DE102024125311B3 - Inverter and method for controlling an electric motor, as well as working machine - Google Patents

Abstract

An inverter for controlling an electric motor, preferably an electric motor of a machine, is disclosed, wherein the inverter comprises an inverter input (4), an inverter output (5), an inverter control unit (9), and an inverter output stage (8). The inverter control unit (9) is configured to generate at least one control signal and to output the at least one control signal to the inverter output stage (8), wherein the at least one control signal has a control signal frequency (f _st ). The inverter output stage (8) is configured to generate at least one inverter output voltage from at least one inverter input voltage applied to the inverter input (4) based on control signals received from the inverter control unit (9) and to output it to the inverter output (5) for controlling the electric motor (3). Furthermore, the inverter control unit (9) has a current input (10) configured to receive an instantaneous current value, wherein the instantaneous current value is representative of an instantaneous current delivered to the electric motor (3). The inverter control unit (9) is configured to adjust the control signal frequency ( _fst ) of the at least one control signal based on an instantaneous current value input to the current input (10), wherein the inverter output voltage is formed by a single-phase or multi-phase, time-varying voltage, and the inverter control unit (9) is configured to perform several adjustments of the control signal frequency (fst) within one period of the inverter output voltage.
Furthermore, a corresponding machine and a corresponding method are disclosed.

Description

The present disclosure relates to an inverter and a method for controlling an electric motor, preferably an electric motor of a working machine, as well as a corresponding working machine.

Machines such as pumps, compressors, agitators, hoists, fluid power machines, conveyor systems, and the like often incorporate an electric motor to drive the machine. These electric motors are typically controlled by an inverter, which generates one or more alternating voltages with variable amplitude and/or frequency and outputs them to the electric motor. This can, for example, generate a rotating magnetic field in the stator of the electric motor, which in turn sets the rotor in motion. By combining an electric motor with an inverter, drives are created that can be flexibly controlled and regulated.

Such inverters typically have an inverter input, an inverter output, an inverter control unit, and an inverter output stage. The inverter control unit generates at least one control signal and outputs this signal to the inverter output stage. Each control signal has a specific frequency. Using the control signal(s) received from the inverter control unit, the inverter output stage generates at least one inverter output voltage from at least one inverter input voltage applied to the inverter input and outputs this voltage to the electric motor via the inverter output. Multiple inverter output voltages can form a multi-phase voltage system, such as a three-phase system.

The inverter output stage typically includes several power semiconductors that play a crucial role in generating the inverter output voltage. At least one control signal then defines how the semiconductor switching elements are activated, such that the switching frequency of the semiconductor switching elements correlates with the control signal frequency. In at least some inverter topologies, control signals from the inverter control unit can be used directly (possibly via drivers or other amplifier stages) to control the power semiconductors. In other inverter topologies, the control signals are further processed.

Determining the control signal frequency of at least one control signal is not a trivial task. A low control signal frequency, for example in the range below a few kilohertz, often offers the advantage of keeping the switching losses of the power semiconductors low. This is because switching losses are usually approximately proportional to the control signal frequency. On the other hand, this can lead to pronounced deviations of the motor current from an ideal curve and significant harmonic content, which regularly results in increased losses and uneven torque in the electric motor. Furthermore, acoustic emissions with clearly perceptible components in the audible range can occur. A higher control signal frequency, for example above 10 kilohertz, offers the advantage that the motor current is closer to an ideal curve and the harmonic content in the inverter output voltage can be reduced. This usually reduces torque ripple and losses in the electric motor. However, this also increases the switching losses in the inverter output stage. It is evident that these conflicting requirements cannot be met simultaneously.

In practice, the control signal frequency is therefore set in a range where the negative effects of the dimensioning on all components involved in the inverter-electric motor system or the driven machine are minimized and/or where predefined operating parameters can be met, for example, regarding heat dissipation, maximum torque ripple, maximum acoustic emission, etc. Common control signal frequencies lie between 2 and 16 kilohertz. At these frequencies, the power loss in the inverter, the power loss in the electric motor, the torque ripple, the acoustic emission, and other parameters are typically within an acceptable range. However, in practice, this dimensioning rarely represents more than a compromise.

Printed matter DE 10 2022 125 169 A1 This describes a motor control system. The motor control system includes an inverter that is controlled by a switching module. The switching module is controlled by a target frequency module that sets a target frequency for the inverter. To determine the Target frequency takes motor phase currents into account. (From the printed document) DE 10 2012 212 318 A1 A method for reducing the noise of a vacuum pump is known. In this method, the switching frequency of a frequency converter is adjusted to reduce pump noise. The motor current is taken into account to determine the switching frequency. (Publication) DE 690 03 777 T2 This section deals with a pulse-width modulation (PWM) inverter. The inverter's switching frequency is adjusted based on the motor current.

The present disclosure is based on the objective of providing an inverter, a driven machine, and a method that enable more efficient operation of the electric motor and the driven machine, respectively. It is advantageous if the power loss of the inverter-electric motor system can be reduced.

This task is solved by the combinations of features in the dependent claims. Further embodiments are disclosed in the respective subclaims.

It should be noted that the features listed individually in the claims can be combined with one another in any technically meaningful way (even across category boundaries, for example between method and apparatus) and reveal further embodiments. The description further characterizes and specifies the present disclosure, particularly in conjunction with the figures.

It should also be noted that the conjunction “and/or” used herein, which stands between two characteristics and links them together, is always to be interpreted in such a way that in a first embodiment only the first characteristic can be present, in a second embodiment only the second characteristic can be present, and in a third embodiment both the first and the second characteristic can be present.

It should further be noted that the present disclosure – despite the subsequent use of the term “inverter” – is not limited to units that generate one or more alternating voltages from a direct voltage. Rather, the configuration of the inverter input voltage is not decisive for the present disclosure. Thus, the inverter input voltage can comprise one or more direct voltages as well as one or more alternating voltages. This applies accordingly to the claims.

It has been recognized that more efficient operation is possible when the control signal frequency is not set to a constant, but rather adjusted according to the current operating condition of the electric motor. The instantaneous current has proven to be one of the most important parameters for the current operating condition and can be used to set the control signal frequency. Therefore, according to the present disclosure, the inverter control unit has a current input into which an instantaneous current value can be entered. Based on a received instantaneous current value, the inverter control unit then adjusts the control signal frequency according to predefined rules. In this way, a control signal frequency can be set that is appropriate to the current operating condition of the electric motor. This can increase the efficiency of the inverter-electric motor system. Since high currents occur primarily during start-up or acceleration of the machine in many application scenarios, and not as a continuous condition, the dimensioning of the control signal frequency does not need to cover all possible operating conditions simultaneously, but can be better adapted to the current operating condition.

It is important to note that the "instantaneous current value" is not an average value over one or more periods of an inverter output voltage. Rather, the instantaneous current value should reflect the actual current flowing at a given moment. Since the instantaneous current value also changes continuously with an inverter output voltage, this likely means that a new instantaneous current value should be entered into the current input relatively frequently. "Relatively frequently" means that an instantaneous current value is entered multiple times during one period of an inverter output voltage. In one embodiment, at least 5 instantaneous current values are entered into the current input per period, in another embodiment at least 10 instantaneous current values, and in yet another embodiment at least 20 instantaneous current values. The input of the instantaneous current value is preferably periodic.

The method used to obtain the instantaneous current value is irrelevant. The instantaneous current value should simply reflect the actual situation at the moment it is input to the current input with sufficient accuracy. The instantaneous current value can be derived, for example, from a measurement, an estimate, or from other quantities, such as the phase shift in conjunction with known motor parameters. Suitable methods for determining an instantaneous current value are well-established in practice.

It is also not crucial how the instantaneous current value is represented. The instantaneous current value can be an absolute value. However, it is also conceivable that relative values are used, for example, representing an instantaneous current relative to a nominal current or a maximum permissible current. The only important factor is that the inverter control unit can correctly interpret an input instantaneous current value and draw the correct conclusions for setting the control signal frequency.

The following considerations can play a role in dimensioning a relationship between instantaneous current value and control signal frequency.

The switching losses in the inverter output stage are calculated – using the example of an IGBT (Insulated Gate Bipolar Transistor) with an antiparallel diode as the power semiconductor in a symmetrically loaded two-level topology – in a simplified manner according to: $$P_{Schaltverlu}=\left(E_{IGBT,EIN}+E_{IGBT,AUS}+E_{Diode,EIN}+E_{Diode,AUS}\right)\cdot f_{st}\cdot n_{IGBT}$$

In this context, E _..,ON and E _...,OFF describe the energy required to switch the IGBT or diode on or off, f _is the control signal frequency, and n _IGBT is the number of power conductors in the inverter output stage.

Thus, the switching losses are proportional to the control signal frequency f _{_st} . Simultaneously, the switching energies E_{_...,ON} and E_{_...,OFF,} respectively, depend on the instantaneous current flowing at each stage. Therefore, it can be advantageous to choose a lower control signal frequency when the instantaneous current flowing at each stage is high. For other power semiconductors, other control methods, and other topologies, different relationships apply, but these still depend on the control signal frequency.

On the other hand, current harmonics are generated during the production of the inverter output voltage. Their proportions are high at relatively low control signal frequencies (e.g., 1 or 2 kilohertz) and lower at higher control signal frequencies (e.g., 16 kilohertz). These harmonics have different effects on the electric motor. Firstly, the harmonics generate magnetic fields in the iron core, causing iron losses and thus heating the core. Secondly, the current harmonics flow through the winding conductors, generating ohmic losses there. Furthermore, the harmonics cause torque ripple, which can lead to vibration losses. At the same time, the current harmonics also depend on the instantaneous current delivered to the electric motor. The higher the instantaneous current delivered to the electric motor, the higher the current components in the harmonics. Overall, a high proportion of harmonics is therefore undesirable, which is why a high control signal frequency at a high instantaneous current can be advantageous.

The effects of the control signal frequency on the inverter and the electric motor can vary considerably, as factors such as the inverter topology, the type of power semiconductors, the motor type, and others can influence the losses in the individual components of the inverter-electric motor system. For example, some inverter-electric motor systems may tolerate high control signal frequencies at high instantaneous currents without problems, while others may benefit from a lower control signal frequency at high instantaneous currents. In one configuration, a high control signal frequency is selected for low instantaneous current values, and a low control signal frequency for high instantaneous current values. Another configuration uses a low control signal frequency for low instantaneous current values and a high control signal frequency for high instantaneous current values. Crucially, the control signal frequency must be adjusted to the respective instantaneous current value. This typically means that numerous adjustments to the control signal frequency are made every minute or even every second.

What "high" or "low" specifically means in this context depends on various factors. One of these factors is likely the specific design of the inverter-electric motor system. For example, the specific power semiconductor used in the inverter output stage can determine whether the control signal frequency is "high" or "low." If the power semiconductor is an IGBT, a low control signal frequency can be a few kilohertz, for example, 1 or 2 kilohertz, while a high control signal frequency is 16 kilohertz or higher. If a SiC FET (Silicon Carbide Field Effect Transistor) is used as the power semiconductor, the control signal frequencies can be significantly higher. A low control signal frequency can be in the range of 10 kilohertz or a few tens of kilohertz, while a high control signal frequency can be up to 100 kilohertz or more. These few examples demonstrate that a "low" or "high" control signal frequency can depend on the power semiconductor, particularly its switching capabilities and respective power losses. Similar dependencies can exist for other aspects of the technology used. For instantaneous current, the rated current can serve as a reference for a "high" or "low" instantaneous current value. For example, a range between 80% and 100% of the rated current can be considered a high instantaneous current, and a range between 0% and 20% of the rated current a low instantaneous current value. These examples are intended to facilitate the understanding of the terms "high" and "low" but do not limit their application to the aspects mentioned.

Between the boundary values, i.e., between "low" and "high" control signal frequencies, relatively arbitrary intermediate current values can be assumed. Various relationships can apply, such as a linear relationship. However the relationship between instantaneous current value and control signal frequency is specifically defined, the inverter control unit should be given clear rules for how this assignment should be made.

The at least one "control signal" can be formed by a wide variety of signals. The essential requirement for the control signal(s) is that they have a control signal frequency and influence the generation of the inverter output voltage in the inverter output stage. These requirements can be met by a wide variety of signals. Such a control signal can be designed to directly drive power semiconductors in the inverter output stage, possibly with the interposition of a driver or other amplifiers and/or impedance converters. It is also conceivable that a control signal is converted before driving the power semiconductors. As long as the control signal contains a control signal frequency and this frequency influences the generation of the inverter output voltage, such a signal can constitute a control signal within the meaning of this disclosure.

The "control signal frequency" generally describes the reciprocal of the control signal's period. The period describes the time interval until the control signal reaches a comparable state again, such as a zero crossing or a rising or falling edge. The period is unlikely to be constant but will vary, particularly when the control signal frequency is adjusted. For such control signals, the (non-constant) time interval between two adjacent comparable states can also be called an interval, so the control signal frequency would be the reciprocal of this interval. The control signal frequency influences the frequency with which power semiconductors in the inverter output stage are switched. The control signal frequency can directly correspond to the frequency of switching events. However, it is also possible that there are multiple switching events per control signal period, or even multiple control signal periods per switching event. In one configuration, one or more of the control signals have different control signal frequencies. In another configuration, each control signal has the same control signal frequency. In practice, this design in particular should result in especially simple tax conditions.

The present disclosure can be used in connection with a wide variety of working machines and electric motors. The term "working machine" refers to a complete system that may include electrical and mechanical components. The working machine converts input electrical energy into another form of energy, such as kinetic energy, pressure, or other forms of energy. Examples of such working machines include pumps, compressors, agitators, hoists, fluid energy machines, conveyors, and fans. Examples of possible electric motors include permanent magnet synchronous machines (PMSM), synchronous reluctance machines (SRM), permanent magnet synchronous reluctance machines (PMASynRM), and asynchronous machines (ASM). This list The examples given are merely illustrative and not exhaustive, and are not intended to limit the present disclosure to these examples. It is important to note that electrical energy input to an inverter input is fed by the inverter into the electric motor and used to drive the working machine. Particularly preferred embodiments of working machines, however, are pumps and compressors.

The term "inverter" can, in principle, encompass everything required to generate an inverter output voltage for driving an electric motor from an inverter input voltage. This can include power semiconductors, filters, amplifiers, drivers, and safety circuitry, as well as rectifiers, DC link capacitors, mains filters, microprocessors, memory, DC/DC converters for internal power supply, ammeters, voltmeters, input/output interfaces, etc. This list is neither exhaustive nor limiting.

In simplified terms, an inverter has at least one inverter input, one inverter output, one inverter control unit, and one inverter output stage with the functions described in the introduction. An inverter input voltage can be applied to the inverter input to supply the inverter and the electric motor being driven. This "inverter input voltage" can be a DC voltage, possibly from a DC link, or a single-phase or multi-phase AC voltage. Three-phase AC is one example of a multi-phase AC voltage. The inverter output voltage is typically an AC voltage, which can be part of a multi-phase voltage system. For the purposes of this discussion, "AC voltage" generally refers to a voltage with a non-constant amplitude, which can be unipolar or bipolar, have a variable frequency, and exhibit various waveforms. The AC voltage can be neutral or have a non-zero DC value. In some cases, the aim is to generate the inverter output voltage(s) as an approximately sinusoidal AC voltage. In other cases, the inverter output voltage(s) is generated as a rectangular voltage. However, the inverter output voltage can also be triangular or trapezoidal, to name just a few other possible voltage waveforms. It goes without saying that the inverter input and output should be matched to the respective input and output voltages. This may also mean that additional circuitry is present at the inverter input or output, such as filters for electromagnetic compatibility, overvoltage protection, or overcurrent protection.

The inverter itself can have a wide variety of topologies. Examples, but not limited to, include 2-level inverters, multi-level inverters, soft-switching topologies with switching points at zero voltage (ZVS), and soft-switching topologies with switching points at zero current (ZCS). In principle, the present disclosure can be used with all these and other topologies.

The inverter output stage can comprise one or more power semiconductors, which can be connected in various configurations. One or more inverter branches can be used as a half-bridge or full-bridge. The power semiconductor(s) can be formed in a variety of ways. Various transistor designs are common, in particular MOSFET (Metal Oxide Semiconductor Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor), SiC-FET (Silicon Carbide Field Effect Transistor), and GaN-FET (Gallium Nitride Field Effect Transistor). The transistor control can also vary and can be adapted to the specific modulation type used. Space vector modulation, sine wave modulation, and discontinuous (flat-top) modulation are mentioned here as examples only, without being exhaustive or limiting.

The inverter control unit can be designed in a variety of ways. It can be used to control important functions of the inverter and/or be part of a control loop. In the latter case, the inverter control unit participates directly or indirectly in the control process. A key task of the inverter control unit is to generate and output control signals that instruct the inverter output stage to generate and output the inverter output voltage.

The inverter can be implemented in various ways. In one design, the inverter is implemented entirely in hardware. In another design, the inverter is implemented through a combination of hardware and software, which allows for, in particular, an adaptation of... The adjustment of the control signal frequency to instantaneous current values is made more flexible. The hardware can include power semiconductors, amplifiers, sensors, analog-to-digital converters, filters (e.g., high-pass, low-pass, band-pass filters), processor(s) (e.g., microcontrollers, digital signal processors, or ASICs (application-specific integrated circuits)), and/or a programmable logic circuit (e.g., an FPGA (field-programmable gate array) or CPLD (complex programmable logic device)). Furthermore, one or more memory modules can be present, such as RAM (random access memory), ROM (read-only memory), or flash memory, which other hardware components can access. Software can control the individual components of the inverter. Depending on the inverter implementation, the different units can also be configured differently.

The inverter output voltage is generated by a single- or multi-phase, time-varying voltage, and the inverter control unit is configured to perform multiple adjustments of the control signal frequency (f _{_st} ) within one period of the inverter output voltage. In one embodiment, the inverter control unit is configured to perform a minimum number of adjustments of the control signal frequency within one period of the inverter output voltage, wherein the minimum number of adjustments is preferably at least 5, more preferably at least 10, and particularly preferably at least 20. It can be assumed that, prior to each adjustment, a current instantaneous current value is input into the current input or calculated from one or more previous instantaneous current values (for example, by extrapolation or estimation from one of the preceding periods). In multi-phase voltage systems, it can generally be assumed that each of the inverter output voltages has an identical period length. A good adaptation to the instantaneous current can be achieved by making a minimum number of adjustments to the control signal frequency during one period of the inverter output voltage. Using at least 5 adjustments allows for good adaptation while maintaining manageable resource requirements. With a minimum of 10 or more adjustments, the adaptation to the instantaneous current can be more precise. With a minimum of 20 or more adjustments, the adaptation to the instantaneous current can be even more precise.

In one embodiment, the inverter control unit is configured to adjust the control signal frequency in response to a changing instantaneous current value, preferably in steps. It is assumed that an instantaneous current value that has remained unchanged since the last adjustment of the control signal frequency does not require further adjustment. The same applies if the instantaneous current value has changed by less than a predefined threshold. In this embodiment, an adjustment is made when the instantaneous current value has changed over time. Adjusting in steps offers the advantage of minimizing the number of changes. After the instantaneous current value changes by a predefined threshold, the control signal frequency can be adjusted by a predefined step. The size of this step can depend on external factors, such as the current instantaneous current.

In one embodiment, the inverter control unit is configured to adjust the control signal frequency in the same or opposite direction to changes in the instantaneous current value. An adjustment in the same direction means that the control signal frequency is increased when the instantaneous current value increases and decreased when the instantaneous current value decreases. An adjustment in the opposite direction means that the control signal frequency is decreased when the instantaneous current value increases and increased when the instantaneous current value decreases. These changes are always relative to previous values, particularly the immediately preceding values. An adjustment in the same direction can be advantageous in inverter-electric motor systems where the switching losses in the inverter output stage are less problematic than the effects of harmonics or torque ripple in the electric motor. An adjustment in the opposite direction can be advantageous in inverter-electric motor systems where reducing switching losses at high currents is more beneficial than reducing current harmonics.

In one embodiment, the inverter control unit is configured to adjust the control signal frequency such that a change in the instantaneous current value exhibits a linear, quadratic, polynomial, exponential, hyperbolic, or other functional relationship with a change in the control signal frequency. Utilizing a defined relationship between a change in the instantaneous current value and a change in the control signal frequency enables defined and reproducible adjustments of the control signal frequency. The control signal frequency adjustment can be pre-calculated and stored in memory. This minimizes the load on the inverter control unit's processing resources. Alternatively, the control signal frequency can be recalculated as needed. This is advantageous when the inverter control unit has sufficient processing resources but limited memory capacity. The specific method used depends on the individual characteristics of the inverter-electric motor system.

In one embodiment, the inverter control unit is configured to receive at least one loss parameter and adjust the control signal frequency such that the power loss represented by this parameter is reduced and/or minimized. This allows for particularly efficient operation of the inverter, the electric motor, and/or the driven machine as a whole. In principle, the loss parameter can be anything representative of power loss. Examples include the active power currently consumed by the electric motor, the reactive power currently present, power components in harmonics, or torque ripple.

In one embodiment, the inverter output voltages form a multiphase voltage system, wherein each phase of the multiphase voltage system is configured to drive a motor phase connection, wherein the current input is configured to receive an instantaneous current value that is representative of the sum of the magnitudes of the instantaneous currents in all phases, and wherein the inverter control unit is configured to adjust the control signal frequency for all phases. Such an approach is particularly suitable for inverter topologies in which the control signals for each of the inverter output voltages have an identical control signal frequency. Commonly used H-bridges are examples of this. The sum of the magnitudes of the instantaneous currents provides a good measure of the current flow through the inverter output stage into the electric motor.

In one embodiment, the inverter output voltages form a multiphase voltage system, wherein each phase of the multiphase voltage system is configured to drive a motor phase connection, wherein the current input is configured to receive an instantaneous current value for each phase, and wherein the inverter control unit is configured to adjust the control signal frequency for the phase corresponding to each instantaneous current value. This approach is applicable to inverter topologies in which each inverter output voltage can be generated with an independently adjustable control signal, and in particular, where the multiple control signals can have different control signal frequencies. This embodiment allows for particularly flexible adjustment of the control signal frequency.

In one embodiment, the inverter additionally includes at least one current measuring device, wherein the current measuring device is communicatively connected to the current input and wherein the current measuring device is configured to measure an instantaneous current value and to input the measured instantaneous current value into the current input. This allows an instantaneous current value to be obtained in a simple and resource-efficient manner and entered into the inverter control unit.

In one embodiment, the current meter measures an instantaneous current value for each inverter output voltage and/or an instantaneous current value from an intermediate circuit of the inverter. By measuring the instantaneous current value for each inverter output voltage, a detailed picture of the electric motor's current consumption can be obtained. Measuring an instantaneous current value is particularly advantageous when the sum of the instantaneous current values is used to adjust the control signal frequency.

In one embodiment, the inverter output stage comprises several power semiconductors, and the control signals are each configured to directly control one of the several power semiconductors, with the control signals preferably being pulse-width modulated signals. In this way, influencing the control signal frequency has a particularly clear and reproducible effect on the inverter output voltage. This is especially simple when using pulse-width modulated signals.

In one embodiment, the inverter control unit is configured to perform frequency adjustment of the control signal frequency based on a nominal frequency, wherein the adjustment is preferably made as a percentage of the nominal frequency. The nominal frequency then forms a reference The reference frequency for changing the control signal frequency can be used in various ways. In one approach, the nominal frequency is a center frequency around which the control signal is located, for example, within a band from x% below to x% above this nominal frequency. Here, x is less than or equal to 50 in one approach, less than or equal to 25 in another, and less than or equal to 10 in yet another. In another approach, the nominal frequency lies at the lower or upper end of a band within which the control signal frequency is adjusted. Thus, the control signal frequency could, for example, be between x% and 100%, where x < 100, or between 100% and x%, where x > 100. In yet another approach, the nominal frequency is a frequency that the control signal frequency will not reach. This frequency could, for example, be a maximum permissible frequency for the inverter. The control signal frequency could then, for example, be between 30% and 80% of the nominal frequency. Using a percentage of the nominal frequency offers the advantage that the range of variation of the control signal frequency scales with the nominal frequency.

In one embodiment, the inverter control unit is configured to adjust the nominal frequency in addition to adjusting it based on the instantaneous current value. This nominal frequency adjustment is preferably based on the average current draw of the electric motor, temperature, electromagnetic emissions, and/or acoustic emissions. This allows for the inclusion of further parameters in the control signal frequency adjustment. For example, in the case of a high average current draw, the nominal frequency can be modified to further reduce power loss, for instance, by lowering the nominal frequency. In addition to adjusting the control signal frequency as described in this disclosure, this enables particularly efficient operation of the inverter-electric motor system. Similarly, if electromagnetic emissions are too high in a certain frequency range, the nominal frequency could be varied at a lower frequency (e.g., 10 or 100 Hertz) to spread the interference spectrum. Excessive acoustic emissions can be reduced by increasing the nominal frequency and/or by avoiding resonance ranges.

In one embodiment, the electric motor is part of a working machine, the working machine is formed by a pump or a compressor, and the inverter is designed to control the electric motor of the pump or compressor.

• 1 ein Blockdiagramm mit Funktionsblöcken eines Ausführungsbeispiels eines Wechselrichters gemäß der vorliegenden Offenbarung,
• 2 Diagramme mit beispielhaften Verläufen eines idealisierten dreiphasigen Motorstroms, eines realen Motorstroms sowie einer Abweichung des realen Motorstroms von einem idealisierten Motorstrom, jeweils für eine Steuersignalfrequenz von 2 Kilohertz und 16 Kilohertz,
• 3 ein Diagramm mit einem beispielhaften Spektrum einer Abweichung eines realen Motorstroms von einem idealisierten Motorstrom bei einer Steuersignalfrequenz von 2 Kilohertz und 16 Kilohertz,
• 4 ein Diagramm mit einem idealisierten dreiphasigen Motorstrom, einer Summe der Beträge der Motorströme und einer angepassten Steuersignalfrequenz und
• 5 ein Flussdiagramm eines Ausführungsbeispiels eines Verfahrens zum Ansteuern eines Elektromotors gemäß der vorliegenden Offenbarung.

Further features and advantages of the present disclosure will become apparent from the following description of exemplary embodiments, which are not to be understood as limiting and are explained in more detail below with reference to the drawing. This drawing schematically shows:

• 1 a block diagram with functional blocks of an embodiment of an inverter according to the present disclosure,
• 2 Diagrams showing exemplary curves of an idealized three-phase motor current, a real motor current, and a deviation of the real motor current from an idealized motor current, each for a control signal frequency of 2 kilohertz and 16 kilohertz.
• 3 a diagram showing an exemplary spectrum of the deviation of a real motor current from an idealized motor current at a control signal frequency of 2 kilohertz and 16 kilohertz,
• 4 a diagram with an idealized three-phase motor current, a sum of the magnitudes of the motor currents and an adapted control signal frequency and
• 5 a flowchart of an embodiment of a method for controlling an electric motor according to the present disclosure.

In the different figures, parts that are equivalent in function are always provided with the same reference symbols, so that they are usually only described once.

1 Figure 1 shows a block diagram with functional blocks of an embodiment of an inverter 1 according to the present disclosure. The inverter 1 is supplied with an inverter input voltage from a power supply network 2 and drives an electric motor 3, which in turn drives at least part of a driven machine (not shown). The power supply network 2 is configured as a three-phase AC network and is connected to an inverter input 4 of the inverter 1. The electric motor 3 is connected to an inverter output 5 of the inverter 1. The inverter input voltage input to the inverter input 4 is first fed into an input stage 6. The input stage 6 includes line filters, for example, to reduce network feedback and to protect the inverter 1 from interference. Voltages and overcurrents, and a rectifier for rectifying the inverter input voltage. The output voltage of the input stage 6 is fed into a DC link 7, which includes, in particular, a DC link capacitor for smoothing and buffering the DC link voltage. The DC link voltage at the output of the DC link 7 is fed into an inverter output stage 8, which generates the inverter output voltages from the DC link voltage and outputs them to the inverter output 5. In generating the inverter output voltages, the inverter output stage 8 uses power semiconductors (not shown) and control signals. The control signals are generated by an inverter control unit 9 and output to the inverter output stage 8. The inverter control unit 9 has a current input 10, which is connected to a current meter 11. The current meter 11 detects an instantaneous current within the inverter 1. In this case, this can be the instantaneous current in the DC link 7 or at each phase of the inverter output. Furthermore, inverter 1 can include additional components, such as analog-to-digital converters, filters, a power supply for internal energy, antiparallel diodes of the power semiconductors, drivers, amplifiers, and safety circuitry. Since these components are not crucial in this case, they are omitted. 1 not shown.

The 2 to 4 The diagrams show examples of various quantities that can occur within an inverter-electric motor system. It is assumed that the inverter output stage comprises a full bridge with three branches and that the inverter outputs a three-phase voltage system to the electric motor.

In 2 The upper part shows an example of an ideal current I_{_id} . The values for phase U are indicated by a dot, the values for phase V by an unfilled square, and the values for phase W by a diamond. The current can, for example, have a peak value of ±9 amperes and a period T of 5 milliseconds, to name just one possible scale for the diagrams. In the middle part of the 2 Two exemplary waveforms of a real current I _{_re} for phase U are shown. One of the waveforms is shown with a solid line and represents a real current waveform at a control signal frequency of 16 kilohertz. The second waveform is shown with a dashed line and represents a current waveform at a control signal frequency of 2 kilohertz. In the lower part of the 2 The deviation I _{_re} - I_{_id} of the real current I _{_re} from the ideal current I _{_id} is shown. Again, the solid line represents a control signal frequency of 16 kilohertz, and the dashed line represents a control signal frequency of 2 kilohertz. At the lower control signal frequency, the deviations in this case are, for example, more than 1 ampere. At 16 kilohertz, the deviations are noticeably smaller.

3 The diagram shows an example spectrum of the deviation of a real motor current from an idealized motor current. Again, the dashed line represents the situation at a control signal frequency of 2 kilohertz, and the solid line represents a control signal frequency of 16 kilohertz. The dashed spectrum exhibits very pronounced spectral components at 2 kilohertz, which will lead to significant iron and copper losses and torque ripple in the electric motor. The peak at this frequency can be, for example, 0.35 amperes. It is clearly evident that the spectral components in the harmonics are significantly lower at higher control signal frequencies.

In the upper part of the 4 The ideal currents I _{_id,U} (line with points), I _{_id,V} (line with rectangles), and I _{_id,W} (line with diamonds) are shown again. In the middle part of the 4 The sum of the current values |I _id | for all three phases is shown. The dashed vertical lines indicate the minimum sum of the current values, the dash-dotted vertical lines the maximum currents. The sum of the current values |I _id | can be entered as an instantaneous current value into the current input 10 of the inverter control unit 9. In the lower part of the 4 An example of adjusting the control signal frequency f _{_st} is shown. The control signal frequency f _{_st} is adjusted inversely to the instantaneous current value: a minimum frequency f_{_min} is chosen at a current maximum, and a maximum frequency f _{_max} at a current minimum. The control signal frequency is adjusted linearly between the minimum frequency f _{_min} and the maximum frequency f _{_max} . The minimum frequency f _{_min} and the maximum frequency f_{_max} are symmetrical around a nominal frequency f_{_nom} , where, for example, the minimum frequency f_{_min} can be at 90% of the nominal frequency f_{_nom} , the maximum frequency f _{_max} at 110% of the nominal frequency f_{_nom} , and the nominal frequency f _{_nom} can be 16 kilohertz.

By using this adjustment of the control signal frequency f _{_st} , an instantaneous current value can be entered multiple times per period T of the inverter output voltage, and then the control signal frequency can be adjusted. This could be done, for example, at each minimum or maximum of the sum of the To adjust current values, an instantaneous current value would be entered 12 times, and the control signal frequency would be adjusted 12 times. It is also conceivable that an instantaneous current value could be entered between each minimum and each maximum. This would result in 24 adjustments. It is evident that this can be extended as needed. Furthermore, a temporal dependency between entering the instantaneous current value and adjusting the control signal frequency is not strictly necessary. Rather, this adjustment can be performed completely independently of specific points in the current flow to the electric motor, for example, 32 or 50 times per period T. The number of adjustments does not necessarily have to be a natural number. It is evident that a continuous adjustment of the control signal frequency f _{_st} , as in 4 This is not how it will be done in practice, as depicted. Instead, the control signal frequency f _st will be adjusted in stages depending on the time elapsed. It is also conceivable that a defined change in the sum of the current values triggers the input of an instantaneous current value to the current value input 10. For example, if the sum changes by 1 or 2 amperes, the current instantaneous current value can be entered into the current input and the control signal frequency f _st adjusted.

5 Figure 1 shows a flowchart of an embodiment of a method according to the present disclosure. For example, an inverter 1 can be used to carry out the method according to Figure 1. 1 The following steps are used: In step S1, the inverter control unit 9 first receives an instantaneous current value via its current value input 10. The instantaneous current value represents an instantaneous current supplied by the inverter 1 to the electric motor 3. This instantaneous current value can be an instantaneous current on a single phase of the inverter output voltage or the sum of the instantaneous currents on all phases of the inverter output voltage. In step S2, the inverter control unit 9 adjusts the control signal frequency f _{_st}. For this purpose, the inverter control unit 9 uses the received instantaneous current value and possibly other information, such as temperature, DC link voltage, user preferences, etc., to name just a few possible examples. In step S3, the inverter control unit 9 generates at least one control signal, at least one of which has the adjusted control signal frequency. In step S4, the at least one control signal generated in step S3 is output to the inverter output stage 8. Using this control signal, in step S5 at least one inverter output voltage is generated from at least one inverter input voltage applied to inverter input 4, which is then output to the electric motor 3 in step S6. The procedure can be executed iteratively, returning to step S1 after completing step S6 and starting a new iteration.

Regarding further advantageous embodiments, reference is made to the general part of the description and to the attached claims to avoid repetition.

Finally, it should be expressly pointed out that the exemplary embodiments described above serve only to illustrate the claimed teaching, but do not limit it to these exemplary embodiments.

Reference symbol list

1

inverter

2

energy supply network

3

electric motor

4

Inverter input

5

Inverter output

6

Rectifier / Input filter

7

Intermediate circle

8

Inverter output stage

9

Inverter control unit

10

Current value input

11

power meter

Claims (14)

Inverter for driving an electric motor, preferably an electric motor of a machine, comprising an inverter input (4), an inverter output (5), an inverter control unit (9), and an inverter output stage (8), wherein the inverter control unit (9) is configured to generate at least one control signal and to output the at least one control signal to the inverter output stage (8), wherein the at least one control signal has a control signal frequency (f _st ), and wherein the inverter output stage (8) is configured to generate at least one inverter output voltage from at least one inverter input voltage applied to the inverter input (4) based on control signals received by the inverter control unit (9) and to output it to the inverter output (5) for driving the electric motor (3), characterized in that the inverter control unit (9) has a current value input (10) configured to receive an instantaneous current value, wherein the instantaneous current value for a current output to the electric motor (3) instantaneous current is representative, and that the inverter control unit (9) is configured to adjust the control signal frequency (f _st ) of the at least one control signal based on an instantaneous current value input into the current value input (10), wherein the inverter output voltage is formed by a single-phase or multi-phase, time-varying voltage and the inverter control unit (9) is configured to make several adjustments of the control signal frequency (f _st ) within one period of the inverter output voltage. Inverters Claim 1 , characterized in that the inverter control unit (9) is designed to adjust the control signal frequency (f _st ) in the case of a time-changing instantaneous current value, wherein an adjustment of the control signal frequency (f _st ) preferably takes place in steps. Inverters after one of the Claims 1 until 2 , characterized in that the inverter control unit (9) is designed to adjust the control signal frequency (f _st ) in the same or opposite direction to a temporal change in the instantaneous current value. Inverters after one of the Claims 1 until 3 , characterized in that the inverter control unit (9) is configured to adjust the control signal frequency (f _st ) such that a temporal change in the instantaneous current value has a linear, quadratic, polynomial, exponential or hyperbolic relationship with a temporal change in the control signal frequency. Inverters after one of the Claims 1 until 4 , characterized in that the inverter control unit (9) is configured to receive at least one loss parameter and to adjust the control signal frequency (f _st ) such that a power loss represented by the loss parameter is reduced and/or minimized. Inverters after one of the Claims 1 until 5 , characterized in that the inverter output voltages form a multi-phase voltage system, wherein each of the phases of the multi-phase voltage system is configured to drive a winding of the electric motor (3), that the current value input (10) is configured to receive an instantaneous current value that is representative of the sum of the magnitudes of instantaneous currents in all phases, and that the inverter control unit is configured to adjust the control signal frequency (f _st ) for all phases jointly. Inverters after one of the Claims 1 until 6 , characterized in that the inverter output voltages form a multi-phase voltage system, wherein each of the phases of the multi-phase voltage system is configured to control a phase terminal of the electric motor (3), that the current value input (10) is configured to receive an instantaneous current value for each of the phases, and that the inverter control unit (9) is configured to adjust the control signal frequency (f _st ) for the phase belonging to each instantaneous current value. Inverters after one of the Claims 1 until 7 , which additionally comprises at least one current measuring device (11), wherein the current measuring device (11) is communicatively connected to the current value input (10) and wherein the current measuring device (11) is configured to measure an instantaneous current value and to input the detected instantaneous current value into the current value input (10). Inverters Claim 8 , characterized in that the current measuring device (11) measures an instantaneous current value for each inverter output voltage and/or an instantaneous current value from an intermediate circuit (7) of the inverter (1). Inverters after one of the Claims 1 until 9 , characterized in that the inverter output stage (8) comprises several power semiconductors and that the control signals are each configured to directly control one of the several power semiconductors, wherein the control signals are preferably formed by pulse width modulated signals. Inverters after one of the Claims 1 until 10 , characterized in that the inverter control unit (9) is configured to perform frequency adjustment of the control signal frequency (f _st ) based on a nominal frequency (f _nom ), wherein an adjustment preferably takes place as a percentage of the nominal frequency (f _nom ). Inverters Claim 11 , characterized in that the inverter control unit (9) is configured to perform an adjustment of the nominal frequency (f _nom ) in addition to the frequency adjustment based on the instantaneous current value, wherein the adjustment of the nominal frequency (f _nom ) is preferably based on an average current consumption of the electric motor (3), a temperature, an electromagnetic emission and/or an acoustic emission. Working machine, preferably a pump or a compressor, comprising an electric motor (3) driving the working machine and an inverter (1) controlling the electric motor, wherein the inverter according to one of the Claims 1 until 12 is trained. Method for controlling an electric motor, preferably an electric motor of a working machine, using an inverter, preferably an inverter according to one of the Claims 1 until 12 , wherein the inverter (1) comprises an inverter input (4), an inverter output (5), an inverter control unit (9), and an inverter output stage (8), comprising: receiving (S1) an instantaneous current value by means of a current value input (10) of the inverter control unit (9), wherein the instantaneous current value is representative of an instantaneous current delivered to the electric motor (3), adjusting (S2) a control signal frequency (f _st ), generating (S3) at least one control signal having an adjusted control signal frequency (f _st ), outputting (S4) the at least one control signal to the inverter output stage (8), generating (S5) at least one inverter output voltage from at least one inverter input voltage applied to the inverter input (4) based on the at least one control signal, wherein the inverter output voltage is formed by a single-phase or multi-phase, time-varying voltage, wherein within one period of the inverter output voltage several adjustments of the control signal frequency (f st ) occur _st ) are carried out, and output (S6) the at least one inverter output voltage via the inverter output (5) to the electric motor (3).