Abstract:
In a motor control system, a controller generates a single control signal for a motor control unit in a first electric motor and another motor control unit in a second electric motor. The motor control units in the first and second electric motors operate the first and second electric motors at a first rate in response to the control signal being at a first level. The motor control unit in the first electric motor operates the first electric motor at a second rate and the motor control unit in the second electric motor operates the second electric motor at a third rate in response to the control signal being at a second level, the third rate being different than the second rate.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to control systems for electric motors, and, in particular, to operating multiple electric motors in a system at different speeds. 
       BACKGROUND 
       [0002]    Some commercial heating, ventilation, and air-conditioning (HVAC) systems include multiple fans and electric motors in multiple units that work together to provide heating and cooling to a building. For example, the air conditioning or refrigeration systems in many large buildings include multiple condenser units that are located on the roof of the building. Each condenser unit includes an electric motor that drives a fan to direct air over a radiator to cool and condense a refrigerant from a vapor phase to a liquid phase. 
         [0003]    In many HVAC systems, multiple motorized units, such as condensers, operate in tandem to provide sufficient cooling capacity for a building or other facility. A central control unit is connected to the fan motors in each of the condenser units and is configured to activate the fan motors, deactivate the fan motors, and adjust the operating speed of the fan motors based on the cooling requirements of the building. 
         [0004]    One challenge confronting HVAC systems that include multiple condenser units or other units that include electric motors is the operation of the individual units in an energy efficient manner. For example, in one existing HVAC system, a central controller can operate the fans in multiple condenser units at different speeds, but all of the fans must operate at the same speed. In some operating conditions, the HVAC system could operate more efficiently if only some of the motors operated simultaneously. In another embodiment, a single controller operates the fan in a single condenser unit, then the control signal from the controller is propagated to a second fan motor in a second condenser unit at a lower level, to a third motor in a third fan unit at still a lower level, etc. to enable a single control unit to operate the fans in multiple condenser units at different speeds. One drawback of the aforementioned system is that the controller is unable to operate the fans in all of the condenser units at a maximum speed in situations where the HVAC system is required to operate at high capacity. 
         [0005]    One approach that controls multiple fan motors at different speeds includes a controller that communicates with each motor individually, either through individual control lines or through a digital control system that communicates using, for example, wired or wireless digital networking. While such systems are known to the art, the added complexity required in the controller and the added infrastructure required to run individual control wires or add digital control systems to the motors in existing HVAC units adds to the cost and maintenance burden for building and operating the HVAC system. Consequently, improvements to HVAC control systems that enable operating different motors in the HVAC system over a full range of different operating speeds during operation without requiring generation of individual control command signals for each motor would be beneficial. 
       SUMMARY 
       [0006]    In one embodiment, a motor control system for controlling multiple electric motors has been developed. The system includes a first electric motor including a first electric motor control unit, a second electric motor including a second electric motor control unit, the second electric motor control unit, and a controller operatively connected to the first electric motor control unit and the second electric motor control unit. The first electric motor control unit is configured to operate the first electric motor at a first rate in response to receiving a control signal at a first level and at a second rate in response to receiving the control signal at a second level, and operate the first electric motor at a plurality of intermediate operating rates between the first rate and the second rate in accordance to a first predetermined control curve in response to receiving the control signal at an intermediate level between the first level and the second level. The second electric motor control unit is configured to operate the second electric motor at the first rate in response to receiving the control signal at the first level and at a third rate in response to receiving the control signal at the second level, the third rate being different than the second rate, and operate the second electric motor at another plurality of intermediate operating rates between the first rate and the third rate in accordance to a second predetermined control curve in response to receiving the control signal at the intermediate level between the first level and the second level. The controller is configured to generate a single control signal to operate both the first electric motor and the second electric motor. The control signal is at one of the first level, the second level, and one of a plurality of intermediate levels between the first level and the second level. 
         [0007]    In another embodiment, a method for controlling multiple electric motors has been developed. The method includes generating a single control signal at one of a first level, a second level, and a plurality of intermediate levels between the first level and the second level, operating a first electric motor at a first rate in response to receiving the single control signal at the first level, operating the first electric motor at a second rate in response to receiving the single control signal at the second level, operating the first electric motor at a plurality of intermediate rates between the first rate and the second rate in accordance to a first predetermined control curve in response to receiving the control signal at one of the plurality of intermediate levels, operating a second electric motor at the first rate in response to receiving the single control signal at the first level, operating the second electric motor at a third rate in response to receiving the single control signal at the second level, the third rate being different than the second rate, and operating the second electric motor at another plurality of intermediate operating rates between the first rate and the third rate in accordance to a second predetermined control curve in response to receiving the control signal at one of the plurality of intermediate levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is a schematic diagram of a control system that generates a single control signal to operate multiple motors in an HVAC system at different rates. 
           [0009]      FIG. 1B  is a schematic diagram of the control system of  FIG. 1A  where the single control signal operates all of the motors at either a maximum or minimum operating speed. 
           [0010]      FIG. 2  is a block diagram of a process for configuring and operating the motors in the HVAC system depicted in  FIG. 1A  and  FIG. 1B . 
           [0011]      FIG. 3  is a graph of predetermined control curves that are used to operated different motors in an HVAC system at different rates. 
           [0012]      FIG. 4  is another graph of predetermined control curves that are used to operated different motors in an HVAC system at different rates. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the term “control curve” refers to data that a motor control device references to identify an operating rate for the motor that corresponds to the level of a control signal. The controller deactivates the motor or operates the motor over a range of operating speeds between a minimum operating rate and a maximum operating rate with reference to the control curve and the control signal. 
         [0014]      FIG. 1A  and  FIG. 1B  are block diagrams that depict a motor control system that is used, for example, in an HVAC system.  FIG. 1A  and  FIG. 1B  include a controller  104  that is operatively connected to motor control units  118 A,  118 B,  118 C and  118 D. The motor control units  118 A- 118 D are each configured to adjust the rate of rotation of motors  116 A,  116 B,  116 C, and  116 D, respectively. In the system  100 , the controller  104  includes a control signal module  108  that generates a single control signal for all of the motor control units  118 A- 118 D that is distributed through, for example, a single electrical connection  112 . In one embodiment the motor control units  118 A- 118 D are also digital control units or are hybrid analog/digital control units in an alternative embodiment. In many HVAC systems, the motors  116 A- 116 B are electric motors that operate, for example, using alternating current (AC) or direct current (DC) electrical power. While  FIG. 1A  and  FIG. 1B  depict the system  100  with four motors  116 A- 116 D and four motor control units  118 A- 118 D, respectively, alternative configurations include two or more motors that each receive a single control signal from a controller such as the controller  104 . 
         [0015]    In one embodiment, the controller  104  is a digital controller that is, for example, operatively connected to one or more thermostats (not shown) and generates a control signal to operate the motors  116 A- 116 D at different speeds to maintain a predetermined temperature within a building. In one embodiment, the control signal module  108  generates one electrical signal at a selected voltage level for each of the motor control units  118 A- 118 D. During operation, the controller  104  changes the voltage level within a predetermined range, such as 0 V to 10 V, to increase and decrease the operating rates of the motors  116 A- 116 D. As described below, the motor control units  118 A- 118 D are each configured to respond to a single control signal differently to enable the motors  116 A- 116 D to operate at different rates for some control signals, as depicted in  FIG. 1A , while still enabling all of the motors to operate at a single rate for at least one predetermined control signal, as depicted in  FIG. 1B . 
         [0016]    In the system  100 , each of the motor control units  118 A- 118 D is configured to respond to a single control signal voltage using a predetermined control curve that is stored within a memory of each of the control units  118 A- 118 D. In one embodiment, the memory in each of the control units  118 A- 118 D stores multiple control curves and an operator reconfigures one or more mechanical switches, such as the switches  120 A- 120 D, in each of the motor control units  118 A- 118 D, respectively, to select one control curve. In the embodiment of  FIG. 1A  and  FIG. 1B , each one of the motor control units  118 A- 118 D stores four different control curves in memory, and the switches  120 A- 120 D are dual-inline package (DIP) switches with two individual switch elements that enable selection between the four different control curves. In another embodiment, each one of the motor control units  118 A- 118 D is programmed with a single control curve that is selected for use with the corresponding control unit through, for example, a software or firmware programming process. 
         [0017]      FIG. 3  is a graph depicting control curves in one embodiment of the system  100 . In  FIG. 3 , a chart  300  depicts control curves  304 ,  308 ,  312 , and  316 . The chart  300  depicts analog voltage control signal levels in a range from 0 to 10 volts on the horizontal axis and the operational rate of each motor corresponding to the control signal that are expressed as percentages of the maximum operating rate of the motor is depicted on the vertical axis. In the chart  300 , the control curves  304 - 316  converge at 100% operational speed when the analog voltage control signal reaches 10 volts. The control curves  303  diverge from each other as the analog input voltage decreases, with, for example, each control curve having a linear segment with a different slope in the chart  300 . In the example of the chart  300 , each control curve has a minimum operational rate, which is a 30% operational rate depicted on the cutoff threshold line  328 . At any control voltage level below the cutoff point for each one of the control curves, the corresponding motor controller deactivates the motor (e.g. runs the motor at a rate of 0%). In  FIG. 3 , the cutoff threshold line  328  intersects the control curve  304  at approximately 0.5 volts, the control curve  308  at approximately 1.0 volt, the control curve  312  at approximately 1.5 volts, and the control curve  316  at approximately 2.0 volts. 
         [0018]    Referring to  FIG. 1A  and  FIG. 3 , the motor control unit  118 A is configured to use the control curve  304 , the motor control unit  118 B is configured to use the control curve  308 , the motor control unit  118 C is configured to use the control curve  312 , and the motor control unit  118 D is configured to use the control curve  316 . In  FIG. 1A , the control signal module  108  in the controller  104  generates a single voltage control signal of approximately 2.0 volts. Each of the motor control units  118 A- 118 D receives the single control voltage signal through, for example, control wires  112 . As depicted in  FIG. 3 , the 2.0 volt control voltage is depicted as vertical line  320  that extends through each of the control curves  304 ,  308 , and  312 . The operating speed for each of the motors  116 A- 116 C corresponds to the intersection between the vertical line  320  and the corresponding control curves  304 - 312  along the vertical axis. For example, the line  320  intersects the control curve  304  at a motor rate of approximately 41%, and in  FIG. 1A  the motor  116 A operates at approximately 41%. Similarly, the line  320  intersects control curve  308  at approximately 38%, and the motor  116 B operates at a rate of 38%, and the line  320  intersects the control curve  312  at approximately 34%, and the motor  116 C operates at a rate of 34%. For exemplary purposes, the control voltage line  320  is set to be slightly below the cutoff line for the control curve  316 . Thus, the line  320  does not intersect the control curve  320  and the motor control unit  318 D deactivates the motor  316 D. 
         [0019]    In  FIG. 1A , the controller  104  generates the control signal to operate the motors  116 A- 116 C at reduced rates and to completely deactivate the motor  116 . In some operating conditions, the three motors  116 A- 116 C provide sufficient airflow to operate, for example, condensers in an air conditioning or refrigeration system while the motor  116 D is deactivated. When deactivated, the motor  116 D consumes minimal electrical energy and the control system  100  can operate in an efficient manner. 
         [0020]    Referring to  FIG. 1B  and  FIG. 3 , the individual motor controllers  118 A- 118 D are configured to enable each of the motors  116 A- 116 D to operate at a rate of 100% in response to a single control signal from the controller  104 . In the system  100 , the control signal module  108  generates a control voltage of 10 volts, which is depicted as the vertical line  324  in the chart  300 . The vertical line  324  intersects each of the control curves  304 - 316  at the 100% operating rate level on the vertical axis of the chart  300 . Consequently, in  FIG. 1B  each of the motor controllers  318 A- 318 D operates one of the motors  316 A- 316 D, respectively, at a 100% operating rate. Thus, in the system  100 , the control curves  304 - 316  enable the controller  104  to generate a single control signal to operate all of the motors at 100% when the HVAC system runs at maximum capacity. During operation, the controller  104  also generates a plurality of control voltages in addition to the exemplary control voltages  320  and  324  that are shown in  FIG. 3 . The motor control units  318 A- 318 D operate the respective motors  316 A- 316 D at different rates that are determined by the intersection of the control voltage and the corresponding control curve. 
         [0021]      FIG. 4  depicts a chart  400  with an alternative set of control curves  404 ,  408 ,  412 , and  416 . In the chart  400 , the control curves  404 - 416  intersect at a control voltage of approximately 0.5 volts, which corresponds to a minimum cutoff operating rate for the motors of, for example, 30% as depicted by the cutoff line  428 . In the configuration of  FIG. 4 , multiple motor control units that are configured to use the different control curves  404 - 416  each operate motors at the minimum 30% operating rate in response to receiving a single control voltage of approximately 0.5 volts as depicted on the vertical line  420 . If the control voltage drops below the 0.5 volt threshold, then each of the motor controllers deactivates the corresponding motor. As the control voltage level increases, the individual motor controllers operate the motors at different rates. In the chart  400 , a maximum control voltage of 10 volts depicted along the line  424  intersects the control curve  404  at an operating rate of 100%, the control curve  408  at an operating rate of 90%, the control curve  412  at an operating rate of 80%, and the control curve  41  at an operating rate of 70%. Thus, in the configuration of  FIG. 4 , only motors that are configured to operate according to the control curve  404  reach a 100% operating rate when the control voltage is at a maximum level. 
         [0022]    While  FIG. 3  and  FIG. 4  depict illustrative embodiments of control curves, alternative configurations of the system  100  include different control curve configurations. For example, alternative configurations include a different number of control curves, such as two or more control curves. While  FIG. 1A  and  FIG. 1B  depict four motors that are each configured to use a different control curve as an example, many HVAC systems include a different number of motors and two or more motors can be configured to operate using a single control curve. While the control curves depicted in  FIG. 3  and  FIG. 4  are linear, other control curve shapes including exponential or quadratic control curves are used in alternative embodiments. The control curves as illustrated in  FIG. 3  and  FIG. 4  have a positive slope, which is to say that the operating rate of the motors on the control curves increases as the voltage of the control signal increases. In an inverted control signal embodiment, the control curves have negative slopes where the operating rate of the motors on the control curves decreases as the voltage of the control signal increases. In still another embodiment, the control curves include a combination of positive and negative slopes over for different ranges of the control signal between the minimum and maximum control signal voltage. 
         [0023]    The embodiments of the control curves that are depicted in  FIG. 3  and  FIG. 4  are shown as graphics for illustrative purposes. In one embodiment of the software in the motor control units  118 A- 118 D, each control unit stores data corresponding to, for example, the slope of the control curve, the minimum cutoff threshold, and at least one point on the control curve, such as the 100% utilization point at the maximum control signal voltage depicted in  FIG. 3 . The control software then identifies the rate for operating the motor given the analog voltage of the control signal using, for example, algebraic techniques that are well-known in the art for finding a value of a dependent variable (the motor rate) on a curve given the value of the independent variable (the analog control signal voltage level). 
         [0024]    In the embodiment of system  100 , the controller  104  and control signal generation module  108  generate analog voltage control signals in a predetermined voltage range of, for example, 0 volts to 10 volts. In one alternative configuration, the control signal generation module  108  generates voltages at a plurality of predetermined levels, such as for example at 0.5 volt increments between 0 volts and 10 volts. In another embodiment, the analog control signal is based on the amplitude of an electrical current instead of voltage. In still another embodiment, the analog control signal is a modulated signal. While analog control signals are common in many HVAC control systems, in an alternative embodiment of the system  100  the controller sends a single command signal that is encoded in a digital data format to all of the motor controller units  118 A- 118 D. For example, in one embodiment the control system  100  sends a digital data frame including a numeric value in a range of 0 to 100 to all of the controller units  118 A- 118 D to select an operating rate for the motors in the system  100 . Regardless of the form of the control signal, the controller  104  sends a single control signal to all of the motor controller units  118 A- 118 D and the motor controller units  118 A- 118 D each operate the corresponding motor based on the predetermined control curve and the command signal. 
         [0025]      FIG. 2  is a block diagram of a process  200  for configuring and operating a plurality of motors in a motor control system with the motors operating at different rates using a single control signal for all of the motors. In the discussion below, a reference to the process  200  performing an action or a function refers to a controller, such as the system controller  104  or the motor controller units  118 A- 118 D, executing stored instructions to perform the action or function with one or more components in the system. Process  200  is described in conjunction with the motor control system  100  of  FIG. 1A  and  FIG. 1B  for illustrative purposes. 
         [0026]    Process  200  begins with selection of control curves for individual motor controllers (block  204 ). In the motor control system  100 , a technician or other operator configures the switches  120 A- 120 D in each of the motor control units  118 A- 118 D to select one of the control curves for use with each motor. In the examples of  FIG. 1A  and  FIG. 1B , the switches  120 A- 120 D in the motor control units  118 A- 118 D are configured in one of four different configurations to select one of four different control curves. In alternative configurations, the motor control units are programmed with data corresponding to predetermined control curves via software or firmware updates. 
         [0027]    Process  200  continues as a central controller, such as the controller  104  in the system  100 , generates a single control signal for all of the motor control units (block  208 ). As depicted in  FIG. 1A  and  FIG. 1B , the control signal generation module  108  in the controller  104  generates a voltage signal at a selected level and each of the motor control units  118 A- 118 D receives the same control voltage signal. If the motor control units are configured with the control curves  304 - 316  that are depicted in  FIG. 3 , then voltage control signal may intersect one or more of the control curves  304 - 316 , or be below the minimum cutoff threshold  328 . Using the example control voltage of 2.0 volts described above in conjunction with the line  320  in  FIG. 3 , the control signal is below the cutoff level for the control curve  316  and the motor control unit  318 D in  FIG. 1A  (block  212 ) and the motor control unit  318 D deactivates the motor  316 D (block  216 ). 
         [0028]    For the remaining control curves  304 - 312 , the control voltage signal is above the cutoff threshold (block  212 ), and each of the motor controllers  318 A- 318 C identifies an operating rate for one of motors  316 A- 316 C, respectively, with reference to the intersection between the control signal  320  and the corresponding control curves  304 - 312  (block  220 ). For example, in the configuration of  FIG. 1A  the controller  118 A operates the motor  116 A at a rate of 41% compared to the maximum operating rate of the motor in accordance with the control curve  304 , the controller  118 B operates the motor  116 B at a rate of 38% in accordance with the control curve  308 , and the controller  118 C operates the motor  116 C at a rate of 34% in accordance with the control curve  312 . The motor controllers  118 A- 118 D continue to operate the motors  116 A- 116 D at the identified rates as long as the control signal remains at the selected voltage level (block  224 ). 
         [0029]    Process  200  continues as the controller  104  generates control signals at various levels for the motor controllers in the motor control system  100  (block  208 ). During operation, the controller  104  can change the level of the single analog voltage signal to increase or decrease the total operational rate of the motors in the system  100 . As depicted above in  FIG. 3 , if the controller  104  increases the control signal to 10 volts, then each of the motor controllers  118 A- 118 D operates the corresponding motor  116 A- 116 D at a 100% rate in accordance with the control curves  304 - 316 . The controller  104  can also generate control signals at intermediate levels to operate the motors  116 A- 116 D at various intermediate rates, to deactivate some of the motors while operating others at intermediate rates, or to deactivate all of the motors. 
         [0030]    It will be appreciated that variations of the above-disclosed apparatus and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.