Motor control circuit and method for monitoring at least one dc link capacitor of motor control circuit

Abstract

The invention relates to a motor control circuit (10) and to a method for monitoring at least one DC link capacitor (1) in an electrical DC link of a motor control circuit (10) of an electrically commutated (EC) motor (4) having a motor winding (41) operating at a voltage source (6), in which rectifier diodes of a rectifier (2) are closed in a predetermined manner, in order to prevent energy from being drawn from the voltage source (6) in a defined period of time, a DC link capacitance (CZK) of the DC link capacitor (1) is determined; and determining the remaining service life, the end of service life and / or the end of availability of the DC link capacitor (1) from the determined DC link capacitance (CZK) by means of an evaluation circuit (7).

Description

TECHNICAL FIELD

[0001] The present invention relates to a motor control circuit and a method for monitoring at least one DC-link capacitor in an electrical DC-link of a motor control circuit for a commutated electrical machine with machine windings operating under a voltage source. BACKGROUND

[0002] Various methods for determining the instantaneous capacitance and / or internal loss resistance as an indicator for the aging of one or more DC-link capacitors during operation of a commutated electrical machine are known from the prior art.

[0003] The document EP 0 652 445 A2 describes a method for charging or discharging a DC-link capacitor using a target switching operation of an inverter, so that the DC-link capacitance can be calculated using the measured voltage and current curves.

[0004] DE 10 2019 117 369 A describes a method for calculating the DC-link capacitor current using the supply current and the machine current. The calculated DC-link current can be used and the formula u dc = 1 / C ZK ∫i dc dt can be used to calculate the DC-link voltage. The capacitance value C ZK is calculated by comparing the calculated DC-link ripple of the voltage and the measured DC-link ripple, wherein the integrator adjusts the capacitance until a comparison between the model and the measured DC-link voltage is made.

[0005] CN 105 717 368 B describes a method for monitoring the capacitance and the equivalent series resistance (ESR) as an indicator for the aging of a DC-link capacitor in a three-phase inverter. The capacitor current is reconstructed using the formula idc = Sa · ia + Sb · ib + Sc · ic, wherein the machine phase currents ia, ib, ic and the switching states Sa, Sb, Sc of the inverter are already present in the machine controller. In addition, the voltage drop on the DC-link capacitor at a specific point in time is used.

[0006] EP 3 477 314 B1 describes a method for detecting the capacitance of a DC-link capacitor in real time, which detects the DC-link current and the DC-link voltage by means of already present sensors in a specific switching state of the inverter with a current of 0 A through the diode rectifier of the inverter and then determines the capacitance using an approximation formula.

[0007] Furthermore, EP 3 555 644 B1 describes a method for measuring the discharge curve in the DC-link ripple of the voltage and determining the capacitance and the remaining useful life of the capacitor by means of digital evaluation. SUMMARY

[0008] It is therefore an object of the present application to overcome the above-mentioned disadvantages and to provide an electric machine control circuit and a method in which the determination of the instantaneous capacitance and / or the internal loss resistance as an indicator of the aging of one or more DC-link capacitors is optimized during the operation of the electric commutated electric machine.

[0009] This object is achieved by the combination of features of the technical solution of the first aspect of the present application.

[0010] The underlying idea of the present application is that in the control of electric machines the so-called field-oriented control is often used. The electric machine phase currents i a , i b , i c The transformation from the stator-fixed coordinate system into the rotating rotor-fixed coordinate system in the component i d , i q In general, the q-component of the current is used to control the rotational speed. For example, a proportional-integral controller (PI controller) is used to minimize the deviation between the target rotational speed and the actual rotational speed. From this deviation, a target value for the q-current, which generates the torque, is calculated, so that, simply speaking, the controller changes the torque of the electric machine until the target rotational speed is reached. On the other hand, the d-current component does not contribute to the generation of torque, but forms an electric machine winding field that does not influence the electric machine torque. In order to achieve a high efficiency of the electric machine, the d-current is usually adjusted from 0 A to a target value, for example by means of a PI controller. In order to achieve a higher rotational speed, the d-current is also usually adjusted to a negative target value in order to deliberately weaken the magnetic field of the electric machine (field weakening operation). The coils of the electric machine thus store energy in the magnetic field of their windings, which can be influenced deliberately by means of the d-current component.

[0011] For electric machines operated with a three-phase power supply, it is usually sufficient to provide a small energy buffer, for example a DC-link capacitor, between the power supply and the frequency converter in order to ensure stable electric machine operation. The DC-link capacitor reduces the ripple of the DC-link voltage, which is caused, for example, by the pulsed power output of a PWM converter. The voltage ripple therefore usually depends on the capacitance of the DC-link capacitor. However, for three-phase electric machines, this dependency is relatively small, since the 3 power supply voltages shifted by 120° produce a more continuous power flow of the power supply after rectification than, for example, in the case of an electric machine connected to only a single power supply phase. The aging of the capacitor is identified, usually, with a decrease in the capacitance or an increase in the internal loss resistance (ESR; equivalent series resistance) as an indicator of the aging.

[0012] The basic idea of the invention is to increase the DC link voltage ripple for a short time in a three-phase motor by targeted storage and release of energy in the magnetic field of the motor winding, the resulting DC link voltage ripple is usually recorded in the device and from the resulting DC link voltage ripple conclusions are drawn about the aging of the DC link capacitor. These charging and discharging processes are achieved by targeted presetting of the d current component to the target value of the existing controller.

[0013] According to the invention, therefore, a method is proposed for monitoring at least one DC link capacitor in an electrical DC link of a motor control circuit of an electrically commutated motor having a motor winding, which is operated under a voltage source and / or a power source. The motor control circuit comprises a rectifier and an inverter, in particular a frequency converter. Furthermore, at least one DC link capacitor to be monitored is located between the rectifier and the inverter. In the method, a motor phase current is detected and a d current component for controlling and / or regulating the motor is determined, in particular in a phasor representation in a rotating or rotor-fixed coordinate system. Furthermore, a voltage ripple of a DC link voltage occurring at the DC link capacitor is detected and / or measured. Furthermore, the d current component is controlled by a controller such that energy from the voltage source and / or the motor control circuit is stored in a magnetic field of the motor winding of the EC motor, and the d current component is controlled by the controller such that the energy stored in the magnetic field of the motor winding of the EC motor from the voltage source and / or the motor control circuit is released back to the motor control circuit, in particular to the at least one DC link capacitor. When the stored energy is released from the magnetic field of the motor winding of the EC motor, the DC link voltage is greater than the input voltage of the voltage source, so that the rectifying diodes of the rectifier are blocked in a predetermined manner to prevent energy being taken from the voltage source for a defined time. Furthermore, a DC link capacitance of the DC link capacitor is determined, and by means of an evaluation circuit, in particular having a microcontroller, the remaining useful life, the end of useful life and / or the end of availability of the DC link capacitor is determined from the determined DC link capacitance.

[0014] The advantage of the invention compared to the prior art is that the capacitance evaluation can be carried out during motor operation and has only a small influence on the motor operation. Furthermore, since the drop in the DC link capacitance due to the aging effect usually occurs over a period of several weeks to several months, the process cannot be carried out continuously, but only at fixed maintenance intervals. Furthermore, the method does not require additional hardware, but makes use of the existing sensor system of the motor. The current and voltage curves can be evaluated with the existing evaluation methods, since the motor current and the DC link voltage are recorded by the motor controller anyway.

[0015] Energy stored in the motor winding can be converted into corresponding power where L denotes the winding inductance and iL represents the current through the winding.

[0016] In an advantageous embodiment it is provided that the d current component for storing energy from the voltage source and / or the motor control circuit in the magnetic field of the motor winding is controlled by the controller such that: L > 0 and or i L < 0 and

[0017] In an embodiment of the application it is provided that the d current component for storing energy from the motor control circuit in the motor winding is controlled by the controller such that the storing is performed continuously over more than one power supply cycle of the voltage source. In this way the influence on the motor operation of the EC motor is reduced.

[0018] Furthermore, in an advantageous embodiment the d current component for releasing energy stored in the magnetic field of the motor winding from the magnetic field of the motor winding back into the motor control circuit is controlled by the controller such that: L > 0 and or i L < 0 and Preferably, after the discharge the d current component i d is adjusted by the controller 5 to a predetermined negative target value or to the target value 0 A.

[0019] In another advantageous variant it is provided according to the application that the d current component is controlled by the controller such that it has a sawtooth curve and / or at least partially a sinusoidal curve and / or a rectangular jump curve.

[0020] Advantageously, the motor control circuit has a sensor system for operating the EC motor and this sensor system is simultaneously used for power detection and / or voltage detection of the voltage ripple at the DC link capacitor and / or for determining the motor phase current

[0021] In an advantageous embodiment it is provided that the motor phase current is determined at least intermittently during the operation of the EC motor.

[0022] In a preferred embodiment the power measurement and the voltage measurement at the DC link capacitor are performed to determine the capacitance of the DC link capacitor to determine the DC link voltage.

[0023] In an embodiment of the application it is provided that when determining the capacitance the entire voltage curve of the DC link voltage and / or a filtered and / or correlated signal curve is used.

[0024] Furthermore, advantageously, the DC link capacitance is determined by an observer system. Therein the DC link current is emulated from recorded input measured variables of the motor control circuit required for controlling the operation of the motor, such as for example the power taken from the power supply, the power output of the motor, the power supply input voltage, the motor current and the phase voltage of the motor. The DC link current is then multiplied by the inverse DC link capacitance and integrated to calculate the estimated DC link voltage. The difference between the estimated DC link voltage and the actually measured DC link voltage is also integrated and then considered as the inverse DC link capacitance, thereby creating an observer loop. This control loop regulates the estimated DC link voltage to the measured DC link voltage, thereby enabling the estimation of the DC link capacitance.

[0025] Furthermore, in an advantageous variant, the observer is activated only when the voltage ripple of the DC link voltage is greater than the input voltage of the voltage source, after which deactivation takes place. Therein in particular the activation and / or deactivation is achieved by multiplication by 0 or not performing the calculation of the observer within the control algorithm.

[0026] In another advantageous embodiment, the decrease of the DC link capacitance or the increase of the internal loss resistance of the DC link capacitor is inferred using the maximum of the voltage ripple and / or a comparison between the detected voltage curve and a Look-Up-Table. Within the scope of the present invention, a Look-Up-Table refers to a data structure for efficient retrieval of values of a function or other data set. It consists of a set of key-value pairs, each key being assigned a corresponding value. When a specific key value is needed, the corresponding value can be quickly retrieved using the Look-Up-Table without having to recalculate the function or data.

[0027] In a preferred embodiment of the present invention, the controller is a PI controller. In particular, the target value of the d current component is calculated using a superimposed PI controller. Therein the difference between the target DC link voltage and the measured DC link voltage is switched to the input of the controller, wherein in particular a step excitation is added to the measured DC link voltage as the target DC link voltage and / or a constant value is used, so that the controller forms a curve of the d current component.

[0028] In another preferred embodiment, the present invention provides that, in order to evaluate the DC link capacitance, a journey representing the course of the change of the DC link voltage, in particular the curve shape, is stored at a predetermined and / or time- and / or time interval-dependent d current component and power, in particular in a controller or a data memory or a control unit, and is evaluated, preferably by a neural network and / or an artificial intelligence stored in a controller or a data memory or a control unit.

[0029] In a further advantageous variant, it is further provided that the curve shape of the d current component is adjusted depending on the operating point by a neural network and / or an artificial intelligence and / or depending on the motor power and / or the rotational speed during operation.

[0030] Furthermore, in an advantageous embodiment, the spectrum of the predetermined target current is adjusted by pre-filtering, preferably by a low-pass filter, in order to avoid noise generation by the motor windings, wherein in particular parts of the spectrum and / or required control reserves and / or bandwidths are reduced in a targeted manner.

[0031] Furthermore, in an advantageous embodiment of the application, a temperature prediction of the EC motor temperature, preferably a winding temperature of the motor windings, is determined, in particular by means of a superposed neural network and / or a temperature model. A corresponding predicted temperature jump can be at least partially compensated by the controller regulating the d- current component, in particular by increasing the d-current component.

[0032] According to the application, an electric machine control circuit of an electrically commutated electric machine having motor windings is also proposed, for monitoring at least one DC link capacitor in an electrical DC link of the electric machine control circuit of an EC motor operating under a voltage source, preferably in accordance with the method disclosed above. The electric machine control circuit comprises a rectifier and an inverter, in particular a frequency converter. The at least one DC link capacitor to be monitored is located between the rectifier and the inverter. Furthermore, a controller is provided for controlling the d-current component of the DC link current, such that energy from the electric machine control circuit is stored in the magnetic field of the motor windings of the EC motor and / or energy stored in the magnetic field of the motor windings from the electric machine control circuit is discharged back into the electric machine control circuit. Furthermore, an evaluation circuit is provided for determining the remaining service life, the end of service life and / or the end of availability of the DC link capacitor from the determined DC link capacitor.

[0033] The features disclosed above can be combined as desired, as long as this is technically feasible and they do not contradict each other. BRIEF DESCRIPTION OF DRAWINGS

[0034] Further advantageous further developments of the application are characterized in other aspects of the application or are described in more detail below in connection with the preferred embodiments of the application with reference to the drawings, in which:

[0035] Figure 1 a schematic circuit diagram of the electric machine control circuit is shown;

[0036] Figure 2 a time curve of the motor phase current and the q- and d-current components of the electric machine control circuit in a rotating coordinate system is shown;

[0037] Figure 3 a time curve of the rectified input voltage and the DC link voltage and the d- current component of the electric machine control circuit and the motor current is shown

[0038] Figure 4A time-temperature curve of an inverter of a motor control circuit is shown. DETAILED DESCRIPTION

[0039] The above figures are exemplary schematic diagrams, in which identical reference signs denote identical functional and / or structural features.

[0040] Figure 1 A schematic circuit diagram of a motor control circuit 10 of an electrically commutated motor 4 with motor windings 41 is shown for monitoring at least one DC link capacitor 1 in an electrical DC link of the motor control circuit 10 of an EC motor 4 operating under a voltage source 6. The motor control circuit 10 comprises a rectifier 2 and an inverter 3. Furthermore, the DC link capacitor 1 to be monitored is located between the rectifier 2 and the inverter 3. Furthermore, a PI controller 5 is provided for controlling a d current component idof the DC link current i d , such that energy from the motor control circuit 10 is stored in the magnetic field of the motor windings 41 of the EC motor 4 and / or correspondingly energy from the motor control circuit 10 stored in the magnetic field of the motor windings 41 is released back to the motor control circuit 10. Furthermore, an evaluation circuit 7 is provided, which determines a DC link capacitor C ZK The remaining useful life, the end of useful life and / or the end of availability of the DC link capacitor is determined.

[0041] The motor control circuit 10 has a sensor system for operating the EC motor 4 and for simultaneously carrying out a detection of power and / or voltage at the electrical DC link capacitor 1 for voltage ripple and / or for determining at least intermittently during operation of the EC motor 4 a motor phase current i a , i b , i c .

[0042] Figure 2 A time curve of the motor phase current i a , i b , i c and the q current component idand the d current component i q of the motor control circuit in a rotating coordinate system is shown. d

[0043] Figure 3 A time curve of the rectified input voltage Ui, U2, U3, the DC link voltage U ZK , the d current component idand the motor current i d of the motor control circuit 10 during application of the method according to the application is shown. Motor

[0044] Due to the fact that during normal operation of the motor the DC link voltage U ZK ​​The voltage ripple of the DC link capacitor 1 will temporarily increase, so the energy stored in the magnetic field of the motor winding 41 must be sufficient for the continued operation of the motor and fed back to the DC link capacitor 1 so that no current is drawn from the voltage source 6 during this period. For this purpose, the d current component i d is controlled by the controller 5 to have a sawtooth curve.

[0045] It can be seen that during the time period t0 to t1 energy is stored in the magnetic field of the motor winding 41 because i d <0 and In order to reduce the influence on the operation of the motor as far as possible, the slope of the edge during this period is chosen so that this charging can take place continuously over more than one mains cycle. In the following time period, from t1 to t2, the d current component i d is increased with as steep an edge as possible so that i d <0 and and the energy from the motor winding 41 is fed back into the DC link. The d current component i d is controlled by the controller 5 to a predetermined negative target value.

[0046] In order to control the d current component i d and the q current component i q , the motor phase current must be measured during operation. Because in the feedback process the voltage at the DC link capacitor U ZK is greater than the voltage of the previously fed-in input voltages U1, U2, U3, the diodes of the rectifier 2 are blocked and temporarily no energy is drawn from the voltage source 6, see time period t1 to t3. The current and voltage curves of the DC link capacitor 1 are therefore known and the inverse capacitance value can be determined by means of an evaluation circuit 7 by observing the system.

[0047] In the method according to the application the detection of the motor phase currents i a , i b , i c and the determination of the d current component i d for the control and / or regulation of the motor 4 in the phasor representation in the rotating coordinate system takes place. In addition, the voltage ripple of the DC link voltage U ZK occurring at the DC link capacitor 1 is detected and / or measured. Furthermore, the d current component i d is controlled by the controller 5 so that energy from the motor control circuit 10 is stored in the magnetic field of the motor winding 41 of the EC motor 4 and the d current component i d is controlled by the controller 5 so that the energy stored in the motor winding is released back into the motor control circuit 10. When the stored energy is released from the magnetic field of the motor winding 41 of the EC motor 4, the DC link voltage U ZKthe voltage ripple of the voltage source 6, U1, U2, U3, such that the rectifier diodes of the rectifier 2 close in a predetermined manner to prevent energy being taken from the voltage source 6 for a defined period of time. Furthermore, the DC link capacitor 1 is charged with a DC link current i ZK determined and, by means of the evaluation circuit 7, the remaining useful life, the end of useful life and / or the end of availability of the DC link capacitor 1 is determined from the determined DC link capacitor C ZK

[0048] In the method, the d current component i d is controlled by the controller 5 such that: i L > 0 and or i L < 0 and In this case, the d current component i d is controlled by the controller 5 such that the aforementioned storing takes place continuously over one or more supply cycles of the voltage source 6. Furthermore, the d current component i d is controlled by the controller 5 such that: i L > 0 and or i L < 0 and

[0049] During operation of the EC motor 4, the motor phase currents i a , i b , i c are determined at least intermittently by a sensor system. Furthermore, in order to determine the DC link capacitor C ZK of the DC link capacitor 1, power measurements and voltage measurements are carried out at the DC link capacitor 1 to determine the voltage ripple.

[0050] In an observer system for determining the DC link capacitor C ZK , the DC link current is simulated from input measured variables of the motor control circuit which are already necessary for controlling the motor operation, for example the power taken from the supply, the power output by the motor, the supply input voltage, the motor current, the phase voltage of the motor. The DC link current is then multiplied by the inverse DC link capacitor and integrated to calculate the estimated DC link voltage. The difference between the estimated DC link voltage and the actually measured DC link voltage is also integrated and then taken as the inverse DC link capacitor, thus creating an observer loop. This control loop adjusts the estimated DC link voltage to the measured DC link voltage, thus enabling the DC link capacitor to be estimated.​

[0051] Among them, in the DC link U ZK The observer is only activated when the voltage ripple of the voltage source 6 is greater than the input voltage U1, U2, U3 of the voltage source 6. Thereafter, it is deactivated, wherein activation and / or deactivation is achieved in particular by multiplying by 0 or not performing calculations of the observer in the control algorithm.

[0052] Alternatively, the DC link capacitance C can be derived using the maximum value of the voltage ripple and / or a comparison between the recorded voltage curve and a lookup table. ZK or an increase in the internal loss resistance of the DC link capacitor 1.

[0053] In addition, a superimposed PI controller can be used to calculate the d current component i d The target DC link voltage is the same as the measured DC link voltage U ZK The difference between is switched to the input of the controller 5. In addition, a step excitation is added to the measured DC link voltage U ZK As the target DC link voltage and / or using a constant value, the controller 5 forms the current component i d curve.

[0054] Figure 4 The time-temperature curve of the inverter 3 of the motor control circuit 10 , in particular the power module of the inverter 3 , is shown.

[0055] Because the current component i d The brief increase in the amount of leads to the inverter 3 introducing more power into the EC motor 4, so that the method according to the invention briefly increases the temperature of the module. In a further variant, the current component i d The injection time point of is intentionally selected so that the aging of the inverter 3 can be reduced. Since the inverter 3, in particular its power modules, generally ages with the number of temperature cycles or jumps, it is possible to use, for example, a temperature prediction using a superposition neural network or a temperature model (e.g. Figure 4 As shown) to predict the temperature jump, such as time point t D1 and t D2 If the controller 5 is at time point t D1 and t D2 Set the larger d current component i between d , then the inverter 3 will go through the temperature curve T D Instead of temperature curve T N This reduces the number of temperature cycles during operation of the inverter 3 and increases its service life. It is also conceivable to select the current component i d The amount and time curve of the inverter 3 allows the inverter 3 to pass through a specific temperature curve in a targeted manner without affecting the operation of the motor.

[0056] Thus, in one embodiment of the method, a temperature prediction of the inverter 3 is determined, in particular a temperature jump is predicted by a superposition neural network and / or a temperature model, wherein the respective predicted temperature jump is at least partially compensated by a control of the controller 5 of the d current component i d , in particular by an increase of the d current component i d .

[0057] The application in its implementation is not limited to the preferred embodiments given above. Rather, even in fundamentally different embodiments, various variants of the use of the shown solutions can be envisaged.

Claims

1. A method for monitoring at least one DC-link capacitor (1) in an electrical DC-link of a machine control circuit (10) of an electric commutated machine, EC machine (4), having machine windings (41), operating under a voltage source (6), the machine control circuit comprising a rectifier (2) and an inverter (3), wherein, The at least one DC link capacitor (1) to be monitored is located between the rectifier (2) and the inverter (3), the method comprising the following steps: a. detecting the motor phase currents (i a , b , c ) and determining the d current component (i d ) for controlling and / or regulating the motor (4); b. detecting and / or measuring a voltage ripple of a direct current link voltage (U ZK ) occurring at the direct current link capacitor (1); c. Control the d current component (i d ), so that energy from the voltage source and / or the motor control circuit (10) is stored in the magnetic field of the motor winding (41) of the EC motor (4); d. Control the d current component (i d ), so that the energy stored in step c is released back to the motor control circuit (10), in particular to the at least one DC link capacitor (1); wherein said DC-link voltage (U ZK ) is greater than the input voltage (Ul, U2, U3) of said voltage source (6) when stored energy is released from said magnetic field of said motor winding (41) of said EC motor (4), so that rectifying diodes of said rectifier (2) are blocked in a predetermined manner to prevent energy acquisition from said voltage source (6) for a defined time, e. determining a direct current link capacitor (1) of a direct current link capacitor (C ZK ); f. determining, with the evaluation circuit (7), a remaining useful life, an end of useful life, and / or an end of availability of the DC link capacitor (1) from the determined DC link capacitor (C ZK ) wherein the d current component (i d ) for storing energy from the voltage source and / or motor control circuit (10) in the magnetic field of the motor winding (41) is controlled by the controller (5) such that: L i > 0 and or i L < 0 and 2. The method according to claim 1, wherein the d current component (id) for storing energy from the motor control circuit (10) in the motor winding (41) is controlled by the controller (5) such that the storing is continuous over a plurality of supply periods of the voltage source (6). d ), making the storing continuous over a plurality of supply periods of the voltage source (6).

3. The method according to claim 1 or 2, wherein the d current component (id) for releasing the energy stored in the magnetic field of the motor winding (41) in step c from the magnetic field of the motor winding (41) back to the motor control circuit (10) is controlled by the controller (5) so that: i L >0 and or i L <0 and 4. The method according to claim 3, wherein the d current component (id) is controlled by the controller (5) to a predetermined negative target value or target value 0A after the release. d ) 5. The method according to any one of claims 1 to 4, wherein the d current component (id) is controlled by the controller (5) such that it has a sawtooth curve and / or at least partially a sinusoidal curve and / or a rectangular jump curve. d ), making it have a sawtooth curve and / or at least partially a sinusoidal curve and / or a rectangular jump curve.

6. The method according to any one of the preceding claims, wherein the motor control circuit (10) has a sensor system for operating the EC motor (4) and the sensor system is simultaneously used for power detection and / or voltage detection of voltage ripple at the DC link capacitor (1) and / or for determining the motor phase current (i a b c ) at least intermittently during operation of the EC motor (4), wherein in particular the motor phase current (i a b c ) is determined at least intermittently during operation of the EC motor (4).​​​​ 7. The method according to claim 6, wherein for determining the capacitance (C ZK ) of the DC link capacitor, power detection and voltage detection are performed at the DC link capacitor (1), thereby determining the DC link capacitor.

8. The method according to any of the preceding claims, wherein the determination of the direct current link capacitance (C ZK ) is carried out by an observer system, wherein preferably the observer is activated only when the voltage ripple of the direct current link voltage (U ZK ) is greater than the input voltage (U0) of the voltage source (6), wherein in particular the activation is followed by a deactivation, wherein in particular the activation and / or deactivation is realized by multiplication with zero or no calculation of the observer within the control algorithm.​​ 9. The method according to any of the preceding claims, wherein a maximum of the voltage ripple and / or a comparison between a detected voltage curve and a lookup table is utilized to infer a decrease of the DC-link capacitance (C ZK ) or an increase of an internal loss resistance of the DC-link capacitor (1).

10. The method according to any one of the preceding claims, wherein the d current component (i d ), wherein the difference between the target DC link voltage and the measured DC link voltage is switched to the input of the controller (5), wherein in particular a step excitation is added to the measured DC link voltage (U ZK ) as the target DC link voltage and / or using a constant value so that the controller (5) forms the d current component (i d ) curve.

11. The method according to any of the preceding claims, wherein the curve shape of the d current component (i d ) is adjusted according to the operating point by a neural network and / or artificial intelligence and / or depending on the motor power and / or rotational speed during operation.

12. The method according to any one of the preceding claims, wherein the frequency spectrum of the predetermined target current is adjusted by pre-filtering, preferably by a low-pass filter, to avoid noise generation by the motor winding (41), wherein in particular parts of the frequency spectrum and / or required control reserves and / or bandwidths are reduced in a targeted manner.

13. The method according to claim 1, wherein a temperature prediction of the temperature of the inverter (3), in particular of a power module of the inverter (3), is determined, in particular by means of a superposition neural network and / or a temperature model, for predicting temperature jumps, wherein, The controller (5) adjusts the d current component (id), in particular by increasing the d current component (i d ) to at least partially compensate for the corresponding predicted temperature jump.

14. A motor control circuit (10) for an electrically commutated (EC) motor (4) having a motor winding (41), for monitoring at least one DC link capacitor (1) in an electrical DC link of the motor control circuit (10) of the EC motor (4) operated under a voltage source (6), preferably monitoring according to the method of any of the preceding claims, the motor control circuit comprising a rectifier (2) and an inverter (frequency converter) (3), wherein the at least one DC link capacitor (1) to be monitored is located between the rectifier (2) and the inverter (3), wherein a controller (5) is also provided for controlling the d current component (i d ), so that energy from the motor control circuit (10) is stored in the magnetic field of the motor winding (41) of the EC motor (4), and / or the energy from the motor control circuit (10) stored in the magnetic field of the motor winding (41) is released back to the motor control circuit (10), wherein an evaluation circuit (7) is also provided, which is used to evaluate the DC link capacitance (C ZK ) to determine the remaining service life, end of service life and / or end of availability of the DC link capacitor (1).

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