Method for monitoring the operation of a pump, preferably a centrifugal pump

Abstract

The invention relates to a method for monitoring the operation of a pump, preferably a centrifugal pump (1), with a multi-phase, in particular three-phase drive motor, comprising the following method steps: establishing whether the pump is working in a stable operating state; in a stable operating state, monitoring at least one characteristic variable of the drive motor in order to determine whether there is an impeller block; if an impeller block is identified, activating a free wheel of the pump impeller, otherwise evaluating the frequency spectrum of the motor current to identify an impairment of the pump; and shutting down the motor by running a reducing speed ramp, if an impairment of the pump is identified.

Description

[0001] Method for monitoring the operation of a pump, preferably a centrifugal pump

[0002] The invention relates to a method for monitoring the operation of a pump, preferably a centrifugal pump, with a multi-phase, in particular three-phase drive motor.

[0003] Wastewater pumps, also known as dirty water pumps, enable the pumping of heavily contaminated water, which often contains solid components of various organic, inorganic, or mineral origins. Wastewater pumps are preferably single-stage and are generally not self-priming. The type of impeller used depends on the fluid being pumped. Channel impellers are often used as impellers, particularly in the form of a single-, double-, or triple-channel impeller, either closed or open. However, there are also designs with open single-channel and diagonal impellers, as well as free-flow impellers.

[0004] The solids carried in the fluid can cause deposits to adhere to the impeller structure, impairing pump function and thus reducing pump efficiency. In the worst case, the impeller becomes completely blocked, preventing further rotation. If the drive is not switched off in such a case, this can lead to mechanical damage to the pump hydraulics or overloading of the electric drive motor. Against this backdrop, it is important that pumps, which often operate independently for long periods, are continuously monitored and automatically deactivated in an emergency.

[0005] However, since emergency shutdowns represent a significant intervention in pump operation, it is desirable to be able to differentiate between acute and less acute failures and to take appropriate, individual countermeasures in each case. Especially in the case of impeller fouling, the pump can continue to operate, but the achievable efficiency is impaired, which worsens the energy balance.

[0006] Therefore, measures are being sought to be able to differentiate between faults during pump operation and to initiate fault-dependent countermeasures.

[0007] This object is achieved by a method according to the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims. Furthermore, the object is also achieved by a pump, in particular a centrifugal pump, with a pump control according to the features of claim 14 or by a pump control with the features of claim 15.

[0008] According to the invention, it is initially proposed to wait for a stable pump operating state before detecting a malfunction. A stable operating state can exist, for example, when the pump operates at a constant operating point for a certain period of time. In other words, monitoring for a pump malfunction should not be active during commissioning, but should only be carried out when the desired operating point is reached. If the pump detects a stable operating state, monitoring of at least one characteristic variable of the drive motor is started in order to be able to determine, based on the monitoring of this characteristic variable, whether an impeller blockage has occurred. If an impeller blockage has occurred, the pump impeller either no longer rotates or at least rotates so slowly that pumping work can no longer be performed effectively.

[0009] If an impeller blockage is detected, the invention proposes switching the pump into freewheel mode, allowing the impeller to rotate freely. Ideally, this can be achieved by mechanically decoupling the pump shaft from the drive motor. However, it can also be provided that the motor is de-energized in freewheel mode, and only the motor's braking torque is applied to the pump shaft.

[0010] If an impeller blockage is not present or is not detected, the method instead evaluates the frequency spectrum of the motor current, in particular of at least one phase of the stator current. By evaluating the frequency spectrum, a possible impairment of pump operation, in particular of the pump impeller, can be detected, since certain impairments, such as impending bearing failure or impeller blockage or impeller contamination due to solid deposits on the impeller, can lead to imbalances that can cause oscillating harmonics of the motor current or an amplification of these harmonics. This can be reliably detected using spectral analysis, also known as spectral analysis.

[0011] If the spectral analysis of the motor current, particularly of at least one phase of the stator current, detects a pump impairment, the motor is deactivated by following a defined speed ramp down to a motor speed of 0. This means that there is no abrupt shutdown of the drive and immediate braking of the pump impeller; instead, the speed is gently reduced until the pump comes to a complete standstill.

[0012] The method according to the invention is preferably executed on the pump's integral microprocessor unit, particularly during the pump's regular operating time. However, execution on an external processing unit is equally conceivable and is intended to be encompassed by the invention. The pump is preferably a centrifugal pump, particularly a pump for conveying media containing solids.

[0013] A stable operating state can be detected, for example, by observing at least one characteristic operating parameter of the pump or drive motor. Monitoring an electrical variable of the drive motor, in particular the motor current, has proven particularly advantageous. If the observed, characteristic parameter remains constant or approximately constant over a definable period of time, or is subject only to tolerable fluctuations, a stable operating state of the pump is assumed, and the method begins by detecting pump impairments using spectral analysis.

[0014] It is conceivable, for example, that in the event of a detected impeller blockage and / or pump impairment, in particular impeller contamination, a cleaning program is initiated during which the drive motor, in particular the pump impeller, is controlled in a defined manner. Ideally, definable speed changes and reversals of the direction of rotation are carried out cyclically in order to clear impeller blockages or impeller contamination. Ideally, individual cleaning programs can be executed depending on the detected error. For example, it is conceivable to carry out a first cleaning procedure if an impeller blockage is detected and a second cleaning procedure if, on the other hand, a pump or impeller impairment is detected. In particular, the cleaning procedures are specifically tailored to the respective error case in order to achieve optimal cleaning results.

[0015] Impeller blockage can be detected, for example, by monitoring at least one characteristic operating parameter of the pump drive, in particular the motor current drawn. A blockage is assumed, for example, when the observed characteristic value exceeds a corresponding limit value or lies outside a definable interval. Ideally, an impeller blockage is assumed when the motor current drawn is a certain value greater than the rated current of the drive unit, in particular, a multiple of the rated current, for example, more than twice the rated current.

[0016] For spectral analysis, it may be sufficient to determine and monitor the spectral amplitude at at least one definable, characteristic frequency. The spectral amplitude is also referred to below as the harmonic amplitude. For example, it is conceivable that certain pump impairments lead to harmonics with specific frequencies, which are subsequently referred to as error frequencies.

[0017] According to an advantageous embodiment of the method, the at least one definable error frequency is calculated as a function of the stator frequency of the drive motor and / or the number of pole pairs of the stator and / or as a function of the motor slip. The corresponding error frequency is particularly characteristic of possible contamination of the pump impeller.

[0018] If the drive motor is a synchronous machine, the slip and the calculation formula for the error frequency f are omitted.r,pump can be defined as follows: where p is the number of pole pairs of the stator and f s is the stator frequency. For asynchronous motors, especially induction motors, the slip s of the drive motor must also be taken into account, and the formula is preferably as follows: For spectral analysis, the motor current / stator current measured over a period of time can be transformed into its frequency spectrum using a Fast Fourier Transformation (FFT) or Discrete Fourier Transformation (DFT) to obtain the spectral amplitude at at least one fault frequency. The spectral analysis is preferably performed for at least one phase of the motor current in the steady state.

[0019] Since performing an FFT or DFT is very resource-intensive, the spectral amplitude of the motor current for at least one fault frequency can alternatively be determined by coordinate transformation. For this purpose, the transformation of the multi-phase, in particular three-phase, motor current into a two-axis dq current coordinate system is proposed. The resulting current coordinate system rotates with the fault frequency of the fault-indicating harmonic or the corresponding angular velocity. The resulting current vector in the dq coordinate system is therefore composed of a rotating current vector and a stationary current vector. The latter corresponds to the portion of the motor current attributable to the harmonic, which is constant over time in the selected representation and thus represents a DC component of the currents i d and i qIn the coordinate representation, the amplitude of the fault-indicating harmonic can be determined by calculating the geometric sum of these DC components. The proposed approach requires significantly fewer operations and resources than, for example, executing an FFT or DFT and, due to its comparatively low resource requirements, can therefore be easily implemented on an internal microprocessor unit of a pump. This allows the solution to be implemented entirely software-based on an existing microprocessor unit for controlling a centrifugal pump. Existing sensors for measuring the motor currents can be used; additional hardware expansion is not required.

[0020] As already explained above, the DC components of the currents i d and i q, which are determined by transformation, provide the necessary information for determining the harmonic amplitude. A simple procedure for determining these direct current components is the application of a low-pass filter, whereby the time-varying alternating component of the corresponding currents i d , i q Ideally, a first-order low-pass filter is used. A first-order Butterworth filter is particularly preferred, the transfer function of which is can be defined, where T preferably corresponds to the sampling rate of the processor unit. The cutoff frequency ω c must be chosen relatively small in order to remove the vibration as much as possible.

[0021] According to an advantageous embodiment of the invention, the Park transformation is used to transform the motor currents into the dq current coordinate system, in particular according to the formula where is a space vector representation of the three-phase motor current in a stator coordinate system and ω F the angular velocity corresponding to the fault-indicating oscillation frequency according to ω F = 2πf r,pump The trigonometric functions required for applying the Park transformation can be implemented within the microprocessor unit using look-up tables with a defined number of value pairs to minimize the microprocessor unit's memory requirements. The use of 300 to 400 value pairs, especially 360 value pairs, is conceivable.

[0022] The aforementioned Park transformation is often also used in a field-oriented (FOC) speed control of an electric motor, where the determination of the i qThe current coordinate system is not determined as a function of a specific harmonic frequency, but instead as a function of the current rotor speed, resulting in a coordinate system that is stationary with respect to the rotor. If this is the case, the method according to the invention can already rely on an existing control module for implementing an FOC to execute the park transformation.

[0023] Since the Park transformation requires a space vector representation of the three-phase motor current, the three-phase motor current must first be converted into a two-dimensional space vector representation. This can be done by transforming it into a stator coordinate system using the Clarke transformation. Here, too, an existing control module of an FOC can theoretically be reused, or instead, only the information about the space vector representation can be retrieved from the control module.

[0024] According to an advantageous embodiment, the method is particularly designed for a wastewater pump, since impeller contamination and the associated impairment of the pump are common, especially when pumping fluids containing solids. The method according to the invention enables wastewater pumps, which are often operated independently, to detect an undesirable impairment of their pump efficiency at an early stage and to take appropriate measures to automatically rectify the corresponding impeller impairment. It is also possible to detect and distinguish between a complete impeller blockage and impeller contamination.

[0025] The present invention relates not only to a method, but also to a pump, preferably a centrifugal pump, which is designed in particular for operation as a wastewater pump. The pump according to the invention has a pump control which is configured to carry out the method according to the present invention. Accordingly, the pump has the same advantages and properties as have already been shown above with reference to the method according to the invention. A repeated description is therefore omitted. The invention also relates to a pump control for use in or with a pump, in particular a centrifugal pump, particularly preferably with a wastewater pump, wherein the control is configured to carry out the method according to the present invention.Such a pump controller can, for example, be an integral component of a pump, but can also be designed as an external module that is merely communicatively connected to a corresponding pump unit in order to carry out the method according to the invention. It is also conceivable for such a pump controller to be connected to more than one pump unit, allowing monitoring, in particular parallel monitoring, of multiple pump units.

[0026] Further advantages and features of the invention will be explained below with reference to the exemplary embodiments illustrated in the figures. They show:

[0027] Figure 1: a flow diagram of a first embodiment of the method according to the invention,

[0028] Figure 2: a diagrammatic representation of the spectrum of one phase of the absorbed motor current,

[0029] Figure 3: a second flow diagram to illustrate a second embodiment of the method according to the invention,

[0030] Figure 4: a comparison of the stationary stator coordinate system and the rotating dq coordinate system,

[0031] Figure 5: a representation of the dq coordinate system rotating with the error frequency for error analysis and

[0032] Figure 6: A block diagram illustrating the individual method steps for error monitoring. Figure 1 shows the flowchart of a first embodiment of the method according to the invention for monitoring the operation of a wastewater pump. The pump has at least one impeller driven by a multi-phase electric motor.

[0033] When the pump is commissioned, it is first ramped up to the desired operating speed according to a defined speed ramp (block 100). Once the operating speed is reached, the pump switches to regular pump operation (block 200). During regular pump operation, a check is carried out to determine whether the pump is in a stable operating state (block 300). For this purpose, the motor current drawn by the motor is recorded. If the motor current remains constant or nearly constant for a certain period of time, a stable operating point is assumed, and the process proceeds to step 400.

[0034] In block 400, the motor current consumed by at least one phase of the drive motor is compared against a limit value, which here is twice the rated motor current 2*l NennIf the motor current drawn exceeds this limit, it is concluded that the pump impeller is blocked, and the pump control system initiates an immediate pump stop. To do this, the motor is immediately de-energized and freewheeling of the pump impeller is forced (block 500), reducing the risk of impeller damage caused by the detected blockage. The process then proceeds to block 600 to start a pump cleaning process specifically tailored to an impeller blockage. The cleaning process can include a sequence of changes in the direction of rotation of the impeller to force the blockage to be released. If the cleaning process was successful, the process returns to the starting point for pump commissioning (block 100).

[0035] However, if the measured motor current in block 400 remains below the limit value 2*l Nenn, i.e., there is no impeller blockage, a spectral analysis of the motor current of at least one phase is performed instead (block 700). This is done, for example, by a high-frequency and high-sampled current measurement and second-by-second evaluation of the frequency band of the current signal. Based on this, the spectral amplitude iF is calculated and monitored at the fault frequency f r,pump . In the embodiment of Figure 1, the spectral amplitude is calculated by transforming the time profile of the detected motor current into its frequency spectrum, either by Discrete Fourier Transformation (DFT) or by Fast Fourier Transformation (FFT).

[0036] Figure 2, for example, shows the frequency spectrum of one phase of the absorbed stator current, specifically for the error-free case (“free”) and for the case of impeller contamination (“clogged”). The two marked circles 80 indicate the amplitudes of the sidebands of the current spectrum, which are characteristic of certain impairments of the pump function and are therefore referred to as error or damage frequencies. Oscillating harmonics of the motor current with these frequencies are to be considered an indication of the presence of an impairment, e.g., an imbalance or a mechanical misalignment in the hydraulic or drive parts of the pump. This imbalance and alignment error can be caused by a clogged impeller, a bearing fault, or even dry running of the pump.In this case, it is clearly evident that the spectral amplitudes at the identified damaging frequencies 80 increase significantly when the impeller of the pump has significant buildup.

[0037] The mentioned relevant error frequency f r,pump can be calculated using an error model that calculates the error frequency according to formula (1 ) as a function of the stator frequency (rotor speed n), the motor slip s and the number p of pole pairs of the drive motor:

[0038] The slip s is obviously only to be used when an asynchronous machine is used as the drive motor. If a synchronous machine is used as the drive motor instead, a slip s = 0 can be used in the above formula. The characteristic fault frequency f for at least one r,pumpA specific spectral amplitude iF is compared in block 800 against a limit value iF, Lim. If the limit value is exceeded, the method concludes that the pump impeller is impaired, particularly impeller contamination that impairs efficiency, and initiates a shutdown of the drive unit as a reaction. However, the second prerequisite here is that the limit value is exceeded for at least 3 seconds (block 900).

[0039] However, unlike in the case of an impeller blockage, the detection of impeller contamination does not trigger an immediate shutdown of the drive motor. Instead, the pump is moderately decelerated by executing a speed ramp down to 0 speed (Block 1000) until the impeller comes to a standstill. Only then is a second cleaning procedure (Block 1100) executed, which differs from the first cleaning procedure (Block 600).

[0040] Figures 3-6 show a modified embodiment of the method according to the invention. The block diagrams of Figures 2 and 3 are identical except for blocks 700, 800, 700', 800', i.e., the spectral analysis of the motor current. Therefore, identical reference numerals are used for identical method steps in both figures, and only the difference in blocks 700', 800' will be discussed below. Figures 4 and 5 show a diagrammatic representation to clarify the procedure. Figure 6 shows a block diagram that describes the sequence in block 700' again in detail.

[0041] For fault monitoring at specific frequencies of the current spectrum, the principle of multiple reference frame theory is used instead of using FFT or DFT to minimize memory requirements and the number of operations. The idea, similar to field-oriented control (FOC), is to rotate a coordinate system. While in FOC the coordinate system rotates at the frequency of the rotor, for fault detection it rotates at the frequency of a fault. As already explained above, at least one fault frequency is determined using the formula given under (1). This is indicated by reference number 10 in the diagram in Figure 6.

[0042] In a three-phase motor, the motor currents can be summarized in a space vector. For this purpose, it is assumed that the sum of the phase currents is zero. The real part of the space vector is called the a-current, and the imaginary part is called the ß-current. The α-β coordinate system (see Figure 4) is called the stator-fixed coordinate system. The transformation of the three-phase stator currents into the two-phase α-β current is called the Clarke transformation.

[0043] To drive an AC motor, the controller transforms the stator-fixed α-β current into the rotor-fixed dq current, a process known as the Park transformation. Mathematically, a coordinate system is caused to rotate according to the rotor speed n. As a result, the dq current is a DC value that can be used for motor control. The interesting aspect is that the vector sum of the d and q currents exactly corresponds to the amplitude of the fundamental oscillation of the motor current. This principle is utilized in the modified version of the method for fault detection.

[0044] If we consider a real motor, the phase current and thus the current space vector are superimposed with oscillations, the magnitude of which increases during faulty operation of the pump or drive motor. For the method according to the invention, it is assumed that the motor current is the sum of the torque-generating current with the amplitude î T and the speed ω S and a harmonic with amplitude î F and the speed ω F The motor currents of the three phases can be calculated using the following equations (2): i a (t) = î T cos(ω S t) + î F cos(ω F t + θ) In this case, î contains F Information about the pump condition and the severity of the fault. For example, ω F can be calculated based on equation (1 ).

[0045] As shown in Figure 4, the current space vector in the stator coordinate system tem is equal to the sum of the torque-forming component , which with the speed ω S rotates, and the error component ^ß, which with the speed ω F rotates. The calculation of the current space vector of the three-phase motor current is carried out according to the following equation (3):

[0046] In the block diagram shown in Figure 6, this step is already carried out by the existing field-oriented control 20 of the pump control, which uses the two currents i as output variables. a and i ß delivers.

[0047] In the sense of the method according to the invention, the length of of interest. Now the dq coordinate system is calculated with the speed of the harmonic frequency ( ω K = ω F) is rotated. To calculate the current vector in dq coordinates, the standard equation for the Park transformation is used, which is indicated in the block diagram by step 30. The Park transformation can be implemented mathematically according to the following equation:

[0048] If formula (3) is inserted into formula (4), formula (5) results for the current vector in the dq coordinate system: The three-phase current vector is equal to the sum of the vectors that are speed (ω S - ω F ) rotate, and the stationary vector see Figure 5. If ω F greater than ω S is, both rotate in the other direction tion.

[0049] If we consider time-dependent quantities, there are d and i qfrom a DC component and an AC component, as can be seen in equations (6) and (7). i d = i F|d + i T|d . cos((ω S - ω F )t) (6) i q = i F|q + i T|q . sin((ω S - ω F )t) (7)

[0050] The initial amplitude î f can be calculated from the geometric sum of i F|d and i F|q be calculated, see equation (8) below.

[0051] In the block diagram of Figure 6, this process step is identified by the reference numeral 50. If the direct current components of i d and i q can be determined, the amplitude î f can be calculated from it. Thus, by applying simple transformations, the amplitude of a harmonic can be calculated. A simple and memory-friendly method for calculating the DC components of i d and i qis a first-order filter, which is identified by the reference numeral 40 in the block diagram of Figure 6.

[0052] For example, a first-order Butterworth filter can be chosen, whose transfer function can be determined as follows according to equation (9) where T is the sampling time of the microprocessor unit. The filter allows for a simple implementation. However, the cutoff frequency ω c The filter's time constant must be chosen relatively small to eliminate the oscillation as much as possible. This makes the filter's time constant relatively high, which slows the system and can be a problem in dynamic systems. However, this is not critical when used in a pump, since rapid load changes are not expected.

[0053] The value for the spectral amplitude î determined as described above fcould then be compared in block 800' as an alternative to the representation in Figure 3 against a limit value iF, Lim, whereby if this limit value is exceeded for a period of at least 3s (block 900), the pump is also stopped (block 1000) and the cleaning procedure (block 1100) is then triggered.

[0054] As an alternative to this variant, however, as shown in Figure 3, in block 700' not only the spectral amplitude î f, as described above, but instead also the damage factor SF. Especially with controlled pumps, e.g. pressure-controlled pumps, the load and the speed of the pump can change during operation, which simultaneously also means a change in the current consumption of the pump. To take this into account, the damage factor ("Severity Factor" SF) is calculated for a fault that relates to the current consumption. This is shown in the block diagram of Figure 6 with the reference number 60. If the motor control of the pump has an FOC 20, the information about the current consumption is available. To ensure load independence, the damage factor SF is calculated from the ratio of the fault-indicating spectral amplitude î f and the amplitude î T the torque-forming component, which is equal to the q-current in the FOC used, with the d-current controlled to zero:

[0055] Based on the damage factor SF, a decision can then be made in block 800' (see Figures 3 and 6) as to whether or not the pump has impeller contamination by comparing it with a limit value SF, Lim. If contamination is detected, and persists for more than 3 seconds, the pump is slowed down to zero speed (block 1000), and then cleaning procedure 2 is started (block 1100).

Claims

Claims 1. Method for monitoring the operation of a pump, preferably a centrifugal pump (1), with a multiphase, in particular three-phase, drive motor, comprising the following steps: - Determining whether the pump is operating in a stable operating state, - If a stable operating state is present, monitoring at least one characteristic parameter of the drive motor to determine whether an impeller blockage is present, - if an impeller blockage is detected, activating a freewheel of the pump impeller, - otherwise, evaluating the frequency spectrum of the motor current to detect an impairment of the pump and switching off the motor by traversing a decreasing speed ramp if an impairment of the pump is detected.

2. Method according to claim 1, characterized in that the impairment of the pump is an impairment of the pump impeller, in particular impeller contamination.

3. Method according to claim 1 or 2, characterized in that a cleaning program is initiated upon detection of an impeller blockage and / or detection of an impeller clogging.

4. Method according to claim 3, characterized in that a first cleaning program is executed after detection of an impeller blockage and a second cleaning program is executed after detection of an impeller clogging.

5. Method according to one of the preceding claims, characterized in that an impeller blockage is carried out by monitoring the recorded motor current, wherein a blockage is detected when the motor current exceeds a definable limit value, in particular exceeds the rated current by a certain value, preferably by a multiple of the rated current.

6. Method according to one of the preceding claims, characterized in that the evaluation of the frequency spectrum comprises monitoring a spectral amplitude of the motor current at at least one definable error frequency.

7. Method according to claim 6, characterized in that the at least one error frequency is determined as a function of the pole pairs of the stator and / or the stator frequency and / or a possible motor slip, in particular according to for an induction motor and for a synchronous motor, where p is the number of pole pairs of the stator, s the motor slip and f s the stator frequency.

8. Method according to claim 6 or 7, characterized in that the determination of the spectral amplitude is carried out by Fast Fourier Transform or by Discrete Fourier Transform.

9. Method according to one of the preceding claims 6 or 7, characterized in that the spectral amplitude î f of the motor current for at least one specific error frequency f r,pump by transforming the three-phase motor current into a system with an error frequency f r,pump rotating d- / q-current coordinate system with the currents i d and i q is calculated, whereby the geometric sum of the DC components of the currents i d and i q in the d- / q-current coordinate system of the spectral amplitude î f corresponds.

10. Method according to one of the preceding claims, characterized in that the DC components of the transformed currents i d and i q by applying a low-pass filter, preferably a first-order low-pass filter, particularly preferably a first-order Butterworth filter.

11. Method according to claim 9 or 10, characterized in that the transformation into the dq current coordinate system is carried out by means of a Park transformation, in particular according to where is a space vector representation of the three-phase motor current in a stator coordinate system and the angular velocity ω F from the error frequency f r,pump according to ω F = 2π f r,pump is calculated.

12. Method according to claim 11, characterized in that the transformation of the three-phase motor current into the space vector representation in the stator coordinate system is carried out by a Clark transformation, wherein the space vector is preferably determined by an existing control module of the pump control, which performs a field-oriented speed control (20).

13. Method according to one of the preceding claims, characterized in that the pump is a wastewater pump.

14. Pump, preferably centrifugal pump, particularly for operation as a wastewater pump, with a pump control configured to carry out the method according to one of the preceding claims.

15. Pump control for a pump, in particular a centrifugal pump, configured to carry out the method according to one of the preceding claims 1 to 13.

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