Motor control circuit and method for monitoring at least one intermediate circuit capacitor of the motor control circuit

Abstract

The invention relates to a motor control circuit (10) and to a method for monitoring at least one intermediate circuit capacitor (1) in an electrical intermediate circuit of a motor control circuit (10) of an electrically commutated (EC) motor (4) with motor windings (41) operated at a voltage source (6), in which rectifier diodes of the rectifier (2) close as intended in order to prevent energy consumption from the voltage source (6) for a defined period of time, wherein a determination of an intermediate circuit capacitance (C ZK ) of the intermediate circuit capacitor (1); and determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor (1) from the determined intermediate circuit capacitance (C ZK ) by means of an evaluation circuit (7).

Description

[0001] The invention relates to a motor control circuit and a method for monitoring at least one intermediate circuit capacitor in an electrical intermediate circuit of a motor control circuit of an electrically commutated motor with motor windings, which motor control circuit is operated at a voltage source.

[0002] A variety of methods for determining the instantaneous capacitance and / or the internal loss resistance as an aging indicator of one or more intermediate circuit capacitors during operation of an electrically commutated motor are known from the prior art.

[0003] The document EP 0 652 445 A2 describes a method of charging or discharging the intermediate circuit capacitor using targeted switching operations of the inverter, so that the intermediate circuit 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 mains current and the motor current. Using the calculated DC link current and the formula u dc = 1 / C ZK ∫i dc dt the intermediate circuit voltage can be calculated. The capacitance value C ZK is formed from a comparison between the calculated and the measured DC link voltage ripple, whereby an integrator adjusts the capacitance until a comparison between the model and the measured DC link voltage has taken place.

[0005] CN105717368B describes a method for monitoring the capacitance and equivalent series resistance (ESR) of a three-phase inverter as an aging indicator of the DC link capacitor. The capacitor current is reconstructed using the formula: idc = Sa ia + Sb ib + Sc - ic, where ia, ib, and ic are the motor phase currents and Sa, Sb, and Sc are the switching states of the inverter, which are already present in the motor controller. The voltage drop across the DC link capacitor at a specific time is also taken into account.

[0006] EP 3 477 314 B1 describes a method for real-time detection of the intermediate circuit capacitor capacitance, which detects the intermediate circuit current and the intermediate circuit voltage with the aid of the existing sensors in certain switching states of the inverter in which the current through the diode rectifier of the inverter is 0A and then uses an approximate formula to determine the capacitance.

[0007] Furthermore, EP 3 555 644 B1 describes a method for measuring the discharge curve in the intermediate circuit voltage ripple and for determining the capacitance and remaining service life of the capacitor with the aid of a digital evaluation.

[0008] Further printed prior art in the present technical field is disclosed in the documents KR 10 1 605 990 B1, US 2009 / 0 072 982 A1 and DE 10 2022 107 523 A1.

[0009] The invention is therefore based on the object of overcoming the aforementioned disadvantages and providing a motor control circuit and a method in which a determination of the instantaneous capacitance and / or the internal loss resistance as an aging indicator of one or more intermediate circuit capacitors is optimized during operation of an electrically commutated motor.

[0010] This problem is solved by the combination of features according to patent claim 1.

[0011] The basic idea of ​​the present invention is that the so-called field-oriented control is often used in the control of electric motors. The motor phase currents i a , i b , i c are converted from the stator-fixed coordinate system into a rotating rotor-fixed coordinate system into the components i d , i atransferred. In general, the q-component of the current is used to control the speed. The deviation between the setpoint and actual speed is minimized, for example, with the help of a proportional-integral controller (PI controller). From this deviation, a setpoint for the q-current, which generates the torque, is calculated so that the controller, to put it simply, changes the motor torque until the setpoint speed is reached. The d-current component, on the other hand, does not contribute to generating the torque, but rather forms a magnetic field in the motor windings that does not influence the motor torque. To achieve high levels of efficiency in electric motors, the d-current is usually controlled to a setpoint of 0 A, for example, with the help of a PI controller. To achieve higher speeds, the d-current is often even controlled to a negative setpoint in order to deliberately weaken the magnetic field of the electric motor (field weakening operation).The coils of the electric motor therefore store energy in the magnetic field of their windings, which can be specifically influenced via the d-current component.

[0012] For electric motors operated on a three-phase mains supply, typically only a small energy buffer, such as a DC link capacitor, is required between the mains supply and the frequency converter to ensure stable motor operation. The DC link capacitor reduces the ripple in the DC link voltage caused by the pulsed power draw by, for example, a PWM converter. The voltage ripple is therefore generally dependent on the size of the capacitance of the DC link capacitor. For three-phase electric motors, however, this dependence is comparatively small, since the three mains voltages, which are 120° out of phase, result in a more continuous power flow from the mains after rectification than would be the case, for example, when connected to only a single mains phase.Typically, a decrease in capacitance or an increase in internal loss resistance (ESR; equivalent series resistance) is used as an aging indicator to detect the aging of capacitors.

[0013] The underlying idea of ​​the invention is to increase the DC link voltage ripple in a three-phase electric motor for a short period of time by selectively storing and discharging energy in the magnetic field of the motor windings. From the resulting DC link voltage ripple, which is usually recorded by the devices anyway, conclusions can be drawn about the aging of the DC link capacitor. These charging and discharging processes are achieved by selectively specifying the d-current component as a setpoint for the existing controller.

[0014] According to the invention, a method is therefore proposed for monitoring at least one intermediate circuit capacitor in an electrical intermediate circuit of a motor control circuit of an electrically commutated motor with motor windings, said motor control circuit being operated at a voltage source and / or a mains voltage. The motor control circuit comprises a rectifier and an inverter, in particular a frequency converter. Furthermore, the at least one intermediate circuit capacitor to be monitored is located between the rectifier and the inverter. The method involves detecting the motor phase currents and determining a d-current component for controlling and / or regulating the motor, in particular in phasor notation in the rotating or rotor-fixed coordinate system. Furthermore, a voltage ripple of an intermediate circuit voltage occurring at the intermediate circuit 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 motor control circuit is stored in a magnetic field of the motor windings of the EC motor, and the d-current component is controlled by the controller such that the energy stored from the voltage source and / or motor control circuit in the magnetic field of the motor windings of the EC motor is discharged back into the motor control circuit, in particular the at least one intermediate circuit capacitor. When discharging the stored energy from the magnetic field of the motor windings of the EC motor, the intermediate circuit voltage is greater than an input voltage of the voltage source, such that rectifier diodes of the rectifier are blocked as intended to prevent energy absorption from the voltage source for a defined period of time.In addition, an intermediate circuit capacitance of the intermediate circuit capacitor is determined and a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor is determined from the determined intermediate circuit capacitance by means of an evaluation circuit, which in particular has a microcontroller.

[0015] An advantage over the state of the art is that the capacitance can be evaluated while the motor is running, with only a minor impact on motor operation. Furthermore, the decrease in DC link capacitance due to aging effects often takes place over a period of several weeks to months, meaning the process cannot be carried out continuously but only at fixed maintenance intervals. Furthermore, the process does not require any additional hardware; instead, it uses the motor's existing sensors. Existing evaluation methods can be used to evaluate the current and voltage curves, since the motor current and DC link voltage are already recorded by the motor controller.

[0016] The energy stored in the motor windings W=12L∗iL2 can be converted into a corresponding service PWinding=12L⋅iL⋅diLdt where L is the winding inductance and i L the current through the winding.

[0017] According to the invention, it is further provided that the d-current component for storing energy from the voltage source and / or motor control circuit in the magnetic field of the motor windings is controlled by means of the controller in such a way that: i L > 0 and diLdt>0; or i L < 0 and diLdt<0.

[0018] In one embodiment of the invention, the d-current component for storing energy from the motor control circuit in the motor windings is controlled by the controller in such a way that the storage occurs continuously over more than one mains period of the voltage source. In this way, any influence on the operation of the EC motor is reduced.

[0019] Furthermore, an embodiment is advantageous in which the d-current component for discharging the energy stored in the magnetic field of the motor windings from the magnetic field of the motor windings back into the motor control circuit is controlled by the controller in such a way that: i L > 0 and diLdt<0; or i L < 0 and diLdt>0. Preferably, the d-current component i d after discharge, it is regulated by means of the regulator 5 to a predetermined negative setpoint or to the setpoint 0 A.

[0020] In a further advantageous variant, the invention provides that the d-current component is controlled by means of the controller in such a way that it has a sawtooth-shaped curve and / or an at least partially sinusoidal curve and / or a rectangular step-like curve.

[0021] It is further advantageous if the motor control circuit has a sensor system for operating the EC motor and this sensor system is used simultaneously for power detection and / or voltage detection of the voltage ripple at the intermediate circuit capacitor and / or for at least intermittent determination of the motor phase currents during operation of the EC motor.

[0022] In an advantageous embodiment, it is provided that the motor phase currents are determined at least intermittently during operation of the EC motor.

[0023] In a preferred embodiment, the power measurement and the voltage measurement at the intermediate circuit capacitor are carried out to determine the capacitance of the intermediate circuit capacitor in order to determine the intermediate circuit voltage.

[0024] In one embodiment of the invention, it is provided that the entire voltage curve and / or a filtered and / or correlated signal curve of the intermediate circuit voltage are used when determining the capacitance.

[0025] Furthermore, it is advantageous if the DC link capacitance is determined using an observer system. The DC link current is simulated from previously recorded input measured variables of the motor control circuit, which are required anyway to regulate motor operation, such as the power drawn from the mains, the power output by the motor, the mains input voltage, the motor current, and the phase voltages of the motor. The DC link current is then multiplied by the inverse DC link capacitance and integrated to calculate an 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, creating an observer loop.This control loop adjusts the estimated DC link voltage to the measured DC link voltage and is thus able to estimate the DC link capacitance.

[0026] Another advantageous variant is one in which the observer is only activated while the voltage ripple of the intermediate circuit voltage is greater than the input voltage of the voltage source, after which it is deactivated. In this case, activation and / or deactivation are achieved by multiplying by 0 or, within a control algorithm, by not executing any calculations of the observer.

[0027] In a further advantageous embodiment, a maximum value of the voltage ripple and / or a comparison between a detected voltage curve and a look-up table are used to infer a decrease in the DC link capacitance or an increase in the internal loss resistance of the DC link capacitor. In the context of the invention, a look-up table refers to a data structure used to efficiently retrieve the value of a function or other data set. It consists of a set of key-value pairs, with each key assigned a corresponding value. When a specific key value is required, the look-up table can be used to quickly retrieve the corresponding value instead of recalculating the function or data.

[0028] In a preferred embodiment of the invention, the controller is a PI controller. In particular, a setpoint for the d-current component is calculated using a superimposed PI controller. A difference between the setpoint intermediate circuit voltage and the measured intermediate circuit voltage is applied to an input of the controller. In particular, a step-like excitation is added to the measured intermediate circuit voltage as the setpoint intermediate circuit voltage and / or a constant value is used, so that a curve for the d-current component is formed by the controller.

[0029] In a further advantageous embodiment, the invention provides that, for the evaluation of the intermediate circuit capacitance, a stroke which represents a course, in particular a curve shape, of the intermediate circuit voltage, at a predetermined and / or varying d-current component and power over time and / or time intervals, in particular in the controller or a data memory or a control unit, is stored and evaluated, preferably by means of a neural network and / or an artificial intelligence stored in the controller or the data memory or the control unit.

[0030] In a further advantageous variant, it is further provided that the curve shape of the d-current component is adapted by means of a neural network and / or artificial intelligence and / or depending on the engine power and / or the speed during operation depending on the operating point.

[0031] Furthermore, an embodiment is advantageous in which a spectrum of a predetermined target current is adapted by means of pre-filtering, preferably by means of a low-pass filter, in order to avoid noise generation of the motor windings, wherein in particular parts of the spectrum and / or a required control reserve and / or a bandwidth are specifically reduced.

[0032] Furthermore, in an advantageous embodiment of the invention, a temperature prediction of the temperature of the EC motor, preferably a winding temperature of the motor windings, is determined, in particular by means of a superimposed neural network and / or a temperature model, to predict temperature jumps. The correspondingly predicted temperature jumps are at least partially compensated by controlling the d-current component by means of the controller, in particular by increasing the d-current component.

[0033] According to the invention, a motor control circuit of an electrically commutated motor with motor windings is also proposed for monitoring at least one intermediate circuit capacitor in an electrical intermediate circuit of the motor control circuit of the EC motor, which is operated at a voltage source, preferably according to a method according to the above disclosure. The motor control circuit comprises a rectifier and an inverter, in particular a frequency converter. The at least one intermediate circuit capacitor to be monitored is located between the rectifier and the inverter. Furthermore, a controller is provided for regulating a d-current component of an intermediate circuit current such that energy from the motor control circuit is stored in a magnetic field of the motor windings of the EC motor and / or the energy correspondingly stored from the motor control circuit in the magnetic field of the motor windings is discharged back into the motor control circuit.In addition, an evaluation circuit is provided for determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor from a determined intermediate circuit capacitance.

[0034] The features disclosed above can be combined as desired, as long as this is technically possible and they do not contradict each other.

[0035] Other advantageous developments of the invention are characterized in the subclaims or are presented in more detail below, together with the description of the preferred embodiment of the invention, with reference to the figures. They show: Fig. 1 a schematic diagram of a motor control circuit; Fig. 2 a time course of motor phase currents as well as a q-current component and a d-current component of the motor control circuit in the rotating coordinate system; Fig. 3 a time course of rectified input voltages and an intermediate circuit voltage as well as a d-current component and a motor current of the motor control circuit; Fig. 4 a temporal temperature curve of an inverter of the motor control circuit.

[0036] The figures are schematic examples. Identical reference numerals in the figures indicate identical functional and / or structural features.

[0037] In Fig. 1 shows a schematic circuit diagram of a motor control circuit 10 of an electrically commutated motor 4 with motor windings 41 for monitoring at least one intermediate circuit capacitor 1 in an electrical intermediate circuit of the motor control circuit 10 of the EC motor 4, which is operated at a voltage source 6. The motor control circuit 10 comprises a rectifier 2 and an inverter 3. Furthermore, the intermediate circuit capacitor 1 to be monitored is located between the rectifier 2 and the inverter 3. In addition, a PI controller 5 for controlling a d-current component i dof an intermediate circuit current is provided such that energy from the motor control circuit 10 is stored in a magnetic field of the motor windings 41 of the EC motor 4 and / or the energy stored from the motor control circuit 10 in the magnetic field of the motor windings 41 is discharged back into the motor control circuit 10. In addition, an evaluation circuit 7 is provided for determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor 1 from a determined intermediate circuit capacitance C ZK provided.

[0038] The motor control circuit 10 has a sensor system for operating the EC motor 4 and this sensor system is simultaneously used for power detection and / or voltage detection of the voltage ripple at the intermediate circuit capacitor 1 and / or for at least intermittent determination of the motor phase currents i a , i b , i c used in the operation of the EC motor 4.

[0039] Fig. 2 shows a time course of motor phase currents i a , i b , i c and a q-current component i q and a d-current component i d the motor control circuit 10 in the rotating coordinate system.

[0040] In Fig. 3 is a time course of rectified input voltages U1, U2, U3 and an intermediate circuit voltage U ZK and a d-current component i d and a motor current i Motor the motor control circuit 10 during the application of a method according to the invention.

[0041] Since the voltage ripple of the intermediate circuit voltage U ZKis to be increased briefly during regular motor operation, the energy stored in the magnetic field of the motor windings 41 must be sufficient for the continued operation of the motor, as well as for the feedback to the intermediate circuit capacitor 1, so that in the meantime no current is drawn from the voltage source 6. For this purpose, the d-current component i d controlled by means of the controller 5 in such a way that it has a sawtooth-shaped curve.

[0042] It can be seen that during the period t0 to t1 energy is stored in the magnetic field of the motor windings 41, since i d < 0 and diddt<0. To minimize the impact on motor operation, the slope of the edge during this period is selected so that this charging occurs continuously over more than one mains period. In the subsequent period, from t1 to t2, the d-current component i dwith a flank as steep as possible, so that i d < 0 and diddt>0 and energy from the motor windings 41 is fed back into the intermediate circuit. The d-current component i d regulated by the controller 5 to a predetermined negative setpoint.

[0043] To control the d-current component i d and q-current component i q It is necessary to measure the motor phase currents during operation. Since during regeneration the voltage at the intermediate circuit capacitor U ZKis greater than the voltage of the previously supplied input voltages U1, U2, U3, the diodes of rectifier 2 are blocked and no energy is drawn from voltage source 6 for a short time, see period t1 to t3. Thus, both the current and voltage curves of the intermediate circuit capacitor 1 are known, and the inverse capacitance value can be determined via the evaluation circuit 7 with the aid of an observation system.

[0044] In the method according to the invention, the motor phase currents i a , i b , i c and determining a d-current component i d for controlling and / or regulating the motor 4 in phasor notation in the rotating coordinate system. Furthermore, a voltage ripple of an intermediate circuit voltage U occurring at the intermediate circuit capacitor 1 is detected and / or measured. ZK . In addition, the d-current component i dby means of the controller 5 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, as well as regulating the d-current component i d by means of the controller 5, such that the energy stored in the motor windings is discharged back into the motor control circuit 10. When discharging the stored energy from the magnetic field of the motor windings 41 of the EC motor 4, the voltage ripple of the intermediate circuit voltage U ZK greater than the input voltage U1, U2, U3 of the voltage source 6, such that the rectifier diodes of the rectifier 2 close as intended to prevent energy absorption from the voltage source 6 for a defined period of time. In addition, the intermediate circuit capacitance C is determined. ZKof the intermediate circuit capacitor 1 and determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor 1 from the determined intermediate circuit capacitance C ZK by means of an evaluation circuit 7.

[0045] In this method, the d-current component i d for storing energy from the motor control circuit 10 in the magnetic field of the motor windings 41 is controlled by the controller 5 in such a way that: i L > 0 and diLdt>; or i L < 0 and diLdt<0. The d-current component i d for storing energy from the motor control circuit 10 in the motor windings 41 is controlled by the controller 5 in such a way that the storage takes place continuously over more than one mains period of the voltage source 6. In addition, the d-current component i dfor discharging the energy stored in the magnetic field of the motor windings 41 from the magnetic field of the motor windings 41 back into the motor control circuit 10 is regulated by means of the controller 5 in such a way that: i L > 0 and diLdt<0; or i L < 0 and diLdt>0.

[0046] The motor phase currents i a , i b , i c are determined at least intermittently during operation of the EC motor 4 by means of the sensors. Furthermore, to determine the intermediate circuit capacitance C ZK of the intermediate circuit capacitor 1 the power measurement and the voltage measurement at the intermediate circuit capacitor 1 to determine the voltage ripple.

[0047] In the observer system for determining the intermediate circuit capacitance C ZKThe DC link current is simulated from previously recorded input measurements of the motor control circuit, which are required anyway to control motor operation, such as the power drawn from the mains, the power output by the motor, the mains input voltage, the motor current, and the phase voltages of the motor. The DC link current is then multiplied by the inverse DC link capacitance and integrated to calculate an 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, creating an observer loop. This control loop adjusts the estimated DC link voltage to the measured DC link voltage and is thus able to estimate the DC link capacitance.

[0048] However, the observer is only activated while the voltage ripple of the intermediate circuit voltage U ZK is greater than the input voltage U1, U2, U3 of the voltage source 6. After that, deactivation takes place, whereby in particular the activation and / or deactivation are achieved by multiplying by 0 or within a control algorithm by not carrying out a calculation of the observer.

[0049] Alternatively, a maximum value of the voltage ripple and / or a comparison between a recorded voltage curve and a look-up table will be used to indicate a decrease in the intermediate circuit capacitance C ZK or an increase in the internal loss resistance of the intermediate circuit capacitor 1.

[0050] In addition, it is possible to set a setpoint for the d-current component i dwith a superimposed PI controller. A difference between the target DC link voltage and the measured DC link voltage U is applied to an input of controller 5. ZK Furthermore, a step-like excitation to the measured intermediate circuit voltage U ZK added and / or a constant value is used, so that a course of the d-current component i d formed by controller 5.

[0051] In Fig. 4 shows a temporal temperature profile of the inverter 3, in particular a power module of the inverter 3, of the motor control circuit 10.

[0052] Since the short-term increase in the magnitude of the d-current component i dAs more power is introduced into the EC motor 4 by the inverter 3, the temperature of the module is briefly increased by the method according to the invention. In a further variant, the time of injection of the d-current component i d deliberately chosen so that the aging of the inverter 3 can be reduced. Since the inverters 3, in particular their power modules, usually age with an increasing number of temperature cycles or jumps, for example, a temperature prediction of a superimposed neural network or a temperature model, as described in Fig. shown, can be used to detect temperature jumps, such as between the times t D1 and t D2 shown. If then between the times t D1 and t D2 a larger d-current component i d set by the controller 5, the inverter 3 passes through the temperature curve T Dinstead of the temperature curve T N . The number of temperature cycles during operation of the inverter 3 is thus reduced and the service life increased. It is also conceivable that the magnitude and the temporal progression of the d-current component i d be selected so that inverter 3 specifically goes through a certain temperature curve without affecting the motor running.

[0053] Accordingly, in one embodiment of the method, a temperature prediction of a temperature of the inverter 3 is determined, in particular by means of a superimposed neural network and / or a temperature model, for predicting temperature jumps, wherein the correspondingly predicted temperature jumps are controlled by a d-current component i d by means of the controller 5, in particular an increase of the d-current component i d , at least partially compensated.

[0054] The invention is not limited to the preferred embodiments described above. Rather, a number of variants are conceivable that utilize the solution presented even in fundamentally different embodiments.

Claims

[1] Method for monitoring at least one intermediate circuit capacitor (1) in an electrical intermediate circuit of a motor control circuit (10) of an electrically commutated motor (4) with motor windings (41) operated at a voltage source (6), comprising a rectifier (2) and an inverter (3), wherein the at least one intermediate circuit capacitor (1) to be monitored is located between the rectifier (2) and the inverter (3), which comprises the steps: a. Determining the motor phase currents (i a , i b , i c ) and determining a d-current component (i d ) for controlling and / or regulating the engine (4); b. Detecting and / or measuring a voltage ripple of an intermediate circuit voltage (U ZK ); c. Control of the d-current component (i d) by means of a controller (5) such that energy from the voltage source and / or motor control circuit (10) is stored in a magnetic field of the motor windings (41) of the EC motor (4); d. Control of the d-current component (i d ) by means of the controller (5), such that the energy stored in step c) is discharged back into the motor control circuit (10), in particular the at least one intermediate circuit capacitor (1); wherein, upon discharging the stored energy from the magnetic field of the motor windings (41) of the EC motor (4), the intermediate circuit voltage (U ZK ) is greater than an input voltage (U1, U2, U3) of the voltage source (6), such that rectifier diodes of the rectifier (2) block as intended in order to prevent energy absorption from the voltage source (6) for a defined period of time, e. Determine an intermediate circuit capacitance (C ZK ) of the intermediate circuit capacitor (1); f. Determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor (1) from the determined intermediate circuit capacitance (C ZK ) by means of an evaluation circuit (7), 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 windings (41) is controlled by means of the controller (5) in such a way that: i L > 0 and diLdt>0; or i L < 0 and diLdt<0. [2] A method according to claim 1, wherein the d-current component (i d ) for storing energy from the motor control circuit (10) in the motor windings (41) is controlled by means of the controller (5) in such a way that the storage takes place continuously over more than one mains period of the voltage source (6). [3] A method according to claim 1 or 2, wherein the d-current component (i d) for discharging the energy stored in step c) in the magnetic field of the motor windings (41) from the magnetic field of the motor windings (41) back into the motor control circuit (10) is controlled by means of the controller (5) in such a way that: i L > 0 and diLdt<0; or i L < 0 and diLdt>0. [4] A method according to claim 3, wherein the d-current component (i d ) after discharge is regulated by means of the controller (5) to a predetermined negative setpoint value or to the setpoint value 0 A. [5] Method according to one of claims 1 to 4, wherein the d-current component (i d ) is controlled by means of the controller (5) in such a way that it has a sawtooth-shaped curve and / or an at least partially sinusoidal curve and / or a rectangular jump-shaped curve. [6] Method according to one of the preceding claims, wherein the motor control circuit (10) has a sensor system for operating the EC motor (4) and this sensor system is simultaneously used for power detection and / or voltage detection of the voltage ripple at the intermediate circuit capacitor (1) and / or for at least intermittent determination of the motor phase currents (i a , i b , i c ) is used in the operation of the EC motor (4), wherein in particular the motor phase currents (i a , i b , i c ) during operation of the EC motor (4) are determined at least intermittently. [7] Method according to claim 6, wherein for determining the capacity (C ZK ) of the intermediate circuit capacitor (1) the power measurement and the voltage measurement at the intermediate circuit capacitor (1) are carried out to determine the intermediate circuit capacitance. [8] Method according to one of the preceding claims, wherein the determination of the intermediate circuit capacitance (CZK ) by an observer system, whereby preferably the observer is only activated during the voltage ripple of the intermediate circuit voltage (U ZK ) is greater than the input voltage (U0) of the voltage source (6), wherein in particular a deactivation takes place thereafter, wherein in particular the activation and / or deactivation are achieved by a multiplication by 0 or within a control algorithm in that a calculation of the observer is not carried out. [9] Method according to one of the preceding claims, wherein a maximum value of the voltage ripple and / or a comparison between a detected voltage curve and a look-up table are used to indicate a decrease in the intermediate circuit capacitance (C ZK ) or an increase in an internal loss resistance of the intermediate circuit capacitor (1). [10] Method according to one of the preceding claims, wherein a setpoint value for the d-current component (i d ) is calculated with a superimposed PI controller, wherein a difference between the target intermediate circuit voltage and the measured intermediate circuit voltage is switched to an input of the controller (5), wherein in particular a step-like excitation to the measured intermediate circuit voltage (U ZK ) is added and / or a constant value is used, so that a curve of the d-current component (i d ) is formed by the controller (5). [11] Method according to one of the preceding claims, wherein a waveform of the d-current component (i d ) by means of a neural network and / or artificial intelligence and / or depending on the engine power and / or the speed during operation depending on the operating point. [12] Method according to one of the preceding claims, wherein a spectrum of a predetermined target current is adapted by means of pre-filtering, preferably by means of a low-pass filter, in order to avoid noise generation of the motor windings (41), wherein in particular parts of the spectrum and / or a required control reserve and / or a bandwidth are specifically reduced. [13] Method according to one of the preceding claims, wherein a temperature prediction of a temperature of the inverter (3), in particular of a power module of the inverter (3), is determined, in particular by means of a superimposed neural network and / or a temperature model, for predicting temperature jumps, wherein the correspondingly predicted temperature jumps are controlled by a d-current component (i d ) by means of the controller (5), in particular an increase of the d-current component (i d ), at least partially compensated. [14] Motor control circuit (10) of an electrically commutated (EC) motor (4) with motor windings (41) for monitoring at least one intermediate circuit capacitor (1) in an electrical intermediate circuit of the motor control circuit (10) of the EC motor (4) operated at a voltage source (6) according to a method of the preceding claims, comprising a rectifier (2) and an inverter (frequency converter) (3), wherein the at least one intermediate circuit capacitor (1) to be monitored is located between the rectifier (2) and the inverter (3), wherein a controller (5) is provided for regulating a d-current component (id) of an intermediate circuit current such that energy from the motor control circuit (10) is stored in a magnetic field of the motor windings (41) of the EC motor (4) and / or the energy correspondingly stored from the motor control circuit (10) in the magnetic field of the motor windings (41) is discharged back into the motor control circuit (10),wherein an evaluation circuit (7) for determining a remaining service life or an end of service life and / or useful life of the intermediate circuit capacitor (1) from a determined intermediate circuit capacitance (C, ZK ) is provided.

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