Method for controlling a pump drive of a centrifugal pump, and centrifugal pump

Abstract

The invention relates to a method for controlling a pump drive of a centrifugal pump, wherein the centrifugal pump comprises an electric drive motor that is controlled by means of frequency converters, and wherein a motor controller is provided which controls rotational speed and / or current and which specifies at least one input value for the PWM generator of the frequency converter per switching cycle of the frequency converter for the purposes of setting the motor voltage, characterized in that the motor controller generates the at least one input value for the PWM generator at an update rate lower than the switching frequency of the frequency converter, and the at least one input value is kept constant over at least two switching cycles of the frequency converter.

Description

[0001] KSB SE & Co. KGaA 67227 Frankenthal

[0002] Method for controlling a pump drive of a centrifugal pump and centrifugal pump

[0003] The invention relates to a method for controlling a pump drive of a centrifugal pump, wherein the centrifugal pump comprises an electric drive motor which is controlled by means of a frequency converter, and wherein a motor control is provided which carries out a speed and / or current control and provides the PWM generator of the frequency converter with an input value for setting the motor voltage per switching cycle of the frequency converter.

[0004] Modern centrifugal pumps, especially circulation pumps in closed circuits, are speed-controlled to operate the pump at the optimal operating point, depending on the current power requirements of the system, and especially in an energy-efficient manner. The electric drive motor is powered by a frequency converter with variable motor voltage to dynamically adjust the pump speed. The inverter, i.e., the controlled bridge circuit, comprises a multitude of power switches, such as IGBT modules, which are switched at a constant switching frequency. By modulating the pulse width of the switching signal, the generated output voltage, i.e., the motor voltage, can be varied.

[0005] The required switching signal (PWM signal) is generated by the frequency converter's PWM generator. The PWM generator uses a setpoint (input value) applied to the input for each switching cycle to set the PWM signal for the bridge switches.

[0006] The motor control receives a setpoint, for example, a speed specification, either from a user input or from a higher-level hydraulic controller. Based on the speed specification and the measured or estimated electrical motor variables, the control value is then generated, which is then made available to the PWM generator as a suitable input value. In addition to speed control, the motor control can also perform subordinate current control. The corresponding control value for the PWM generator must therefore be available and updated at the switching frequency of the frequency converter.

[0007] Switching operations of the frequency converter's built-in circuit breakers can be audible to the human ear and may be perceived by people as an unpleasant whistling noise. If the pumps are installed outside of the usual human habitation area, the acoustic impact is not significant. Otherwise, however, there is a desire to reduce the acoustic impact caused by the circuit breakers' whistling as much as possible.

[0008] The subjective noise of the frequency converter can be reduced by increasing the switching frequency to a frequency range beyond the human perceptible range. This frequency range varies depending on the installed hardware. However, experience has shown that for centrifugal pumps, especially circulating pumps, the switching frequency should be higher than 15 kHz. However, the switching frequency cannot be increased indefinitely, as an increasing number of switching operations also means higher switching losses. Due to the increased switching frequency, the motor control system must also calculate at a higher clock rate, which requires more powerful processing components.

[0009] Therefore, an optimized approach for controlling an electric drive motor of a centrifugal pump is sought for the initial problem described above. This problem is solved by a method according to the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims.

[0010] According to the invention, it is proposed that the at least one input value for the PWM generator is generated by the motor control system at an update rate that is lower than the switching frequency of the frequency converter. However, since at least one input value must be provided to the PWM generator for each switching cycle of the frequency converter, it is proposed to keep the at least one input value of the PWM generator constant over at least two switching cycles of the frequency converter. The motor control system therefore does not have to calculate the input value(s) at an update rate that corresponds to the switching frequency of the frequency converter; instead, the update rate can be reduced, thereby relieving the load on the motor control system and, if necessary, freeing up computing capacity of the motor control system that can be used elsewhere. The use of a control module with lower computing power is sufficient.

[0011] The centrifugal pump, for example, is a heating circulation pump. The installed electric drive motor can be a permanent magnet synchronous motor, although other motor types can also be used. The switching frequency of the frequency converter can be in the range between 14 kHz and 20 kHz. Experience has shown that switching noise in this frequency range is no longer perceptible to humans, or at least less disturbing. Ideally, the switching frequency should be between 15 kHz and 18 kHz. The switching losses occurring in this range are acceptable for pump applications.

[0012] A preferred variant provides for the update rate of the motor control to be selected so that it corresponds to half the switching frequency of the frequency converter, i.e. the motor control only provides the PWM generator with an updated input value every other switching cycle. In the specific example, this means that the motor control calculates an input value for the PWM generator based on the control strategy being pursued. The calculated input value is made available to the PWM generator for the next switching cycle, and the PWM signal for controlling the half-bridge is generated accordingly. However, no updated value is generated by the motor control for the next switching cycle; instead, the previously calculated input value is reused as the current input value for the PWM generator. As a result, the generated PWM signal (i.e. the pulse width) remains constant over two consecutive switching cycles.Only in the following step is the motor control executed again and calculates an updated input value for the PWM generator, so that the pulse width is then adjusted.

[0013] As already explained above, it can be particularly advantageous if the motor controller temporarily suspends the process for calculating the input value due to the lower update rate. The resources freed up in the motor controller's control module can then be used for alternative tasks. For example, it is conceivable that one or more optional monitoring functions are executed by the motor controller during a phase in which motor control is suspended. A specific example of optional functions is the execution of temperature monitoring of the motor and / or the motor controller and / or the frequency converter.Since direct temperature measurement is not always possible for thermal monitoring due to a lack of sensors, one or more thermal models can be used to estimate the temperature, particularly for estimating the temperature of the conveyed medium and / or the motor temperature (e.g., winding temperature). The computing capacity freed up by the method according to the invention can be used effectively to calculate corresponding thermal models.

[0014] However, alternative monitoring functions can also be implemented, for example for monitoring the motor current and / or the motor voltage. It is also conceivable to execute any optimization and / or calibration functions in order to check and / or adjust the current controller parameters of the motor control. However, the computing capacity freed up by temporarily suspending the motor control does not necessarily have to be used for other purposes; in principle, the measure according to the invention can also simply reduce the energy consumption of the motor control. Typically, a reduction in control quality is to be expected due to the reduced update rate of the motor control, since keeping the pulse width constant over several switching cycles of the frequency converter can lead to a certain distortion of the generated output signal, i.e. the generated voltage curve of the motor voltage.However, the inventors have recognized that a certain distortion of the output signal can be compensated for by the inductances of the motor used, so that at least with a limited reduction in the update rate, no or only minor restrictions in the control of the motor are to be accepted. The proposed method is particularly attractive for the specific pump application, since higher-level hydraulic controllers are typically used here, whose time constants are chosen to be significantly larger than the electrical time constants of the motor control. Due to this fact, the limitation or impairment of the control quality feared by the method according to the invention is negligible.

[0015] The at least one input value provided at the input of the PWM generator can be the so-called "duty cycle," i.e., the ratio of pulse width to period duration of the PWM signal. It is also conceivable that the at least one input value is at least a setpoint for the motor voltage.

[0016] For the concrete implementation of the method according to the invention, it can be advantageous to define a control cycle. For each control cycle, which corresponds to a defined period of time, a measurement procedure is executed at the beginning to measure and / or calculate or estimate one or more electrical drive variables relevant to the motor control. Based on the recorded drive variables, the motor control can calculate the updated input value for the PWM generator, which is made available to the PWM generator and, based on this, then generates and outputs the PWM signal for switching the power switches. In each control cycle, the measurement procedure as well as the generation and output of the PWM signal are preferably executed; the calculation of the at least one input value by the motor control can be suspended for every nth control cycle.

[0017] The implementation of the motor control can vary. Field-oriented control is preferred. By using a motor model, a sensorless motor can be used to determine the position and / or speed of the rotor based on the detected motor currents. The setpoint for the speed is preferably provided to the motor control by a higher-level hydraulic controller.

[0018] In addition to the method according to the invention, the present invention relates to a centrifugal pump comprising an electric drive motor, a frequency converter, and a control module for executing the motor control, wherein the control module is configured to implement the method according to the invention. However, the control module and frequency converter do not have to be a fixed or integral component of the pump, but can be a separate device that is merely communicatively connected to the pump. Accordingly, the invention also provides a system consisting of a centrifugal pump, a frequency converter, and a control module configured to implement the method.

[0019] Further advantages and features of the invention will be explained in more detail below using an exemplary embodiment illustrated in the figures. They show:

[0020] Figure 1 : a schematic representation of the embodiment of the method according to the invention,

[0021] Figure 2: the flow diagram of the method according to the invention and

[0022] Figure 3: An exemplary representation of the generated PWM signal for controlling the pump motor according to the method according to the invention. Figure 1 shows the design diagram of a possible embodiment of the method according to the invention. The exemplary embodiment involves a heating circulation pump with a permanent magnet synchronous motor. A higher-level hydraulic controller, e.g., a pressure or differential pressure controller, specifies a target speed to the motor controller, which then regulates the motor current and, using the power switches of the frequency converter, controls the motor voltage in order to regulate the target speed or the desired motor current. Even though the method is described in more detail below for a heating circulation pump, its application to various pump applications is conceivable.

[0023] The installed frequency converter, especially the IGBT module, should be operated at the highest possible switching frequency to minimize the acoustic disturbance to any people in the vicinity of the system. An integral PWM generator in the frequency converter generates the pulse width modulation based on a target motor voltage and switches the power switches of the bridge switches with the generated pulse duration. The IGBT module, i.e., the bridge circuit of the installed frequency converter, can be switched, for example, at a fixed switching frequency of approximately 16 kHz.

[0024] Figure 1 shows the duty cycle, hereinafter also referred to as the control cycle, of a pump microprocessor for implementing motor control. A duty or control cycle has a duration TR. At the beginning of the control cycle, a measurement procedure 10 is executed by the microprocessor to record electrical variables of the drive motor. Specifically, the motor currents are recorded here and made available for motor control. Block 20 corresponds to the execution of the calculation steps for the actual motor control by the microprocessor. The measured motor currents are processed, and the required voltage space vectors for the motor voltage are calculated using field-oriented control.Using a software model of the machine that is as realistic and accurate as possible, the motor control can estimate the parameters required for vector control, such as the rotor angle and / or the speed, thus enabling sensorless motor operation. After execution of the calculation process 20 of the motor control, the current voltage space vectors are available, which are then passed as input values ​​to the PWM generator. The process of setting the PWM generator is identified by reference numeral 30. The frequency converter's PWM generator generates the pulse width modulation based on the voltage values ​​in order to supply the motor with the desired motor voltage, i.e., the frequency converter's power switches are switched with the appropriate pulse width of the generated modulation signal.

[0025] At the end of the control cycle TR, a residual time Ti remains, which can be used in the microprocessor to execute further tasks. Depending on the performance of the processor chip, the computing time Ti can be longer or shorter. After the first control cycle has been completed, the measurement procedure is repeated at the beginning of the next cycle. However, according to the method according to the invention, the motor control 20 is suspended during this cycle, and the PWM generator is instead supplied with the same input value for the desired motor voltage. Consequently, the generated modulation signal, i.e., the pulse width, also remains constant.

[0026] By suspending the motor control 20, the computational effort is significantly reduced, so that the remaining time Tr at the end of the cycle is significantly longer and offers sufficient headroom for other pump control functions. As shown in Figure 1, the calculation by the motor control is suspended again in the next control cycle and the motor control 20 is not executed again until the fourth control cycle in order to supply the PWM generator with an updated input value. How often the process 20 for calculating the input value is actually suspended depends, among other things, on the hardware used. Typically, however, the calculation 20 is executed for every second or third control cycle, i.e. the update rate is 0.5 or 0.33 of the switching frequency of the frequency converter.

[0027] Figure 2 shows the concrete flow chart for the implementation of the method according to the invention. The process described in Figure 2 is triggered by the measurement procedure 10 (block 40). In the next step (block 41), a query is made as to whether or not the motor control 20 should be executed for the current control cycle. If the "Scheduler" flag is set to the value "runCoreControl", the calculation 20 of the motor control is executed for this cycle and an updated input value is generated for the PWN generator (block 21). The PWM signal is ultimately set based on the updated input values ​​(block 30). The described procedure is also executed if the variable "coreMotorControlSlowingDownFactor" is not defined, which corresponds to a deactivation of the method according to the invention.

[0028] If, however, the "Scheduler" flag is set to "runOverhead," the motor control 20 is suspended, and the microcontroller can instead perform other tasks ("overhead tasks") (block 41). These include, for example, monitoring the motor temperature, possibly using a thermal motor model for temperature estimation, as well as starting channel conversions and / or any calibration tasks.

[0029] Subsequently, in block 42, the variable "slowingDownCounter" is incremented and checked (block 43) whether the variable value is less than or equal to the predefined limit value "coreMotorControlSlowingDownFactor". If the variable value is still below the limit value, the "Scheduler" flag remains at the value "runOverhead" (block 43) and in the next work cycle, motor control 20 is suspended again (block 41). If, instead, the variable value has reached the limit value (block 44), the "Scheduler" flag is set to the value "runCoreControl" and the variable "slowingDownCounter" is reset to "1", which means that motor control 20 is executed again in the following control cycle. The variable "coreMotorControlSlowingDownFactor" thus specifies the maximum number of consecutive control cycles for which motor control may be suspended. Figure 3 finally shows a temporal representation of the generated PWM signal of the PWM generator.It can be seen here that an identical PWM signal, i.e. a PWM signal with an identical pulse width, was always generated for two consecutive control cycles.

Claims

KSB SE & Co. KGaA 67227 Frankenthal Method for controlling a pump drive of a centrifugal pump and centrifugal pump Claims 1. Method for controlling a pump drive of a centrifugal pump, wherein the centrifugal pump comprises an electric drive motor which is controlled by means of a frequency converter, wherein a motor control is provided which carries out a speed and / or current control and specifies at least one input value for setting the motor voltage to the PWM generator of the frequency converter per switching cycle of the frequency converter, characterized in that the motor control generates the at least one input value for the PWM generator with an update rate which is lower than the switching frequency of the frequency converter, and the at least one input value is kept constant over at least two switching cycles of the frequency converter.

2. Method according to claim 1, characterized in that the switching frequency of the frequency converter is in the range between 14kHz and 20kHz, preferably in the range between 15kHz and 18kHz.

3. Method according to one of the preceding claims, characterized in that the switching frequency of the frequency converter corresponds to an integer multiple of the update rate of the motor control.

4. Method according to one of the preceding claims, characterized in that the motor control is temporarily suspended for one cycle by the update rate being lower than the switching frequency in order to provide computing resources for alternative tasks.

5. Method according to claim 4, characterized in that in phases of suspended motor control one or more monitoring functions are carried out by the motor control, in particular the execution of a calculation of one or more thermal models for media and motor temperature estimation and / or the execution of at least one monitoring function of the motor current and / or the motor voltage, and / or the implementation of an optimization algorithm for calibrating one or more parameters of the motor control.

6. Method according to one of the preceding claims, characterized in that the pump comprises a hydraulic controller which is superior to the motor controller, wherein the one or more hydraulic time constants of the hydraulic controller are larger than the one or more electrical time constants of the motor controller.

7. Method according to one of the preceding claims, characterized in that the at least one input value for the PWM generator generated by the motor control corresponds to the duty cycle of the pulse width modulation and / or is at least one setpoint value for the motor voltage.

8. Method according to one of the preceding claims, characterized in that a control cycle is defined which comprises the execution of a measurement procedure for detecting one or more electrical drive variables relevant for the control, the optional calculation of an updated input value for the PWM generator and the provision of the existing or updated input value to the PWM generator.

9. Centrifugal pump comprising an electric drive motor together with a frequency converter and a control module for executing the motor control, characterized in that the control module is configured to carry out the method according to one of the preceding claims.

10. System comprising a centrifugal pump with an electric drive motor, a frequency converter and a control module configured to carry out the method according to one of claims 1 to 8.

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