Abstract:
A pump control system includes a plurality of different variable speed pumps. Each of the pumps can be operated at a different speed to equalize pump loads. A system wide proportional integral differential control loop increases and decreases the speed of all pump motors to maintain a desired system output pressure. A second proportional integral differential control loop associated with each pump adjusts pump speed to equalize a load profile for each respective pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 61/034,076 filed Mar. 5, 2008 and entitled “Constant Pressure Variable Speed Pump Control System with Load Equalization for Dissimilar Pumps” which is also incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The invention pertains to systems and methods of control of variable speed pumps. More particularly, the invention pertains to such systems and methods which take into account characteristics of dissimilar pumps. 
       BACKGROUND 
       [0003]    Variable speed pumping systems vary the speed of the pumps using variable frequency drives to maintain a constant system pressure. Where multiple pumps are required to maintain the desired pressure, pumps with dissimilar pumping and load curves can experience an undesirable imbalance in the demand made on each pump to maintain the desired pressure if all pumps operate at the same speed. It would be desirable to be able to dynamically equalize load profile for all operating pumps while at the same time maintaining system pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a block diagram of a system which embodies the invention; and 
           [0005]      FIG. 2  is a flow diagram illustrating aspects of a method that embodies the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated. 
         [0007]    Embodiments of the invention adjust the speed of all operating pumps based on desired system pressure while equalizing load profile for all operating pumps. While operating, variable frequency drives sense various parameters such as operating frequency, current and voltage. Such data is available to embodiments of the invention in analog or digital form. 
         [0008]    Data can be acquired from displaced pumps via a data link, such as a serial data link implemented using a publicly available telecommunications system, or via a computer network, for example the Internet. In an aspect of the invention, a determination can be made as to the load on the variable frequency drive, hence the load on the pump. The current load for each pump of interest can be established and compared with the maximum load for that particular pump. The percent of full load at which that particular unit is operating can be established for each member of the plurality of operating pumps. 
         [0009]    In a disclosed embodiment of the invention, an average percent of full load is established based on characteristics of all members of the plurality of operating pumps. If a selected pump has a load that is below the average for all operating pumps, then the speed of that pump can be increased. If a selected pump has a load that exceeds the average, the speed of that pump would not be increased. If desired, a speed required parameter, or set point, can be established for each member of the plurality of pumps. 
         [0010]    An embodiment of the invention can incorporate several proportional, integral, differential (PID) control loops. A primary PID loop processes speed required control outputs for all pump motors in a system. This loop will increase or decrease the speed of all pump motors to maintain a desired system fluid pressure output. Each of the operating pumps and associated driving motor contribute to output flow and pressure. The primary loop has a shorter response time than other control loops operating in the system. 
         [0011]    A secondary PID control loop is associated with each of the operating pumps in the system. This control loop generates a speed required control output for the respective drive/pump motor. This control loop increases the speed of the respective motor to track the average percent of load value that is generated from data received from all active drive/pump motor combinations in the system. The members of the plurality of pump specific PID loops each has a longer response time than does the primary control loop noted previously. 
         [0012]    FIGS.  1 , 2  illustrate various aspects of systems and methods in accordance with the invention. A system  10  includes a plurality of variable speed pump units  14 . Members of the plurality  14  are coupled to and in communication with system control circuits  18 . Communication between members of plurality  14  and control circuits  18  can be by hard wiring, modems and wired or wireless switched telephone networks, or by computer based networks such as intranets or the Internet, generally indicated at  20 . The details of such communications are not limitations of the invention. 
         [0013]    Control circuits  18  can be implemented with one or more programmable processors, such as  18   a , and associated executable control software  18   b , stored on a computer readable medium. Inputs to circuits  18  can include a system pressure setpoint  18   c , a feedback pressure indicator  18   d  and one or more feedback parameters  18   e  from members of the plurality  14 . Control circuits  18  can output pump speed setpoints, indicated generally at  18   f , for each of the members of the plurality  14 . 
         [0014]    In the disclosed embodiment, each of the members of the plurality, such as  14   n  include a variable frequency drive, such as VFDn, and an associated pump Pn. A pump PID feedback loop, implemented in control circuits  18  is indicated generally at  24 . It will be understood that speed control systems different than variable frequency drives also come within the spirit and scope of the invention. 
         [0015]    Outputs from the pumps Pi are combined and coupled to a system fluid output conduit such as conduit  28 . System pressure can be sensed at a pressure sensor  30  and a signal indicative thereof  18   d  can be coupled to the control circuits  18 . A pressure based PID loop  18   g  can be implemented in control circuits  18 . 
         [0016]      FIG. 2  illustrates aspects of a system pressure control method  100  which embodies the invention. As illustrated at  102 , on a per-pump basis, actual percent load values for each pump P 1  . . . Pn can be determined by control circuits  18 . As at  104 , a system average desired percent load can be established. 
         [0017]    As illustrated at  106 - i , for each operating pump, actual VFD percent load can be compared to average, desired VFD percent load. Where actual VFD percent load is less than the desired average, speed at the respective pump Pi can be increased by first producing a new speed parameter or indicium as at  108 - i . As at  110 - i , that updated value can be sent via communications link  20  to the respective variable speed pump Pi. 
         [0018]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.