Pump assembly and controlling method

Abstract

A pump assembly (1) includes a pump unit (2) capable of providing a desired head (H.sub.0) at zero flow rate, a brushless speed-controlled permanent-magnet AC drive motor (205) for driving the pump unit (2), and a control unit for controlling the drive motor (205). The control unit includes a frequency converter configured to receive an input voltage (U.sub.in). The drive motor (205) is operable in a field-weakening mode and non-field-weakening mode. The drive motor (205) is undersized for driving the pump unit (2) at a design input voltage (U.sub.0) to provide a lower head (H) than the desired head (H.sub.0) at zero flow rate in the non-field-weakening mode and for driving the pump unit (2) to provide the desired head (H.sub.0) at zero flow rate in the field-weakening mode.

Claims

1. A pump assembly comprising: a pump unit configured to provide a desired pressure head at zero flow rate; a brushless speed-controlled permanent-magnet AC drive motor for driving the pump unit; and a control unit for controlling the drive motor, the control unit comprising a frequency converter configured to receive an input voltage, wherein: the drive motor is operable in a field-weakening mode and non-field-weakening mode; and the drive motor is undersized for driving the pump unit at a design input voltage to provide a lower pressure head than the desired pressure head at zero flow rate in the non-field-weakening mode and for driving the pump unit to provide the desired pressure head at zero flow rate in the field-weakening mode.

2. The pump assembly according to claim 1, wherein the frequency converter is configured to provide a pulse width modulated AC output voltage at a modulation index to the drive motor.

3. The pump assembly according to claim 2, wherein the AC output voltage is limited by the input voltage at a maximum modulation index M.sub.max1.

4. The pump assembly according to claim 1, wherein the design input voltage is a lowest possible input voltage for providing the lower pressure head at zero flow rate in the non-field-weakening mode.

5. The pump assembly according to claim 1, wherein: the control unit further comprises a voltage converter for providing the input voltage to the frequency converter; and the input voltage is adjustable within a voltage range between a minimum input voltage and a maximum input voltage.

6. The pump assembly according to claim 1, wherein the drive motor comprises a stator with at least 10% more windings of a wire having at least a 10% smaller cross-section compared to a reference drive motor sized for driving the pump unit to provide the desired pressure head at zero flow rate in the non-field-weakening mode.

7. The pump assembly according to claim 1, wherein the drive motor may demand an at least 10% higher output voltage from the frequency converter when operated in non-field-weakening mode compared to a reference drive motor sized for driving the pump unit to provide the desired pressure head at zero flow rate in the non-field-weakening mode.

8. The pump assembly according to claim 1, wherein the frequency converter is configured to receive an input voltage below 60 V.

9. The pump assembly according to claim 5, wherein: the voltage converter is configured to provide the input voltage within a voltage range between the minimum input voltage and a reference voltage in the field-weakening mode; the voltage converter is configured to provide the input voltage within a voltage range between the reference voltage and the maximum input voltage in the non-field-weakening mode; and the minimum input voltage is less than the reference voltage which is less than the maximum input voltage.

10. The pump assembly according to claim 9, wherein the frequency converter is configured to operate at a maximum modulation index M.sub.max1 when the input voltage approximately equals the reference voltage.

11. The pump assembly according to claim 5, wherein the design input voltage is the maximum input voltage for providing the pressure head at zero flow rate in the non-field-weakening mode.

12. The pump assembly according to claim 5, wherein: the control unit is configured to determine an actual power consumption of at least one of the drive motor and the frequency converter during operation of the pump unit; and the control unit is configured to tune the input voltage so that the determined actual power consumption is minimized.

13. The pump assembly according to claim 1, wherein the pump unit comprises a wet rotor circulation pump for a heating or cooling system.

14. A method for controlling a brushless speed-controlled permanent-magnet AC drive motor via a frequency converter for driving a pump unit for providing a desired pressure head at zero flow rate, the method comprising: driving the pump unit at a design input voltage to provide a lower pressure head than the desired pressure head at zero flow rate in a non-field-weakening mode; driving the pump unit to provide the desired pressure head at zero flow rate in a field-weakening mode.

15. The method according to claim 14, further comprising providing a pulse width modulated AC output voltage at a modulation index to the drive motor.

16. The method according to claim 14, wherein the design input voltage is the lowest possible input voltage for providing the pressure head at zero flow rate in the non-field-weakening mode.

17. The method according to claim 14, wherein driving the pump unit at a design input voltage to provide a lower pressure head than the desired pressure head at zero flow rate in a non-field-weakening mode comprises driving the pump unit with an at least 10% higher output voltage from the frequency converter to the drive motor compared to driving the pump unit with a reference drive motor sized for driving the pump unit to provide the desired pressure head at zero flow rate in the non-field-weakening mode.

18. The method according to claim 14, further comprising providing an input voltage to the frequency converter, wherein the input voltage is adjustable within a voltage range between a minimum input voltage and a maximum input voltage.

19. The method according to claim 18, wherein the design input voltage is the maximum input voltage for providing the pressure head at zero flow rate in the non-field-weakening mode.

20. The method according to claim 18, wherein providing an input voltage comprises providing a DC input voltage below 60 V.

21. The method according to claim 18, wherein: providing the input voltage comprises providing the input voltage within a voltage range between the minimum input voltage and a reference voltage in the field-weakening mode; providing the input voltage includes providing the input voltage within a voltage range between the reference voltage and the maximum input voltage in the non-field-weakening mode; and the minimum input voltage is less than the reference voltage which is less than the maximum input voltage.

22. The method according to claim 21, further comprising operating the frequency converter at a maximum modulation index M.sub.max1 when the input voltage approximately equals the reference voltage.

23. The method according to claim 14, further comprising determining an actual power consumption of at least one of the drive motor and the frequency converter during operation of the pump unit, and wherein the control unit is configured to tune the input voltage so that the determined actual power consumption is minimized.

Description

SUMMARY OF THE DRAWINGS

(1) Embodiments of the present disclosure will now be described by way of example with reference to the following figures of which:

(2) FIG. 1 is a perspective view on an example of a pump assembly disclosed herein;

(3) FIG. 2 is a schematic power circuit diagram of a control unit according to an example of a pump assembly disclosed herein;

(4) FIG. 3 is a circuit diagram of a three phase equivalent circuit of a control unit according to an example of a pump assembly disclosed herein;

(5) FIG. 4 is a circuit diagram a single phase equivalent circuit of a control unit according to an example of a pump assembly disclosed herein;

(6) FIG. 5 is a vector diagram for operating a control unit according to an example of a pump assembly disclosed herein in a non-field-weakening mode;

(7) FIG. 6 is a vector diagram for operating a control unit according to an example of a pump assembly disclosed herein in a field-weakening mode;

(8) FIG. 7 is a graph view showing a design phase current, a design motor speed, a design motor power loss and a design electronics power loss for three different fixed design torques over the number of stator windings;

(9) FIG. 8 is a graph view showing a HQ-diagram, a PQ-diagram and two eQ-diagrams for full and part load operation, respectively, of a frequency converter according to an example of a pump assembly disclosed herein; and

(10) FIG. 9 is a UH-diagram according to an example of a pump assembly disclosed herein compared to a standard motor design.

DETAILED DESCRIPTION

(11) Referring to the drawings, FIG. 1 shows a pump assembly 1 with a centrifugal pump unit 2, an input port 3 and an output port 5, wherein the input port 3 and an output port 5 are coaxially arranged on a pipe axis A on opposing sides of the pump unit 2. The input port 3 and the output port 5 comprise connector flanges 7, 9 for a connection to pipes (not shown). The pump unit 2 comprises a rotor axis R essentially perpendicular to the pipe axis A. A pump housing 11 of the pump unit 2 is essentially arranged between the input port 3 and the output port 5. The pump housing 11 comprises an impeller (not shown) for rotating around the rotor axis R and pumping fluid from the input port 3 to the output port 5. The impeller is driven by a motor (not shown) located in a motor housing 13 extending from the pump housing 11 along the rotor axis R to an electronics housing 15. The electronics housing 15 comprises an inverter circuit 201 (see FIG. 2) for controlling a three-phase synchronous permanent magnet drive motor 205.

(12) The circuit diagram of FIG. 2 illustrates the basic principle of the inverter circuit 201 of a frequency converter of a motor control unit located within the electronics housing 15, the inverter circuit 201 comprising six switches 203 in form of insulated-gate bipolar transistors (IGBT) or metal oxide semiconductor field effect transistors (MOSFET). A microcontroller (not shown) controls the six switches 203 to produce a desired pulse width modulated AC output voltage U.sub.out for each phase of the three-phase motor 205. The three phases are phase-shifted by 120 relative to each other for driving the motor 205. An input voltage U.sub.in, which may be referred to as DC link voltage, may be provided as input to the converter 201 by a rectifier (not shown) or a voltage converter (not shown). The input voltage U.sub.in may be adjustable to maintain a maximum modulation index M in part load operation of the frequency converter.

(13) The three phase equivalent circuit of FIG. 3 simplifies the main components for controlling the motor. The three phases are identical and only phase-shifted by 120 with respect to each other. For each phase, the output voltage U.sub.out results from a resistive component U.sub.R, an inductive component U.sub.L and a counter-electromotive force U.sub.E (abbreviated counter EMF or back EMF). The back EMF U.sub.E is an induced voltage caused on the stator windings by the motion of the permanent magnet rotor, and is thus dependent on the magnetic flux and the motor speed by U.sub.E=.Math.. As further simplified in FIG. 4 as a single phase equivalent circuit, the main components may only be considered for one phase.

(14) The vector diagram in a rotating reference frame of FIG. 5 illustrates the phase relation between the main components in non-field weakening mode. The magnetic flux .sub.pm of the permanent magnet is 90 phase-shifted with respect to the phase current I. The output voltage U.sub.out results from the resistive voltage drop U.sub.R=R.Math.I, the back EMF U.sub.E and the inductive voltage drop U.sub.L=.Math.L.Math.I, where an effective inductance L includes both a self-inductance and a phase coupling inductance. The resistive voltage drop U.sub.R and the back EMF U.sub.E are in phase, and the inductive voltage drop U.sub.L is 90 phase-shifted with respect to the resistive voltage drop U.sub.R. In the example shown in FIG. 5, the modulation index M is maximal so that the output voltage U.sub.out essentially equals a maximum output voltage U.sub.out,max illustrated by the circle. The undersized drive motor of the pump disclosed herein is designed to operate at a maximum modulation index M.sub.max, a maximum output voltage U.sub.out, max and a speed in non-field weakening mode to provide a head H at a zero flow rate as shown in FIG. 5.

(15) In order to achieve a higher desired H.sub.0 at a zero flow rate, the speed must be increased in field weakening mode as shown in FIG. 6. The switches 201 are controlled in such a way that the phase current I is phase shifted by an angle , whereby a magnetic flux .sub.m=I.Math.L.Math.sin is induced by the stator windings weakening the resulting magnetic flux in the motor =.sub.pm+.sub.m. The reduced magnetic flux in the motor results in less torque and hydraulic output power of the pump, but can provide (at lower efficiency) the desired H.sub.0 at zero flow rate. The reduced efficiency is the reason why conventional motor design for pumps would foresee motor parameters, such as the number of stator windings and/or the wire cross-section for achieving the desired H.sub.0 at a zero flow rate through the non-field weakening mode as depicted in FIG. 5. However, the undersized drive motor of the pump disclosed herein may not predominantly be operated in such a full load operation of the frequency converter. Most of the time, the frequency converter is operated at part load in a non-field weakening mode for which the undersized armature is more efficient.

(16) Thus, field-weakening mode means that the phase current partly reduces the total magnetic flux, because it is phase-shifted with respect to the rotor magnetic flux by more than 90. In non-field-weakening mode, the phase current has a phase-shift of 90 or less with respect to the rotor magnetic flux such that no component of the phase current reduces the total magnetic flux. In order to measure such a phase shift, position sensors may be used. As an alternative or in addition to using position sensors measuring the phase shift angle , the output voltage U.sub.out may be measured to determine whether a motor is running in field-weakening mode or non-field weakening mode. Having determined the magnetic flux , the resistance R, the inductance L, the motor speed and the phase current I, an output voltage U.sub.out,calc may be calculated as
U.sub.out,calc={square root over ((U.sub.E+U.sub.R).sup.2+U.sub.L.sup.2)}={square root over ((.Math.+I.Math.R).sup.2+(.Math.L.Math.I).sup.2)}
under the assumption of non-field-weakening mode, i.e. a 90 phase-shift between U.sub.L and U.sub.R. If the measured output voltage U.sub.out is lower than the calculated output voltage U.sub.out,calc, the motor is running in field-weakening mode. Otherwise, it is running in non-field-weakening mode.

(17) Thus, a drive motor of a pump may be tested on whether it is undersized or not by operating the pump at the specified minimal input voltage to provide a head H.sub.0 at zero flow according to the specification of the pump. The magnetic flux , the resistance R, the inductance L, the motor speed and the phase current I may be measured to calculate U.sub.out,calc under the assumption of non-field-weakening mode as outlined above. If the measured output voltage U.sub.out is lower than the calculated output voltage U.sub.out,calc, the motor is running in field-weakening mode and is therefore undersized. Otherwise, it is running in non-field-weakening mode and is thus normally sized.

(18) FIG. 7 shows different operational parameters for three fixed torques T.sub.1, T.sub.2 and T.sub.3, where T.sub.1>T.sub.2>T.sub.3, as a function of the number of stator windings N as design parameter. In a), the phase current I decreases in the non-field weakening region A with the number of stator windings N. Conventional motor design would aim to reduce the phase current for a given torque T.sub.1 by the maximum number of windings up to the borderline between the non-field weakening region A and the field weakening region B (see point C). In contrast to that, the undersized drive motor disclosed herein has at least 10% more windings than that and has an operational point P in the field weakening region B for achieving a head H.sub.0 at a zero flow rate. In b), the maximum speed .sub.max is similarly limited by the number of windings N. The undersized drive motor with more windings at point P has a reduced maximum speed .sub.max compared to a conventional reference motor at point C. The power loss in the motor as shown in c) is constant in the non-field weakening region A and rises with the number of windings N in the non-field weakening region B. Thus, conventional motor design would not exceed the number of stator windings into the field weakening region B, because it results in power loss in the motor. The undersized drive motor at point P has thus higher motor power loss for full load operation. The electronic power loss is shown in d) with a minimum at the borderline between the non-field weakening region A and the field weakening region B (see point C). A deviation like the undersized drive motor at point P results in more electronic power loss.

(19) FIG. 8 a) shows in a head-flow diagram, i.e. HQ-diagram, as a solid line the characteristic curve of the pump with the undersized drive motor operating in non-field weakening mode, in which only a head H is achievable at a zero flow rate. The dashed curve displays the undersized pump's characteristic curve in field weakening mode, in which it achieves the desired H.sub.0. Conventional pump design would choose design parameters to follow the dashed curve in non-field weakening mode. The reason for this becomes clear in FIG. 8 b) showing a higher power consumption P.sub.in for running in field weakening mode (dashed line) compared to the non-field weakening mode at full load of the frequency converter. FIG. 8 c) shows the efficiency h=P.sub.in/P.sub.out at full load of the frequency converter in comparison between a conventional motor design (solid line) and the undersized design (dashed-dotted line). At full load, the conventional motor design is more efficient for lower flow rates and essentially the same for higher flow rates. However, as shown in FIG. 8 d), at part load of the frequency converter in non-field weakening mode only, the efficiency h=P.sub.in/P.sub.out is lower for a conventional motor design (solid line) compared to the undersized design (dashed-dotted line) over a wide range of flow rates. So, the undersized drive motor has its advantage in particular when the frequency converter is operated at part load.

(20) FIG. 9 shows the output voltage U.sub.out as a function of the head H for a standard motor design (solid line) and an undersized motor design (dashed-dotted line). In part load of the frequency converter for providing a head in a range between H.sub.1 to H.sub.0, where H.sub.1<H.sub.0, the standard motor control would vary the output voltage U.sub.out adapted to the needed motor speed. In order to maintain a high modulation index M, the input voltage U.sub.in to the frequency converter may be adjusted accordingly. The standard motor design would achieve the desired head H.sub.0 at a maximum output voltage U.sub.out,max with the maximum modulation index M.sub.max.

(21) In part load for providing a head in a range between H.sub.1 to H, where H.sub.1<H, the undersized drive motor operates at a higher output voltage compared to the standard motor design for providing the same head. The minimal output voltage U.sub.out,min undersized for providing a head H.sub.1 is higher than the minimal output voltage U.sub.out,min standard for a standard motor. Therefore, the undersized motor can be operated more efficiently with a lower phase current I to provide the same power P.sub.out in part load of the frequency converter. However, when the maximum output voltage U.sub.out,max is reached at a head H, the undersized motor must go into the less efficient field weakening mode for providing a higher head than H.

(22) Where, in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as optional, preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

(23) The above embodiments are to be understood as illustrative examples of the disclosure. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. While at least one exemplary embodiment has been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art and may be changed without departing from the scope of the subject matter described herein, and this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

(24) In addition, comprising does not exclude other elements or steps, and a or one does not exclude a plural number. Furthermore, characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. Method steps may be applied in any order or in parallel or may constitute a part or a more detailed version of another method step. It should be understood that there should be embodied within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of the contribution to the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the disclosure, which should be determined from the appended claims and their legal equivalents.