US20030138327A1 - Speed control for a pumping system - Google Patents

Abstract

A wastewater pumping system includes a centrifugal pump operatively connected to an electric motor via a coupling. The speed of the electric motor is automatically varied responsive to the head or load realized by the pump of the system as it varies with the composition of the slurry mixture. A variable frequency drive is operatively connected to the motor and the electrical power supply that affects a shift or change in electrical frequency delivered to the electrical motor so that the speed of the motor and consequently the pump is varied. This allows the electrical power deliver to the system to remain substantially unchanged for varying loads realized by the pump.

Description

I. BACKGROUND OF THE INVENTION
• A. Field of Invention[0001]
• The present invention relates to the art of wastewater pumps and more specifically to a speed control device for operating the wastewater pumps.[0002]
• B. Description of the Related Art[0003]
• Wastewater pumping systems are known to use centrifugal pumps. Centrifugal pumps work well to maintain flow rate. However, when pressure or head is increased in a wastewater system the flow rate decreases. Therefore, the centrifugal pump does not lend itself to higher head applications.[0004]
• A typical centrifugal pump functions based on speed of the impeller or kinetic energy of the fluid being pumped based on the impeller speed. This is contrasted with positive displacement pumps, such as piston pumps or the like. Typically, the centrifugal pump is powered by an electric motor. That is to say that the motor, generally an AC motor, rotates the impeller of the pump at a specific speed based upon the operating characteristics of the motor, such as the number poles in the motor and the frequency of the power being supplied to the motor. If the load realized by the impeller increases, the impeller will want to slow down. However, the motor will naturally try to maintain the characteristic speed of the motor by drawing additional current to compensate for the increase in load seen by the impeller. This results in increased current and power being drawn by the centrifugal pumping system.[0005]
• What is needed is a self-regulating speed controlling device, for maximizing pump performance and maintaining constant current, which allows the centrifugal pump to be installed in a variety of environments where higher heads, as well as lower operating heads are exhibited.[0006]
• Other objects and advantages of the invention will appear from the following detailed description of the preferred embodiment of the invention with reference being made to the accompanying drawings.[0007]
II. BRIEF SUMMARY
• There is provided a centrifugal pump that is operatively connected to slurry conduits as part of a wastewater pumping system. The pump has an inlet for connecting to a first slurry conduit, which in turn is communicated to a reservoir or source for providing an associated slurry mixture to the system. The pump also includes an outlet connected to second slurry conduit through which the slurry mixture is propelled to a discharge tank. An electric AC motor is directly coupled to the pump for use in providing rotational power to drive the pump thereby propelling the slurry mixture. Inherent to the pumping system, head is developed and realized by the pump that is based upon the elevation or distance that the slurry mixture is being pumped, as well as inconsistencies in the composition of the slurry mixture. Still other characteristics of the system contribute to the head, which are well known in the art but will not be discussed further at this point. As the head increases, which may be due to other pumping system outputs being dumped into the same conduit or inconsistencies in the slurry mixture, a self regulating speed controlling device is operatively electrically communicated between an electrical power source or electrical mains and the motor that alters the speed of the motor and hence the pump in response to the change in load or head realized at the pump. The controlling device includes a variable frequency drive that may alter the frequency of the electrical power thereby changing the speed of the motor and the pump respectively. In this manner, as the load realized by the pump increases, the controlling device alters the electrical power frequency to speed up the motor thereby holding constant the electrical current drawn by the pumping system and vice versa for decreases in pump loads. In other words, the pumping system automatically changes the speed that the motor rotates a propeller in the pump responsive to the load realized by the pump.[0008]
• It is an object on the present invention to provide a wastewater pumping system including a pump, a motor and a controlling device, wherein the controlling device alters the speed of the motor responsive to the load realized by the pump.[0009]
• It is another object of the present invention to provide a wastewater pumping system including a pump, a motor, a controlling device and a load current sensor that automatically adjusts the frequency of the electrical power received by the controlling device for delivery to the motor responsive to the change in load realized by the pump as determined by a load current sensor.[0010]
• It is yet another object of the present invention to provide a wastewater pumping system including a centrifugal pump, an AC Induction motor, a variable frequency drive, a load current sensor, reference signal controlling means and selectively adjustable switching means, for use in selecting one of a plurality of reference signals whereby substantially constant current is maintained with respect to the selected reference signal, while the speed of the motor is automatically adjusted responsive to the output from the sensor.[0011]
III. BRIEF DESCRIPTION OF THE DRAWINGS
• The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:[0012]
• FIG. 1 is a schematic representation of a wastewater pumping system.[0013]
• FIG. 2 is a perspective view of an electric motor and wastewater pump with a control panel.[0014]
• FIG. 3 is a schematic representation of a reference signal storing means.[0015]
• FIG. 4 is a pump characteristic comparison chart.[0016]
• FIG. 5 is a schematic representation of a PWM signal.[0017]
• FIG. 6 is a partial schematic representation of a centrifugal with internal rotating impeller.[0018]
IV. DESCRIPTION OF THE PREFERRED EMBODIMENT
• Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the invention only and not for purposes of limiting the same, FIG. 1 depicts a schematic representation of a[0019]wastewater pump1 operatively connected to asupply source3 ofslurry mixture4 via aslurry inlet conduit6. Thewastewater pump1 may include aninlet7 for use in receiving theslurry mixture4 from theslurry conduit6. Thewastewater pump1 may also include animpeller19, shown in FIG. 6, for use in pumping theslurry mixture4 via anoutlet9 to adischarge tank13 or other storage means. In this manner, theslurry mixture4 is transmitted through thebody2 of thepump1. Theoutlet9 of thewastewater pump1 may be connected to thedischarge tank13 viaslurry outlet conduit16. It is noted that any configuration of supply source and discharge tank storage means for containing the slurry mixture may be chosen with sound engineering as is appropriate for wastewater pumping systems. In the preferred embodiment, thewastewater pump1 may be acentrifugal pump18. However, any pumping means may be chosen with sound engineering judgment as is appropriate for a wastewater pumping system including progressive cavity pumps. It is expressly noted that the invention as described herein is not intended to be limiting by the type of pumping device or configuration of the pumping system depicted in the embodiments but shall be construed as applicable to any configuration of pumping mechanism as is appropriate for wastewater pumping systems and other pumping systems as well.
• With reference now to FIG. 2, a[0020]wastewater pump1 is depicted, which as stated may be acentrifugal pump18. Anelectric motor21 is shown coupled to thewastewater pump1, via acoupler22, for use in supplying power to drive thewastewater pump1 at a specified rotational speed. In that the coupling of electric motors to pumping devices is well known in the art no further explanation will be offered at this point. In the preferred embodiment, theelectric motor21 may be anAC Induction motor23 as will be discussed further in a subsequent paragraph. Acontrol panel27 is also shown havingcontrol componentry28 contained therein for use in controlling the operation of the motor and hence the pump. Thecontrol panel27 may include a main disconnect, one or more power transformers, over-current circuit protecting means, as well as other additional control componentry, not shown, as is appropriate for use in a control panel. In that the make-up of such control panels is well known in the art, no further explanation will be offered at this point.
• With continued reference to FIGS. 1 and 2, as is well known in the art, power supplied to the[0021]electric motor21 may be communicated viaelectrical conductors31 that transmit electrical power from thecontrol componentry28 disposed in thecontrol cabinet27, which is further supplied from the electrical mains. It is expressly noted that any type (DC or AC), magnitude or frequency, of electrical power utilized by the motor may be chosen with sound engineering judgment as is appropriate for operating theelectrical motor21. It is also noted that by changing the speed of themotor21, the speed of the impeller of thepump1 may also change accordingly. Thecontrol panel27 may include a motorspeed control module34. The motorspeed control module34 may be mounted within thecontrol panel27. It is also conceived in an alternate embodiment that the motorspeed control module34 may be contained in a separate enclosure mounted exterior to thecontrol panel27 such as in proximity to the pump and operatively electrically connected to the control componentry in a manner well known in the art. In the preferred embodiment, the motorspeed control module34 may include a selectively variable frequency controlling means29 for use in selectively varying the frequency of electrical power that is delivered to themotor21. The selectively variable frequency controlling means29 may receive AC electrical power from atransformer30 located within thecontrol panel27, which may initially receive power from the electrical mains. It is noted that main power may be connected to thetransformer30 for adjusting the magnitude of voltage being communicated to the selectively variable frequency controlling means29. Additionally, the power may also be converted to DC power for use by the selectively variable frequency controlling means29. However, any means of conditioning or communicating power to the selectively variable frequency controlling means29 may be chosen with sound engineering judgment.
• With continued reference to FIG. 2 and now to FIG. 5, as previously noted, the selectively variable frequency controlling means[0022]29 receives electrical power. The selectively variable frequency controlling means29 subsequently delivers a conditionedpower signal42, which is communicated viaelectrical conductors31, to themotor21. In the preferred embodiment, theconditioned signal42 may be Pulse Width Modulated (PWM)signal42, represented in FIG. 5. Any duty cycle, frequency and magnitude combinations may be chosen with sound engineering judgment as is appropriate for controlling amotor21 and as will be discussed in a subsequent paragraph. In other words, the power delivered from the mains is received by the selectively variable frequency controlling means29 and selectively conditioned for use in controlling themotor21. It is expressly noted here that selectively variable frequency controlling means29 may selectively vary the frequency of thePWM signal42. In other words, the selectively variable frequency controlling means29 may be aVariable Frequency Drive29aorVFD29a. The variable frequency controlling means29 may include a sensing means43 for sensing the current load drawn by themotor21. In the preferred embodiment, the sensing means43 automatically determines when a change in the magnitude of current of thesignal42 has occurred and theVFD29aautomatically adjusts the frequency of thePWM signal42 so as to vary the speed of themotor21 such that substantially constant power is drawn by themotor21 during operation of the system. In other words, theVFD29aautomatically selectively adjusts the speed of themotor21, and consequently thepump1, dependent upon the load of thepump1 and hence themotor21 in order to maintain substantially constant current being utilized by the system. Inasmuch, as Variable Frequency Drives are well known in the art, no further explanation will be discussed at this point. It is noted that thepump1,motor21 andcontrol panel27 withVFD29aand supporting components constitute a wastewater pump assembly1a.
• With reference now to FIGS. 4 and 6, a typical centrifugal pump functions based on speed of the impeller or kinetic energy of the fluid being pumped based on the impeller speed. This is contrasted with positive displacement pumps, such as piston pumps or the like, which may function to provide constant fluid pressure. Typically, the centrifugal pump is powered by an electric motor. That is to say that the motor, generally an AC motor, rotates the impeller of the pump at a specific speed based upon the operating characteristics of the motor, such as the number poles in the motor and the frequency of the power being supplied to the motor. If the load realized by the impeller increases, the impeller will want to slow down. However, the motor will naturally try to maintain the characteristic speed of the motor by drawing additional current to compensate for the increase in load seen by the impeller. This results in increased current and power being drawn by the centrifugal pumping system. In the preferred embodiment, it is desirable to maintain constant current and constant voltage over the operating range of the wastewater pump assembly[0023]1a. Therefore, the sensing means43 senses increased current drawn by themotor21 when thepump1 realizes increased head H. The sensing means43 then feeds back signals to theVFD29a, wherein theVFD29amaintains the magnitude of current presently being drawn while increasing the frequency of thePWM signal42. Similarly, with a decrease in Head H, the sensing means43 senses a decrease in current and theVFD29amay consequently decrease the frequency of thePWM signal42 while maintaining substantially the same magnitude of current. In this manner, an increase in Head H results in the automatic adjustment of theVFD29ato increase the frequency ofpower signal42 seen by the motor, which increases the speed of the motor and the impeller. Likewise, decreasing the head H decreases the frequency of thesignal42. In this manner, when a change in the head H occurs, the magnitude of electrical current is substantially constant although the frequency of the motor varies commensurate with the magnitude and direction of change in the head H. In essence, for a given magnitude of current being drawn by the system, there is a corresponding Head versus Flow chart, such as is depicted in FIG. 4, relative to the given magnitude of current.
• With reference now to FIGS. 2 and 3, it is desirable to selectively operate the[0024]pump1 at different levels of substantially constant current in a manner consistent with the previous discussion. Therefore, a reference signal representing a given magnitude of substantially constant operating current must be available for reference by theVFD29ato maintain a selectively chosen constant current during operation. As it is desirable to utilize the present invention in a plurality of environments, a method for selecting one of a plurality of reference signals will be discussed presently. In the preferred embodiment, theVFD29amay include a reference signal storing means45, which may be anIntegrated Circuit45 shown in FIG. 3, that may store a plurality of reference signals used by theVFD29a. It is noted that each signal in the storing means45 corresponds to magnitude of constant operating current. In this manner, theVFD29amay reference one of the signals for use in maintaining a specific magnitude of constant operating current as previously discussed. TheIntegrated Circuit45 may be a Programmable Read-Only-Memory chip45 orPROM45 wherein a plurality of reference signals may be selectively “burned” into thePROM45. However, any means of storing reference signals for use by theVFD29ain maintaining constant operating power may be chosen with sound engineering judgment. In this manner, thepump1 may be operatively configured to operate in various modes or in various environments by referencing different signals stored in the reference signal storing means45. That is to say that theVFD29a, during operation, automatically maintains a substantially constant magnitude of current with respect to one of the reference signals stored in the reference signal-storingmeans45. A selectively adjustable switching means46, shown in FIG. 2a, may also be included in the control panel circuitry and operatively communicated with theVFD29a, which may be selectively adjusted by an operator to cause theVFD29ato reference one of the different signals stored in the reference signal storing means45. In the preferred embodiment, the selectively adjustable switching means46 may be aDip Switch46 having eight (8) binary signal switches. TheDip Switch46 may be operatively connected to theVFD29aso as to select one of the signals stored therein for reference by theVFD29aduring operation. In other words, when an operator adjusts theDip Switch46, the internal VFD circuitry will detect the Dip Switch setting and, in a predetermined manner, reference one of the signals stored in the reference signal storing means45. Thereafter, the wastewater pump assembly1ais selectively configured to automatically substantially maintain the operating current of the motor with respect to the reference signal chosen by the operator, which may correspond to a specific operating environment. In this manner, the wastewater pump assembly1amay be selectively configured for use in a variety of wastewater pumping scenarios by switching into or out-of engagement the individual binary signal switches.
• While specific embodiments of the invention have been described and illustrated, it is to be understood that these embodiments are provided by way of example only and that the invention is not to be construed as being limited thereto but only by proper scope of the following claims.[0025]

Claims (12)

I/we claim:

1. A fluid pumping system, comprising:

a pump operable to pump associated fluid;

a motor operatively coupled to drive the pump; and,

a controlling device electrically communicated between an associated electrical power source and the motor, wherein the controlling device is operable to change the speed of the motor responsive to a change in the load of the pump, while maintaining substantially constant voltage and constant current.

2. The pumping system ofclaim 1, wherein the speed of the motor is changed by adjusting the frequency of electrical power being supplied to the motor.

3. The pumping system ofclaim 1, wherein the controlling device includes:

variable frequency controlling means operably communicated to deliver variable frequency electrical power to the motor.

4. The pumping system ofclaim 3, further comprising:

a sensor operatively communicated to the variable frequency controlling means, and,

wherein the variable frequency controlling means delivers variable frequency electrical power to the motor responsive to the sensor output.

5. The pumping system ofclaim 4, wherein the sensor is a load current sensor for use in feeding back load current being drawn by the pumping system.

6. The pumping system ofclaim 1, further comprising:

a reference signal storing means for use in storing a plurality of reference signals, the reference signal storing means being operatively communicated with the controlling device; and,

wherein the magnitude of constant current is maintained relative to one of the plurality of reference signals stored in the reference signal storing means.

7. The pumping system ofclaim 6, further comprising:

selectively adjustable switching means for use in selecting said one of the plurality of reference signals stored in the reference signal storing means, the selectively adjustable switching means being operatively communicated with the reference signal storing means.

8. A wastewater pumping system, comprising:

a centrifugal pump operable to pump an associated wastewater slurry;

an AC motor operatively coupled to drive the pump; and,

a control panel operatively connected drive to the AC motor, the control panel being operable to receive associated electrical input power, the control panel including:

a load current sensor,

a variable frequency drive operatively communicated to the AC motor and the load current sensor, and,

wherein the variable frequency drive is operable to change the speed of the motor responsive to the output of the sensor, while maintaining substantially constant voltage and constant current.

9. The pumping system ofclaim 8, further comprising:

a reference signal storing means for use in storing a plurality of reference signals, the reference signal storing means being operatively communicated with the variable frequency drive; and,

wherein the magnitude of constant current is maintained relative to one of the plurality of reference signals stored in the reference signal storing means.

10. The pumping system ofclaim 9, further comprising:

selectively adjustable switching means for use in selecting said one of the plurality of reference signals stored in the reference signal storing means, the selectively adjustable switching means being operatively communicated with the reference signal storing means.

11. A method for controlling a pumping system, the steps comprising:

providing a centrifugal pump operable to pump an associated slurry,

an AC motor operatively coupled to drive the pump,

a load current sensor; and,

variable frequency controlling means operatively communicated to the AC motor and the load current sensor,

operating the pumping system;

sensing a change in the load current of the motor;

automatically varying the frequency of the electrical power responsive to the change in the load of the pump while maintaining substantially constant current supplied to the AC motor.

12. A method for controlling a wastewater pumping system, the steps comprising:

providing a centrifugal pump operable to pump an associated wastewater slurry,

an AC Induction motor operatively coupled to drive the pump,

a load current sensor,

a variable frequency drive operatively communicated to the AC motor and the load current sensor,

a reference signal storing means for use in storing a plurality of reference signals, the reference signal storing means being operatively communicated with the variable frequency drive, and,

selectively adjustable switching means for use in selecting one of said plurality of reference signals stored in the reference signal storing means, the selectively adjustable switching means being operatively communicated with the reference signal storing means;

selecting one of a plurality of reference signals;

operating the pumping system;

sensing a change in the load current of the motor;

automatically varying the frequency of the electrical power responsive to the change in the load of the pump, while maintaining substantially constant current supplied to the AC Induction motor relative to said one of a plurality of reference signals.

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