Vector control method and control unit for synchronous electric machine

Abstract

The invention relates to a vector control method for a synchronous machine, in particular a permanent magnet synchronous machine, in which a Q-axis target current and a D-axis target current (Iq, Id) are determined by respective distributions (4) linking a target torque (MTgt) and a target flow rate (psi TgtMTPA) to the Q-axis target current (Iq) and the D-axis target current (Id), respectively, a voltage change in a voltage to be applied to a stator winding of the synchronous motor is not taken into account. The voltage change is pre-adjusted or compensated for by adaptively adjusting the target flow rate (psi TgtMTPA). The invention also relates to a control unit designed and programmed to execute the vector control method.

Description

Technical Field

[0001] The present invention relates to a vector control method for a synchronous motor, in particular a permanent magnet synchronous motor (PMSM), and a control unit designed and programmed to perform the vector control method. Background Art

[0002] In order for a synchronous motor to operate efficiently, it is important to apply the optimal current for efficiency to the stator winding of the synchronous motor according to the current operating point. As the speed of the rotor of the synchronous motor increases, the voltage needs to be increased due to the back electromagnetic force (back EMF). In order to be able to operate the synchronous motor even at high speeds, when the maximum voltage of the power electronics unit driving the stator winding is reached, the magnetic field of the synchronous motor is weakened by adjusting the current accordingly. Otherwise, the operation of the synchronous motor may become unstable or even uncontrollable. In the generator operation of the synchronous motor, if the maximum voltage is exceeded, current peaks that may damage the power electronics unit and oscillatory torques that significantly deviate from the required target torque may occur. For example, oscillatory torques lead to unsafe driving behavior in electric vehicles, so this behavior must be prevented at all costs. Due to the hardware limitations of the control unit that controls or regulates the operation of the synchronous motor, the control or regulation method must also be designed to be computationally and memory-efficient.

[0003] Typically, the optimal efficiency currents for synchronous motor operation are stored in lookup tables. One table is used for the D-axis currents and one table is used for the Q-axis currents, wherein the currents are stored as a function of the target torque (positive and negative), the speed, the voltage that can be applied to the stator windings by the power electronics unit, in particular the applicable DC voltage, and the rotor temperature. Alternatively, the currents can also be stored as a function of the target torque, speed, and rotor temperature. In this case, the speed is scaled according to the ratio of the voltage that can be applied to the stator windings by the power electronics unit to the nominal voltage of the power electronics unit (i.e., the maximum voltage that can be applied by the power electronics unit). For example, with a ratio of 0.5, the speed is doubled.

[0004] The input variables target torque and speed are usually written off at a much higher level than the input variables temperature and voltage because they vary significantly more during operation of the synchronous machine and have a greater impact on the current.

[0005] The relationship between current and speed is approximately inversely proportional. The extremely uneven distribution of the support points must be taken into account, and this can lead to large interpolation errors.

[0006] Various methods are known for field weakening in order to increase the speed of the rotor above the base speed. Typically, when a specified voltage, such as the maximum voltage or a specified proportion of the maximum voltage (e.g., 90%), is exceeded, a negative D-axis current is additionally applied to weaken the stator magnetic field at many operating points. The Q-axis current can then be adjusted to keep the torque constant. For D-axis currents less than the short-circuit current, the additional negative D-axis current no longer reduces the required voltage, but in fact even causes the voltage at the stator winding to increase. This leads to a reversal of the control path and unstable operation of the synchronous motor. Therefore, this field weakening method cannot weaken the stator magnetic field of a synchronous motor operated with a D-axis current less than the short-circuit current during motor operation, and also has the disadvantage that the controlled variable has a strongly nonlinear relationship with the actuating variable.

[0007] T.Gemaβmer in his paper "Effiziente und dynamische In "Efficient and dynamic torque injection in highly utilized synchronous machines with embedded magnets" (Efficient and dynamic torque injection in highly utilized synchronous machines with embedded magnets), a superimposed voltage regulator is described as an alternative to field weakening, which adjusts the speed input to a lookup table towards a higher value or the supply voltage input towards a lower value if the voltage to be applied to the stator winding is greater than a specified voltage. However, this method has the disadvantage that it is difficult to correctly limit the use of field weakening towards low voltages. This is because, depending on the current speed, each torque has a different minimum voltage at which field weakening is not required. In practice, this regulation must therefore be limited so that the field is only weakened. It cannot be prevented or reversed if the field weakening stored in the table is too strong, for example, for optimal generator operation with respect to the efficiency of the synchronous machine. Summary of the Invention

[0008] Therefore, the object of the present invention is to provide an improved vector control method and a control unit for a synchronous motor. This object is achieved by a vector control method and a control unit having the features according to the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0009] In a vector control method for a synchronous motor, particularly a permanent magnet synchronous motor (PMSM), according to the present invention, the Q-axis target current and the D-axis target current are determined by corresponding associations linking the target torque and target magnetic flux to the Q-axis target current and the D-axis target current, respectively, without taking into account voltage variations in the voltage applied to the stator windings of the synchronous motor. The associations can be stored, in particular, as a lookup table. Voltage variations are pre-adjusted or compensated for by adaptively adjusting the target magnetic flux. Voltage variations are primarily due to the voltage drop across the resistors in the stator windings. However, voltage variations are also affected by losses in the power electronics unit used to control the synchronous motor and voltage drops in the wiring used. Therefore, voltage variations weaken the magnetic field in the synchronous motor. The method according to the present invention can compensate for voltage variations without expanding the associations. The method according to the present invention also utilizes the fact that the ideal Q-axis current and the D-axis current are approximately linearly related to the magnetic flux. It should be noted that the magnetic flux is the quotient of the voltage and the electrical frequency. This linear relationship significantly reduces the number of support points compared to conventional lookup tables. For example, a short-circuit current point can be represented as a point where the magnetic flux is 0 Vs.

[0010] According to one aspect of the present invention, the corresponding associations used to determine the Q-axis target current and the D-axis target current can additionally link a characteristic parameter related to changes in the synchronous motor's magnetic and electrical characteristics to the Q-axis target current or the D-axis target current. The characteristic parameter can preferably be the rotor temperature. This allows for more accurate determination of the Q-axis target current and the D-axis target current.

[0011] According to another aspect of the present invention, the voltage change can be adjusted based on the magnetic flux difference between the maximum magnetic flux and the current magnetic flux. The maximum magnetic flux corresponds to the maximum voltage that can be applied by the power electronics unit for controlling the synchronous motor, and the current magnetic flux corresponds to the voltage currently applied to the stator winding. Thus, the voltage change can be appropriately adjusted. It should be noted that the maximum magnetic flux can be determined as the quotient of the maximum voltage that can be applied to the stator winding by the power electronics unit, that is, the maximum voltage, and the electrical frequency. The current magnetic flux can be determined as the quotient of the voltage currently applied to the stator winding and the electrical frequency.

[0012] According to an additional aspect, the flux difference can be input into the I controller to determine the correction flux. Therefore, the correction flux can be easily determined.

[0013] According to a further advantageous aspect, the sum of the maximum flux and the correction flux or the best efficiency flux in case this sum exceeds the best efficiency flux can be used as the target flux. This ensures that the target flux is always limited to the best efficiency flux.

[0014] According to a preferred aspect, the optimum efficiency flux can be determined using the maximum torque per ampere (MTPA) method.

[0015] According to one aspect of the present invention, the target torque can be normalized to the maximum torque, that is, normalized to the maximum torque achievable under the target magnetic flux, and the corresponding association for determining the Q-axis target current and the D-axis target current can receive the normalized target torque as the target torque. In a conventional lookup table, the value of the torque is stored as an absolute value. This means that the magnetic flux value is determined at a specified torque interval (e.g., 10 Nm) and stored in the table. However, in a field weakening operation, the maximum available torque may be significantly reduced. Therefore, a portion of the lookup table is unavailable, and the resolution of the effective available area is reduced. In addition, the field weakening in the maximum torque area of ​​the synchronous motor may cause serious interpolation errors that are difficult to correct. By normalizing the target torque to the maximum torque and determining the current value at a specified interval of a certain proportion of the maximum torque (e.g., 0.05 or 0.1), the current characteristics within the maximum torque range can be interpolated accurately enough, and high resolution associated over the entire operating range of the synchronous motor can be ensured.

[0016] According to another aspect, the maximum torque can be determined based on the target magnetic flux and the characteristic parameter. Therefore, the maximum torque can be determined in a simple manner.

[0017] According to a preferred aspect, the corresponding associations used to determine the Q-axis target current and the D-axis target current can be parameterized for motor operation of the synchronous machine, and the generator operation of the synchronous machine can be taken into account by inverting the Q-axis target current. Consequently, the memory space required to store the corresponding associations can be reduced by half compared to parameterization for motor operation and generator operation. Compared to conventional lookup tables, the required memory space can even be reduced to approximately one-sixth.

[0018] The control unit according to the present invention is designed and programmed to perform a vector control method according to one of the aforementioned aspects. The control unit can receive the required input variables directly, for example, from sensors, or indirectly from other control units, and perform any necessary transformations, such as Clarke and Park transformations. The control unit can also output parameters for operating the power electronics unit driving the synchronous motor to the power electronics unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the accompanying drawings:

[0020] Figure 1 shows a schematic diagram of a control unit;

[0021] Figure 2 A diagram showing current points of an ammeter for determining a Q-axis target current and a D-axis target current is shown, the diagram showing an MTPA line;

[0022] Figure 3A graph showing the current point changes with target flux and target torque and the MTPA line is shown;

[0023] Figure 4 A diagram showing current points determined for a value of a target torque normalized to a maximum torque according to a target magnetic flux and a target torque and an MTPA line is shown;

[0024] Figure 5 A schematic diagram showing the target flux adjustment portion of the control unit.

[0025] The drawings are merely schematic in nature and are only useful for understanding the present invention. Identical elements are provided with identical reference numerals. DETAILED DESCRIPTION

[0026] Figure 1 A schematic diagram of a control unit 1 according to an embodiment of the present invention is shown. The control unit is used to control the operation of a synchronous motor or a power electronics unit that in turn controls the synchronous motor. The control unit 1 includes functional components for performing the processing operations of the method according to the present invention. These functional components are implemented by executing software stored in the control unit 1 or by retrieving data stored in the control unit 1.

[0027] like Figure 1 As shown, the control unit 1 has a current determination section 2 and a target magnetic flux adjustment section 3. The current determination section 2 has a Q-axis current I q Or D-axis current I d The current meter 4 has at least the target magnetic flux Ψ as an input parameter. Tgt_MTPA and target torque M Tgt This means that the ammeter 4 sets the target magnetic flux Ψ Tgt_MTPA and target torque M Tgt and Q-axis current I q Or D-axis current I d Links. These links can be stored in a common table or in different tables. If different tables are used, the tables can have different resolutions.

[0028] Particularly preferably, the ammeter 4 can also have as input a characteristic parameter related to a change in the electrical or magnetic properties of the synchronous machine or its components. In this embodiment, the ammeter 4 has as an additional input parameter the rotor temperature T Rotor Therefore, the current I can be determined more accurately. q and I d .

[0029] In the current table 4, only the stationary operating point of the synchronous motor is stored, so that the current I q and I dThe term "current table 4" ignores transient variations. Furthermore, current table 4 only considers the magnetic flux responsible for generating the stator magnetic field. Therefore, voltage variations due to, for example, the voltage drop across the stator windings, losses in the line electronics, and voltage drops in the used lines are ignored in current table 4. Thanks to this approach, current table 4 covers all voltage levels and speeds. Figure 2 The graph in the figure shows the rotor temperature at 20 ° C with the corresponding I q and I d The current point of the value and the resulting flux and achievable torque. Therefore, Figure 2 The graph in shows the values ​​stored in the current meter 4.

[0030] It should be noted that the magnetic flux corresponds to the quotient of the voltage applied to the stator winding and the electrical frequency. Therefore, a change in voltage corresponds to a decrease in the magnetic flux due to losses. The electrical frequency can be calculated based on the mechanical speed and number of poles of the synchronous motor's rotor.

[0031] In addition, if Figure 1 As shown, if the target torque M Tgt It is not input into the current meter 4 as an absolute value but is pre-normalized to the maximum torque M max_MTPA , is found to be advantageous. This is due to the fact that as speed increases and therefore as the available flux, caused by the maximum voltage that can be applied in field weakening operation, decreases, the maximum adjustable torque decreases. Therefore, when using absolute torque values ​​for low flux, fewer current points along the torque axis can be used, and therefore only a smaller portion of the ammeter 4 can actually be used.

[0032] Figure 3 shows the case where a target magnetic flux Ψ is applied Tgt_MTPA and target torque M Tgt A diagram shows different combinations of current points. Each current point is determined for a given torque interval (e.g., 10 Nm). In addition, the maximum torque per ampere (MTPA) line is plotted. The current points actually used to control the synchronous motor are marked with an x ​​in the diagram. As can be seen, for low magnetic flux, almost no current point can be used because the achievable maximum torque is significantly reduced.

[0033] In order to overcome this shortcoming, the current determining part 2 may preferably include a maximum torque determining part 5 and a normalizing part 6, as shown in FIG. Figure 1 The maximum torque determination part 5 can be designed as a lookup table and can be based on the target magnetic flux Ψ Tgt_MTPA and rotor temperature T Rotor Determine the corresponding achievable maximum torque M max_MTPA . Maximum torque M max_MTPA Corresponding to Figure 3 The highest available current point in the target torque MTgt It is then normalized to the determined maximum torque M max_MTPA And is input into the ammeter 4, which can receive the normalized target torque M Relativ as input parameters. Figure 4 The figure shows the target magnetic flux Ψ Tgt_MTPA and target torque M Tgt The current points of different combinations of the current points are normalized to the target torque M Relative It should be noted that the torque axis in the figure indicates the actual power value. Therefore, it can be seen from the figure that by setting the target torque M Tgt Normalized to the maximum torque M max_MTPA , the density of current points for low flux can be significantly increased, making it possible to control synchronous machines more efficiently in this range.

[0034] As already mentioned, the ammeter 4 ignores the voltage variation of the voltage to be applied to the stator winding. Therefore, the target flux Ψ needs to be adjusted in the superimposed or upstream target flux adjustment section 3. Tgt_MTPA , to adjust or compensate for voltage changes.

[0035] Figure 5 The target flux adjustment part 3 is shown as a schematic diagram. The target flux adjustment part has a part 7 for determining the best efficiency flux and a voltage regulation part 8. The target flux adjustment part 3 receives the target torque M Tgt , rotor temperature T Rotor , Maximum voltage U Max , the voltage currently applied to the stator winding U ctrl_req and the current electrical frequency ω el It should be noted that the maximum voltage U Max Determined by the limitations of the power electronics unit that controls the synchronous machine.

[0036] First, the voltage regulating part 8 is adjusted according to the maximum voltage U Max and the currently applied voltage U ctrl_req , by dividing the above voltage by the electrical frequency ω el To calculate the corresponding magnetic flux, that is, the maximum magnetic flux Ψ Max and current flux Ψ ctrl_req Use these values ​​to calculate the maximum magnetic flux Ψ Max and current magnetic flux Ψ ctrl_req The difference in magnetic flux between Error Then the magnetic flux difference Ψ Error Input to the I controller 10 to obtain the correction flux Ψ correction The correction flux is then compared with the maximum flux Ψ Max Add.

[0037] In addition, Section 7 uses the MTPA method at the rotor temperature T Rotor The target torque M Tgt Determine the optimal efficiency flux Ψ Max_MTPA , and inputs the optimal efficiency flux into the voltage regulation part 8.

[0038] Then, the comparison section 11 of the voltage regulation section 8 compares the best efficiency magnetic flux Ψ Max_MTPA and maximum magnetic flux Ψ Max and correction flux Ψ correction The sum of the two values ​​is compared and the minimum value of the two values ​​is output. Therefore, the target flux Ψ Tgt_MTPA Limited by the optimal efficiency flux Ψ Max_MTPA This means that the control of the synchronous motor is neglected in the Figure 4 Finally, adjust the target magnetic flux Ψ Tgt_MTPA Output to the current determination section 2, which then uses the ammeter 4 to determine the Q axis current I q and D-axis current I d .

[0039] In addition, the voltage regulating portion 8 may have an additional comparison portion 12 which compares the maximum torque flux Ψ corresponding to the maximum flux that can be applied at the current operating point. AtMaxTorque and maximum magnetic flux Ψ Max and correction flux Ψ correction The sum of the two values ​​is compared and the minimum value of the two values ​​is output as the theoretical maximum target magnetic flux Ψ Tgt_Max . Then, as Figure 1 As shown, for the theoretical maximum target flux Ψ Tgt_Max and rotor temperature T Rotor The part 9 determining the theoretical maximum torque outputs the theoretical maximum torque M Max Theoretically maximum torque M Max This is an important factor, for example, regarding the utilization of synchronous motors.

[0040] It is also particularly advantageous if ammeter 4 is created only for the motor operation of the synchronous machine and stored in control unit 1. In this case, the generator operation of the synchronous machine is taken into account by inverting the Q-axis current. When the target torque is negative, this is taken into account by inverting the sign of the output Q-axis current in current determination section 2. This allows for a smaller ammeter, thus saving memory space in control unit 1.

[0041] Reference Signs List

[0042] 1 control unit

[0043] 2 Current determination part

[0044] 3 Target magnetic flux adjustment part

[0045] 4 Ammeter

[0046] 5 Maximum torque determination part

[0047] 6 Normalization part

[0048] 7 Part for determining the optimal efficiency flux

[0049] 8 Voltage regulation part

[0050] 9 Part used to determine the theoretical maximum torque

[0051] 10I controller

[0052] 11 Comparison Section

[0053] 12Comparison section.

Claims

1. A vector control method for a synchronous motor, in particular a permanent magnet synchronous motor, wherein the Q-axis target current and the D-axis target current (I q , I d ) is obtained by respectively converting the target torque (M Tgt ) and target flux (Ψ Tgt_MTPA ) is linked to the Q-axis target current (I q ) and the D-axis target current (I d ) is determined without taking into account voltage variations of the voltage to be applied to the stator winding of the synchronous machine, wherein, By adaptively adjusting the target magnetic flux (Ψ Tgt_MTPA ) to pre-adjust or compensate for the voltage variation.

2. The method according to claim 1, wherein For determining the Q-axis target current (I q ) and the D-axis target current (I d ) in addition to the corresponding relationship (4) of the characteristic parameters related to the change of the magnetic and electrical properties of the synchronous machine, in particular the rotor temperature (T Rotor ) and the Q-axis target current (I q ) or the D-axis target current (I d )Link.

3. The method according to claim 1 or 2, wherein: With the help of the maximum magnetic flux (Ψ Max ) and the current magnetic flux (Ψ ctrl_req ) between the magnetic flux difference (Ψ Error ) to adjust or compensate for the voltage change.

4. The method according to claim 3, wherein: The magnetic flux difference (Ψ Error ) is input to the I controller (10) to determine the correction flux (Ψ correction ).

5. The method according to claim 4, wherein The maximum magnetic flux (Ψ Max ) and the correction flux (Ψ correction ) or the sum of the above and above the optimum efficiency flux (Ψ Error ) under the condition of the optimal efficiency flux (Ψ Max_MTPA ) is used as the target flux (Ψ Tgt_MTPA ).

6. The method according to claim 5, wherein: The maximum torque per ampere (MTPA) method is used to determine the optimum efficiency flux (Ψ Max_MTPA ).

7. The method according to claim 1, wherein: The target torque (M Tgt ) is normalized to the maximum torque (M MAX_MTPA ), and is used to determine the Q-axis target current (I q ) and the D-axis target current (I d ) of the corresponding associated receiving normalized target torque (M Relativ ) as the target torque.

8. The method according to claim 2 and one of claims 5 or 6, wherein Based on the target magnetic flux (Ψ Tgt_MTPA ) and the characteristic parameters to determine the maximum torque (M MAX_MTPA ).

9. The method according to claim 1, wherein: The motor operation for the synchronous motor will be used to determine the Q-axis target current and the D-axis target current (I q , I d ) is parameterized by the corresponding association (4) and the generator operation of the synchronous machine is taken into account by inverting the determined Q-axis current.

10. A control unit designed and programmed to carry out the vector control method according to one of claims 1 to 9.

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