Method for estimating the resistance of stator windings, control unit and electric machine

Abstract

A method for estimating the resistance of stator windings of an electric machine used in a vehicle, in particular a permanent magnet synchronous machine that can be driven via a power output stage, is disclosed. Furthermore, a control unit configured and programmed to execute the method and an electric machine, in particular a permanent magnet synchronous machine, comprising the power output stage and the control unit, are disclosed.

Description

[0001] The present invention relates to a method for estimating the resistance of stator windings, preferably to derive the temperature of the stator windings from this estimated resistance. The present invention further relates to a control unit configured and programmed to execute the method according to the invention, and to an electric machine equipped with this control unit.

[0002] For the effective and safe operation of a synchronous machine, such as a permanent magnet synchronous machine (PMSM), knowledge of the resistance of stator windings and preferably of the temperature of the stator windings is important.

[0003] A thorough understanding of the stator winding resistance can improve the control and regulation of an electric motor and allows for a better estimation of operating losses. In a hybrid electric vehicle, this can, for example, lead to more effective support of the combustion engine. In a battery-electric vehicle, it enables more precise control and regulation of the electric motor. Furthermore, knowing the temperature of the stator windings can protect an electric motor, and especially its stator windings, from thermal damage.

[0004] In the prior art, there are various methods for determining the resistance during the operation of an electric machine. For example, additional currents are injected into the stator windings of the electric machine, and the resistance of the stator windings is determined by measuring the applied voltage.

[0005] AT 514 356 B1 discloses a method and a device for detecting contact errors on an electrical machine.

[0006] German patent DE 10 2014 200 337 A1 describes such a method. This method utilizes additional degrees of freedom of an electric machine with more than three connections, e.g., a three-phase electric machine with a neutral conductor, to inject an additional current without affecting the torque. In addition to the phase currents that generate the rotating field of the machine, so-called bias currents are selectively introduced. These bias currents do not produce torque but can be used for targeted heating, e.g., for self-tests or calibration. For this purpose, a DC component (or at least a sufficiently low-frequency component) is superimposed on the phase currents of such a rotating field machine. This component does not contribute to torque generation. By measuring the current and voltage, a resistance can be determined, and the temperature can be deduced from this.

[0007] In German patent DE 10 2020 117 906 A1, the individual phase resistance of an electric motor is determined. For this purpose, a DC signal is first applied in one phase and then in another. The resulting voltages and currents are measured. Based on these measured values, the phase resistance of each phase is determined.

[0008] The object of the present invention is to provide an advanced technique for estimating the resistance of stator windings. In particular, the technique should not cause any noticeable change in the behavior of the electric machine, i.e., the driving behavior of an electric vehicle in which the electric machine is used.

[0009] The problem is solved by the method, the control unit, and the electric machine with the features according to the independent claims. Advantageous embodiments are the subject of the dependent claims.

[0010] According to the invention, a method for estimating the resistance of stator windings of an electric motor used in an electric vehicle, in particular a permanent magnet synchronous motor that can be driven via a power output stage, is provided. An electric vehicle is understood to be a purely battery-electric vehicle or a hybrid electric vehicle that, in addition to the electric motor, has an internal combustion engine. The method is carried out when the electric vehicle is stationary. It should be noted that "stationary" is understood to mean a state in which the electric vehicle is briefly stopped or not moving. Examples of this are a stop at a traffic light or a road sign, or another short stop after which driving is to resume. The method comprises the following steps: Applying a short initial voltage test signal to the stator windings via the power output stage prevents heating of the stator windings. This initial voltage is sufficiently high to ensure that the voltage drop in the power output stage caused by an initial current remains linear or constant. In other words, the initial current flowing in response to the applied voltage causes no, or no noticeable, change in the temperature of the stator windings. Furthermore, the magnitude of the applied initial voltage is chosen such that the voltage drop in the power output stage caused by the initial current remains linear or constant. This is necessary because the voltage drop in power output stages is highly non-linear at low voltages.

[0011] The next step involves capturing and storing the initial current flowing through the stator windings. This measured and stored value can then be used for further analysis.

[0012] In the next step, a second voltage test signal is applied to the stator windings via the power output stage. This second signal has a short duration and a voltage higher than the first test signal. Consequently, the second current flowing due to this second voltage also causes no, or at least no noticeable, change in the temperature of the stator windings. Since the second voltage of the second test signal is higher than the first voltage of the first test signal, the second voltage also lies within a linear or constant range of the voltage drop in the power output stage.

[0013] A step then follows to detect and store a second current flowing through the stator windings. Consequently, the value of this second current is available for further processing.

[0014] The next step involves calculating the resistance of the stator windings as the quotient of the difference between the second and first voltages and the difference between the second and first currents. Therefore, the difference between the second and first voltages is divided by the difference between the second and first currents to determine the resistance of the stator windings.

[0015] Of course, to improve the estimation, the described procedure can be carried out with several voltage test signals at different voltage levels in order to increase the reliability of the resistance determination.

[0016] For cost reasons, conventional power output stages used in vehicles usually do not implement a measurement of the voltage applied to the stator windings. However, especially at low voltages and currents, the power output stage's own power consumption results in a non-linear relationship between current and voltage, which can distort the results of measurements taken using the methods described above.

[0017] More precisely, in the (linear) ideal case, the effective output voltage of a phase can be calculated directly from the PWM level and the DC voltage U_DC according to the following voltages: U_a=U_DC*(PWM_a−0.5)=I_a*R U_b=U_DC*(PWM_b−0.5)=I_b*R U_c=U_DC*(PWM_c−0.5)=I_c*R

[0018] Here, U_a, U_b, and U_c are the respective voltages applied to three stator windings, I_a, I_b, and I_c are the corresponding currents, R is the (ohmic) resistance, and PWM_a, PWM_b, and PWM_c are the duty cycles of the individual phases. In practice, however, a voltage drop U_drop, dependent on the phase current, occurs in the power output stage for each phase. U_a_eff=U_a−U_drop(I_a) U_b_eff=U_b−U_drop(I_b) U_c_eff=U_c−U_drop(I_c)

[0019] This voltage drop is extremely non-linear in the low current range, but is almost constant for higher currents.

[0020] In order to use a resistance measurement for an effective temperature estimation, the measurement must be extremely accurate, as even a 4% error would cause a temperature error of 10 K.

[0021] This method prevents the stator windings from heating up and thus avoids any change in resistance during the resistance estimation. Furthermore, the voltages of the first and second voltage test signals are selected to lie within a linear or constant range of the voltage drop in the power output stage. By calculating the difference between the second and first voltages, the influence of the voltage drop in the power output stage is eliminated. Consequently, an accurate resistance estimation can be obtained. By performing the method while the electric machine is stationary, any impact on the driving behavior of the electric vehicle can also be avoided.

[0022] The short duration can be set between 20 ms and 40 ms. Setting the short duration to a value within this range prevents the stator windings from heating up.

[0023] The first voltage of the first voltage test signal can be in the range of 30% to 50%, preferably 40%, of the rated voltage of the power output stage. The second voltage of the second voltage test signal can be in the range of 70% to 90%, preferably 80%, of the rated voltage of the power output stage. By setting the first and second voltages to these ranges, it is ensured that they lie within a linear range of the voltage drop in the power output stage.

[0024] Preferably, the first and second voltages can be predefined in offline tests. This allows for the definition of a relatively accurate ideal current-voltage curve profile before the actual application, and the selection of the voltage test signals to lie within a linear range of the voltage drop in the power output stage. Furthermore, the signals can be set to achieve the most accurate possible determination of the current-voltage curve. For example, the first voltage test signal is set close to a minimum permissible voltage, and the second voltage test signal close to a maximum permissible voltage, at which the curve is already or still linear (each with a sufficient safety margin from the actual extreme values). In this way, the linear portion of the current-voltage curve, and thus the resistance of the stator windings, can be precisely determined.

[0025] Furthermore, the calculated resistance can be advantageously compared with a resistance calculated based on a measured actual voltage drop across the stator windings for verification purposes. The calculated resistance can thus be used to monitor the measurement of the actual voltage drop across the stator windings.

[0026] The calculated resistance can be used to derive the temperature of the stator windings. This requires knowledge of the material used in the stator windings.

[0027] Furthermore, the temperature derived from the calculated resistance can be compared with a measured temperature for the purpose of verifying the measurement. Therefore, the temperature derived from the calculated resistance can be used to monitor the temperature measurement at the stator windings.

[0028] Furthermore, a control unit is provided, which is designed and programmed to execute the method according to one of the preceding aspects. The control unit therefore has corresponding inputs for receiving analog or digital measurement signals. The measurement signals are then processed by the control unit according to the method according to the invention, which is implemented as program code. The control unit can also have an output for outputting the resistance and / or temperature of the stator windings.

[0029] Furthermore, an electric machine, in particular a permanent magnet synchronous machine, is provided, which has a power output stage and a control unit according to the preceding section. Consequently, the power output stage and thus the electric machine can be controlled according to the method described in the preceding sections.

[0030] The present invention is described in detail below with reference to the figures. These show: Fig. 1. A qualitative current-voltage diagram showing a voltage drop across the stator windings of an electric machine, a voltage drop in a power output stage, and a resulting composite voltage drop; and Fig. 2 An exemplary resistance line obtained from two measurements.

[0031] An embodiment of the present invention is described below with reference to the figures.

[0032] Fig. Figure 1 shows a qualitative current-voltage diagram in which a voltage drop 1 across stator windings of an electric machine, a voltage drop 2 in a power output stage of an electric machine, and a resulting composite voltage drop 3 are shown.

[0033] As from Fig. As can be seen, the voltage drop 1 across the stator windings is linear due to the ohmic nature of the material used for the stator windings, e.g., copper. The voltage drop 2 in the power output stage exhibits a non-linear behavior at low currents. At higher currents, however, the voltage drop is linear or constant. The sum of the two voltage drops 1 and 2 thus results in the combined voltage drop 3. It should be noted that in the vast majority of synchronous machines or their control units, separate measurement of voltage drops 1 and 2 is not implemented.

[0034] To take this circumstance into account, the following will be discussed with reference to Fig. The two described methods were carried out. Again, the compound voltage drop 3 is in Fig. 2 shown.

[0035] The procedure is executed when the electric motor is stationary. It should be noted that "stationary" is understood to mean a state in which the electric vehicle is briefly stopped or not moving. Examples include stopping at a traffic light or road sign, or any other brief stop after which the vehicle is to resume driving.

[0036] In a first step, an initial voltage test signal is applied to the stator windings via the power output stage. This signal is of short duration to avoid heating the stator windings, and the initial voltage is sufficiently high so that any voltage drop caused by an initial current in the power output stage remains within the linear or constant range. The operating point obtained by applying the initial voltage test signal is then determined. Fig. 2 with the reference number 4.

[0037] The short duration is set between 20 ms and 40 ms. The initial voltage of the first voltage test signal can be in the range between 30% and 50%, preferably 40%, of the nominal voltage of the power output stage.

[0038] The next step involves capturing and storing the initial current passing through the stator windings.

[0039] A second voltage test signal is then applied to the stator windings with a short duration and a second voltage that is higher than the first voltage of the first test signal. The operating point obtained by applying the second voltage test signal is in Fig. 2 with the reference number 5.

[0040] The short duration is also set between 20 ms and 40 ms. The second voltage of the second voltage test signal is in the range of 70% to 90%, preferably 80%, of the nominal voltage of the power output stage.

[0041] It should be noted that the first voltage and the second voltage can be determined beforehand by offline tests to ensure that they lie within the linear and constant range of the voltage drop 2, respectively.

[0042] A step then follows to capture and store a second current that is imprinted through the stator windings.

[0043] The next step involves calculating the resistance of the stator windings as the quotient of the difference between the second and first voltages and the difference between the second and first currents. The resistance thus corresponds to the slope of a slope in Fig.The line 6 shown in Figure 2 is defined by the two operating points 4 and 5. Since the two operating points 4 and 5 lie within a linear or constant region of the voltage drop 2 in the power output stage, the influence of the voltage drop 2 is eliminated when calculating the resistance. Consequently, an accurate estimate of the resistance of the stator windings is achieved.

[0044] After calculating the resistance, the temperature of the stator windings can also be derived. This only requires knowledge of the material used for the stator windings.

[0045] The result of estimating the resistance and / or temperature of the stator windings can also be used, for example, to verify measured values.

[0046] It should be noted that the method described above can also be implemented in a control unit. The control unit is then designed and programmed to execute the method described above. For this purpose, the control unit has corresponding inputs for receiving analog or digital measurement signals. These measurement signals are then processed by the control unit according to the method according to the invention. The control unit can also have an output for outputting the resistance and / or temperature of the stator windings.

[0047] Furthermore, the control unit is used to control a power output stage, which is used to drive an electric machine. Consequently, the power output stage, and thus the electric machine, can be operated according to the procedure described above.

Claims

[1] Method for estimating the resistance of stator windings of an electric machine used in an electric vehicle, in particular a permanent magnet synchronous machine which can be driven via a power output stage, wherein the method is carried out when the vehicle is stationary and comprises the following steps: - Applying a short initial voltage test signal to the stator windings via the power output stage to avoid heating the stator windings, and an initial voltage that is sufficiently high so that any voltage drop in the power output stage caused by an initial current is within a linear or constant range, - Capturing and storing the initial current imprinted by the stator windings, - Applying a second voltage test signal to the stator windings via the power output stage with a short duration and a second voltage that is higher than the first voltage of the first test signal, - Capturing and storing an imprinted second current through the stator windings, and - Calculating the resistance of the stator windings as a quotient of a difference between the second and the first voltage and a difference between the second and the first current. [2] Method according to claim 1, wherein the short duration is between 20 ms and 40 ms. [3] Method according to one of claims 1 or 2, wherein the first voltage of the first voltage test signal is in the range between 30% and 50%, preferably 40%, of the nominal voltage of the power output stage, and the second voltage of the second voltage test signal is in the range of 70% to 90%, preferably 80%, of the nominal voltage of the power output stage. [4] Method according to any one of the preceding claims 1 to 3, wherein the voltages of the first and second voltage test signals are determined in advance in offline tests. [5] Method according to any one of the preceding claims 1 to 4, wherein the calculated resistance is compared with a resistance calculated on the basis of a detected actual voltage drop across the stator windings. [6] Method according to any one of the preceding claims 1 to 5, which further comprises, after calculating the resistance, a step for deriving a temperature of the stator windings from the calculated resistance. [7] Method according to claim 6, wherein the derived temperature is compared with a detected temperature for the purpose of checking the detection. [8] Control unit which is designed and programmed to execute the method according to any one of the preceding claims 1 to 7. [9] Electric machine, in particular permanent magnet synchronous machine, comprising a power output stage and a control unit according to claim 8.

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