operating state in which the rotor of the electric motor does not move or moves only slowly, for example more slowly than a preset speed threshold value, relative to the stator of the electric motor is designated as a measuring operating state of the electric motor. This embodiment of the invention takes into account the above-mentioned criteria for selecting the measuring operating state of the electric motor as an operating state with a vanishing or low rotor speed. The measuring operating state is additionally designated by the fact that the magnitude of the current space vector of the current flowing in the winding at the end of the measuring period does not differ significantly from the magnitude at the beginning of the measuring period, for example by less than a preset deviation threshold value, and the winding resistance is determined as an average value of a plurality of measurements, for example approximately 200 measurements, performed during the measuring period. By determining the winding resistance as an average value of a plurality of measurements under the described conditions, measurement errors are reduced and the accuracy of the measurement is increased.

[0020] According to the invention, an operating state in which the current flowing in the winding is modulated with a modulation frequency is designated as a measuring operating state, and the winding resistance is calculated from the modulation of the current and the associated voltage applied to the winding. In the case of a three-phase electric motor, for example, the current along the d-axis and / or the current along the q-axis of a d / q coordinate system is modulated with the modulation frequency.

[0021] The above-mentioned embodiment of the present invention is a measuring method for measuring the winding resistance, which can be used as an alternative or in addition to the above-mentioned measuring method (measurement at low rotor speed under the above-mentioned premise). The winding resistance and the winding temperature are determined here by the modulated current and the associated modulated voltage. This measuring method has the advantage that it can also be used in the operation of the electric motor at higher rotor speeds. However, this measuring method requires modulation of the current in the winding, which does not affect the operation of the electric motor and nevertheless allows the winding resistance to be measured. In this measuring method, a thermal model is preferably also used in order to determine the temperature of the winding bundle with the highest temperature of the winding from the measured values.

[0022] In a further embodiment of the present invention, a winding temperature of a stator for an electric motor is determined.

[0023] In a further embodiment of the invention, at least one winding temperature determined in the measuring operating state of the electric motor is used as a starting value by the thermal model when calculating the winding temperature in an operating state different from the measuring operating state. This embodiment of the invention takes into account that the thermal model usually calculates the winding temperature iteratively starting from the starting values. Since the winding temperature determined in the measuring operating state of the electric motor is a measured value or is calculated from the measured value, it is suitable as an actual starting value for calculating the winding temperature using the thermal model.

[0024] In a further embodiment of the invention, when calculating the winding temperature in an operating state different from the measuring operating state, the thermal model uses at least one winding temperature determined in the measuring operating state of the electric motor for updating the thermal model. This embodiment of the invention makes it possible, for example, by means of a so-called observer to update the thermal model with the actual value of the winding temperature determined in the measuring operating state and thus improve the accuracy of the thermal model.

[0025] The current converter according to the invention is provided for controlling an electric motor and comprises:

[0026] a current measuring device, which is arranged to measure at least one current in a winding of the electric motor,

[0027] - a voltage measuring device, which is arranged to measure at least one voltage corresponding to the measured current, and

[0028] a calculation unit which is configured to calculate a winding temperature of a winding of the electric motor from measured values ​​detected by the current measuring device and the voltage measuring device in a specific measuring operating state of the electric motor and to calculate the winding temperature from at least one operating parameter of the electric motor using a thermal model in an operating state of the electric motor which differs from the measuring operating state.

[0029] The current converter according to the invention enables the method according to the invention to be carried out. The advantages of the current converter according to the invention therefore correspond to the above-described advantages of the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above-described characteristics, features and advantages of the present invention and the ways and methods to achieve these characteristics, features and advantages will become clearer and more clearly understandable in conjunction with the following description of embodiments, which are explained in more detail in conjunction with the accompanying drawings.

[0031] Here it is shown:

[0032] Figure 1 A block diagram showing an embodiment of a current converter for controlling an electric motor,

[0033] Figure 2 Schematic diagram of the windings of an electric motor,

[0034] Figure 3 A flow chart showing an embodiment of the method according to the present invention,

[0035] Figure 4 A PT1 element is shown for calculating the winding temperature of an electric motor. DETAILED DESCRIPTION

[0036] Figure 1 The block diagram of an exemplary embodiment of a current converter 1 for controlling an electric motor 5 is shown. 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 the winding temperature is determined in different operating states of the electric motor 5 as described below.

[0037] Figure 2 The winding 6 of the stator of the electric motor 5 is schematically shown. The winding 6 has a winding bundle 7, 8, 9 for each phase of the electric motor 5. The winding bundles 7, 8, 9 are electrically connected to one another in a star connection and are each guided, for example, by a slot of a laminated core of the stator.

[0038] The winding bundle 7 of the first phase, called the U phase, has a phase resistance RU. The winding bundle 8 of the second phase, called the V phase, has a phase resistance RV. The winding bundle 9 of the third phase, called the W phase, has a phase resistance RW. The phase resistances RU, RW, RV are temperature-dependent, that is, they are dependent on the temperature of the corresponding winding bundles 7, 8, 9.

[0039] During operation of the electric motor 5 , a phase current IU flows in the U-phase winding bundle 7 , a phase current IV flows in the V-phase winding bundle 8 , and a phase current IW flows in the W-phase winding bundle 9 .

[0040] The current converter 1 has, for example, in a known manner, a half bridge (not shown here) with two semiconductor switches for each phase of the electric motor 5, which are actuated in a pulse width modulated manner. In addition, the current converter 1 comprises a current measuring device 2 and a voltage measuring device 3, which are provided for measuring each phase current IU, IV, IW. The voltage measuring device 3 is provided for measuring, for each winding bundle 7, 8, 9, a phase voltage UU, UV, UW between the corresponding winding bundle 7, 8, 9 and a reference electrical line 10, which is at a reference potential. The reference potential is, for example, the potential of the DC voltage of the intermediate circuit of the current converter 1, from which the voltage is supplied to its semiconductor switches.

[0041] The current converter 1 further comprises a calculation unit 4 which is provided for determining the winding temperature of the winding 6 of the electric motor 5 according to the method described below. The calculation unit 4 can be integrated in particular into a control unit of the current converter 1 and the control unit controls the semiconductor switches of the current converter 1. For example, the calculation unit 4 can be implemented as a computer program executed in the control unit, which can be executed by the control unit and is provided for determining the winding temperature of the winding 6 of the electric motor 5.

[0042] Figure 3 The flow chart shows an exemplary embodiment of a method according to the invention having method steps 11 , 12 , 13 for determining a winding temperature of a winding 6 .

[0043] In a first method step 11, at least one measuring operating state of the electric motor 5 is specified for measuring the winding resistance during a measuring period. For example, an operating state is specified as a measuring operating state when the rotor of the electric motor 5 does not move or moves only slowly, for example more slowly than a predefined speed threshold, relative to the stator of the electric motor 5 during the measuring period, and the magnitude of the current space vector formed by the phase currents IU, IV, IW at the end of the measuring period does not differ significantly from the magnitude at the beginning of the measuring period, for example differs by less than a predefined deviation threshold.

[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] In order to measure the winding resistance, the phase currents IU, IV, IW and the phase voltages UU, UV, UW are measured several times in a measuring time period, for example approximately 200 times respectively, and an average value is formed 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 formed, which in a rotor-fixed d / q coordinate system has a component id along the d-axis of the coordinate system and a component iq along the q-axis. Furthermore, from the average values ​​of the phase voltages UU, UV, UW, a voltage space vector is formed, which in the d / q coordinate system has a component ud along the d-axis and a component uq along the q-axis. From the current space vector and the voltage space vector, the winding resistance is calculated as the space vector resistance R RZS , for example, according to RRZS = [ud·iq+(uq-uL)·iq)] / (id 2 +iq 2 ),

[0046] Therein, uL denotes the voltage induced by the rotor in the q direction, which voltage depends on the rotor speed of the rotor and is processed, for example, to be proportional to the rotor speed.

[0047] Using the thermal model of the electric motor 5, the temperature of the winding bundle 7, 8, 9 having the highest temperature in the measured operating state is determined as the winding temperature. The winding temperature is calculated, for example, from the winding resistance according to

[0048] T=T0+(R RZS / R0-1)·(1 / k)·K uses the following names:

[0049] T0: reference temperature,

[0050] R0: Space vector resistance R at reference temperature RZS The value of

[0051] k: Coefficient for the temperature dependency of the winding resistance, which depends on the winding material,

[0052] K: correction factor.

[0053] The correction factor K is used to calculate the vector resistance R from the space RZS The temperature of the winding bundle 7, 8, 9 having the highest temperature in the measuring operating state is calculated as the winding temperature T. For example, the correction factor K is calculated according to

[0054] K=(3 / 2)·I 2 ·max i {PT1(I i 2 )} / sum i {PT1(I i2 )·I i 2}

[0055] Use the following names:

[0056] I: the magnitude of the current space vector,

[0057] i: index covering the three phases U, V, and W,

[0058] PT1(I i 2 ): initial value of the PT1 element 14 of phase i (see Figure 4 and its description),

[0059] max i {PT1(I i 2 )}: PT1(IU 2 )、PT1(IV 2 ) and PT1(IW 2 ),

[0060] sum i {PT11((I i 2 )·I i 2}=PT1(IU 2 )·IU 2 +PT1(IV 2 ))·IV 2 +PT1(IW 2 ))·IW 2 .

[0061] The correction factor K takes into account that the measurement of the phase currents IU, IV, IW and the phase voltages UU, UV, UW generally does not allow the determination of the phase resistances RU, RV, RW of the winding bundles of the individual phases, since the measurement of the phase voltages UU, UV, UW, respectively measured relative to the reference potential of the reference line 10, does not allow the determination of the voltage of the phase windings 7, 8, 9 relative to the star point of the star point circuit of the phase windings 7, 8, 9 (see Figure 2 ). The average value of the phase voltages UU, UV, UW only produces the star point voltage if all phase resistances RU, RV, RW are of equal magnitude, which, however, cannot be assumed here due to the different heating of the individual phase windings 7 , 8 , 9 .

[0062] Figure 4 The PT1 element 14 for the U phase of the winding 6 is schematically shown. The input value of the PT1 element 14 is substantially the square of the phase current IU 2 Multiply by the ratio ΔT / I0 2, wherein ΔT represents the temperature increase ΔT of the winding bundle 7 of the U phase at a reference current I0, for example the quiescent current of the electric motor 5. The initial value PT1 (IU 2 )=ΔT U is the temperature increase ΔT of the winding bundle 7 of the U phase relative to the initial value of the reference temperature U The PT1 elements for phases V and W are relative to the PT1 element for phase U, wherein the initial values ​​of these PT1 elements contain the values ​​instead of IU 2 Square IV 2 or IW 2 .

[0063] according to Figure 2 and Figure 3 The determination of the winding temperature described can be modified in different ways, in particular it can be improved. For example, the calculation of the winding temperature can also take into account the influence of the laminated core of the stator and / or rotor of the electric motor 5 .

[0064] In a third method step 13, in an operating state of the electric motor 5 that differs from the measured operating state, the winding temperature is calculated from at least one operating parameter of the electric motor 5 using the thermal model. In this case, for example, the motor current of the electric motor 5 is used as an operating parameter for calculating the winding temperature. Alternatively or additionally, for example, the speed of the rotor of the electric motor 5 relative to the stator of the electric motor 5 is used as an operating parameter for calculating the winding temperature. For example, the winding temperature determined in the measured operating state of the electric motor 5 is used as a starting value by the thermal model when calculating the winding temperature. Furthermore, the winding temperature determined in each measured operating state of the electric motor 5 can be used to update the thermal model.

[0065] Although the present invention has been shown and described in more detail by means of preferred embodiments, the present invention is not restricted to the disclosed examples and a person skilled in the art can derive other variations therefrom without departing from the scope of protection of the present invention.

Claims

1. A method for determining the winding temperature of a winding (6) of an electric motor (5), wherein: - assigning at least one measuring operating state of the electric motor (5) to measuring the winding resistance of the winding (6), - measuring the winding resistance in a measuring operating state of the electric motor (5) and calculating the winding temperature from the measured winding resistance, and - calculating the winding temperature from at least one operating parameter of the electric motor (5) using a thermal model in an operating state of the electric motor (5) different from the measured operating state, - wherein an operating state in which the current flowing in the winding (6) is modulated at a modulation frequency is designated as a measuring operating state, and the winding resistance is calculated from the modulation of the current and the associated voltage applied to the winding (6).

2. The method according to claim 1, wherein: In the case of a multiphase electric motor (5), in an operating state of the electric motor (5), the temperature of the winding bundle (7, 8, 9) of the winding (6) of the electric motor (5) having the highest temperature in this operating state is determined as the winding temperature.

3. The method according to claim 2, wherein: The temperature of the winding bundle (7, 8, 9) having the highest temperature in the operating state is determined using the thermal model.

4. A method according to any one of the preceding claims, wherein: The motor current of the electric motor (5) is used as an operating variable for calculating the winding temperature in an operating state of the electric motor (5) that differs from a measuring operating state.

5. A method according to any one of the preceding claims, wherein: The speed of the rotor of the electric motor (5) relative to the 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) that differs from a measuring operating state.

6. A method according to any one of the preceding claims, wherein: The winding resistance of the winding (6) of the electric motor (5) is measured by recording at least one current (IU, IV, IW) flowing in the winding (6) and a voltage (UU, UV, UW) corresponding to the current, and the winding resistance of the winding (6) of the electric motor (5) is calculated from the measured values.

7. A method according to any one of the preceding claims, wherein: An operating state in which the magnitude of the current space vector of the current flowing in the winding (6) is greater than a preset magnitude threshold is designated as a measured operating state of the electric motor (5).

8. A method according to any one of the preceding claims, wherein: An operating state in which the rotor of the electric motor (5) does not move or moves only slowly, for example slower than a preset speed threshold, relative to the stator of the electric motor (5) is designated as a measured operating state of the electric motor (5).

9. The method according to claim 7 or 8, wherein: The measuring operating state is additionally specified by the fact that the magnitude of the current space vector of the current flowing in the winding (6) at the end of the measuring period does not differ significantly from the magnitude at the beginning of the measuring period, for example by less than a preset deviation threshold, and the winding resistance is determined as an average value of a plurality of measurements performed during the measuring period.

10. A method according to any one of the preceding claims, wherein: In the case of a three-phase electric motor (5), the current along the d-axis and / or the current along the q-axis of the d / q coordinate system is modulated at the modulation frequency.

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

12. A method according to any one of the preceding claims, wherein: When calculating the winding temperature in an operating state different from the measuring operating state, at least one winding temperature determined in the measuring operating state of the electric motor (5) is used as a starting value by the thermal model.

13. A method according to any one of the preceding claims, wherein: When calculating the winding temperature in an operating state different from the measured operating state, at least one winding temperature determined in the measured operating state of the electric motor (5) is used by the thermal model in order to update the thermal model.

14. A current converter (1), the current converter being configured to control an electric motor (5), the current converter (1) comprising: - a current measuring device (2) arranged to measure at least one current (IU, IV, IW) in a winding (6) of the electric motor (5), - a voltage measuring device (3) arranged to measure at least one voltage (UU, UV, UW) corresponding to the measured current (IU, IV, IW), and a calculation unit (4) which is configured to calculate the winding temperature of the winding (6) of the electric motor (5) from the measured values ​​recorded by the current measuring device (2) and the voltage measuring device (3) in a specified measuring operating state of the electric motor (5), and to calculate the winding temperature from at least one operating parameter of the electric motor (5) using a thermal model in an operating state of the electric motor (5) which is different from the measuring operating state, - wherein an operating state in which the current flowing in the winding (6) is modulated at a modulation frequency is designated as a measuring operating state, and the winding resistance is calculated from the modulation of the current and the associated voltage applied to the winding (6).

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