Method for Monitoring the Operation of a Pump, Preferably a Centrifugal Pump

Abstract

A method for monitoring the operation of a centrifugal pump includes multiple steps. In one step, the method establishes whether the centrifugal pump is in a stable operating state. The centrifugal pump has a three-phase drive motor. In another step, the method monitors, when the stable operating state exists, at least one characteristic variable of the three-phase drive motor in order to establish whether there is an impeller blockage. When the impeller blockage is identified, in a further step the method activates the impeller to free-wheel. When the impeller blockage is not identified, the method also includes analyzing a frequency spectrum of a motor current to identify impairment of the centrifugal pump, and shuts down the three-phase drive motor by following a downward speed ramp if the impeller blockage is identified.

Claims

1-15. (canceled)

16. A method for monitoring the operation of a centrifugal pump, the method comprising: establishing whether the centrifugal pump is in a stable operating state, wherein the centrifugal pump has a three-phase drive motor; when the stable operating state exists, monitoring at least one characteristic variable of the three-phase drive motor in order to establish whether there is an impeller blockage; when the impeller blockage is identified, activating the impeller to free-wheel; and when the impeller blockage is not identified, analyzing a frequency spectrum of a motor current to identify impairment of the centrifugal pump, and shutting down the three-phase drive motor by following a downward speed ramp if the impeller blockage is identified.

17. The method as claimed in claim 16, wherein the impairment of the centrifugal pump involves an impairment of the impeller, or impeller fouling.

18. The method as claimed in claim 17, further comprising: initiating a cleaning program on identification of the impeller blockage and/or identification of the impeller fouling.

19. The method as claimed in claim 18, further comprising: performing a first cleaning program, after identifying the impeller blockage, and performing a second cleaning program after identifying the impeller fouling.

20. The method as claimed in claim 19, wherein the impeller blockage is identified by monitoring a drawn motor current, and the impeller blockage is identified when the drawn motor current lies above a multiple of a rated current.

21. The method as claimed in claim 20, wherein the analyzing the frequency spectrum comprises monitoring a spectral amplitude of the motor current at least at one fault frequency.

22. The method as claimed in claim 21, wherein the at least one fault frequency is determined as a function of pole pairs of the stator and/or a stator frequency and/or a motor slip, as given by $f_{r,\mathrm{pump}}=\left(1\frac{1}{p}\left(1-s\right)\right).Math.f_s$ for an induction motor, and for a synchronous motor as given by $f_{r,\mathrm{pump}}=\left(1\frac{1}{p}\right).Math.f_s,$ where p is the number of pole pairs of the stator, s is the motor slip, and f.sub.s is the stator frequency.

23. The method as claimed in claim 22, wherein the spectral amplitude is monitored by the fast Fourier transform or by a discrete Fourier transform.

24. The method as claimed in claim 22, wherein spectral amplitude .sub.f of the motor current for the at least one determined fault frequency f.sub.r,pump is calculated by transforming the three-phase motor current into a d/q current coordinate system rotating at the fault frequency f.sub.r,pump and containing the currents i.sub.d and i.sub.q, where a geometric sum of the DC components of the currents i.sub.d and i.sub.q in the d-/q current coordinate system equals the spectral amplitude .sub.f.

25. The method as claimed in claim 24, wherein DC components of the transformed currents i.sub.d and i.sub.q is determined by applying a a first-order Butterworth filter.

26. The method as claimed in claim 25, wherein the transformation into the d-q current coordinate system is performed by a Park transform, given by ${\underset{}{\stackrel{.fwdarw.}{l}}}_{\mathrm{dq}}={\underset{}{\stackrel{.fwdarw.}{l}}}_a.Math.e^{-i\left({}_{F}t\right)},$ where {right arrow over (l)}.sub. is a space vector representation of the three-phase motor current in a stator coordinate system, and an angular velocity .sub.F is calculated from the fault frequency f.sub.r,pump in accordance with .sub.F=2f.sub.r,pump.

27. The method as claimed in claim 26, wherein the transformation of the three-phase motor current into the space vector representation in the stator coordinate system is performed by a Clark transform, where the space vector {right arrow over (l)}.sub. is determined preferably by an existing control chip of the pump controller, wherein the control chip performs field oriented speed control.

28. The method as claimed in claim 27, wherein the centrifugal pump is a wastewater pump.

29. A centrifugal pump, for operation as a wastewater pump, having a pump controller which is configured to perform the method as claimed in claim 28.

30. A pump controller for a centrifugal pump, which is configured to implement the method as claimed in claim 28.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a flow diagram of the method according to the disclosure;

[0033] FIG. 2 shows a diagram of the spectrum of one phase of the drawn motor current;

[0034] FIG. 3 shows a second flow diagram for illustrating the method according to the disclosure;

[0035] FIG. 4 shows the stationary stator coordinate system in relation to the rotating d-q coordinate system;

[0036] FIG. 5 shows a representation of the d-q coordinate system rotating at the fault frequency for the fault analysis; and

[0037] FIG. 6 shows a block diagram for illustrating the individual method steps for fault monitoring

DETAILED DESCRIPTION

[0038] FIG. 1 shows the flow diagram of a first embodiment of the method according to the disclosure for monitoring the operation of a wastewater pump. The pump has at least one impeller, which is driven by a multiphase-fed electric motor.

[0039] When the pump is first put into operation, the pump is initially run up to the desired operating speed according to a defined speed ramp (block 100). After reaching the operating speed, the pump shifts into normal pump operation (block 200). In normal pump operation, a check is made to ascertain whether the pump is in a stable operating state (block 300). For this purpose, the motor current drawn by the motor is detected. If the motor current remains constant or nearly constant during a certain timespan, a stable operating point is presumed to exist, and the method proceeds to step 400.

[0040] In block 400, the drawn motor current in at least one phase of the drive motor is compared with a limit value, which here equals twice the rated motor current 2*I.sub.rated. If the drawn motor current exceeds this limit value, a blockage of the pump impeller is inferred, and the pump controller initiates an immediate pump stop. This is done by immediately disconnecting the electrical supply from the motor and making the pump impeller free-wheel (block 500), thereby reducing the risk of the identified blockage damaging the impeller. The method then proceeds to block 600, in order to start a process for cleaning the pump that is designed specifically for an impeller blockage. The cleaning process can contain a series of changes in the direction of rotation of the impeller in order to force the blockage to release. If the cleaning process was successful, the method returns to the starting point for putting the pump into operation (block 100).

[0041] If, on the other hand, in block 400 the detected motor current remains below the limit value 2*I.sub.rated, i.e. there is no impeller blockage, then instead a spectral analysis is performed on the motor current in at least one phase (block 700). This is done, for example, by a high-frequency and highly sampled current measurement and analyzing every second the frequency band of the current signal. This is used as the basis for calculating and monitoring the spectral amplitude iF at the fault frequency f.sub.r,pump. In the embodiment of FIG. 1, the spectral amplitude is calculated by transforming the temporal waveform of the detected motor current into its frequency spectrum, either by the discrete Fourier transform (DFT) or by the fast Fourier transform (FFT).

[0042] FIG. 2 shows by way of example the frequency spectrum of one phase of the drawn stator current, specifically for the fault-free case free and for the impeller fouling case clogged. The two labeled circles 80 mark the amplitudes of the sidebands of the current spectrum, which are characteristic of specific impairments in the pump function and consequently are referred to as fault or damage frequencies. Oscillation harmonics in the motor current at these frequencies can be considered as an indicator of the existence of an impairment, for example an imbalance or a mechanical misalignment in the hydraulics and also in the drive section of the pump. Said imbalance and misalignment can be produced by a clogged impeller, a bearing fault or even dry-running of the pump. It is clearly evident in this case that the spectral amplitudes at the identified damage frequencies 80 increase markedly when there is significant build-up on the impeller of the pump.

[0043] The aforementioned relevant fault frequency f.sub.r,pump can be calculated using a fault model that calculates the fault frequency according to equation (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:

[00005]$\begin{array}{cc}{f}_{r,\mathrm{pump}}=\left(1\frac{1}{p}\left(1-s\right)\right).Math.f_s.& \left(1\right)\end{array}$

[0044] The slip s clearly has to be used only when an asynchronous machine is being used as the drive motor. If instead a synchronous machine is employed as the drive motor, a drag s=0 can be inserted into the above equation.

[0045] In block 800, The spectral amplitude iF determined for the at least one characteristic fault frequency f.sub.r,pump is compared with a limit value iF, Lim. If the limit value is exceeded, the method infers an impairment of the pump impeller, in particular impeller fouling that impairs the efficiency, and initiates a shutdown of the drive unit in response. It is required here as a second precondition, however, that the limit value is exceeded for at least 3 seconds (block 900).

[0046] Unlike in the case of an impeller blockage, however, on detection of impeller fouling, an immediate shutdown of the drive motor is not initiated, but instead the pump is decelerated in a measured way along a defined speed ramp to a speed of 0 (block 1000) until the impeller comes to a standstill. Only afterwards is a second cleaning procedure (block 1100) carried out, which differs from the first cleaning procedure 600.

[0047] FIGS. 3-6 show a modified embodiment of the method according to the disclosure. The block diagrams in FIGS. 2 and 3 are identical except for blocks 700, 800, 700, 800, i.e. the spectral analysis of the motor current, and therefore the same reference signs are used for the same method steps in both figures, and the following discussion deals only with the difference in the blocks 700, 800. FIGS. 4 and 5 show a diagram for illustrating the procedure. FIG. 6 shows a block diagram, which describes the process in block 700 again in detail.

[0048] For the fault monitoring at the specific frequencies of the current spectrum, the principle of the multiple reference frame theory is used instead of an implementation by means of FFT or DFT, with a view to minimizing memory usage and the number of operations. Similar to the case of field oriented control (FOC), the idea is to make a coordinate system rotate. Whereas in FOC the coordinate system rotates at the frequency of the rotor, for the purpose of fault identification it rotates at the frequency of a fault. As was already explained above, at least one fault frequency is determined using the equation given by (1). This is labeled in the diagram of FIG. 6 by the reference sign 10.

[0049] For a three-phase motor, the motor currents can be combined 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 denoted as the current, and the imaginary part as the current. The - coordinate system (see FIG. 4) is referred to as a stator-referenced coordinate system (stator coordinate system). The transformation of the three-phase stator currents into the two-phase - current is called a Clarke transform.

[0050] In order to drive an AC motor, the stator-referenced - current is transformed by the controller into the rotor-referenced d-q current, which process is called a Park transform. From the mathematical viewpoint, a coordinate system is made to rotate at a speed equal to the speed n of the rotor. As a result, the d-q current is a DC value, which can be used for the motor control. The interesting aspect is that the vector sum of the d-current and q-current is exactly equal to the amplitude of the fundamental harmonic of the motor current. The modified embodiment of the method makes use of this principle for the fault identification.

[0051] If an actual motor is considered, then superimposed on the phase current and hence the current space vector are oscillations, the extent of which increase during faulty operation of the pump or drive motor. For the method according to the disclosure, it is now assumed that the motor current is the sum of the torque-producing current of amplitude .sub.T and speed .sub.S and of a harmonic of amplitude .sub.F and speed .sub.F. The motor currents of the three phases can be calculated according to the following equations (2):

[00006]$\begin{array}{cc}{i}_a\left(t\right)={\widehat{\mathrm{.Math.}}}_T\cos \left({}_{S}t\right)+{\widehat{\mathrm{.Math.}}}_F\cos \left({}_{F}t+\right)& \left(2\right)\end{array}$$i_b\left(t\right)={\widehat{\mathrm{.Math.}}}_T\cos \left({}_{S}t-\frac{2}{3}\right)+{\widehat{\mathrm{.Math.}}}_F\cos \left({}_{F}t+-\frac{2}{3}\right)$$i_c\left(t\right)={\widehat{\mathrm{.Math.}}}_T\cos \left({}_{S}t-\frac{4}{3}\right)+{\widehat{\mathrm{.Math.}}}_F\cos \left({}_{F}t+-\frac{4}{3}\right)$

[0052] In this case, .sub.F contains information about the state of the pump and about the severity of the fault. As an example, .sub.F can be calculated on the basis of equation (1).

[0053] As shown in FIG. 4, the current space vector {right arrow over (l)}.sub. in the stator coordinate system equals the sum of the torque-producing component {right arrow over (l)}.sub.T|, which rotates at the speed .sub.S, and the fault component {right arrow over (l)}.sub.F|, which rotates at the speed .sub.F. The current space vector {right arrow over (l)}.sub. of the three-phase motor current is calculated according to the following equation (3):

[00007]$\begin{array}{cc}{\underset{}{\stackrel{.fwdarw.}{l}}}_a={\widehat{\mathrm{.Math.}}}_T.Math.e^{i\left({}_{S}t\right)}+{\widehat{\mathrm{.Math.}}}_F.Math.e^{i\left({}_{F}t+\right)}& \left(3\right)\end{array}$

[0054] In the block diagram shown in FIG. 6, this step is already implemented by the existing field oriented control 20 of the pump controller, which supplies as output values the two currents i.sub.a and i.sub..

[0055] For the purpose of the method according to the disclosure, the length of {right arrow over (l)}.sub.F| is of interest. The d-q coordinate system is then rotated at the velocity of the harmonic frequency (.sub.K=.sub.F). The standard equation for the Park transform is used to calculate the current vector in d-q coordinates, which is labeled by step 30 in the block diagram. The Park transform can be implemented mathematically according to the following equation:

[00008]$\begin{array}{cc}{\underset{}{\stackrel{.fwdarw.}{l}}}_{\mathrm{dq}}={\underset{}{\stackrel{.fwdarw.}{l}}}_a.Math.e^{-i\left({}_{F}t\right)}& \left(4\right)\end{array}$

[0056] Inserting equation (3) into equation (4) yields the equation (5) for the instantaneous vector {right arrow over (l)}.sub.dq in the d-q coordinate system:

[00009]$\begin{array}{cc}{\underset{}{\stackrel{.fwdarw.}{l}}}_{\mathrm{dq}}={\widehat{\mathrm{.Math.}}}_F.Math.e^i+{\widehat{\mathrm{.Math.}}}_T.Math.e^{i\left[\left({}_S-{}_F\right)t\right]}& \left(5\right)\end{array}$

[0057] The rotating-current vector {right arrow over (l)}.sub.dq is equal to the sum of the vectors {right arrow over (l)}.sub.T|dq, which rotate at the velocity (.sub.S-.sub.F), and the stationary vector {right arrow over (l)}.sub.F|dq; see FIG. 5. If .sub.F is greater than .sub.S, both {right arrow over (l)}.sub.dq and {right arrow over (l)}.sub.T|dq rotate in the other direction.

[0058] If time-dependent variables are considered, i.sub.d and i.sub.q consist of a DC component and an AC component, as presented in equations (6) and (7).

[00010]$\begin{array}{cc}{i}_d=i_{F|d}+i_{T|d}.Math.\cos \left(\left({}_S-{}_F\right)t\right)& \left(6\right)\end{array}$$\begin{array}{cc}{i}_q=i_{F|q}+i_{T|q}.Math.\sin \left(\left({}_S-{}_F\right)t\right)& \left(7\right)\end{array}$

[0059] The initial amplitude .sub.f can be calculated from the geometric sum of i.sub.F|d and i.sub.F|q; see equation (8) below.

[00011]$\begin{array}{cc}{\widehat{\mathrm{.Math.}}}_f=\sqrt{i_{F.Math.d}^2+i_{F.Math.q}^2}& \left(8\right)\end{array}$

[0060] This method step is labeled by the reference sign 50 in the block diagram of FIG. 6. If the DC components of i.sub.d and i.sub.q are determined, the amplitude .sub.f can be calculated therefrom. Thus the amplitude of a harmonic can be calculated by applying simple transforms. A simple and memory-friendly method for calculating the DC components of i.sub.d and i.sub.q is a first-order filter, which is labeled by the reference sign 40 in the block diagram of FIG. 6.

[0061] For example, a first-order Butterworth filter can be chosen, the transfer function of which can be defined as follows by equation (9):

[00012]$\begin{array}{cc}H\left(z\right)=\frac{1-e^{{}_{c}T}}{z-e^{{}_{c}T}},& \left(9\right)\end{array}$ [0062] where T is equal to the sampling interval of the microprocessor unit. The filter allows a simple implementation. The cutoff frequency .sub.c must be chosen to be relatively small, however, in order to to remove the oscillation as far as possible. This means that the time constant of the filter is relatively high, which makes the system slow, and this can be a problem in dynamic systems, When employed in a pump, however, this is uncritical because rapid changes in load are not expected.

[0063] Then, in block 800, as an alternative to that presented in FIG. 3, the value determined as described above for the spectral amplitude .sub.f could be compared with a limit value iF, Lim, where if this limit is exceeded for the length of time of at least 3 s (block 900), again the pump is stopped (block 1000) and then the cleaning procedure (block 1100) is initiated.

[0064] As an alternative to this variant, however, as shown in FIG. 3, in block 700, instead of determining just the spectral amplitude .sub.f, as described above, the severity factor SF is also determined. In particular in the case of pumps that are controlled, for example pressure-controlled pumps, the load and speed of the pump can vary during operation, which at the same time also means a change in the current drawn by the pump. In order to take account of this, the severity factor (SF) for a fault is calculated, which is related to the current draw. This is labeled in the block diagram of FIG. 6 by the reference sign 60. If the motor control of the pump has FOC 20, the information about the current draw is available. To ensure load-independence, the severity factor SF is formed from the ratio of the fault-indicating spectral amplitude .sub.f and the amplitude .sub.T of the torque-producing component, which is equal to the q-current in the FOC being used, where the d-current is controlled to zero:

[00013]$SF=\frac{{\stackrel{}{\mathrm{.Math.}}}_f}{{\stackrel{}{\mathrm{.Math.}}}_T}.Math.100\%.$

[0065] Based on the severity factor SF, a comparison with a limit value SF, Lim can be used in block 800 (see FIGS. 3 and 6) to decide whether or not impeller fouling is present in the pump. If this is identified, and identified for a length of time of more than 3 seconds, then the pump is slowed down to a speed of zero (block 1000), and then the cleaning procedure 2 is started (block 1100).

[0066] The foregoing disclosure has been set forth merely to illustrate the disclosure and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the disclosure may occur to persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims and equivalents thereof.