Method and device for speed control of an asynchronous machine

Abstract

Method for speed control of an asynchronous machine (1), wherein a target speed (ns) and an actual speed (n) of the asynchronous machine (1) are determined, wherein a control deviation (e) is determined, wherein at least one controller output variable is determined as a function of the control deviation (e) by a control function, wherein a target phase voltage (V1,S) is determined as a function of the controller output variable, wherein a target stator frequency (ω1,S) is determined as a function of the actual speed (ns) and the controller output variable, wherein the target stator frequency (ω1,S) is determined as the sum of a quantity proportional to the actual speed (n) and a constant or as the sum of a quantity proportional to the actual speed (n), a quantity proportional to the control deviation (e) and the constant, characterized in that one sign of the constant corresponds to the sign of the controller output variable.

Description

[0001] The invention relates to a method and a device for speed control of an asynchronous machine.

[0002] Asynchronous machines are used in a wide variety of applications to generate torque. One example is the generation of so-called assist torque in power steering systems of vehicles.

[0003] An alternating voltage or current for operating the asynchronous machine, in particular for energizing the phase windings of a stator of the asynchronous machine, is generally provided by a power converter, especially an inverter. The inverter can comprise a number of controllable switching elements, whereby a desired phase voltage or phase current profile can be set by switching the switching elements at specific switching times.

[0004] Methods for speed control of an asynchronous machine utilize, for example, the so-called field-oriented vector control. However, such methods are usually based on current sensor signals from at least two phase currents of the asynchronous machine.

[0005] Other well-known methods include current-sensorless speed control, such as the so-called U / f control. This is disclosed, for example, in A. Munoz-Garcia et al., "A new induction motor V / f control method capable of high-performance regulation at low speeds," in Industry Applications, IEEE Transactions on, vol. 34, no. 4, pp. 813-821, July / August 1998. A disadvantage of such speed control is its generally inconsistent behavior, which deviates from the desired dynamic response, and its comparatively low efficiency. Furthermore, it is difficult to reliably prevent overcurrent faults.

[0006] The technical problem therefore arises of creating a method and a device for speed control of an asynchronous machine that enable simple and sufficiently dynamic speed control and efficient operation of the asynchronous machine. The solution to this technical problem is provided by the objects with the features of claims 1 and 10. Further advantageous embodiments of the invention are described in the dependent claims.

[0007] A basic idea of ​​the invention is to determine a phase voltage and a rotor frequency as control variables, wherein both the phase voltage and the rotor frequency are determined as a function of a common controller output variable.

[0008] A method for speed control of an asynchronous machine is proposed. The asynchronous machine can comprise at least one phase, but preferably several phases, in particular three phases. Furthermore, the asynchronous machine can comprise a stator with phase windings. The asynchronous machine can also comprise a rotor with rotor windings. The construction of asynchronous machines, in particular three-phase asynchronous machines, is known to those skilled in the art.

[0009] The stator's phase windings can be electrically connected to the AC output terminals of a power converter, particularly an inverter. The inverter can provide phase voltages and / or phase currents with a desired time profile to generate a rotating electromagnetic field with the desired properties in the stator. The stator frequency is a rotational frequency that is proportional to the rotational frequency of the electromagnetic field via the number of pole pairs in the stator.

[0010] The frequency and amplitude (or RMS value) of a phase voltage or phase current are set by appropriate switching, i.e., on / off switching, of the converter's switching elements. Speed ​​control of the asynchronous machine can thus be achieved by defining a target amplitude or RMS value of the phase voltage and a target stator frequency as input variables for regulating or controlling the converter's operation. The stator frequency can be identical to the frequency of the respective phase voltage.

[0011] The inventive method for speed control of one phase of an asynchronous machine is described below. However, the method described for one phase can also be applied analogously to at least one further phase or to all phases of the asynchronous machine. In particular, the method can be carried out, based on symmetry conditions of the asynchronous machine, to determine phase-specific parameters of all phases, especially a target phase voltage and a target stator frequency.

[0012] In the speed control process for an asynchronous machine, a target speed is determined. This target speed can be determined, for example, by a higher-level system. For instance, the target speed can be fixed or determined based on a desired operating scenario.

[0013] Next, the actual rotational speed of the asynchronous machine is determined. In particular, the actual rotational speed of the asynchronous machine can be measured, for example, by a speed sensor. The actual rotational speed here refers to the rotational speed of an output shaft or the rotor of the asynchronous machine. Of course, it is also possible to calculate the actual rotational speed, for example, as a function of other parameters, such as rotor position, rotor acceleration, or electrical parameters of the asynchronous machine.

[0014] Next, a control deviation is determined. The control deviation can be calculated as the difference between the target speed and the actual speed.

[0015] Furthermore, at least one controller output variable is determined by a control function depending on the control deviation. The control deviation serves as an input variable for the control function, and the execution of the control function determines the at least one controller output variable as the output variable of the control function. In other words, the control function serves to determine the controller output variable as a function of the control deviation. The controller can preferably be a so-called PI controller. Thus, the control function can be a PI control function.

[0016] Furthermore, a target phase voltage is determined based on the controller output. The target phase voltage can be defined, for example, as the target amplitude or target RMS value. The target phase voltage can thus be determined as the first manipulated variable in the speed control method for the asynchronous machine. It is possible, but not preferred, for the target phase voltage to correspond to the controller output.

[0017] According to the invention, a target stator frequency is determined as a function of the actual rotational speed and the controller output. The target stator frequency can thus be determined as a further manipulated variable. Furthermore, the rotational speed of the asynchronous machine is adjusted as a function of the target phase voltage and the target stator frequency. This can be achieved, for example, by minimizing the deviation between the actual phase voltage and the target phase voltage, as well as the deviation between the actual stator frequency and the target stator frequency. Alternatively, this can be achieved by operating the converter in such a way that the target phase voltage and a phase voltage frequency proportional to the target stator frequency are provided.

[0018] As explained previously, the target phase voltage and target stator frequency can be input variables for a method of operating the power converter, whereby duty cycles of switching elements of the power converter are determined depending on the input variables. Thus, the target phase voltage and target stator frequency can serve as input variables for a pulse width modulation method.

[0019] To determine the target stator frequency as a function of the actual speed and the controller output, the actual speed can be multiplied by the factor 2πp, where p denotes the number of pole pairs of the asynchronous machine.

[0020] Through pulse width modulation or the method for operating the power converter, the power converter can provide a phase voltage or phase current depending on the target phase voltage and the target stator frequency, whereby the desired target speed is set when or after this phase voltage or phase current is set.

[0021] Advantageously, the controller output is used to determine both the target phase voltage and the target stator frequency. This enables, in particular, a simple speed control system that allows for speed adjustment with a desired dynamic range. Advantageously, no determination, especially no detection or measurement, of a phase current is necessary. Thus, the speed control method can also be described as a current-sensorless or phase-current-independent speed control method. It is advantageous that standard methods for determining parameters of the control function, especially a PI control function, can be used, thereby simultaneously enabling simple parameter determination and speed adjustment with the desired dynamic range. One such method could be, for example, the method of symmetrical optimum.

[0022] Furthermore, the proposed speed control advantageously allows for scalability on asynchronous machines with different rated power outputs.

[0023] In a preferred embodiment, the target stator frequency is determined as the sum of a quantity proportional to the actual rotational speed, e.g., the actual rotational speed multiplied by a factor of 2πp, and a constant. Alternatively, the target stator frequency can be determined as the sum of a quantity proportional to the actual rotational speed, a quantity proportional to the control deviation, e.g., the control deviation multiplied by a factor of 2πp, and the constant. Furthermore, one sign of the constant corresponds to the sign of the controller output. Thus, the constant has a constant value over time and is independent of the value of the controller output; however, one sign of the constant, and therefore one result of the sum, changes when the sign of the controller output changes.

[0024] Due to the physical operating principle of the asynchronous machine, it is necessary, for example, that in motor operation of the asynchronous machine the stator frequency is higher than the rotor speed multiplied by a factor of 2πp. This is achieved by adding constants with a positive sign, since the resulting stator frequency is then higher than the rotor speed multiplied by 2πp. Similarly, in generator operation of the asynchronous machine, it may be necessary that the rotor speed multiplied by 2πp is higher than the stator frequency. This is accounted for by summing with a constant with a negative sign.

[0025] Overall, this results in a speed control of the asynchronous machine that is very easy to implement computationally and requires little computing power.

[0026] In another embodiment, the target stator frequency is determined as a predetermined maximum frequency if the sum of the phase currents is greater than this maximum frequency. Furthermore, the target stator frequency is determined as a predetermined minimum frequency if the sum of the phase currents is less than this minimum frequency. The maximum and minimum frequencies can be equal in magnitude. This effectively limits the target stator frequency to predetermined maximum values. This limitation advantageously prevents undesirably high phase currents with a high probability, while simultaneously ensuring the desired dynamic behavior, particularly in transient states of the asynchronous machine.

[0027] The magnitude of the maximum and minimum frequencies can be greater than the actual rotational speed multiplied by the number of pole pairs of the stator. Preferably, the magnitude of the maximum and minimum frequencies is greater than the sum of the actual rotational speed multiplied by the number of pole pairs of the stator and a multiple of the quotient of rotor resistance and rotor inductance.

[0028] The higher the value, the better the control quality when adjusting the speed in transient states and thus the dynamic behavior of the asynchronous machine.

[0029] In a further embodiment, the constant is determined as a function of the rotor resistance and the rotor inductance. In particular, the constant is determined as the quotient of rotor resistance and rotor inductance. This advantageously results in this constant corresponding to the rotor frequency, i.e., the electrical frequency in the rotor, of the asynchronous machine during steady-state operation. At this rotor frequency, the asynchronous machine can advantageously be operated during steady-state operation with optimal efficiency or with an efficiency that deviates as little as possible from the optimal efficiency.

[0030] In another embodiment, the target phase voltage is determined as the product of the at least one controller output and a stator frequency-dependent factor. For example, the target stator frequency, as explained above, can be determined as the sum of several summands, with the stator frequency-dependent factor then being determined as a function of the target stator frequency thus determined. The controller output can then be multiplied by this factor to determine the target phase voltage. This can also be referred to as adaptation of the controller output and advantageously enables the use of control functions with constant parameters.

[0031] Furthermore, nonlinearities of the controlled system, in particular a transfer function between phase voltage and speed, can be advantageously taken into account, which in turn results in improved control quality, especially in transient states.

[0032] In a preferred embodiment, the stator frequency-dependent factor is determined as the quotient of the squared target stator frequency and the constant. This advantageously results in a particularly good adaptation of the determination of the target phase voltage to the nonlinear control system.

[0033] In a further preferred embodiment, the target phase voltage is determined as the square root of the controller output or as the square root of the previously described product of the at least one controller output and the stator frequency-dependent factor. This results in a further improved adaptation of the phase voltage determination to the nonlinear controlled system.

[0034] In another embodiment, the target phase voltage is determined as a maximum voltage when it exceeds the maximum voltage. Conversely, the target phase voltage is determined as a minimum voltage when it falls below the minimum voltage. This effectively limits the target phase voltage. This limitation advantageously prevents undesirably high phase currents with a high degree of probability. Consequently, the operational reliability of the asynchronous machine using the described speed control method is significantly improved.

[0035] In another embodiment, the maximum and minimum voltages, in particular their magnitudes, are determined as a function of the target stator frequency. For example, the magnitudes of the maximum and minimum voltages can be determined as a function of a linear function.

[0036] The y-intercept of the linear function can be determined as a function of the stator resistance. In particular, the y-intercept can be proportional to the stator resistance. The slope of the linear function can be determined as a function of the stator rated voltage. In particular, the slope can be proportional to the stator rated voltage.

[0037] For example, the linear function could have a product of the stator's rated current and resistance as its y-intercept value, and a quotient of the difference between the stator's rated voltage and the y-intercept value as the dividend and the stator's rated frequency as the divisor as its slope. For instance, the magnitude of the maximum and minimum voltages could be determined as the output of the linear function, with the target stator frequency or the actual rotational speed multiplied by 2πρ serving as the input.

[0038] This advantageously results in a particularly reliable avoidance of undesirably high phase currents with minimal impairment of dynamic properties.

[0039] Furthermore, a device for speed control of an asynchronous machine is proposed. This device serves to carry out a method according to one of the embodiments described in this disclosure. Therefore, the device can be designed accordingly.

[0040] The device comprises at least one means for determining the actual rotational speed of the asynchronous machine. The means for determining the rotational speed can, in particular, be designed as a speed sensor or include one. Furthermore, the device comprises at least one computing unit. The computing unit can, for example, be designed as a microcontroller or include one.

[0041] Furthermore, a target speed and an actual speed of the asynchronous machine can be determined. The target speed can, for example, be determined by a higher-level system. The actual speed can, for example, be determined using the actual speed determination device.

[0042] Furthermore, a control deviation can be determined, particularly using the computing device. Furthermore, at least one controller output variable can be determined, depending on the control deviation, by a control function, particularly using the computing device. The computing device can provide the previously described controller, in particular the PI controller, or its function. Furthermore, a target phase voltage can also be determined, particularly using the computing device, depending on the controller output variable.

[0043] According to the invention, a target stator frequency can be determined, in particular also by means of the at least one computing device, depending on the actual rotational speed and the controller output variable.

[0044] Furthermore, the speed of the asynchronous machine can be adjusted depending on the target phase voltage and the target stator frequency, e.g., by the computer or another computer. For example, the device can include a power converter, which can be operated in such a way that the target phase voltage and a phase voltage frequency identical to or proportional to the target stator frequency are provided. As explained above, duty cycles of switching elements of the power converter can be determined depending on the target phase voltage and target stator frequency.

[0045] The device thus advantageously enables the implementation of an embodiment described in this disclosure.

[0046] The invention is explained in more detail using an exemplary embodiment. The figures show: Fig. 1 a schematic equivalent circuit diagram of a phase of an asynchronous machine, Fig. 2 a schematic block diagram of a method according to the invention for speed control and Fig. 3 a schematic block diagram of a device according to the invention.

[0047] In the following, identical reference symbols denote elements with the same or similar technical characteristics.

[0048] In Fig. Figure 1 is a schematic equivalent circuit diagram for one phase of an asynchronous machine. 1 (see Fig. 3) shown. Shown is a phase voltage V1 and a phase current I1 . A stator resistor is also shown. R1 and a stator leakage inductance L1 A coupling or main inductance is also shown.L 1h A rotor leakage inductance is also shown. L 2' and rotor resistance R 2' A rotor current is also shown. I 2‛ The tension and the currents V1 , I1 , I 2‛ Here, denotes complex quantities. The rotor resistance R 2‛ , the rotor leakage inductance L 2' and the rotor current I 2‛ Here, denotes quantities related to the stator side. These can be determined in a known manner depending on the value not related to the stator side and the winding factor.

[0049] In Fig. Figure 2 is a schematic block diagram of a method according to the invention for controlling a rotational speed n an asynchronous machine 1 (see Fig. 3) is shown. Here, a target speed is specified. n s of a rotor of an asynchronous machine 1 Specified, for example by a higher-level system. Furthermore, an actual rotational speed is recorded.n the asynchronous machine 1 determined, for example by means of a speed sensor 2 (see Fig. 3).

[0050] Furthermore, a control deviation e is defined as the difference between the target speed and the target speed. n s and the actual rotational speed n The control deviation e serves as the input variable for a control function whose transfer function is in the Laplace domain. G R ( s ) = V R × ( 1 + ( 1 / ( T N * s ) ) ) corresponds to the following parameters. V R and T N Parameters of a PI controller, in particular the gain factor and the reset time of the PI controller. The parameters of the PI controller can be determined using methods known to those skilled in the art, for example according to the so-called symmetric optimum. For example, the parameter V R as V R = ( 2 π / 3 ) × ( Θ × R 2 ' ) / ( p × a × t 1 ) are determined where p is the number of pole pairs of the stator, a is a matching parameter, Θ is the moment of inertia of the asynchronous machine, and t1 t 1 = ( L 1 + L 2 ' ) + t M <?page 7=""?> corresponds. Here, t corresponds to M a time constant of any measurement filtering of the actual rotational speed. However, this does not necessarily have to be taken into account. The parameter T N can be T N = ( a ) 2 × t 1 The adjustment parameter can be determined. It can be greater than or equal to 2.

[0051] It is further shown that a sign operation sgn() determines the sign of the controller output. Furthermore, a constant is defined as the quotient of rotor resistance. R 2' and rotor inductance L 2' The constant is determined and multiplied by the sign of the controller output. This constant is added to the control deviation, where the control deviation before addition is multiplied by the factor. 2 πp was multiplied.

[0052] A first boundary element is shown further. 3 , which reduces the previously explained sum to a maximum frequency ω 2max limited if the sum is greater than a predetermined maximum frequency ω 2max is. Likewise, the first limiting element limits 3 the sum to a given minimum frequency -ω 2max of the rotor, if the explained sum is smaller than this specified minimum frequency -ω 2max of the rotor.

[0053] The sum thus limited then becomes the actual rotational speed. n added, where the actual rotational speed n before adding with the factor 2 The value was multiplied by πp. This sum forms the target stator frequency. oh 1,S , which forms a manipulated variable in the proposed speed control method and an input variable for a pulse width modulation control device 4 (see Fig. 3) serves. The PWM control unit 4This can involve a pulse width modulation process and switching elements of a power converter. 5 control in such a way that the stator frequency, i.e. the rotational frequency of the electromagnetic rotating field, is determined according to the invention. oh 1,S corresponds to it or deviates from it as minimally as possible.

[0054] It is further shown that the controller output variable is multiplied by a factor that is a quotient of the squared target stator frequency. oh 1,S and the signed constant R 2‛ / L 2’ is. A square root function is applied to this product using a square root operation. The resulting value is then applied to a maximum voltage V. 1,max by a further limiting element 7 The resulting value is limited if it exceeds the maximum voltage. The resulting value is also limited to a minimum voltage -V. 1,maxlimited if the resulting value is less than this minimum voltage -V 1,max This potentially limited value corresponds to a target phase voltage. V 1,S The magnitude of the maximum and minimum voltages is determined as the output value of a linear function, where the linear function is defined as a product of its y-intercept value. V 10 between a stator rated current and a stator resistance, and as a slope a quotient between a difference between a stator rated voltage V 1,ref and the ordinate section value as dividend and a stator nominal frequency oh 1,ref as a divisor and as an input value the target stator frequency ω 1,s or the actual rotational speed multiplied by 2πp n serves.

[0055] Also shown is a so-called anti-wind-up functionality. This involves a difference between the unlimited target phase voltage and the actual phase voltage. V 1,S and the further limiting element 7 limited target phase voltage V 1,S This difference is squared and divided by the constant factor R. 2‛ / L 2' The initial value is divided. The resulting value is then multiplied by a gain factor. k multiplied and by the parameter T N subtract the divided summands of the PI transfer function (see formula 1).

[0056] By means of the block diagram in Fig. The two methods for speed control shown can therefore use a target phase voltage as manipulated variables. V 1,S and a target stator frequency oh 1,S These are determined as control variables. They serve as input variables for the previously described PWM control device. 4 , the duty cycles of switching elements of a power converter 5 such that the stator voltage is adjusted to the target stator voltage determined as the control variable V 1,S corresponds to or does not deviate from it by more than a predetermined amount, as well as the rotational frequency of the stator rotating field of the target stator frequency determined as a control variable. oh 1,S The speed corresponds to or does not deviate from it by more than a predetermined amount. By adjusting these parameters, the control deviation between the target speed and the actual speed is determined. n s and the actual rotational speed n minimized.

[0057] In Fig. Figure 3 is a schematic block diagram of a device according to the invention. 8 The device is shown for speed control. 8 includes a computing facility 9 as well as a speed sensor 2 The speed sensor 2 records an actual rotational speed n of a rotor of an asynchronous machine 1 The target rotational speed is shown below. n s The computer system 9 determined how in relation to the in Fig. The block diagram shown explains the control variables as a target phase voltage. V 1,S and a target stator frequency oh 1,S This signal is then sent to a PWM control unit. 4 transmitted, which then determine the duty cycles of switching elements of a power converter. 5 Adjust accordingly.

[0058] The differential equation describing a simplified three-phase system can be formulated according to dn / dt = ( ( 3 lV 1 l 2 ) / 2 π R 2 ' ) × ( ω 2 / ω 1 2 ) ) × ( p / Θ ) to be determined, where ω2 is the factor 2 πp multiplied actual rotational speed n of the rotor and Θ corresponds to the moment of inertia of the asynchronous machine. According to the block diagram in Fig. 2 mathematical operations applied to the controller output to determine the stator voltage V1 These correspond to controller adaptations that were introduced based on this simplified model.

[0059] A sinusoidal triangular comparison function can be used as a pulse-width modulation (PWM) method, for example. Of course, other well-known PWM methods can also be used. In the Fig. The block diagram shown in section 2 shows the target phase voltage. V 1,S to denote an effective value.

[0060] Experiments and simulations have shown that with the proposed speed control method, a permissible phase current is either not reached or exceeded only for an acceptably short period. Furthermore, it was demonstrated that the response time for compensating for speed jumps is comparable to existing field-oriented control methods. Simulations and experiments have also shown that maximum efficiency operating points are automatically and load-independently established in steady state, and consequently, field weakening operation (speed values ​​above rated speed) under partial load is possible without further modifications. Reference symbol list V1 Phase voltage I1 Phase current R1 Stator resistor L1 Stator inductance L 2' Rotor inductance R 2' Rotor resistance I 2‛ Rotor current L 1h Coupling inductance VR Controller parameters T N Controller parameters k amplification factor n s Target speed n Actual rotational speed p number of pole pairs V 1,S Target phase voltage ω 1,S Target stator frequency ω 1,ref Stator rated frequency V 1,ref Stator rated voltage V 10 ordinate intercept value 1 Asynchronous machine 2 Speed ​​sensor 3 first limiting element 4 PWM controller 5 power converters 6th root segment 7 further limiting element 8 Device 9 Computer equipment QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0000] A. Munoz-Garcia et al., “A new induction motor V / f control method capable of high-performance regulation at low speeds,” in Industry Applications, IEEE Transactions on, vol. 34, no. 4, pp. 813-821, July / August 1998

[0005]

Claims

[1] Method for speed control of an asynchronous machine (1), wherein a target speed (n s ) and an actual rotational speed (n) of the asynchronous machine (1) is determined, wherein a control deviation (e) is determined, wherein at least one controller output variable is determined by a control function as a function of the control deviation (e), wherein a target phase voltage (V) is determined as a function of the controller output variable 1,S ) is determined, characterized by , that depending on the actual rotational speed (n s ) and the controller output variable a target stator frequency (ω 1,S ) is determined. [2] Method according to claim 1, characterized by , that the target stator frequency (ω 1,S) is determined as the sum of a quantity proportional to the actual rotational speed (n) and a constant, or as the sum of a quantity proportional to the actual rotational speed (n), a quantity proportional to the control deviation (e) and the constant, where one sign of the constant corresponds to the sign of the controller output. [3] Method according to claim 2, characterized by , that the target stator frequency (ω 1,S ) is determined as a predetermined maximum frequency if the sum is greater than the predetermined maximum frequency, where the target stator frequency (ω) 1,S )is determined as a given minimum frequency if the sum is smaller than the given minimum frequency. [4] Method according to claim 2 or 3, characterized by , that the constant depends on a rotor resistance (R) 2‛ ) and depending on a rotor inductance (L 2‛ ) is determined. [5] Method according to any of the preceding claims, characterized by , that the target phase voltage (ω 1,S ) is determined as the product of at least one controller output variable and a stator frequency-dependent factor. [6] Method according to claim 5, characterized by , that the stator frequency-dependent factor is the quotient of the squared target stator frequency (ω) 1,S ) and the constant is determined. [7] Method according to any of the preceding claims, characterized by , that the target phase voltage (V 1,S ) is determined as the square root of the controller output or as the square root of the product. [8] Method according to any of the preceding claims, characterized by , that the target phase voltage (V 1,S ) is determined as a maximum voltage when the target phase voltage (V 1,S ) is greater than the maximum voltage, where the target phase voltage (V 1,S ) is determined as a minimum voltage if the sum is less than the minimum voltage. [9] Method according to claim 8, characterized by , that the maximum voltage and the minimum voltage depend on the target stator frequency (ω 1,S ) are determined. [10] Device for speed control of an asynchronous machine (1), wherein the device (8) comprises at least one device for determining an actual speed (n) of the asynchronous machine (1) and at least one computing device (9), wherein a target speed (n) s ) and an actual rotational speed (n) of the asynchronous machine (1) can be determined, wherein a control deviation (e) can be determined, wherein at least one controller output variable can be determined as a function of a control function, wherein a phase voltage (V) can be determined as a function of the controller output variable 1,S ) can be determined, characterized by , that depending on the actual rotational speed (n) and the controller output variable, a target stator frequency (ω) 1,S ) can be determined.