DE102019204934B4 - Method for controlling an electric motor, control device and corresponding systems - Google Patents

Abstract

Method for controlling an electric motor, in particular an electronically commutated electric motor, in which a speed (n) of the electric motor (M) is controlled to a desired speed by means of a speed controller (2), the desired speed being specified by a desired value, the electric motor (M) is loaded with a load moment (ML) during its operation, comprising the steps:detecting an actual motor current (Iact) which flows at the load moment (ML),detecting an actual speed (nact) at which a rotor of the electric motor is rotating (M) rotates,calculating a parameter based on the actual motor current (Iact) and the actual speed (nact), the parameter being representative of an operating point of the electric motor (M),determining a maximum speed (nmax) based on the Parameter and a characteristic, the characteristic indicating a dependence on a maximum current (Imax) and a maximum speed (nmax), generating a scaled setpoint by scaling the Setpoint based on the maximum speed (nmax) and inputs of the scaled setpoint into the speed controller (2), characterized in that the parameter is calculated as the quotient of the actual current (Iact) and the square of the actual speed (nact).

Description

The invention relates to a method for controlling an electric motor, in particular an electronically commutated electric motor (EC motor). The invention also relates to a corresponding control device for an electric motor, a system made up of such a control device and an electric motor, and a system made up of such a control device and a fan.

Electric motors generate drive torque during operation. This drive torque is usually counteracted by a load torque, with the load torque and drive torque being directed in opposite directions. If the drive torque is greater than the load torque, the electric motor will accelerate. If the drive torque is less than the load torque, the electric motor is braked. If the load torque and drive torque are the same, the speed remains constant so that there is a stable operating point. The load torque is usually generated by the element that is driven by the electric motor.

If the electric motor is part of a fan and the electric motor drives an impeller of the fan, the load moment is caused by the impeller and the gaseous medium - usually air - moved by the impeller. The ventilator conveys the gaseous medium from an inflow side to an outflow side, thereby generating an increase in pressure. Depending on the technology of the fan and the basic structure, the increase in pressure results in a volume flow of the gaseous medium moved by the fan. A mutual dependency of volume flow and pressure increase occurs, which can be described in a fan characteristic curve (also referred to as fan characteristic curve and also referred to below as pressure characteristic curve). The fan characteristic is specified in data sheets and measured by a fan manufacturer on a fan test bench. The measured fan is usually operated as a free-standing fan, so that the volume flow generated by the fan can flow unhindered. A real installation situation can influence this characteristic.

When a fan is installed in a system or device, the fan must work against a flow resistance. The operating point (also referred to as the operating point) of such a fan results from the intersection of the fan characteristic curve with the system/device characteristic curve. If there is a system/device with a high flow resistance, the system/device characteristic rises steeply.

With a low flow resistance, the system/device characteristic curve is flatter. At the operating point, the fan creates a pressure increase that exactly compensates for the pressure loss in the device or system.

It turns out that depending on the pressure increase at a specific drive torque and - because of the proportionality between drive torque and motor current - at a specific motor current, different speeds are set. Thus, in different situations in which the fan is used, depending on the flow resistance of the system in which the fan is installed, a pressure is set that determines the speed. If a maximum possible motor current flows, this results in a maximum possible speed for the current installation situation. Since system resistances can change over time in practice, for example due to increasing contamination of filters, the maximum possible speed can also change during operation. This means that the maximum possible speed for a specific fan or, more generally, for a specific electric motor cannot be determined without knowledge of the current application situation. However, parameters of this installation situation can only be determined with considerable effort, for example by using pressure sensors to determine the pressure difference.

When controlling the speed of a motor or a fan, it is common to assign a control signal (for example a voltage between 0 volts and 10 volts) specifying the speed setpoint to a range between minimum speed (for example 0 revolutions/minute) and maximum speed. However, since the maximum speed is unknown when the fan is manufactured, the difficulty arises in choosing a suitable setpoint value. If the setpoint specification is scaled to the lowest possible maximum speed, a large part of the possible working range would not be used in most installation situations, since the electric motor then - regardless of the actual installation situation - cannot be regulated beyond this lowest possible maximum speed. If the greatest possible maximum speed were used to scale the setpoint specification, this maximum speed would never be reached in most installation situations, which is why part of the value range of the setpoint is not exhausted. This is undesirable for control engineering reasons.

Because of these difficulties, speed control is usually dispensed with in practice and torque control is used instead. A speed is automatically set that corresponds to the system resistance and the motor current specifications via the control signal. This achieves the goal that, for example, with a control signal of 0 to 10 volts, the entire range up to the respective maximum speed can be covered.

Such a torque control can, for example, the U.S. 5,447,414A be removed. For this purpose, an electric motor of a fan is controlled in such a way that it generates a target torque to bring about a desired air flow C. The target torque is calculated from the desired air flow C, a measured speed S of the electric motor and constants K1 to K4. The target torque is linked to a measured voltage V at the electric motor and a measured current I in the electric motor via summing elements. The value obtained in this way is entered as a modified control signal in the inverter for the electric motor. Torque-speed characteristics are used to determine the constants K1 to K4.

However, doing without speed control leads to a number of disadvantages in practical operation. For example, start-up problems as a result of the electric motor getting stuck can be managed much better with a speed control, since the speed control - in contrast to the torque control - "follows". In addition, many engine properties are based on speed values, such as step operation or the hiding of certain speeds to avoid resonances. In addition, instabilities can occur in combination with upstream process controls due to the lack of speed control. This list of potential disadvantages, which is a non-exhaustive example, shows that speed control would be preferable to pure torque control.

Also in the document U.S. 2005/0 280 384 A1 describes a method and a control device for controlling the volume flow of a fan based on the torque

The present invention is therefore based on the object of designing and developing a method, a control device and systems of the type mentioned at the outset in such a way that a flexible speed specification based on an operating situation of the electric motor is possible.

• Erfassen eines Ist-Motorstroms, der bei dem Lastmoment fließt,
• Erfassen einer Ist-Drehzahl, mit der sich ein Rotor des Elektromotors dreht,
• Berechnen einer Kenngröße basierend auf dem Ist-Motorstrom und der Ist-Drehzahl, wobei die Kenngröße repräsentativ für einen Arbeitspunkt des Elektromotors ist,
• Ermitteln einer maximalen Drehzahl basierend auf der Kenngröße und einer Kennlinie, wobei die Kennlinie eine Abhängigkeit von einer maximalen Stromstärke und einer maximalen Drehzahl angibt,
• Erzeugen eines skalierten Sollwerts durch Skalieren des Sollwerts basierend auf der maximalen Drehzahl und
• Eingaben des skalierten Sollwerts in den Drehzahlregler,

wobei die Kenngröße als Quotient aus der Ist-Stromstärke und dem Quadrat der Ist-Drehzahl berechnet wird.According to the invention, the above object is achieved by the features of claim 1. Then, in the method under discussion, a speed of the electric motor is regulated to a desired speed by means of a speed controller, the desired speed being specified by a desired value, with the electric motor being loaded with a load torque during its operation. This procedure includes the steps:

• detecting an actual motor current flowing at the load torque,
• detecting an actual speed at which a rotor of the electric motor rotates,
• calculating a parameter based on the actual motor current and the actual speed, the parameter being representative of an operating point of the electric motor,
• Determining a maximum speed based on the characteristic and a characteristic curve, the characteristic curve indicating a dependency on a maximum current intensity and a maximum speed,
• Generating a scaled setpoint by scaling the setpoint based on the maximum speed and
• Entering the scaled setpoint in the speed controller,

where the parameter is calculated as the quotient of the actual current and the square of the actual speed.

• einen Drehzahlregler,
• einen Sollwerteingang zum Eingeben eines Sollwerts,
• einen Stromeingang zum Eingeben eines Ist-Motorstroms, der bei dem Lastmoment in den Elektromotor fließt,
• einen Drehzahleingang zum Eingeben einer Ist-Drehzahl des Elektromotors, mit der sich ein Rotor des Elektromotors dreht,
• einen Speicher mit einer Kennlinie, die eine Abhängigkeit zwischen einer maximalen Stromstärke und einer maximalen Drehzahl angibt,
• eine Berechnungseinheit, die dazu ausgebildet ist, basierend auf dem Quotienten aus dem Ist-Motorstrom und dem Quadrat der Ist-Drehzahl eine Kennzahl zu berechnen und basierend auf der Kennzahl und der Kennlinie eine maximale Drehzahl zu ermitteln, und
• eine Skalierungseinheit, die basierend auf der maximalen Drehzahl den Sollwert skaliert und als skalierten Sollwert an den Drehzahlregler ausgibt.

With regard to a control device, the above object is achieved by the features of claim 11. According to this, the control device for the electric motor, which is subjected to a load torque during its operation, comprises:

• a speed controller,
• a setpoint input for entering a setpoint,
• a current input for inputting an actual motor current that flows into the electric motor at the load moment,
• a speed input for inputting an actual speed of the electric motor, at which a rotor of the electric motor rotates,
• a memory with a characteristic that indicates a relationship between a maximum current and a maximum speed,
• a calculation unit which is designed to calculate an index based on the quotient of the actual motor current and the square of the actual speed and to determine a maximum speed based on the index and the characteristic curve, and
• a scaling unit that scales the setpoint based on the maximum speed and outputs it to the speed controller as a scaled setpoint.

With regard to the systems, the above object is achieved by the features of claims 15 and 17. Accordingly, these systems include a control device according to the invention and an electric motor or a fan.

In a manner according to the invention, it has first been recognized that a parameter that is representative of an operating point of the electric motor in the current operating situation can be determined based on an actual motor current and an actual speed. This parameter can in turn be used to determine a characteristic curve that indicates a relationship between a maximum current and a maximum speed. By using this knowledge, a maximum speed of the current operating point can be determined and thus a scaled setpoint can be generated without this maximum speed actually being present and/or the maximum motor current actually having to flow.

In order to use this knowledge, in the method according to the invention, an actual motor current is first detected, which flows during operation of the electric motor when loaded with a load torque. Furthermore, an actual speed is recorded, with which a rotor of the electric motor rotates under the load of the load moment. Based on this actual motor current and this actual speed, a parameter that is representative of the current operating point of the electric motor is calculated in a next step. As explained above, the operating point is dependent on the actual motor current flowing or the drive torque generated thereby and the load torque. In a further step, a maximum speed is determined based on the parameter using a characteristic curve that indicates a relationship between a maximum current intensity and a maximum speed. Finally, the maximum speed is used to generate a scaled setpoint. This scaled setpoint is finally entered into the speed controller. In this way, the setpoint value that was originally entered can be defined completely independently of the actual operating situation of the electric motor. In general, for example, a value between 0 and 100% can be specified. The speed that this ultimately corresponds to is determined by the scaling to the maximum speed. In this way, the speed controller is always subjected to a setpoint that lies between a minimum speed value and the maximum speed value. As a result, the value range for the setpoint specification can always be fully utilized.

In principle, the parameter can be formed in a wide variety of ways, as long as the parameter is dependent on the actual motor current and the actual speed and a maximum speed can be determined therefrom. The parameter must be representative of the operating point of the electric motor, which occurs when the load torque is applied. According to the invention, the parameter is calculated as the quotient of the actual current and the square of the actual speed. This choice allows a particularly favorable parameter to be obtained, particularly in the case of an electric motor which is part of a fan and drives an impeller of this fan. Because with fans, the motor current is proportional to the square of the speed.

In principle, the load torque applied to the electric motor can be generated in a wide variety of ways. As long as the rotor of the electric motor is loaded in a defined manner by the load moment, the prerequisites for using the method according to the invention are in principle met. In a preferred embodiment, however, the load moment is generated by an impeller of a fan, with the electric motor driving this impeller. Since the impeller will move an air flow, a load torque that depends on the speed is generated. In this case, the load moment is preferably influenced by a flow resistance which opposes an air flow moved through the impeller. The higher the flow resistance, the higher the load moment.

If the electric motor is part of a fan, it is basically irrelevant how the fan is constructed. The validity of the method according to the invention could be proven for axial fans, for diagonal fans and for radial fans (with forward-curved blades on the impeller), so that these represent preferred configurations.

The characteristic used to calculate the maximum speed can be defined in a variety of ways. For example, the characteristic curve can be defined via a mathematical relationship in the form of a formula, which means that relatively any values of the characteristic curve can be calculated very easily by solving the formula. In a preferred embodiment, however, the characteristic curve is stored in a memory as a table of values (also known as a look-up table) and an interpolation is carried out when the maximum speed is determined. In this way, even mathematically complex relationships can be mapped with sufficient precision without the need for complex calculations.

The setpoint value is preferably scaled to a range between a minimum speed and the maximum speed. The minimum speed is particularly preferably equal to zero revolutions per minute. In principle, it is irrelevant how the entered setpoint is scaled. Ultimately, relatively any behavior can be mapped via scaling. Scaling with an exponential relationship is just as conceivable as quadratic or polynomial relationships. However, an essentially linear scaling is preferably carried out, i.e. a range of the input reference value is mapped to a range of the scaled reference value by means of a linear relationship. It is also conceivable that speeds to be omitted are also taken into account in the setpoint scaling, so that when an impermissible setpoint speed is entered, a scaled speed is output which is a differential value Δn above or below the entered speed value, with Δn indicating a small speed difference and preferably based on the gap to be left out and/or the maximum speed. Thus, in a preferred embodiment, Δn is at most 10% of the maximum speed, particularly preferably at most 5% of the maximum speed and very particularly preferably at most 1% of the maximum speed. However, it should be ensured that the shifted scaled setpoint is outside of a correspondingly "forbidden" range.

In principle, the actual motor current and the actual speed can be determined in a wide variety of ways. It is thus conceivable that a dedicated current sensor is present for determining the actual motor current, which current values are recorded and made available—preferably in digitized form—for further processing in the method according to the invention. A dedicated speed sensor, which detects the speed of the rotor or of an element connected to the rotor, can be provided to determine the actual speed. Corresponding current or speed sensors are well known from practice. In the case of electronically commutated motors in particular, however, motor electronics are present which already contain information about the actual motor current and the actual speed. In a preferred development, the step of determining an actual motor current includes the step of querying this value from the motor electronics of the electric motor and the step of determining the actual speed includes a step of querying this value from the motor electronics of the electric motor. In this way, the actual motor current and/or the actual speed can be determined in a particularly simple manner, without the need for additional sensors.

If the electric motor drives an impeller of a fan, the method according to the invention can additionally be used to determine a volume flow generated by the fan, a mass flow moved by the fan and/or a pressure increase generated by the fan. Since the parameter is representative of the operating point of the fan and the operating point can be found in a pressure characteristic of the fan, a volume flow and/or an increase in pressure can be determined in this way. If the pressure characteristic shows a relationship between mass flow and pressure increase, a mass flow moved by the fan and/or a pressure increase generated by the fan can be determined in this way. In this way, the fan can also be used as a sensor.

The pressure characteristic can in turn be stored in different ways. Since the pressure characteristic is usually a relationship that is complicated to describe, the pressure characteristic is preferably stored in the form of a value table (look-up table). A volumetric flow and/or a mass flow and/or a pressure increase based on a speed or a motor current is preferably specified in the table of values. The volume flow and/or the mass flow is preferably normalized to the speed and the pressure increase to the motor current. In order to determine the volumetric flow, the mass flow and/or the pressure difference, a value would then be read from the value table or entered calculated an interpolated value between two existing values in the value table. This read out or interpolated value would then - depending on the reference value of the value - be multiplied by the actual speed or the actual motor current. In this way, an additional effect can be created, namely that the electric motor becomes a sensor for volume flow, mass flow and/or pressure increase.

In a development, another additional effect can be generated by passing on one or more values obtained during the method to a user or a higher-level system. For example, the user/higher-level system can be informed of the current maximum speed. If the method according to the invention is used to determine a volume flow, a mass flow and/or a pressure increase, these results can also be output to the user/superordinate system. How these values are output depends on the respective application. It would be conceivable that the device processing the method, for example a microprocessor, is connected to a display device and that the values are displayed on this display device or can be displayed there. However, it is also conceivable that a communication unit is present via which the values obtained are output. The communication unit can be designed in a wide variety of ways and a wide variety of communication standards and technologies can be used for data transmission from and to this communication unit. Digital transmission techniques can be used as well as analogue ones. Transmission can be wired or wireless. Parallel or serial transmission interfaces can be used. The transmission can take place in packets or in direct connections. The use of Bluetooth, Bluetooth LE (Low Energy), NFC (Near Field Communication), Ethernet, RS485, Modbus, Profibus, CAN bus or USB (Universal Serial Bus) is referred to purely by way of example, but not limited to these. In this case, the communication unit particularly preferably provides a connection to an Industry 4.0 environment.

The characteristic curve, which indicates a dependency on a maximum current intensity and a maximum speed, can be obtained in a wide variety of ways. In one embodiment, the electric motor in which the method according to the invention is carried out is measured in a test stand. This can be done, for example, as part of a final inspection of the electric motor after it has been manufactured. Since electric motors are usually subjected to a final test anyway, the characteristic curve can be determined without too much additional effort. Since the same models of an electric motor are very similar in terms of their behavior, it is also conceivable that an electric motor of the same type is measured and this characteristic curve is then distributed to other electric motors of the same type that have been produced. If the calculations require particular precision, the characteristic curve can be determined in a further development during commissioning of the electric motor in a system, i.e. in its subsequent installation environment. This configuration offers the advantage that influences from the installation environment flow directly into the characteristic curve.

When determining the characteristic curve, a maximum permissible motor current (and thus a maximum possible drive torque) can be impressed into the electric motor and the load torque can be increased in steps up to a maximum value. If the electric motor is part of a fan and drives its impeller, this can be achieved by gradually increasing a flow resistance for an air flow conveyed by the fan, so that there is a gradually increasing pressure.

In principle, only one characteristic curve can be used in the method according to the invention. This is particularly easy to do when the actual motor current and/or the actual speed can be determined relatively precisely. However, it is also conceivable to make the characteristic available as a family of characteristics, with the family of characteristics consisting of a plurality of individual characteristics. In this case, preferably between 5 and 50 individual characteristic curves, very preferably between 10 and 20 individual characteristic curves, are used. The family of characteristics can then be used to determine the maximum speed. The use of a family of characteristic curves is particularly recommended when there are deviations from the maximum characteristic curve at lower speeds or motor currents due to the characteristics of the fan, the system or the measuring device. The use of a family of characteristics increases the accuracy. In the application, the maximum performance of a fan is often not used. For better accuracy in such cases, a map can provide improvements.

The control device according to the invention is designed for controlling an electric motor, in particular an electronically commutated motor, which is loaded with a load torque during its operation. The control device is preferably suitable for carrying out a method according to the invention. The control device according to the invention includes a speed controller, a setpoint input, a current input, a speed input, a memory with a characteristic curve, and a calculation unit and a scaling unit. The setpoint input is designed for entering a setpoint. The current input is designed to input an actual motor current that flows at the load moment. The speed input is designed to input an actual speed of the electric motor, with which a rotor of the electric motor rotates. The memory stores a characteristic that indicates a relationship between a maximum current and a maximum speed. The calculation unit is designed to determine a maximum speed based on the actual motor current, the actual speed and the characteristic. The scaling unit is designed to scale the setpoint entered into the setpoint input based on the maximum rotational speed and to output it to the rotational speed controller as a scaled setpoint.

The control device can be implemented in a wide variety of ways. The functions can be implemented as software, as hardware or as a combination of software and hardware. In a preferred embodiment, the control device is implemented in a suitably programmed microprocessor with associated input interfaces.

The memory can also be implemented in many different ways. However, the memory is preferably in the form of a non-volatile memory. Such a non-volatile memory can be, for example, an EEPROM (Electronically Erasable Programmable Read-Only Memory), a flash memory, an NVRAM (Non-Volatile Random Access Memory) or another semiconductor memory.

The speed controller can also be implemented in various ways. It is important that the speed controller acts on the electric motor in such a way that its speed is brought as close as possible to an entered desired value. How the controller is structured in detail depends on the expected control behavior. Correspondingly suitable approaches are known from practice.

In order to avoid or reduce undesired fluctuations in the scaled rotational speed over a period of time, a smoothing unit for smoothing fluctuations over a period of time can be coupled in between the calculation unit and the scaling unit. A maximum speed value calculated by the calculation unit would then be input into this smoothing unit. The smoothing unit would smooth the input maximum speed value over time and output the smoothed maximum speed value to the speed controller. The smoothing unit can be designed as an integrator.

In principle, the speed controller can output a manipulated variable directly to engine electronics, which then generates and outputs control signals for the electric motor based on this manipulated variable. In a preferred development, however, the manipulated variable output by the speed controller is input to a subordinate control device, with the subordinate control device preferably controlling the torque of the electric motor. In this way, an even more targeted regulation of the electric motor can take place.

In general, the inputs of the control device, i.e. in particular the setpoint input, the current input and/or the speed input, can be designed in different ways. So it is conceivable that the inputs are formed by memory cells of a memory in which an “external” unit stores the values to be entered and from which the control device reads these values. The inputs can also be formed by (parallel or serial) bus lines into which the respective values are entered using logic levels. The individual inputs can also be designed differently. With the setpoint input, the setpoint can be entered as an analog voltage that assumes a value between a minimum voltage and a maximum voltage. In the area of controlling electric motors, a voltage between 0 volts and 10 volts has become established for entering setpoints. Therefore, in a preferred embodiment, the setpoint input is designed such that a setpoint can be entered by such a control signal. The control signal can indicate a percentage of the maximum speed. For example, a voltage of 8.5 volts would correspond to a target value of 85% of the determined maximum speed.

The control device according to the invention can form a system according to the invention together with an electric motor or a fan. In a single fan system, the electric motor would drive an impeller of the fan, with the controller controlling the electric motor. In both cases, the electric motor can be designed as an electronically commutated electric motor and thus include motor electronics. In this case, the control device can be implemented in the motor electronics.

• 1 ein Blockdiagramm einer Drehmomentregelung gemäß Stand der Technik,
• 2 eine Tabelle mit Kenndaten eines aus dem Stand der Technik bekannten Ventilators,
• 3 ein Blockdiagramm eines Ausführungsbeispiels eines erfindungsgemäßen Systems mit einer Drehzahlregelung mit adaptivem Sollwertbereich und
• 4 ein Flussdiagramm für ein zweites Ausführungsbeispiel eines erfindungsgemäßen Verfahrens mit einer Bestimmung eines Volumenstroms und/ oder einer Druckerhöhung.

There are now various possibilities for embodying and developing the teaching of the present invention in an advantageous manner. For this purpose, on the one hand, reference is made to the claims subordinate to the independent claims and, on the other hand, to the following explanation of preferred exemplary embodiments of the invention with reference to the drawing. In connection with the explanation of the preferred exemplary embodiments of the invention based on the drawing, preferred configurations and developments of the teaching are also explained in general. Show in the drawing

• 1 a block diagram of a torque control according to the prior art,
• 2 a table with characteristics of a fan known from the prior art,
• 3 a block diagram of an embodiment of a system according to the invention with a speed control with adaptive setpoint range and
• 4 a flowchart for a second exemplary embodiment of a method according to the invention with a determination of a volume flow and/or a pressure increase.

1 shows a block diagram with a torque control of an electric motor known from the prior art. A torque controller A outputs a manipulated variable to power electronics B, on the basis of which the power electronics generate and output control signals for an electric motor M, in particular an electronically commutated electric motor (EC motor). An actual motor current I, which is entered into a summing element C, is detected between the power electronics B and the electric motor M. The summing element C forms a difference between a specified setpoint value D and the actual motor current I. This difference signal is input into the torque controller A as a control difference. The torque controller A is constructed in such a way that the control difference is brought to zero or at least close to zero. The setpoint D can be set using a control signal, which assumes a value between 0 V and 10 V.

If the electric motor drives the impeller of a fan and the air moved by the fan encounters flow resistance in a system in which the fan is installed, such a control device automatically adjusts a speed according to the system resistance and the current specification via the control signal and in the installation environment, the goal is achieved that the 0-10V control signal corresponds to the range from 0 to maximum speed.

In 2 an example of characteristics of a fan is given, which is known from the prior art and which can be controlled in such a torque control. The characteristics come from a centrifugal fan with forward-curved impeller blades. In the individual columns of the table 2 the input current of the motor electronics I_L1, the power factor cos_phi_ed, the active power P, the speed n, the static pressure increase by the fan p_fs, the dynamic pressure p_f, the motor current and the volume flow q_V1 transported by the fan are specified, with one line each for a measurement is equivalent to.

3 shows an embodiment of a system according to the invention with a control device 1 according to the invention and an electric motor M. This control device 1 includes a speed controller 2, a setpoint input 3, a current input 4, a speed input 5, a maximum speed determination 6 and a scaling unit 7. The maximum speed determination 6 includes a memory with a characteristic (symbolized with a table) and a calculation unit (not shown separately). The speed controller 2 outputs a control signal that is entered into a subordinate torque control 8, which is in 1 is equivalent to. A determined actual motor current I act _is entered into the current input 4, a determined actual speed n _act into the speed input 5. The actual motor current I and the actual speed n act _can be obtained and output by motor electronics of the electric motor M . The actual motor current I and the actual speed n _ist are entered into the maximum speed determination 6 . A parameter is determined there based on the actual motor current I and the actual speed n _ist . This parameter is calculated as the quotient of the actual motor current I and the square of the actual speed n. This parameter is used to determine a maximum permissible speed nmax, using a characteristic curve calculated with the same formula. The characteristic curve results from the characteristic data 2 as follows: n _max reference [1/min] I _max [A] I _max / n _max ² 1 1529 0.71 3.04E-07 2 1625 0.71 2.69E-07 3 1677 0.71 2.52E-07 4 1743 0.71 2.34E-07 5 1823 0.71 2.14E-07 6 1915 0.71 1.94E-07 7 2030 0.71 1.72E-07 8th 2104 0.71 1.60 E-07 9 2198 0.71 1.47E-07 10 2308 0.71 1.33E-07 11 2391 0.71 1.24E-07 12 2499 0.7 1.12E-07

These values of the parameter I _max /n _max ² together with associated speed and motor current values can be stored in the memory as a look-up table and form a characteristic in the sense of the method according to the invention. The parameter is constant for a stable operating point. Due to the quadratic relationship between current and speed for a fan, corresponding maximum speeds for other operating points can be calculated from the maximum values (max. possible operating point).

Since the look-up table will only contain comparatively few values, an interpolation can be used. With the actual value of the parameter I/n ² one looks for the next larger and smaller value of the parameter I/n ² in the value table. Assuming the current value of the parameter I/n ² is 2.26 E-07. This current value is therefore between table entry 4 (range start) and 5 (range end), the maximum speed accordingly in the range between 1743 and 1823 rpm. A normalizing factor F can be calculated with these values: $$\text{f}=1-\left(\left({\text{I}}_{\text{is}}/{\text{<figure-callout id="2" label="n" filenames="DE102019204934B4\_0005.png" state="{{state}}">n</figure-callout>}}^2{}_{\text{is}}-\text{end of range}\right)/\left(\text{range start}-\text{end of range}\right)\right).$$

This factor is in the range from 0 to 1. In this case, it should be noted that the characteristic I/n ² behaves in the opposite direction to the maximum speed, which is already taken into account when calculating F. This factor F can then be used to interpolate between the speeds as follows: $${\text{n}}_{\text{Max}}-\text{reference}={\text{n}}_{\text{Max}}-\text{range start}+\text{f}*\left({\text{n}}_{\text{Max}}-\text{end of range}-{\text{n}}_{\text{Max}}-\text{range start}\right).$$

With the example value 2.26 E-07, you get a factor F = 0.4, which results in an nmax reference of 1775 rpm.

A value for the maximum speed nmax obtained by determining the maximum value 6 is transferred to the scaling unit 7 . This uses the value nmax received to scale a setpoint entered into setpoint input 3. A scaled desired value generated in this way is output to an adder 9, which determines a difference between the scaled desired value and the actual _speed nactual. The difference thus determined is entered into the speed controller as the control difference.

When processing the method according to the present embodiment, after determining the actual motor current I _act and the actual speed n act _, a current parameter I _act /n _act ² from the detected actual motor current I _act and the detected actual speed n _is calculated. An n _max reference value for the maximum speed of the current operating point of the fan is determined from this parameter by means of interpolation from the characteristic curve. This n _max reference value can be smoothed using an integrator in order to be able to compensate for fluctuations over time. This (possibly smoothed) value for the maximum speed nmax is used in the scaling unit 7 to scale the setpoint input entered into the setpoint input 3 . The setpoint specification is a control signal that can assume a voltage between 0 and 10 volts.

The exemplary embodiment shown here uses the knowledge of the working point to determine the permissible maximum speed nmax for the working point. The background to this is that an electronically commutated motor (EC motor) only has a certain maximum torque available. At low pressures, ie high volume flows, the speed must be set lower than at high pressures and low volume flows, otherwise the fan will be overloaded or permanently limited, which has a negative effect on the control behavior. In this exemplary embodiment, the method is used to design the control behavior in an ideal manner at each operating point and to always achieve the greatest possible volume flow at the respective operating point.

In a current EC motor design, a drive unit (power electronics) is integrated in a permanent magnet synchronous motor (PMSM). The core of the motor control works with a field-oriented current control. For this type of motor control, the exact motor currents and the position of the rotor and therefore also the speed must be known. These two values can be used to determine the operating point using the method according to the invention.

4 shows a flowchart of a second embodiment of a method according to the invention. In step 10, the parameter I/n ² _is calculated using current measured _values for the actual motor current I actual and the actual speed n actual. In step 11, the area on the maximum fan characteristic is determined with the determined I/n ² . Since the fan characteristic curve is stored as a table of values with only a few characteristic values in this exemplary embodiment, the next larger or smaller value for the current characteristic variable I/n ² is determined from the table in this step for the following interpolation. In the following step 12, an interpolation is carried out between these two values (see description of the 3 ). In step 12, the operating point is determined from the characteristic by interpolation. In step 13, a Q _v /n, p/l or an nmax is determined for the operating point determined. In step 14, a current volume flow Q _v or a current pressure increase p is calculated from the values determined in this way by multiplying them by the actual speed or the actual motor current. These values can then be output to a user or communicated to a higher-level system.

With regard to further advantageous configurations, to avoid repetition, reference is made to the general part of the description and to the appended claims.

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

Reference List

A

torque control

B

power electronics

C

summing element

D

setpoint specification

M

engine

1

control device

2

speed controller

3

setpoint input

4

power input

5

speed input

6

Maximum speed determination

7

scale unit

8th

torque control

9

summing element

10

Step

11

Step

12

Step

13

Step

14

Step

Claims (17)

Method for controlling an electric motor, in particular an electronically commutated electric motor, with a speed (n) of the electric motor (M) being controlled to a desired speed by means of a speed controller (2), with the desired speed being specified by a desired value, with the electric motor (M) is loaded during its operation with a load moment (M _L ), comprising the steps: detecting an actual motor current (I _ist ) flowing at the load moment (M _L ), detecting an actual speed (n _ist ) with which a rotor of the electric motor (M) rotates, calculating a parameter based on the actual motor current (I _ist ) and the actual speed (n _ist ), the parameter being representative of an operating point of the electric motor (M), determining a maximum speed (nmax) based on the parameter and a characteristic, the characteristic indicating a dependence on a maximum current (Imax) and a maximum speed (nmax), generating a scaled desired value dur ch scaling of the reference value based on the maximum speed (nmax) and inputs of the scaled reference value into the speed controller (2), characterized in that the parameter is the quotient of the actual current (I _act ) and the square of the actual speed (n _is ) is calculated. procedure after claim 1 , characterized in that the load moment (M _L ) is generated by an impeller of a fan, which is driven by the electric motor (M), wherein the load moment (M _L ) is preferably influenced by a flow resistance, which is an air flow moved through the impeller opposes. procedure after claim 1 or 2 , characterized in that the characteristic curve is stored as a table of values in a memory and that the maximum speed (nmax) is determined by means of interpolation. Procedure according to one of Claims 1 until 3 , characterized in that the step of scaling comprises scaling the target value to a range between a minimum speed (nmin) and the maximum speed (nmax), the minimum speed (nmin) preferably being equal to 0. Procedure according to one of Claims 1 until 4 , characterized in that the step of determining an actual motor current (I _ist ) and/or the step of determining an actual speed (n _ist ) comprises a step of requesting values from motor electronics of the electric motor (M). Procedure according to one of Claims 1 until 5 , characterized in that the electric motor (M) drives an impeller of a fan and in that a volume flow (Q _v ) moved by the fan, a mass flow moved by the fan and/or a volume flow moved by the fan from the parameter using a pressure characteristic curve of the fan generated pressure difference (p) is determined. procedure after claim 6 , characterized in that the pressure characteristic is stored in the form of a table of values, with a volume flow (Q _v ), a mass flow and/or a pressure difference (p) relating to a speed (n) or to a motor current (I) being specified in the table of values is, and that when determining the volume flow (Q _v ), the mass flow and/or the pressure difference (p), a value read from the table of values or a value interpolated between two values of the table of values with the actual speed (n _ist ) or the actual -Motor current (I _ist ) is multiplied. Procedure according to one of Claims 1 until 7 , characterized in that the maximum speed (nmax) and/or other values determined while the method is being processed is output to a user and/or to a higher-level system. Procedure according to one of Claims 1 until 8th , characterized in that the characteristic curve is determined in a calibration measurement in a test stand and/or during commissioning of the electric motor in a system. Procedure according to one of Claims 1 until 9 , characterized in that the characteristic is made available as a family of characteristics, the family of characteristics consisting of a plurality of individual characteristics, preferably between 5 and 50 individual characteristics, very particularly preferably between 10 and 20 Individual characteristics, is formed, and that when determining the maximum speed, the family of characteristics is used. Control device for an electric motor, in particular an electronically commutated motor, in particular for carrying out a method according to one of Claims 1 until 10 , wherein the electric motor is loaded with a load torque (M _L ) during its operation, comprising a speed controller (2), a setpoint input (3) for entering a setpoint, a current input (4) for entering an actual motor current (I _ist ), which flows at the load torque (M _L ), a speed input (5) for inputting an actual speed (n _ist ) of the electric motor (M) at which a rotor of the electric motor rotates, a memory with a characteristic curve that shows a dependency between a maximum current (I _max ) and a maximum speed (n _max ), a calculation unit that is designed to do this based on the quotient of the actual motor current (I _act ) and the square of the actual speed (n _act ) a To calculate index and based on the index and the characteristic to determine a maximum speed (n _max ), and a scaling unit (7), based on the maximum speed (n _max ) scales the setpoint and as a scaled setpoint to the speed oil controller (2) outputs. control device claim 11 , characterized in that a smoothing unit is coupled between the calculation unit and the scaling unit (7), the smoothing unit preferably being designed as an integrator and the smoothing unit being designed to smooth a value of the maximum speed (nmax) calculated by the calculation unit over a period of time and output it to the speed controller as a smoothed value. control device claim 11 or 12 , characterized in that the speed controller (2) outputs a manipulated variable which is entered into a subordinate control device, the subordinate control device preferably controlling the torque (8) of the electric motor (M). Control device according to one of Claims 11 until 13 , characterized in that the setpoint input (3) is designed to receive the setpoint by means of a control signal, the control signal preferably being between a minimum voltage and a maximum voltage, particularly preferably between 0 V and 10 V. System consisting of a control device (1) according to one of Claims 11 until 14 and an electric motor (M). system after claim 15 , characterized in that the electric motor (M) has motor electronics and that the control device (1) is implemented in the motor electronics. System of a control device according to one of Claims 11 until 14 and a fan, the fan comprising an electric motor (M) and an impeller driven by the electric motor (M), and the control device (1) controls the electric motor (M).

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