Optimized specification of set currents for externally excited synchronous motors.

Abstract

The set value determining device (10) determines the instantaneous rotation speed (n) and the set torque (M * ) exceeds a maximum value (Pmax) for the power (P). The setpoint determination device (10) sets a first optimization problem (O1) for a current vector (i) in one case, and a second optimization problem (O2) in the other case, and solves them in real time. To solve the first optimization problem (O1), the setpoint determination device (10) determines a current vector (i), which includes a field forming component (Id) and a torque forming component (Iq) of the motor current (I) and an excitation current (Ie), thereby determining a set torque (M * ) is obtained and the losses of the synchronous motor (1) are minimized. In the other case, the setpoint determination device (10) determines the current vector (i) such that the resulting actual torque (M) is maximized. In all cases, the setpoint determination device (10) considers the boundary conditions according to which the magnitude of the motor current (I) is maximum and reaches a maximum value (Imax) for the motor current (I), the magnitude of the excitation current (Ie) is maximum and reaches a maximum value (Iemax) for the excitation current (Ie), and the magnitude of the motor voltage (U) is maximum and reaches a maximum value (Umax) for the motor voltage (U). The setpoint determination device (10) assigns the determined current vector (i) as setpoint to the current control device (9), which operates the converter device (8) accordingly.

Description

[Technical field]

[0001] The present invention is based on a method for operating an externally excited synchronous motor having an excitation winding and a motor winding, a setpoint determination device receives the instantaneous rotational speed of the synchronous motor and a setpoint torque to be applied by the synchronous motor; a setpoint determination device for determining setpoints for an excitation current to be supplied to the excitation winding and for a motor current to be supplied to the motor winding, The set value determination device assigns the determined set value as a set value to a current control device for the converter device, whereby the current control device operates the converter device, and the converter device supplies an excitation current to the excitation winding and a motor current to the motor winding. [Background technology]

[0002] The invention is further based on a computer program for a set value determination device, the computer program comprising machine code processable by the set value determination device, the processing of the machine code by the set value determination device causing the set value determination device to perform the operating method.

[0003] The invention is further based on a set value determination device for determining set values ​​for the excitation current and the motor current of an externally excited synchronous motor, the set value determination device being programmed with a computer program such that the set value determination device executes the operating method during operation.

[0004] The invention is further based on a drive, The drive unit has an externally excited synchronous motor having an excitation winding and a motor winding, the drive comprises a converter device connected to the excitation winding for supplying an excitation current and connected to the motor winding for supplying a motor current; the drive unit has a current control device, which operates the converter device; the drive has such a setpoint determination device, the setpoint determination device has an input for receiving the instantaneous rotational speed of the synchronous motor and a setpoint torque to be applied by the synchronous motor, The setpoint determination device is connected to the current control device for specifying a value of the field-forming component of the motor current and a value of the torque-forming component of the motor current and a value of the excitation current.

[0005] In recent years, the use of externally excited synchronous motors (EESMs) has increased rapidly. This is due to their high efficiency over the entire operating range and their high torque at start-up. A further advantage over permanent magnet synchronous motors (PMSMs) is that they do not require less available materials, especially rare earths. Furthermore, the rotor flux, which is adjustable by the excitation current, represents an additional degree of freedom that allows to obtain the desired torque in an energy-efficient and flexible way.

[0006] The control of the torque in an EESM is generally performed in the field direction coordinate. For this purpose, the set currents have to be calculated from the specification of the torque to be achieved depending on the motor parameters, the converter data and the instantaneous rotation speed. The conditions for these set currents are, among other things, that the required torque is obtained in the best possible way and, moreover, that as few losses as possible occur. The calculations must be completed within one control cycle and therefore must have a real-time nature. Physical constraints, such as the maximum available voltage of the converter device and the maximum current in the motor, must be respected and therefore taken into account in the calculation.

[0007] In the prior art, the current components of the motor current, i.e. the field-forming component and the torque-forming component and the excitation current, are determined partly independently of each other. The set torque to be applied is fed to a current filter, which determines a set value for the torque-forming current component of the motor current (generally called q-current) by means of filtering and assigns this set value to the current controller. Furthermore, the maximum value of the motor voltage and the current motor voltage are fed to a field-weakening controller, which in turn determines a set value for the excitation current and assigns this set value to the current controller. Furthermore, the field-weakening controller determines a provisional set value for the field-forming component of the motor current (generally called d-current). On the basis of the instantaneous rotation speed of the synchronous motor, a correction value for the set value for the field-forming component of the motor current is determined with the aid of a characteristic curve. The final set value for the field-forming component of the motor current is determined by adding this correction value to the provisional set value for the field-forming component of the motor current. The final set value for the field-forming component of the motor current is again assigned to the current controller.

[0008] Other approaches to EESM also attempt to solve this problem. However, only partial problems are always solved. Moreover, these solutions are not real-time capable. Other prior art methods only consider the basic speed range (where operation is possible without field weakening) and use characteristic curves for the field weakening controller in the field weakening range, i.e. at higher speeds. These methods therefore require characteristic curves that must be recorded in advance. In addition, the motor current and the excitation current are considered independently of each other, so the obtained solutions are suboptimal.

[0009] It is also known in the prior art to use look-up tables for currents optimized offline to find the optimal operating point depending on the instantaneous rotation speed and the required torque. However, this solution requires a very large amount of measurement data and some preparation. The look-up tables for motor currents and excitation currents created in this way are inflexible in execution and need to be fully adapted if the machine parameters are changed. Moreover, if the look-up tables are used with high accuracy and depending on the motor parameters, they require a very large memory.

[0010] From the patent document 1, a method for operating an externally excited synchronous motor having an excitation winding and a motor winding is known. In the operating method, a setpoint determination device receives the instantaneous rotational speed of the synchronous motor and a setpoint torque to be applied by the synchronous motor. The maximum values ​​for the excitation current supplied to the excitation winding, the maximum values ​​for the motor voltage driving the motor current, the power of the synchronous motor, and the resistance values ​​of the excitation winding and the motor winding are known to the setpoint determination device. The setpoint determination device sets an optimization problem for a current vector and solves this problem in real time. The current vector has one component each for the field-forming component of the motor current, the torque-forming component of the motor current, and the excitation current. The setpoint determination device determines the current vector such that the losses occurring in the excitation winding and the motor winding are minimized within the scope of solving the optimization problem. The setpoint determination device determines the current vector such that the actual torque of the synchronous motor occurring based on the excitation current and the motor current corresponds to the setpoint torque. The setpoint determination device takes into account as additional conditions that the magnitude of the excitation current reaches a maximum value for the excitation current and that the magnitude of the motor voltage reaches a maximum value for the motor voltage. The setpoint determination device assigns the determined current vector components of a current control device for the converter device as setpoints, whereby the current control device operates the converter device, which supplies the excitation current to the excitation winding and the motor current to the motor winding.

[0011] The prior art procedures do not lead to optimal operation of an externally excited synchronous motor in all operating conditions: in particular, the determination of the field weakening and the corresponding corrections to the setpoints for the field-shaping components of the motor currents is generally only performed when the rotational speed of the externally excited synchronous motor exceeds a limiting rotational speed.

[0012] For PMSMs, procedures exist that allow the optimal operating mode to be determined. However, the complexity and solvability of the problem in EESMs is significantly greater than in PMSMs. This also applies to the decision logic for finding the optimal solution. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] DE 102014223014 [Non-patent literature]

[0014] [Non-Patent Document 1] O. Haala, B. Wagner, M. Hofmann and M. Marz, “Optimal current control of externally excited synchronous machines in automotive traction drive applica-tions”, International Journal of Electrical and Computer Engineering, volume 7 (2013), pp.1133-1139. [Non-Patent Document 2] D. Schroder, “Elektrische Antriebe: Regelung von Antriebssystemen”, Springer-Verlag, 2009. [Non-Patent Document 3] H. Eldeeb, CM Hackl, J. Kullick, and L. Horlbeck, “Analytical solutions for the optimal reference currents for MTPC / MTPA, MTPV and MTPF control of anisotropic synchronous machines”, Proc. 2017 IEEE International Electric Machines and Drives Conference (IEMDC), 2017, pp.1-6. [Non-Patent Document 4] T. Englert and K. Graichen, “Optimal setpoint computation for constrained torque control of PMSMs”, Proc. 2018 European Control Conference (ECC), 2018, pp.2671-2677. Summary of the Invention [Problem to be solved by the invention]

[0015] The task of the present invention is to create the possibility to operate an externally excited synchronous motor in an optimal way. In particular, the losses in the synchronous motor should be minimized. The solution must be executable in real time. [Means for solving the problem]

[0016] This problem is solved by an operating method having the features of claim 1. Advantageous embodiments of the operating method are the subject of dependent claims 2-6.

[0017] According to the present invention, a method for operating an externally excited synchronous motor is provided, the method comprising the steps of: a setpoint determination device receiving the instantaneous rotational speed of the synchronous motor and a setpoint torque to be applied by the synchronous motor; maximum values ​​for the excitation current supplied to the excitation winding, maximum values ​​for the motor current supplied to the motor winding, maximum values ​​for the motor voltage driving the motor current and the power of the synchronous motor are known to the setpoint determination device, Furthermore, the resistance values ​​of the excitation winding and the motor winding are known to the setpoint determination device, the setpoint determination device checks whether the required power of the synchronous motor, given by the product of the instantaneous rotational speed and the setpoint torque to be applied, exceeds a maximum value for the power; the setpoint determination device sets a first optimization problem for the current vector if the required power does not exceed a maximum value for the power and solves the first optimization problem in real time, and / or sets a second optimization problem for the current vector if the required power exceeds the maximum value for the power and solves the second optimization problem in real time; the current vector has one component each for the field-forming component of the motor current, the torque-forming component of the motor current and the excitation current, The setpoint determination device determines a current vector so that losses occurring in the excitation winding and the motor winding are minimized within a range in which the first optimization problem is solved; the setpoint determination device considers, within the scope of the first optimization problem, as a supplementary condition that the actual torque of the synchronous motor resulting from the excitation current and the motor current corresponds to the setpoint torque; The setpoint determination device further considers a first general boundary condition within the first optimization problem, according to which - the magnitude of the motor current is at a maximum and a maximum value for the motor current is reached, --The magnitude of the excitation current is at a maximum and a maximum value for the excitation current is reached; - the magnitude of the motor voltage reaches a maximum value for the motor voltage, the setpoint determination device determines a current vector such that the resulting actual torque of the synchronous motor is maximized within the scope of solving the second optimization problem; the setpoint determination device considers, as an additional condition within the scope of the second optimization problem, that the magnitude of the motor voltage is equal to a maximum value for the motor voltage; The setpoint determination device further considers a second general boundary condition within the scope of the second optimization problem, according to which - the magnitude of the motor current is at a maximum and a maximum value for the motor current is reached, --The magnitude of the excitation current is at a maximum and a maximum value for the excitation current is reached; The set value determination device assigns the component of the current vector determined by solving the first optimization problem or the second optimization problem as a set value to a current control device for the converter device, whereby the current control device operates the converter device, and the converter device supplies an excitation current to the excitation winding and a motor current to the motor winding.

[0018] That is, a homogenized solution is provided for all relevant currents (field-forming components, torque-forming components, excitation currents). This allows the synchronous motor to be operated with minimal losses in all operating conditions where the set torque to be applied is available. In all operating conditions where the set torque to be applied is not available, the synchronous motor can be operated so that its torque is as close as possible to the set torque to be applied.

[0019] The solution according to the invention is therefore based on determining the motor and field currents by solving an appropriate optimization problem.

[0020] The first optimization problem cannot be solved collectively in its generality, at least in real time, with currently available mass production hardware. Therefore, in order to efficiently solve the first optimization problem, it is preferable that, within the scope of solving the first optimization problem, the set value determination device: First, a current vector is provisionally determined without taking into account the first general boundary condition; then checking whether the provisionally determined current vector satisfies a first general boundary condition, - if the provisionally determined current vector satisfies a first general boundary condition, use the provisionally determined current vector as a current vector, otherwise determine the current vector taking into account at least one of the following first special boundary conditions, according to the first special boundary condition: --the magnitude of the motor current is equal to the maximum value for the motor current, -- the magnitude of the excitation current is equal to the maximum value for the excitation current, --the magnitude of the motor voltage is equal to the maximum value for the motor voltage; -Depending on which of the first general boundary conditions is not satisfied, it is determined which of the first special boundary conditions is to be taken into account.

[0021] That is, a partitioning into subproblems that can be solved analytically or at least approximately numerically is performed. If analytically unsolvable, an efficient numerical optimization is used to reach the optimal operating point in just a few iterations. Based on a decision tree, it is decided which subproblems are actually solved and their solutions are used. This procedure, in particular, significantly reduces the decision complexity and can guarantee real-time performance.

[0022] Preferably, the setpoint determination device checks whether the provisionally determined current vector satisfies a first general boundary condition by: First, checking whether the magnitude of the motor current reaches a maximum value for the motor current and whether the magnitude of the excitation current reaches a maximum value for the excitation current; Only then is it provided that a check is made as to whether the magnitude of the motor voltage reaches a maximum value for the motor voltage.

[0023] This procedure quickly yields the correct solution, i.e., the current vector, using a simple, easy-to-process decision tree.

[0024] In order to efficiently solve the second optimization problem, the set point determination device preferably performs the following within the scope of solving the second optimization problem: -First, the current vector is provisionally determined without taking into account the second general boundary condition, then checking whether the provisionally determined current vector satisfies a second general boundary condition, - if the provisionally determined current vector satisfies the second general boundary condition, use the provisionally determined current vector as the current vector; otherwise, determine the current vector taking into account at least one of the following second special boundary conditions, according to the second special boundary condition: --the magnitude of the motor current is equal to the maximum value for the motor current, -- the magnitude of the excitation current is equal to the maximum value for the excitation current, -Depending on which of the second general boundary conditions is not satisfied, it is determined which of the second special boundary conditions is to be taken into account.

[0025] This, in particular, significantly reduces the decision complexity and allows real-time to be guaranteed.

[0026] Preferably, the setpoint determination device checks whether the provisionally determined current vector satisfies a second general boundary condition by: First, it is checked whether the magnitude of the motor current reaches a maximum value for the motor current; Only then is it checked whether the magnitude of the excitation current reaches a maximum value for the excitation current.

[0027] This procedure quickly yields the correct solution, i.e., the current vector, using a simple, easy-to-process decision tree.

[0028] Preferably, the setpoint determination device only considers copper losses as losses. The restriction to copper losses is certainly a simplification, but it is justified since copper losses make up a large proportion of the total losses (at least 70%, often more than 80%) and synchronous motors are usually operated in a basic speed range where copper losses are particularly dominant compared to iron losses.

[0029] The object is furthermore achieved by a computer program having the features of claim 7. According to the invention, by the processing of the computer program by the setpoint determination device, the setpoint determination device carries out the operating method according to the invention.

[0030] The object is further achieved by a setpoint determination device having the features of claim 8. According to the invention, the setpoint determination device is programmed with a computer program according to the invention, whereby during operation the setpoint determination device carries out the operating method according to the invention.

[0031] The object is further achieved by a drive having the features of claim 9. According to the invention, in a drive of the type mentioned at the beginning, the setpoint determination device is designed as a setpoint determination device according to the invention.

[0032] The characteristics, features and advantages of the invention mentioned above, as well as how they can be obtained, will become clearer and more easily understood in relation to the following description of an embodiment, which is given in more detail in conjunction with the drawings, in which: [Brief description of the drawings]

[0033] [Figure 1] FIG. [Diagram 2] FIG. [Diagram 3] FIG. 1 illustrates a first optimization problem. [Figure 4] FIG. [Diagram 5] FIG. 2 illustrates a second optimization problem. [Figure 6] FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] According to FIG. 1, the drive comprises an externally excited synchronous motor 1. The synchronous motor 1 comprises a stator 2 in which a motor winding 3 is arranged. The motor winding 3 is supplied with a motor current I via a converter 4. The motor winding 3 is generally formed as a three-phase winding. The motor current I therefore comprises the phases a, b and c of a three-phase system. The synchronous motor 1 further comprises a rotor 5 with a co-rotating excitation winding 6. The excitation winding 6 is supplied with an excitation current Ie via a further converter 7. The excitation current Ie is generally a direct current. The converter 4 and the converter 7 together form a converter arrangement 8. The converter arrangement 8 is operated by a current control device 9.

[0035] The drive further comprises a setpoint determination device 10, which assigns as setpoints to the current controller 9 the field-forming and torque-forming components Id and Iq of the motor current I and the excitation current Ie. Together the field-forming and torque-forming components Id and Iq form the setpoint of the motor current I. The two components Id and Iq are locally rotated by 90° relative to each other. This means that the motor current I is a vector variable as far as the internal processing of the motor current I in the setpoint determination device 10 and the assignment of setpoints to the current controller 9 are concerned and, in a (co-rotating) dq system, the relationship

[0036]

number

[0037] applies.

[0038] Based on the specification of the above setpoints Id, Iq and Ie, the current control device 9 can operate the converter device 8 such that the converter 7 supplies the excitation current Ie to the excitation winding 6 and the converter 4 supplies the motor current I to the motor winding 3. The conversion from a co-rotating dq system to an abc system is well known to those skilled in the art and does not need to be described in detail.

[0039] The setpoint determination device 10 is the actual central subject of the present invention. The setpoint determination device 10 is programmed with a computer program 11. The computer program 11 contains a machine code 12 which can be processed by the setpoint determination device 10. On the basis of the processing of the machine code 12 by the setpoint determination device 10, the setpoint determination device 10 executes an operating method which is explained in more detail below, first in conjunction with FIG. 2 and then with additional reference to the other figures.

[0040] According to Fig. 2, the setpoint determination device 10 is informed in step S1 of the values ​​Imax, Iemax and Umax. The said values ​​Imax, Iemax and Umax can, for example, be determined by the computer program 11 or can be determined once only when the drive is started up. If necessary, they can also be determined dynamically as a function of the state of the drive, for example the operating temperature of the synchronous motor 1.

[0041] The value Imax is the maximum value for the magnitude of the motor current I. The value Iemax is the maximum value for the magnitude of the excitation current Ie. The value Umax is the maximum value for the magnitude of the motor voltage U that drives the motor current I. The maximum value Umax for the motor voltage U may be determined by the supply voltage of the converter 4, for example the DC link voltage on the input side of the converter 4.

[0042] The motor voltage U, like the motor current I, has d and q components in a co-rotating dq system, which are referred to below as the field-forming and torque-forming voltage components Ud and Uq. Like the motor current I, the relationship

[0043]

number

[0044] applies.

[0045] The setpoint determination device 10 is further informed in step S2 of the resistance R of the motor winding 3 and the resistance Re of the excitation winding 6. The values ​​R, Re can likewise be determined by the computer program 11 or can be determined once only when the drive is started up.

[0046] Based on the variables now given, the components Ud, Uq of the motor voltage U and the excitation voltage Ue, and the components Id, Iq of the motor current I and the excitation current Ie can be related to each other, because in the case of steady operation:

[0047]

number

[0048] is applied approximately,

[0049] In equations (3) to (5), ω is the electrical angular frequency. Ld and Lq are the self-inductances of the motor winding 3 on the d-axis and q-axis. Lm is the mutual inductance of the excitation winding 6.

[0050] It is also indirectly clear from equation (5) that the maximum excitation voltage Ue does not need to be considered in detail, since it is linearly related to the excitation current Ie. Therefore, the maximum value Iemax may be determined to correspond to the previously occurring limit (excitation current Ie or excitation voltage Ue).

[0051] In equations (3) to (5), the effects arising due to the changes in the motor current I and the excitation current Ie over time are not taken into account. This is permissible since these changes and the resulting effects are small. Furthermore, in equations (3) to (5), the resistances R, Re are assumed to be constant over time. However, possible temperature dependencies or dependencies on other states of the drive can be taken into account without any problem.

[0052] The self-inductances Ld, Lq of the motor winding 3 and the mutual inductance Lm of the excitation winding 6 depend on the components Id, Iq and the excitation current Ie of the motor current I. If necessary, specific values ​​of the self-inductances Ld and Lq of the motor winding 3 and the mutual inductance Lm of the excitation winding 6 can be stored in a look-up table in the set point determination device 10.

[0053] The setpoint determination device 10 is further informed of a maximum value Pmax for the power P of the synchronous motor 1. It is possible to explicitly specify the maximum value Pmax for the power P of the synchronous motor 1 to the setpoint determination device 10. Alternatively, the setpoint determination device 10 determines in step S3 the relationship

[0054]

number

[0055] It is also possible to determine the maximum value Pmax itself according to

[0056] In step S4, the setpoint determination device 10 determines the instantaneous (mechanical) rotational speed n of the synchronous motor 1 and the setpoint torque M to be applied by the synchronous prime mover 1. * Receive the set torque M * The method of specifying the instantaneous rotational speed n may be arbitrary. Such a method is not the subject of the present invention. The method of specifying the instantaneous rotational speed n is also not the subject of the present invention. For example, the drive may have a decision block 13 for determining the rotational speed n on the basis of the actual operating state of the synchronous prime mover 1. Alternatively, an electrical angle frequency ω can be determined instead of the rotational speed n, and the rotational speed n can be calculated from the electrical angle frequency ω. Suitable procedures for determining the electrical angle frequency ω and for calculating the rotational speed n from the electrical angle frequency ω are well known to those skilled in the art. Furthermore, in the following, the set torque M * Assume that the set torque M * If I.sub.d, I.sub.q, or I.sub.e were negative, it would only affect the sign and would not change the magnitude of the field forming component Id, the torque forming component Iq, or the excitation current Ie.

[0057] In step S5, the set value determination device determines the instantaneous required power P of the synchronous motor 1. The instantaneous required power P of the synchronous motor 1 is determined by the instantaneous rotation speed n and the set torque M to be applied. * It is given by the product of

[0058] In step S6, the set value determination device 10 checks whether the instantaneous required power P exceeds a maximum value Pmax. If the required power P does not exceed the maximum value Pmax, the set value determination device 10 proceeds to step S7. In step S7, the set value determination device 10 sets a first optimization problem O1 for the current vector i and solves it in real time. On the other hand, if the required power P exceeds the maximum value Pmax, the set value determination device 10 proceeds to step S8. In step S8, the set value determination device 10 sets a second optimization problem O2 for the current vector i and solves it in real time.

[0059] The current vector i includes three components, namely, the field generating component Id and the torque generating component Iq of the motor current I, and the excitation current Ie. Therefore, the solution of the first or second optimization problem O1 or O2 is a value that the setpoint determination device 10 assigns as a setpoint to the current control device 9. This assignment is performed in step S9.

[0060] It is possible to supplement the procedure of FIG. 2 so that the set value determination device 10 outputs a message to a higher-level device (not shown) when the second optimization problem O2 is solved. This allows the requested set torque M * It is possible to inform the host device that no power can be supplied.

[0061] Within the scope of solving the first optimization problem O1, the setpoint determination device 10 according to Fig. 3 determines the current vector i such that the resulting losses V are minimized. Within the scope of the present invention, only the copper losses VK occurring in the excitation winding 6 and in the motor winding 3 are taken into account. In practice, the copper losses VK are significantly larger than the other losses, so that the other losses, in particular the iron losses, can be neglected.

[0062] The copper loss VK is related

[0063]

number

[0064] Within the scope of the first optimization problem O1, the setpoint determination device 10 determines, as an additional condition, that the actual torque M of the synchronous motor 1 is equal to or greater than the setpoint torque M * Consider the basic premise of the first optimization problem O1 that the real torque M is determined by the excitation current Ie and the components Iq, Id of the motor current I. To a good approximation,

[0065]

number

[0066] Z is the number of pole pairs of the synchronous motor 1.

[0067] Furthermore, the setpoint determination device 10 takes into account a first general boundary condition within the first optimization problem O1. According to the first general boundary condition: the magnitude of the motor current I reaches a maximum value Imax for the motor current I, the magnitude of the excitation current Ie reaches a maximum value Iemax for the excitation current Ie, It is required that the magnitude of the motor voltage U reaches a maximum value Umax for the motor voltage U at most.

[0068] Mathematically, the first optimization problem O1 can be formulated as follows:

[0069]

number

[0070] The following will be taken into consideration:

[0071]

number

[0072] Since the actual torque M depends in a complex way on the components of the current vector i and, in addition, the nonlinear constraints according to inequalities (10) to (12) must be taken into account, solving the first optimization problem O1 is not a trivial task. The exact procedure for solving the first optimization problem O1 is explained in detail below in connection with FIG. 4.

[0073] FIG. 4 shows a specific procedure for the set point determining device 10 to determine the current vector i within the range for solving the first optimization problem O1.

[0074] 4, the set value determination device 10 first determines a current vector i=i1 in step S11. The current vector i1 is determined by the set value determination device 10 without considering the first general boundary condition. That is, the set value determination device 10 indeed solves the first optimization problem O1 according to equation (9) and taking equation (13) into consideration. However, at the same time, the set value determination device 10 solves the optimization problem O1 without taking inequalities (10) to (12) into consideration.

[0075] In fact, step S11 can be solved analytically exactly, for example using a Lagrangian approach. This solution is known from the literature. As a pure example, reference can be made to [1], and also to [2]. This solution does not depend on the rotational speed n or the electrical angular frequency ω. In the art, this solution is known as the Maximum Torque Per Current (MTPC).

[0076] The solution determined in step S11 is only provisional. Before the setpoint determination device 10 adopts the current vector i=i1 as the final current vector i, the setpoint determination device 10 first checks in step S12 whether the magnitude of the motor current I has reached a maximum value Imax for the motor current I. If this condition is met, the setpoint determination device 10 then checks in step S13 whether the magnitude of the excitation current Ie has reached a maximum value Iemax for the excitation current Ie.

[0077] If the check in step S12 is already negative, i.e. for the current vector i1, the magnitude of the motor current I is greater than the maximum value Imax for the motor current I, then the setpoint determination device 10 determines the current vector i=i2 in step S14. The setpoint determination device 10 determines the current vector i2 as before by solving equation (9) taking into account equation (13). However, the setpoint determination device 10 additionally determines a first special boundary condition

[0078]

number

[0079] Based on the boundary conditions, the term to be minimized in equation (9) can be obviously simplified. This is because the component R(Id 2 +Iq 2 ) is a constant according to equation (14) and has a value RImax 2 and can therefore be neglected within the scope of the minimization. As a consequence, the resistance Re can also be neglected, since it is only a constant coefficient. Therefore, instead of minimizing the (complete) copper loss VK, the term Ie 2Strictly speaking, it is sufficient to minimize the magnitude of the magnetizing current Ie. Furthermore, based on equation (14), the value of the field-forming component Id can be used to determine the magnitude of the torque-forming component Iq, so that there is one less variable that needs to be changed in the result. In practice, step S14 can be solved analytically by a Lagrangian approach as well.

[0080] If the current vector i passes the check in step S12 but not the check in step S13, the setpoint determination device 10 determines the current vector i=i3 in step S15. The setpoint determination device 10 determines the current vector i3 in the conventional manner by solving equation (9) taking into account equation (13). However, the setpoint determination device 10 additionally takes into account a first special boundary condition

[0081]

number

[0082] Also consider and solve equation (9).

[0083] Step S15 has as solution the MTPC trajectory of the permanently excited synchronous machine. This solution is known from the literature. As a pure example, reference can be made to "Patent Document 3". Also, reference can be made to "Patent Document 4".

[0084] Similar to the procedure for determining the current vector i = i2, when determining the current vector i = i3, it is possible to obviously simplify the terms to be minimized within equation (9), since the components ReIe 2 is a constant according to equation (15), and ReIemax 2 , and can therefore be ignored within the scope of the minimization. As a consequence, the factor 3R / 2 can also be ignored, since it is only a constant factor. Therefore, instead of minimizing the (complete) copper loss VK, the term Id 2 +Iq 2It is sufficient to minimize Iemax. Furthermore, based on equation (15), one less variable needs to be changed because the magnitude of the magnetizing current Ie is fixed. Therefore, the magnetizing current Ie can only have values ​​of +Iemax or -Iemax.

[0085] The current vectors i=i2 and i=i3 in steps S14 and S15 are also only provisional. Before the setpoint determination device 10 adopts the current vector i=i2 or the current vector i=i3 as the final current vector i, the setpoint determination device 10 checks in steps S16 and S17, respectively, whether the remaining current conditions are fulfilled, i.e., whether inequality (11) is fulfilled in the case of the current vector i=i2 or whether inequality (10) is fulfilled in the case of the current vector i=i3. If, on the one hand, the current vector i=i2 does not satisfy inequality (11), or, on the other hand, the current vector i=i3 does not satisfy inequality (10), the setpoint determination device 10 determines the current vector i=i4 in step S18. The setpoint determination device 10 determines the current vector i4 in the conventional manner by solving equation (9) taking into account equation (13). However, the setpoint determination device 10 simultaneously solves equation (9) taking into account the conditions according to equations (14) and (15).

[0086] The solution of step S18 is (almost) trivial, because in the end, only the field-generating component Id needs to be changed. This leaves the torque-generating component Iq fixed except for its sign. The excitation current Ie is also fixed except for its sign.

[0087] There are also possible alternatives to the embodiment of steps S11 to S18. In particular, it is possible to carry out the checks of steps S12 and S13 in the reverse order, thereby preferentially determining the current vector i=i3 instead of the current vector i=i2. Even better, the checks of steps S12 and S13 are combined. In this embodiment, if the current vector i=i1 does not pass only one of the two checks, the setpoint determination device 10 determines two current vectors i=i2 and i=i3 and checks whether the two current vectors i=i2 and i=i3 pass the other check, respectively. That is, the setpoint determination device 10 checks whether the current vector i=i2 satisfies the condition of inequality (11) and whether the current vector i=i3 satisfies the condition of inequality (10). If both the current vector i=i2 and the current vector i=i3 pass their respective inspections, the set value determination device 10 selects, as the current vector i, one of the two current vectors i=i2 and i=i3 that has a smaller copper loss VK. If only one of the two current vectors i=i2 and i=i3 passes the inspection and the other of the two current vectors i=i2 and i=i3 does not pass the inspection, the set value determination device 10 selects, as the current vector i, one of the two current vectors i=i2 and i=i3 that has passed the inspection. If both of the two current vectors i=i2 and i=i3 do not pass their respective inspections, the set value determination device 10 continues to determine the current vector i=i4.

[0088] The current vector i determined here satisfies the current limitations of inequality (10) and inequality (11), regardless of whether it is the current vector i1, i2, i3 or i4. However, it is still not guaranteed that the voltage limitation according to inequality (12) is also satisfied. Therefore, in step S19, the setpoint determination device 10 checks whether the determined current vector i satisfies the condition related to inequality (12). That is, the setpoint determination device 10 checks whether the magnitude of the motor voltage U reaches a maximum value Umax for the motor voltage U at the maximum.

[0089] If the current vector i passes the check in step S19, the set value determination device 10 uses the current vector i determined by the processing in steps S11 to S18. In other cases, the set value determination device 10 determines the current vector i=i5 in step S20. The set value determination device 10 determines the current vector i5 in the same manner as in the conventional method, by solving equation (9) taking equation (13) into account. However, the set value determination device 10 also determines the current vector i5 in the same manner as in the conventional method, by simultaneously taking into account the condition

[0090]

number

[0091] Solve equation (9) taking into account

[0092] To determine the current vector i=i5, it may be necessary to calculate the current vector i=i5 numerically. For this purpose, various determination methods are available to the skilled person.

[0093] It is possible to solve the current vector i=i5 by gradient methods, for example using line search or multidimensional Newton methods. However, there is a risk that these methods will take too much time. However, based on the structure of the optimization problem for the current vector i=i5, it is possible to determine a partial analytical solution and thus derive a very efficient numerical method.

[0094] An example of such a method is the fixed-point iteration of the excitation current Ie. In this case, the necessary first-order optimality conditions of the optimization problem are given by the Lagrangian approach

[0095]

number

[0096] where La is the Lagrange function; λ1 and λ2 are the Lagrange multipliers; R', M', U' are 3x3 matrices that can be determined using the resistances R, Re, the generated torque M, the voltage components Ud, Uq, and the excitation voltage Ue in combination with the boundary conditions to be taken into account (equation (13) and equation (16)).

[0097]

number

[0098] At least the condition

[0099]

number

[0100] This condition can be used to calculate the excitation current Ie and the Lagrange multipliers λ1, λ2 as functions of the components Id, Iq of the motor current I. Specifically, the function

[0101]

number

[0102] exists.

[0103] The solution of this function does not depend on the Lagrange multipliers λ1 and λ2.

[0104] Similarly, this can be applied to the boundary conditions for the torque M (equation (13)) and the motor voltage U (equation (16)). The field-forming component Id and the torque-forming component Iq of the motor current I are expressed as a function of the excitation current Ie by the function

[0105]

number

[0106] and

[0107]

number

[0108] It can be calculated as:

[0109] Function f Ie , f Id , f Iq Based on this, the iterative solution is in the form of a fixed point iteration.

[0110]

number

[0111] where k is each iteration step. The fixed point of the iteration rule is the optimal current vector i=i5, and the Lagrange multipliers λ1 and λ2 can be calculated analytically.

[0112] The iterations are stopped at an appropriate time to guarantee real-time. The stopping criterion is, for example, that a maximum number of iterations have been performed or that the result has only changed slightly. By using an appropriate starting point, the actual solution is obtained in very few iteration steps. The starting point may, for example, be the excitation current Ie last output by the setpoint determination device 10 to the current control device 9. However, other approaches are also possible.

[0113] Alternatively, one can use one-dimensional methods to determine the zeros of the function. These methods are very efficient. In this case, it is assumed that equation (19) must have a nontrivial solution. As a result, the matrix

[0114]

number

[0115] has a kernel and the determinant of the matrix must have a value of 0. This allows us to decompose equation (24) into λ1 or λ2. For example, λ1 can be expressed as a function of λ2. This results in a solution for the optimal current vector i=i5, because the current vector i=i5 must be the kernel of the matrix to satisfy the gradient equation. That is,

[0116]

number

[0117] must be valid.

[0118] where i(λ2) is the current vector of length 1 and therefore determines the "direction" of the current vector i=i5, i.e., the ratio of the three components of the current vector i=i5. The length IL is determined by the conditions of equations (13) and (16), because:

[0119]

number

[0120] and

[0121]

number

[0122] must be valid.

[0123] Equation (26) and equation (27) are 2 By solving and equating according to

[0124]

number

[0125] is obtained.

[0126] In equation (28), only λ2 is unknown. Therefore, equation (28) can be solved from the beginning according to λ2. However, no analytical closed-form solution is known for equation (28). However, to determine the zero point, i.e., the specific value of λ2, commonly known zero point determination methods, such as Newton's method or bisection method, can be used.

[0127] After numerically solving equation (28), the components Id, Iq and λ1 of the motor current I can be analytically calculated. Again, the optimal values ​​obtained in the calculation step can be used in advance as an initial solution.

[0128] Obviously, other solutions can be used.

[0129] Within the described procedure, it must be guaranteed that the solution found lies within the permissible current range. To verify this fact, equation (13) (together with equation (8)), equation (14) and equation (16) can be solved analytically. This results in the calculation of the minimum and maximum values ​​of the magnetizing current Ie that must be maintained. If the calculated minimum value is smaller than zero, it is raised to zero. If the calculated maximum value is larger than the maximum magnetizing current Imax, the maximum magnetizing current Imax is used instead of the calculated maximum value. If the magnetizing current Ie of the current vector i=i5 is outside the operating range thus determined, the calculated minimum value is the optimal solution. This ensures that the current limits are respected, all constraints are met and at the same time the required torque M * It is guaranteed that a valid solution that provides

[0130] Within the scope of solving the second optimization problem O2, the setpoint determination device 10 determines the setpoints M so that the actual torque M of the synchronous motor 1 is maximized, i.e., the actual torque M is smaller than the setpoint torque M * Determine the current vector i according to Figure 5 so that it is as close as possible to . The losses V, in particular the copper losses VK, are not taken into account within the scope of solving the second optimization problem O2.

[0131] Within the scope of the second optimization problem O2, the setpoint determination device 10 takes into account, as an additional condition, that the equation (16) is fulfilled, which corresponds in content to an attempt to operate the converter 4 as far as possible so that the actual torque M of the synchronous motor 1 is maximized.

[0132] Furthermore, the setpoint determination device 10 takes into account second general boundary conditions within the scope of the second optimization problem O2. According to these boundary conditions: - the magnitude of the motor current I reaches a maximum value Imax for the motor current I (inequality (10)); and It is required that the magnitude of the excitation current Ie reaches a maximum value Iemax for the excitation current Ie (inequality (11)).

[0133] Therefore, mathematically, the second optimization problem O2 can be formulated as follows:

[0134]

number

[0135] Here, the setpoint determiner 10 further takes into account equation (16) and inequalities (10) and (11). The exact procedure for solving the second optimization problem O2 is explained in more detail in connection with FIG.

[0136] FIG. 6 shows a specific procedure used by the set point determining device 10 to determine the current vector i within the scope of solving the second optimization problem O2.

[0137] 6, the set value determination device 10 first determines the current vector i as the current vector i6 in step S31. The set value determination device 10 determines the current vector i6 without considering the second general boundary condition. That is, the set value determination device 10 solves the second optimization problem O2 according to equation (29) by additionally considering equation (16). The set value determination device 10 does not consider inequality (10) and inequality (11) in step S31.

[0138] Step S31 can in fact be solved analytically exactly, for example using a Lagrangian approach. The solution is similar to the determination of the current vectors i2 and i3.

[0139] The solution determined in step S31 is only provisional. Before the setpoint determination device 10 adopts the current vector i6 as the final current vector i, the setpoint determination device 10 first checks in step S32 whether the magnitude of the motor current I has reached a maximum value Imax for the motor current I.

[0140] If the current vector i=i6 does not pass the check in step S32, the setpoint determination device 10 determines the current vector i as the current vector i7 in step S33. In step S34, the setpoint determination device 10 again solves equation (29), but also takes into account additionally equation (14) as well as equation (16).

[0141] Next, in step S34, the set value determination device 10 checks whether the currently determined current vector i, regardless of whether it is the current vector i6 or i7, satisfies the inequality (11), i.e., whether the condition that the magnitude of the excitation current Ie reaches a maximum value Iemax for the excitation current Ie is satisfied.

[0142] If the current vector i passes the check in step S34, the setpoint determination device 10 uses the current vector i on which the check was based, i.e., current vector i=i6 or current vector i=i7. If the current vector i does not pass the check in step S34, the setpoint determination device 10 determines the current vector i8 as the current vector in step S35. In step S35, the setpoint determination device 10 solves equation (29) again. However, the setpoint determination device 10 additionally considers equation (15) as well as equation (16).

[0143] Similar to the procedure for current vectors i=i2 and i=i3, in determining current vectors i=i7 and i=i8, the magnitude of the torque-forming component Iq can be determined using the value of the field-forming component Id, or the magnitude of the excitation current Ie is fixed, so there is one less variable that needs to be changed.

[0144] In step S35, a further optimization problem has to be solved, which maximizes the torque M at the maximum value Iemax of the excitation current Ie while respecting the voltage limits according to equation (16). This optimization problem is known for permanent magnet synchronous machines and has an analytical solution. This solution locus is called MTPV (Maximum Torque Per Voltage) in the literature and is known, for example, from the already mentioned non-patent documents 3 and 4. The solution locus can be adopted 1:1.

[0145] Step S35 may in fact be solved exactly analytically, for example using a Lagrangian approach.

[0146] In step S36, the setpoint determination device 10 checks for the current vector i=i8 whether the magnitude of the motor current I has reached a maximum value Imax for the motor current I. If the current vector i passes the check in step S36, the setpoint determination device 10 uses the current vector i=i8 determined in step S35. If the current vector i does not pass the test in step S36, the setpoint determination device 10 determines the current vector i=i9 in step S37. Within the scope of the determination of the current vector i9, the setpoint determination device 10 again solves equation (29). However, the setpoint determination device 10 additionally takes into account not only equation (16) but also equation (14).

[0147] This optimization problem is known for permanent magnet synchronous machines and has an analytical solution, for example from [3] and [4].

[0148] In the embodiment of FIG. 6, the procedure can be modified. This modification is similar to the modification in the embodiment of FIG. 4. In particular, the checks in steps S32 and S34 can be performed in the reverse order, and the current vector i=i8 can be preferentially determined instead of the current vector i=i7. Even better is a procedure that combines the checks in steps S32 and S34. In this embodiment, if the current vector i=i6 does not pass only one of the two checks, the set value determination device 10 determines two current vectors i=i7 and i=i8, and checks whether both the current vectors i=i7 and i=i8 pass the other checks, respectively. That is, the set value determination device 10 checks whether the current vector i=i7 satisfies the condition of inequality (11) and whether the current vector i=i8 satisfies the condition of inequality (10). If both the current vectors i=i7 and i=i8 pass their respective checks, the setpoint determination device 10 selects, as the current vector i, one of the two current vectors i=i7 and i=i8 that provides a higher actual torque M. If only one of the two current vectors i=i7 and i=i8 passes its check and the other of the two current vectors i=i7 and i=i8 does not pass its check, the setpoint determination device 10 selects, as the current vector i, one of the two current vectors i=i7 and i=i8 that passes its test. If both the current vectors i=i7 and i=i8 do not pass their respective checks, the setpoint determination device 10 continues to determine the current vector i=i9.

[0149] The current vectors i1-i4 in Fig. 4 and the current vectors i6-i9 in Fig. 6 can be calculated analytically and therefore very efficiently. For the current vector i5, only a numerical solution is possible, but this solution can also be calculated very fast, so that the current vector i5 can also be determined in real time. By splitting the two optimization problems O1, O2 into several sub-problems that are solved depending on the situation (compliance with current and voltage limits, compliance with only one current limit, compliance with only the other current limit, etc.), real-time properties are obtained, especially for determining the current vector i, which includes both the field-forming component Id and the torque-forming component Iq of the motor current I and the excitation current Ie, and thus all the relevant currents.

[0150] The invention has many advantages. For example, the method according to the invention can be used in any EESM. Expensively pre-recorded or pre-calculated look-up tables for the motor current I and / or excitation current Ie are not necessary. No significant simplifications are made. Only the motor parameters need to be known. This allows the method according to the invention to be quickly adapted to new synchronous motors 1. Compared to methods that only consider one partial operating range of the EESM (in particular the basic rotational speed range) and then use a field-weakening control device to comply with the operating limits, the method according to the invention offers the advantage that the entire operating range is considered in an energy-optimized manner. By calculating the actual optimal operating point, a higher torque is obtained with the same power or the same torque is obtained with less power. By taking into account all current and voltage limitations, the torque behavior at high rotational speeds is improved as well. The operating method according to the invention can be configured and applied in all externally excited synchronous motors 1. It can also be equipped in existing drives. Compared to the solutions according to the prior art, the electrical energy savings can reach at least 3% and up to 10%.

[0151] Although the present invention has been illustrated and described in detail through preferred embodiments, the present invention is not limited to the disclosed examples, and other variations may be derived by those skilled in the art within the scope of the claims, without departing from the scope of protection of the present invention. [Explanation of symbols]

[0152] 1 Synchronous prime mover 2 Stator 3 Motor Windings 4,7 Converter 5 Rotor 6 Excitation Winding 8 Converter device 9 Current Control Device 10. Setting value determination device 11. Computer Programs 12 Machine Code 13 Decision Block a, b, c phase i, i1~i9 current vector I Motor current Id Field forming component Ie Excitation current Iq torque forming component Imax Maximum value (motor current) Iemax Maximum value (excitation current) M Actual torque M * Set torque n Rotational speed O1, O2 optimization problems P power Pmax Maximum value (power) R Resistance (motor winding) Re Resistance (excitation winding) S1~S37 steps U Motor voltage Ue Excitation voltage Ud, Uq Motor voltage components Umax Maximum value (motor voltage) ω Electrical angular frequency

Claims

1. A method for operating an externally excited synchronous motor (1) having an excitation winding (6) and a motor winding (3), comprising the steps of: The setpoint determination device (10) determines the instantaneous rotational speed (n) of the synchronous motor (1) and the setpoint torque (M ＊ ) and the maximum value (Iemax) for the excitation current (Ie) supplied to the excitation winding (6), the maximum value (Imax) for the motor current (I) supplied to the motor winding (3), the maximum value (Umax) for the motor voltage (U) driving the motor current (I) and the power (P) of the synchronous motor (1) are known to the setpoint determination device (10); - furthermore, the resistances (Re, R) of the excitation winding (6) and of the motor winding (3) are known to the setpoint determination device (10), - the setpoint determination device (10) determines the instantaneous rotational speed (n) and the setpoint torque (M ＊ Check whether the required power (P) of the synchronous motor (1), given through the product of the required power (P) and the maximum value (Pmax) for the power (P), the setpoint determination device (10) sets and solves in real time a first optimization problem (O1) for a current vector (i) if the required power (P) does not exceed a maximum value (Pmax) for the power (P) and / or sets and solves in real time a second optimization problem (O2) for a current vector (i) if the required power (P) exceeds the maximum value (Pmax) for the power (P); the current vector (i) has one component each for the field generating component (Id) of the motor current (I), the torque generating component (Iq) of the motor current (I) and the excitation current (Ie); - the setpoint determination device (10) determines the current vector (i) so that losses (V) occurring in the excitation winding (6) and the motor winding (3) are minimized within the scope of solving the first optimization problem (O1); The setpoint determination device (10) determines, within the scope of the first optimization problem (O1), as a supplementary condition, whether the actual torque (M) of the synchronous motor (1) generated based on the excitation current (Ie) and the motor current (I) is equal to or smaller than the setpoint torque (M ＊ ) and said setpoint determination device (10) takes into account, within said first optimization problem (O), furthermore a first general boundary condition, according to which: the magnitude of the motor current (I) reaches a maximum value (Imax) for the motor current (I), the magnitude of the excitation current (Ie) reaches a maximum value (Iemax) for the excitation current (Ie), the magnitude of the motor voltage (U) reaches a maximum value (Umax) for the motor voltage (U), the setpoint determination device (10) determines the current vector (i) so that the resulting actual torque (M) of the synchronous motor (1) is maximized within the scope of solving the second optimization problem (O2); the setpoint determination device (10) takes into account, within the scope of the second optimization problem (O2), as an additional condition that the magnitude of the motor voltage (U) is equal to a maximum value (Umax) for the motor voltage (U); said setpoint determination device (10) further takes into account, within said second optimization problem (O2), a second general boundary condition according to which the magnitude of the motor current (I) reaches a maximum value (Imax) for the motor current (I), the magnitude of the excitation current (Ie) reaches a maximum value (Iemax) for the excitation current (Ie), - the set value determination device (10) assigns the components of the current vector (i) determined by solving the first optimization problem (O1) or the second optimization problem (O2) to a current control device (9) for a converter device (8) as set values, whereby the current control device (9) operates the converter device (8), and the converter device (8) supplies the excitation current (Ie) to the excitation winding (6) and supplies the motor current (I) to the motor winding (3).

2. The set value determination device (10) solves the first optimization problem (O1), - firstly, the current vector (i) is provisionally determined without taking into account the first general boundary condition, then checking whether said provisionally determined current vector (i) satisfies said first general boundary condition, - if the provisionally determined current vector (i) satisfies the first general boundary condition, use the provisionally determined current vector (i) as the current vector (i), otherwise determine the current vector (i) taking into account at least one of the following first special boundary conditions, according to which: the magnitude of the motor current (I) is equal to a maximum value (Imax) for the motor current (I); the magnitude of said excitation current (Ie) is equal to the maximum value (Iemax) for said excitation current (Ie), the magnitude of the motor voltage (U) is equal to a maximum value (Umax) for the motor voltage (U), 2. The method according to claim 1, characterized in that it is determined which of the first special boundary conditions are taken into account depending on which of the first general boundary conditions are not satisfied.

3. Within the scope of the setpoint determination device (10) checking whether the provisionally determined current vector (i) satisfies the first general boundary condition, - first checking whether the magnitude of the motor current (I) reaches a maximum value (Imax) for the motor current (I) and whether the magnitude of the excitation current (Ie) reaches a maximum value (Iemax) for the excitation current (Ie); 3. The method according to claim 2, further comprising the step of checking only afterwards whether the magnitude of the motor voltage (U) reaches at most a maximum value (Umax) for the motor voltage (U).

4. The set value determination device (10) solves a second optimization problem (O2), - firstly, the current vector (i) is provisionally determined without taking into account the second general boundary condition, then checking whether said provisionally determined current vector (i) satisfies said second general boundary condition, - if the provisionally determined current vector (i) satisfies the second general boundary condition, use the provisionally determined current vector (i) as the current vector (i), otherwise determine the current vector (i) taking into account at least one of the following second special boundary conditions, according to which: the magnitude of the motor current (I) is equal to a maximum value (Imax) for the motor current (I); the magnitude of said excitation current (Ie) is equal to the maximum value (Iemax) for said excitation current (Ie), The method according to any one of claims 1 to 3, characterized in that it is determined which of the second special boundary conditions is taken into account depending on which of the second general boundary conditions is not satisfied.

5. Within the scope of the setpoint determination device (10) checking whether the provisionally determined current vector (i) satisfies a second general boundary condition, - first checking whether the magnitude of the motor current (I) reaches a maximum value (Imax) for the motor current (I); 5. The method according to claim 4, further comprising the step of checking only thereafter whether the magnitude of the magnetizing current (Ie) reaches a maximum value for the magnetizing current (Ie).

6. 6. The method according to claim 1, wherein the setpoint determination device (10) takes into account only copper losses (VK) as losses (V).

7. 7. A computer program for a set value determination device (10), the computer program comprising machine code (12) processable by the set value determination device (10), the processing of the machine code (12) by the set value determination device (10) causing the set value determination device (10) to perform an operating method according to any one of claims 1 to 6.

8. A set value determination device (10) for determining set values ​​for an excitation current (Ie) and a motor current (I) of an externally excited synchronous motor (1), the set value determination device (10) being programmed with a computer program (11) according to claim 7, such that the set value determination device (10) executes an operating method according to any one of claims 1 to 6 during operation.

9. A drive unit, the drive comprises an externally excited synchronous motor (1) with an excitation winding (6) and a motor winding (3), the drive comprises a converter device (8) connected to the excitation winding (6) for supplying an excitation current (Ie) and to the motor winding (3) for supplying a motor current (I); - the drive comprises a current control device (9) which operates the converter device (8); the drive comprises a setpoint determination device (10) according to claim 8, The setpoint determination device (10) determines the instantaneous rotational speed (n) of the synchronous motor (1) and the setpoint torque (M ＊ ) and an input for receiving the setpoint determination device (10) is connected to the current control device (9) in order to specify the value of the field-forming component (Id) of the motor current (I) and the value of the torque-forming component (Iq) of the motor current (I) and the value of the excitation current (Ie),

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