Method for controlling a pump arrangement

Abstract

A method for controlling a pump arrangement comprising a pump having a motor and a control unit. The motor, when the pump is in an active state and the motor is driven in a first direction, is associated with a load factor that corresponds to an operating condition of the pump arrangement. The pump arrangement also includes a unit configured to monitor at least one operating parameter from which the load factor of the motor can be derived. The method includes determining a real value of the at least one operating parameter, determining if an externally applied force is acting on the motor to such an extent that an operating condition detrimental to the pump arrangement is initiated, effecting a state shift from the active state to an inactive state of the pump if an operating condition detrimental to the pump arrangement is initiated, the state shift including a step according to which the control unit, immediately after a detrimental operating condition is initiated, abruptly breaks the driving of the motor in the first direction.

Claims

1. A method for controlling a pump arrangement comprising a pump (2) and a controller (6), the pump (2) comprising a motor (7) and the controller (6) being arranged to drive said motor (7), wherein said motor (7), at each individual instant of time, when the pump (2) is in an active state and the motor (7) is driven in a first direction, has a load factor that corresponds to an operating condition of the pump arrangement, the method comprising the steps of: monitoring at least one operating parameter from which the load factor is derived; determining a real value of said at least one operating parameter, when the pump (2) is in said active state, determining, using said real value of said at least one operating parameter, if an externally applied force is acting on the motor (7) to such an extent that one of a plurality of operating conditions of the pump arrangement is initiated when the load factor of the motor (7) exceeds a level such that continued operation will cause the pump to break, and effecting a state shift from the active state of the pump (2) to an inactive state of the pump (2) if said one of the plurality of operating conditions is initiated, wherein said state shift comprises the step of causing the controller (6), immediately after it is determined that said one of the plurality of operating conditions is initiated, to abruptly break the driving of the motor (7) in said first direction by a nominal rotational speed of the motor (7) being set equal to zero in the controller (6).

2. The method according to claim 1, wherein the step of determining, using said real value of said at least one operating parameter, if the externally applied force is acting on the motor (7), comprises the step of: determining if said real value of said at least one operating parameter is equal to or exceeds a predetermined threshold value (GS).

3. The method according to claim 2, wherein another one of the plurality of operating conditions is initiated when based on said real value of said at least one operating parameter, the externally applied force is determined to be acting on the motor (7) to such an extent that when the load factor of the motor (7) exceeds a predetermined level in response to solid matter being caught in the pump or an object being at least partially blocking the pump, and further comprising the step of effecting a state shift from the active state of the pump (2) to a cleaning state of the pump (2) when said another of the plurality of operating conditions is initiated.

4. The method according to claim 1, wherein the step of determining, using said real value of said at least one operating parameter, if the externally applied force is acting on the motor (7), comprises the step of: determining if said real value of said at least one operating parameter is outside a predetermined interval (RS).

5. The method according to claim 4, wherein another one of the plurality of operating conditions is initiated when based on said real value of said at least one operating parameter, the externally applied force is determined to be acting on the motor (7) to such an extent that when the load factor of the motor (7) exceeds a predetermined level in response to solid matter being caught in the pump or an object being at least partially blocking the pump, and further comprising the step of effecting a state shift from the active state of the pump (2) to a cleaning state of the pump (2) when the another of the plurality of operating conditions is initiated.

6. The method according to claim 5, wherein the externally applied force is determined to be acting on the motor (7) by determining if said real value of said at least one operating parameter is outside a predetermined interval (RA) during a predetermined first period of time (T1).

7. The method according to claim 6, wherein the interval (RS) is greater than and includes the interval (RA).

8. The method according to claim 1, wherein the step of determining, using said real value of said at least one operating parameter, if the externally applied force is acting on the motor (7), comprises the step of: determining if a difference between said real value and a predetermined nominal value, when the pump (2) is in the active state, is equal to or exceeds a predetermined difference threshold value (DS).

9. The method according to claim 8, wherein another one of the plurality of operating conditions is initiated when based on said real value of said at least one operating parameter, the externally applied force is determined to be acting on the motor (7) to such an extent that when the load factor of the motor (7) exceeds a predetermined level in response to solid matter being caught in the pump or an object being at least partially blocking the pump, and further comprising the step of effecting a state shift from the active state of the pump (2) to a cleaning state of the pump (2) when said another one of the plurality of operating conditions is initiated.

10. The method according to claim 9, wherein the externally applied force is determined to be acting on the motor (7) by determining if a difference between said real value and a predetermined nominal value, when the pump (2) is in the active state, is equal to or exceeds a predetermined difference threshold value (DA) during a predetermined first period of time (T1).

11. The method according to claim 10, wherein the difference threshold value (DS) is at least two times greater than the difference threshold value (DA).

12. The method according to claim 1, which furthermore comprises the step of: effecting a state shift from the active state of the pump (2) to a cleaning state of the pump (2) if the motor (7) continually has been driven in said first direction during a predetermined period of time (T2).

13. A method for controlling a pump arrangement comprising a pump (2) and a controller (6), the pump (2) comprising a motor (7) and the controller (6) being arranged to drive said motor (7), wherein said motor (7), at each individual instant of time, when the pump (2) is in an active state and the motor (7) is driven in a first direction, has a load factor that corresponds to an operating condition of the pump arrangement, the method comprising the steps of: monitoring at least one operating parameter from which the load factor is derived; determining a real value of said at least one operating parameter, when the pump (2) is in said active state, determining, using said real value of said at least one operating parameter, if an externally applied force is acting on the motor (7) to such an extent that one of a plurality of operating conditions of the pump arrangement is initiated when the load factor of the motor (7) exceeds a level such that continued operation will cause the pump to break, and effecting a state shift from the active state of the pump (2) to an inactive state of the pump (2) if said one of the plurality of operating conditions is initiated, wherein said state shift comprises the step of causing the controller (6), immediately after it is determined that said one of the plurality of operating conditions is initiated, to abruptly break the driving of the motor (7) in said first direction by a nominal rotational speed of the motor (7) being set equal to zero in the controller (6), determining that the externally applied force is acting on the motor (7) to such an extent that when the load factor of the motor (7) exceeds a predetermined level in response to solid matter being caught in the pump or an object is at least partially blocking the pump, initiating another one of the plurality of operating conditions when based on said real value of said at least one operating parameter, and effecting a state shift from the active state of the pump (2) to a cleaning state of the pump (2) when the another one of the plurality of operating conditions is initiated.

14. The method according to claim 13, wherein the externally applied force is determined to be acting on the motor (7) by determining if said real value of said at least one operating parameter is equal to or exceeds a predetermined threshold value (GA) during a predetermined first period of time (T1).

15. The method according to claim 14, wherein the externally applied force is determined to be acting on the motor by determining if said real value of said at least one operating parameter is equal to or exceeds a predetermined threshold value (GS), and wherein the predetermined threshold value (GS) is at least 20% greater than the predetermined threshold value (GA).

16. The method according to claim 13, wherein the externally applied force is determined to be acting on the motor (7) by determining if said real value of said at least one operating parameter is outside a predetermined interval (RA) during a predetermined first period of time (T1).

17. The method according to claim 13, wherein the externally applied force is determined to be acting on the motor (7) by determining if a difference between said real value and a predetermined nominal value, when the pump (2) is in the active state, is equal to or exceeds a predetermined difference threshold value (DA) during a predetermined first period of time (T1).

18. The method according to claim 13, wherein said state shift from the active state of the pump (2) to the cleaning state of the pump (2) comprises the step of: decreasing the rotational speed of the motor (7) in said first direction according to a predetermined down-ramping of the rotational speed of the motor (7).

19. The method according to claim 18, which furthermore comprises the steps of: stopping the motor (7), and keeping the motor (7) stopped during a predetermined third period of time (T3).

20. The method according to claim 19, which furthermore comprises the steps of: increasing the rotational speed of the motor (7) in a second direction opposite the first direction according to a predetermined up-ramping of the rotational speed of the motor (7), driving the motor (7) in said second direction during a predetermined fourth period of time (T4), decreasing the rotational speed of the motor (7) in said second direction according to said down-ramping of the rotational speed of the motor (7), and stopping the motor (7).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the above-mentioned and other features and advantages of the present invention will be evident from the following, detailed description of preferred embodiments, reference being made to the accompanying drawings, wherein:

(2) FIG. 1 is a schematic illustration of a pump station,

(3) FIG. 2 is a flow chart showing a preferred embodiment of the method according to the invention,

(4) FIG. 3 is a flow chart showing the sub-method Cleaning,

(5) FIG. 4 is a diagram that schematically shows how the current consumption I is changed over time T, and when a detrimental operating condition has been detected,

(6) FIG. 5 is a diagram corresponding to the one shown in FIG. 4, when a strained operating condition of a first type has been detected,

(7) FIG. 6 is a diagram corresponding to the one shown in FIG. 5, when a strained operating condition of a second type has been detected,

(8) FIG. 7 is a diagram corresponding to the one shown in FIG. 4, when a time-based cleaning need has been detected,

(9) FIG. 8 is a flow chart showing another embodiment of the method according to the invention,

(10) FIG. 9 is a flow chart showing yet another embodiment of the method according to the invention,

(11) FIG. 10 is a flow chart showing still another embodiment of the method according to the invention, and

(12) FIG. 11 is a flow chart showing still another embodiment of the method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(13) In FIG. 1, a pump station, generally designated 1 is shown, comprising at least one speed controlled pump 2, usually two submersible pumps, arranged to pump liquid from a sump 3 included in the pump station 1 to an outlet pipe 4 and further away from the pump station 1. Furthermore, in a conventional way, the pump station 1 comprises at least one level instrument 5 arranged to determine the liquid level in the pump station 1. It should be pointed out that the level instrument 5 may be a separate device that is operatively connected to an external control unit 6, be operatively connected to said at least one speed controlled pump 2, be built-in in said at least one speed controlled pump 2, etc. Said at least one speed controlled pump 2 is preferably operatively connected to the external control unit 6 with the purpose of allowing regulation of the rotational speed of the pump, alternatively, said at least one speed controlled pump 2 may comprise a built-in control unit (not shown). Hereinbelow, the denomination control unit 6 will be used independently of the physical location of the same.

(14) Together, the pump 2 and the control unit 6 form at least one part of a pump arrangement, wherein the pump 2 comprises an electric motor 7, which is arranged to be driven by said control unit 6, and an impeller 8, which is connected to the motor 7 via a drive shaft 9 in a conventional way.

(15) With the wording speed controlled, all feasible ways to change the rotational speed of a pump, or more precisely the rotational speed of the motor 7, are embraced, above all, reference is made to current feed frequency control by means of a frequency converter (VFD), which is built-in in a pump or which is external, and which is an example of said control unit 6, the rotational speed being proportionate to the current feed frequency. However, internally or externally controlled supply voltage control, internal mechanical brake that preferably acts on the drive shaft of the pump, etc., is also intended. Thus, on a comprehensive level of the invention, it is not central how the rotational speed of the pump is regulated, only that the rotational speed of the pump 2 can be regulated/controlled.

(16) The method according to the invention is aimed at controlling a pump arrangement that comprises a pump 2 having a motor 7 and a control unit 6, with the purpose of obtaining adapted cleaning based on the instantaneous operating condition of the pump 2. In this connection, the pump station 1 should be seen as a delimited plant to which incoming liquid arrives and from which outgoing liquid is pumped. The pump station should, as regards the present invention, be regarded irrespective of the type of liquid and irrespective from where the liquid comes and where the liquid should be pumped. In the case when the pump station comprises a plurality of pumps 2, a suitable alternation may take place between them, which however is not handled herein.

(17) In FIG. 2, there is shown a preferred embodiment of a method, generally designated 10, for controlling a pump arrangement comprising a pump 2 and a control unit 6. It should be pointed out that the method 10 according to the invention can be expanded using one or more sub-methods, and/or be run in parallel with other control methods.

(18) The method 10 according to the invention for controlling a pump arrangement is in practice a cleaning method for a pump, which is entirely or partly clogged. The extent of clogging and/or the character of clogging create a load on the motor 7 of the pump 2 and indicate an operating condition of the pump arrangement. Thus, at each individual instant of time, when the pump 2 is in an active state and the motor 7 is driven in a first direction by the control unit 6, the motor 7 is associated with a load factor that corresponds to an operating condition of the pump arrangement. The pump arrangement also comprises means for, intermittently or continuously, monitoring at least one operating parameter (OP) from which the load factor, torque, and rotational speed of the motor 7 can be derived, either by direct measurement or by derivation from a measurement of another operating parameter/quantity. Said operating parameter (OP) is, for instance, current consumption (I), power consumption (P), torque (M), rotational speed (RPM), etc., or combinations thereof. In reality, the load factor of the motor 7 will be changed, which means that torque and rotational speed are changed, when the hydraulic unit of the pump 2 is entirely or partly clogged. A direct effect of this is that the current consumption, power consumption etc., of the pump are changed to the corresponding extent, wherein the load factor of the motor 7 can be derived from, for instance, the current consumption of the motor. Preferably, the real current consumption I.sub.R of the pump 2, or more precisely of the motor 7, is monitored when the pump 2 is in the above-mentioned active state, and hereinbelow, the invention will be described using this as a starting point. However, it will be appreciated that the invention is not limited to the measurement of current consumption as operating parameter.

(19) Now, the method 10 according to the invention in its most fundamental form will be described in connection with FIGS. 2 and 4.

(20) The method 10 presupposes that the pump 2 is in its active state and the motor 7 is driven in a first direction by the control unit 6. In this connection and in normal operation, said first direction is the direction that makes the impeller 8 to transport liquid from the sump 3 to the outlet pipe 4, i.e., the motor 7 is driven in the forward direction. In the start of the pump 2, i.e., originating from an inactive state of the pump 2, the control unit 6 provides for a controlled, for instance linear, up-ramping of the nominal rotational speed (V.sub.N) of the motor 7 from 0 to a predetermined operating speed (V.sub.D) that, for instance, is approximately 75-85% of the so-called maximum rotational speed (V.sub.MAX) of the motor 7. The maximum rotational speed of the motor 7 is the rotational speed the motor 7 has if the pump 2 would be directly connected to a grid (i.e., usually a current feed frequency of 50 Hz or 60 Hz). The operating speed (V.sub.D) may, for instance, be a manually set value or an automatically optimized value based on instantaneous energy consumption, etc.

(21) When the pump 2 is in said active state, the method 10 comprises the step of determining a real value of said at least one operating parameter; in the described embodiment, real current consumption (I.sub.R) is determined. The real current consumption (I.sub.R) varies during normal operation around a nominal value of the current consumption (I.sub.N) because of solid material found in the pumped liquid entering, affecting and being transported through the hydraulic unit of the pump 2 and thereby instantaneously impacting the load factor of the motor 7.

(22) Next, the step occurs of determining, based on said real value of said at least one operating parameter, if an externally applied force is acting on the motor 7 to such an extent that an operating condition detrimental to the pump arrangement is initiated, which is true when the load factor of the motor 7 exceeds a level detrimental to the pump arrangement 7. With detrimental operating condition, reference is made to an operating condition that immediately or in the short term will cause the pump 2 and/or the control unit 6 to become overworked and break down upon unaltered driving of the motor 7, alternatively will cause the security system/protective equipment to trig. A detrimental operating condition is imminent when a large and/or hard object enters the hydraulic unit of the pump 2 and is wedged between the impeller 8 and the pump housing. Examples of how the step of determining if an externally applied force is acting on the motor 7 are presented below.

(23) Next, the step occurs of effecting a state shift from the active state of the pump 2 to an inactive state of the pump 2 if an operating condition detrimental to the pump arrangement is initiated, said state shift in turn comprising the step of the control unit 6, immediately after it is determined that an operating condition detrimental to the pump arrangement is initiated, abruptly breaking the driving of the motor 7 in said first direction. The feature to abruptly break the driving is realised by the nominal rotational speed (V.sub.N) of the motor 2 being set equal to 0 in the control unit 6, i.e., no down-ramping of the rotational speed of the motor 7 takes place, or by the nominal rotational speed (V.sub.N) of the motor 2 being set equal to 0 by disengagement of the motor 7, i.e., the motor 7 being made entirely currentless. This entails that the foreign object, which has entered the hydraulic unit of the pump 2 and been wedged, is not wedged harder/more severe.

(24) In the preferred embodiment, shown in FIG. 4, the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition detrimental to the pump arrangement 1 is initiated comprises the step of determining if said real value of said at least one operating parameter (OP) is equal to or exceeds a predetermined detrimental threshold value (G.sub.S). In other words, it is checked whether the real current consumption I.sub.R is equal to or exceeds a predetermined detrimental threshold value (G.sub.S) of the current consumption. Preferably, the value of said detrimental threshold value (G.sub.S) is 70-90%, most preferably 75-85%, greater than the nominal value of the operating parameter, e.g., the nominal current consumption (I.sub.N).

(25) According to an alternative embodiment shown in FIG. 8, the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition detrimental to the pump arrangement 1 is initiated comprises instead the step of determining if said real value of said at least one operating parameter (OP) is outside a predetermined detrimental interval (R.sub.S). Said detrimental interval (R.sub.S) may be equilaterally as well as inequilaterally distributed around the nominal value of the operating parameter, e.g., the nominal current consumption (I.sub.N).

(26) According to a further alternative embodiment shown in FIG. 9, the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition detrimental to the pump arrangement 1 is initiated comprises instead the step of determining if a difference between said real value and a predetermined nominal value of the operating parameter (OP.sub.N), when the pump 2 is in an active state, is equal to or exceeds a predetermined detrimental difference threshold value (D.sub.S). In the described embodiment, the difference is determined between the real current consumption (I.sub.R) and the nominal current consumption (I.sub.N).

(27) Reference is once again made primarily to FIG. 2, in combination with FIGS. 5 and 6. The method 10 according to the invention comprises preferably also the step of determining, based on said real value of said at least one operating parameter, if an external force is acting on the motor 7 to such an extent that an operating condition straining the pump arrangement 1 is initiated, which is true when the load factor of the motor 7 exceeds a level straining the pump arrangement 1, and the step of effecting a state shift from the active state of the pump 2 to a cleaning state of the pump 2 if an operating condition straining the pump arrangement 1 is initiated. With a strained operating condition, reference is made to an operating condition that relatively slowly gives rise to a higher current consumption and impaired pump performance/impaired hydraulic properties, wherein the pump 2 and/or the control unit 6 will be unnecessarily strained. A strained operating condition is imminent when solid matter is caught in the hydraulic unit of the pump and slowly adheres to the impeller of the pump 2 as well as to the inside of the pump housing of the pump 2, see FIG. 5. Alternatively, a strained operating condition is imminent when the inlet of the pump 2 entirely or largely is plugged up by a large object that does not enter the hydraulic unit of the pump 2 and thereby prevents liquid flow into the pump 2, see FIG. 6. Examples of how the step of determining if an externally applied force is acting on the motor 7 are presented below.

(28) Said state shift from the active state of the pump 2 to the cleaning state of the pump 2 comprises preferably the step of decreasing the rotational speed of the motor 7 in said first direction according to a predetermined controlled, for instance linear, down-ramping of the nominal rotational speed (V.sub.N) of the motor 7 to 0.

(29) In the preferred embodiment, shown in FIG. 5, the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition straining the pump arrangement 1 is initiated comprises the step of determining if said real value of said at least one operating parameter is equal to or exceeds a predetermined straining threshold value (G.sub.A) during a predetermined first period of time (T.sub.1). In other words, it is checked whether the real current consumption (I.sub.R) is equal to or exceeds a predetermined straining threshold value (G.sub.A) of the current consumption during a predetermined period of time, which preferably is longer than 5 s. Preferably, the value of said strained threshold value (G.sub.A) is 10-30%, most preferably 15-25%, greater than the nominal value of the operating parameter, e.g., the nominal current consumption (I.sub.N).

(30) According to an alternative embodiment shown in FIG. 10, the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition straining the pump arrangement 1 is initiated comprises instead the step of determining if said real value of said at least one operating parameter (OP) is outside a predetermined straining interval (R.sub.A) during a predetermined first period of time (T.sub.1). Said straining interval (R.sub.A) may be equilaterally as well as inequilaterally distributed around the nominal value of the operating parameter, e.g., the nominal current consumption (I.sub.N).

(31) According to a further alternative embodiment shown in FIG. 11), the step of determining if an external force is acting on the motor 7 to such an extent that an operating condition straining the pump arrangement 1 is initiated comprises instead the step of determining if a difference between said real value and a predetermined nominal value of the operating parameter (OP.sub.N), when the pump 2 is in an active state, is equal to or exceeds a predetermined straining difference threshold value (D.sub.A) during a predetermined first period of time (T.sub.1). In the described embodiment, the difference is determined between the real current consumption (I.sub.R) and the nominal current consumption (I.sub.N).

(32) In those cases when the method 10 comprises detection of a detrimental operating condition as well as a strained operating condition, the threshold value (G.sub.S) detrimental to said at least one operating parameter should be at least 20% greater than the threshold value (G.sub.A) straining said at least one operating parameter. Alternatively, the interval (R.sub.S) detrimental to said at least one operating parameter should be greater than and include the interval (R.sub.A) straining said at least one operating parameter. Further alternatively, the difference threshold value (D.sub.S) detrimental to said at least one operating parameter should be at least two times greater than the difference threshold value (D.sub.A) straining said at least one operating parameter.

(33) Reference is once again made primarily to FIG. 2, in combination with FIG. 7. The method 10 according to the invention comprises preferably also the step of effecting a state shift from the active state of the pump 2 to a cleaning state of the pump 2 if the motor 7 continually has been driven in said first direction during a predetermined second period of time (T.sub.2). In other words, the pump 2 should at regular intervals undergo a cleaning even if strained or detrimental clogging has not been detected. Said state shift from the active state of the pump 2 to the cleaning state of the pump 2 comprises preferably the step of decreasing the rotational speed of the motor 7 in said first direction according to a predetermined controlled, for instance linear, down-ramping of the nominal rotational speed (V.sub.N) of the motor 7 to 0.

(34) Reference is now made essentially to FIG. 3. In conclusion of the method 10 according to the invention, a sub-method is carried out that generally is designated 11 and goes under the denomination Cleaning. It should be mentioned that the method 10 according to the invention may comprise different types of sub-methods coupled to cleaning, based on if a detrimental or strained operating condition, respectively, have been detected. Hereinbelow, however, one and the same sub-method 11 is described.

(35) The sub-method 11 comprises the steps of stopping the motor 7, and keeping the motor 7 stopped during a predetermined third period of time (T.sub.3). In this way, an automatic backwash of the hydraulic unit of the pump 2 is obtained, whereupon the solid matter possibly is flushed out of the pump 2.

(36) Furthermore, the sub-method 11 preferably comprises the steps of increasing the nominal rotational speed (V.sub.N) of the motor 7 from 0 to a second cleaning rotational speed (V.sub.2) in a second direction opposite the first direction according to a predetermined up-ramping of the rotational speed of the motor 7, driving the motor 7 in said second direction during a predetermined fourth period of time (T.sub.4), decreasing the nominal rotational speed (V.sub.N) of the motor 7 from said second cleaning rotational speed (V.sub.2) in said second direction to 0 according to a predetermined down-ramping of the rotational speed of the motor 7, and keeping the motor 7 stopped during said predetermined third period of time (T.sub.3). In this way, an increased turbulence is obtained in the hydraulic unit of the pump 2, which causes solid matter to come loose and be pumped rearward out of the pump 2; in addition, the effect is obtained that large and/or hard objects that are caught in the hydraulic unit of the pump 2 will be worked loose.

(37) Next, the sub-method 11 preferably comprises the steps of increasing the nominal rotational speed (V.sub.N) of the motor 7 from 0 to a first cleaning rotational speed (V.sub.1) in the first direction according to a predetermined up-ramping of the rotational speed of the motor 7, driving the motor 7 in said first direction during the predetermined fourth period of time (T.sub.4), decreasing the nominal rotational speed (V.sub.N) of the motor 7 from said first cleaning rotational speed (V.sub.1) in said first direction to 0 according to a predetermined down-ramping of the rotational speed of the motor 7, and keeping the motor 7 stopped during said predetermined third period of time (T.sub.3). In this way, a further increased turbulence in the hydraulic unit of the pump 2 is obtained, which causes solid matter to come loose and be pumped out of the pump 2; in addition, an enhanced effect is obtained in that large and/or hard objects that are caught in the hydraulic unit of the pump 2 will be worked loose.

(38) Said up-rampings are preferably controlled, for instance linear, up-rampings of the nominal rotational speed (V.sub.N) of the motor 7 from 0 to the predetermined second cleaning rotational speed (V.sub.2) in said second direction and to the predetermined first cleaning rotational speed (V.sub.1) in said first direction, respectively. Furthermore, said down-rampings are preferably controlled, for instance linear, down-rampings of the nominal rotational speed (V.sub.N) of the motor 7 from the predetermined second cleaning rotational speed (V.sub.2) in the second direction and from the predetermined first cleaning rotational speed (V.sub.1) in the first direction, respectively, to 0. The magnitude of the first cleaning rotational speed (V.sub.1) is preferably equal to the maximal rotational speed (V.sub.MAX) of the motor 7, and the magnitude of the second cleaning rotational speed (V.sub.2) is preferably equal to 80% of the maximal rotational speed (V.sub.MAX) of the motor. Said third period of time (T.sub.3) and said fourth period of time (T.sub.4) are preferably longer than 5 s each.

(39) Feasible Modifications of the Invention

(40) The invention is not limited only to the embodiments described above and shown in the drawings, which only have the purpose of illustrating and exemplifying. This patent application is intended to cover all adaptations and variants of the preferred embodiments described herein, and consequently the present invention is defined by the wording of the accompanying claims and the equivalents thereof. Accordingly, the equipment may be modified in all feasible ways within the scope of the accompanying claims.

(41) It should be pointed out that even if it is not explicitly mentioned that features from one specific embodiment can be combined with the features of another embodiment, this should be regarded as evident when possible.

(42) Throughout this specification and in the subsequent claims, unless the context indicates something different, it will be appreciated that the word comprise, and variants such as comprises or comprising, means inclusion of indicated unit or step or group of units or steps but not exclusion of other units or steps or groups of units or steps.