CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/193,512, filed Dec. 4, 2008, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present application relates to a variable speed air compressing system, for example, an industrial variable speed air compressing system. 
         [0003]    Industrial air compressors are used in factories and industry to power pneumatic and other devices that require compressed air. Such applications may include hand tools (such as drills or sprays), robotic mechanisms with pneumatic joints, pneumatic lifts, etc. 
       SUMMARY 
       [0004]    In one exemplary embodiment, a variable speed air compressing system includes a compressor, a motor configured to actuate the compressor, and a rectifier configured to receive alternating current from a first power source and to provide rectified direct current having a first voltage. The variable speed air compressing system also includes an inverter configured to receive the rectified direct current and to receive direct current from a second power source having a second voltage. The inverter is further configured to provide alternating current to the motor. The alternating current provided to the motor is based on the rectified direct current if the first voltage is greater than the second voltage and the alternating current is based on the direct current from the second power source if the second voltage is greater than the first voltage. 
         [0005]    In another exemplary embodiment, a variable speed drive for an air compressing system includes a rectifier configured to receive alternating current from a first power source and to provide rectified direct current having a first voltage. The variable speed drive also includes an inverter configured to receive the rectified direct current and to receive direct current from a second power source having a second voltage. The inverter is further configured to provide alternating current to a motor. The alternating current provided to the motor is based on the rectified direct current if the first voltage is greater than the second voltage and the alternating current is based on the direct current from the second power source if the second voltage is greater than the first voltage. 
         [0006]    In another exemplary embodiment, a variable speed air compressing system, includes a compressor, a motor configured to actuate the compressor, and a rectifier configured to receive alternating current from a first power source and to provide rectified direct current having a first voltage. The system also includes a second power source and an inverter configured to receive the rectified direct current and to receive direct current from a second power source having a second voltage. The inverter is further configured to provide alternating current to the motor. The alternating current is based on the rectified direct current if the first voltage is greater than the second voltage and the alternating current is based on the direct current from the second power source if the second voltage is greater than the first voltage. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The features, aspects and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
           [0009]      FIG. 1  is a block diagram illustrating a variable speed air compressing system, according to an exemplary embodiment. 
           [0010]      FIG. 2  is a block diagram illustrating a variable speed air compressing system including a power source, according to an exemplary embodiment. 
           [0011]      FIG. 3  is a graph illustrating voltage at times during operation of an air compressing system, according to an exemplary embodiment. 
           [0012]      FIG. 4  is a block diagram illustrating a variable speed air compressing system including a power source and a controller, according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Hereinafter, various exemplary embodiment will be described in detail with reference to the drawings. 
         [0014]      FIG. 1  shows a variable speed air compressing system  10 , according to an exemplary embodiment. The variable speed air compressing system comprises a variable speed air compressor  22  that uses a variable speed drive  14  to control its speed (RPM). Such a compressor  22  is more energy efficient as compared to a fixed speed air compressor. The variable speed drive  14  for the air compressor  22  is connected to an AC power source  12  providing AC power or voltage V ACS . The variable speed drive  14  converts the AC voltage into DC voltage V DCR  by rectification using a rectifier  16 . The rectified DC voltage V DCR  is then converted back into a variable frequency AC voltage V ACR  using an inverter  18 . The AC voltage V ACR  is fed into a motor  22 , such as an AC induction motor, which powers the compressor  22 . The compressor  22  can be, for example, a 700 hp compressor, which can be used on suitable compressor loads  26 , such as hand tools (such as drills or sprays), robotic mechanisms, pneumatic lifts, etc. 
         [0015]      FIG. 2  shows the variable speed air compressing system  10  including a power source  24 , according to an exemplary embodiment. The power source may be one or more solar panels, wind power generators, one or more fuel cells, one or more batteries, one or more battery banks, a DC generator, other types of power sources, or any combination thereof. The power source  24  may provide a DC voltage V DCS  to the variable speed drive as an additional or alternative source of power to the motor  20 . 
         [0016]    The power line from the power source  24  may be connected to the variable speed drive by being connected to the power line(s)  40  from the rectifier  16  to the inverter  18 , by being connected to the input (DC) bus  28  of the inverter  18 , or any other suitable connection. 
         [0017]    According to one exemplary embodiment of the present invention, the power source  24  is the primary power source for the air compressor  22  when the power source  24  provides a voltage Vocs that is greater than the rectified voltage V DCR  provided by the rectifier  16 . When the motor  22  is being powered by these two different voltages, the motor  22  will draw power from the source with the greater voltage. Thus, the power source  24  is the primary source of power to motor  22  (after the DC voltage being input in the inverter  18  is converted to the variable AC voltage V ACR ).  FIG. 3  shows a graph at times during operation of the air compressing system, according to an exemplary embodiment. Between times t 1  and t 2 , the power source  24  is the primary source of power to motor  22  because V DCS &gt;V DCR . 
         [0018]    The power source  24  is designed to allow a predetermined amount of allowable “sag” (V TH ) in the amount of voltage being supplied to the inverter  18  based on the difference in voltages between the rectified voltage V DCR  and the voltage from the power source  24  V DCS  and the loading down of the power source  24  caused by the motor  20 /air compressor  22 . For example, if V DCR  is designed to provide 550V and V DCS  is designed to provide 600V, the power source  24  will provide the primary voltage V DCS  to the motor/air compressor (via the inverter  18 ) because the voltage will be drawn from the higher voltage of 600V. If the loading of the air compressor becomes greater (for example, more devices or systems are added which require more compressed air), the increased loading of the power source  24  causes the available voltage from the power source  24  to drop. If the voltage of the power source  24  drops such that V DCR  is substantially equal to V DCS  (in this example, V DCS  drops until it reaches about 550V), then the motor/air compressor are powered equally by the AC power source  12  and the power source  24 . Thus, the power from the AC power source  12  is pulled into the inverter  18  such that the AC power source  12  is used as an auxiliary power source when the voltage of the power source  24  drops below a predetermined threshold (that is, the predetermined amount of voltage sag V TH  allowed by the power source  24  is exceeded). In  FIG. 3 , between times t 2  and t 3 , the power source  24  and the AC power source  12  both supply power equally to the motor  22  because V DCS  is substantially equal to V DCR . 
         [0019]    If the loading of the air compressor becomes even greater (for example, more devices or systems are added which require even more compressed air), the increased loading of the power source  24  causes the available voltage from power source  24  to drop even farther. If the voltage of the power source  24  drops such that V DCR  is greater than V DCS  (in this example, V DCS  drops until it reaches 530V while V DCR  remains at 550V), then the motor/air compressor is primarily powered by the AC power source  12  because the higher of the two voltages is utilized. In  FIG. 3 , after time t 3 , the AC power source  12  is the primary source of power to the motor  22  because V DCS &lt;V DCR . 
         [0020]    It should be recognized that the values of the available voltage supplied by the AC power source  12 , the maximum voltage available from the power source  24 , the power requirements of the air compressor  22 , and the predetermined amount of voltage sag V TH  allowed by the power source  24  may have any suitable values depending upon the application, requirements, and design of the overall air compressing system. According to one exemplary embodiment, the maximum voltage available from the power source  24  and the predetermined threshold may be fixed after installation of the entire air compressing system is complete. It is also noted that the power source  24  may be configured to be added to an existing air compressing system already existing in a factory or the entire air compressing system may be one stand alone system comprising the motor  20 , the variable speed air compressor  22 , the variable speed drive  14 , the power source  24 , and/or any combination thereof. 
         [0021]      FIG. 4  shows another exemplary embodiment of the present invention similar to  FIG. 2  but also includes a controller  30 , according to an exemplary embodiment. The power source  24  is the primary power source for the air compressor  22  when a voltage V DCS  is greater than the rectified voltage V DCR  provided by the rectifier  16 . The power source  24  and the AC power source  12  equally supply power when V DCS  is substantially equal to V DCR . The AC power source  12  is the primary power source when V DCS  is less than V DCR . The voltages V DCS  and V DCR  are read or sensed using voltage sensors  32  and  34 , respectively. The sensors  32  and  34  are monitored by the controller  30 . 
         [0022]    The controller  30  may comprise the necessary hardware, software, or other mechanisms necessary to carry out the functions to which the controller  30  was designed, such as one or more microprocessors, CPU, and/or circuitry. The controller may be configured to change the available voltage from the rectifier such that the available V DCR  may be raised or lowered. The effect of changing the voltage V DCR  from the rectifier  16  is to make variable the predetermined amount of allowable sag (V TH ) in the amount of voltage being supplied to the inverter  18  from the power source  24 . Thus, the moment in which the power source  24  switches from being the primary source of power to the motor  20  to sharing the load with the AC power source  12  may be changed because the time span that the voltage V DCR  is pulled in to share the load with the voltage V DCS  may be shortened or lengthened if the voltage V DCR  is raised or lowered relative to the voltage V DCS , respectively. 
         [0023]    According to the exemplary embodiment of  FIG. 4 , if the controller  30  determines that the amount of allowable sag V TH  is to be increased, the controller  30  decreases the amount of available V DCR . If the controller  30  determines that the amount of allowable sag V TH  is to be decreased, the controller  30  increases the amount of available V DCR . The controller may increase or decrease the amount of available voltage V DCR  by any known means or mechanism in the art, such as one or more DC-to-DC converters. The controller  30  may increase or decrease the amount of available voltage V DCR  based on input from a user using an input device  36 , such as a keypad, keyboard, or any other known input device. The controller  30  may also be equipped with one or more displays  38  which output the values of V DCR  and V DCS . It is also noted that the power source  24  and the controller  30  may be configured to be added to an existing air compressing system already existing in a factory or the entire air compressing system may be one stand alone system comprising the motor  20 , the variable speed air compressor  22 , the variable speed drive  14 , the power source  24 , the controller  30 , the sensor  32 , the sensor  30 , and/or any combination thereof. 
         [0024]    According to yet another exemplary embodiment of the present invention, the power source  24  may comprise one or more solar panels. The suitable amount of allowable “sag” (V TH ) for the panels may be determined by using a power point tracking algorithm or PPT to achieve the optimal voltage/operating point for the solar panels. The solar panel may be used as the power source  24  in any of the above exemplary embodiments. 
         [0025]    Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope and spirit of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is to be defined as set forth in the following claims.
A variable speed air compressing system includes a compressor, a motor configured to actuate the compressor, and a rectifier configured to receive alternating current from a first power source and to provide rectified direct current having a first voltage. The system also includes an inverter configured to receive the rectified direct current and to receive direct current from a second power source having a second voltage. The inverter is configured to provide alternating current to the motor. The alternating current provided to the motor is based on the rectified direct current if the first voltage is greater than the second voltage and the alternating current is based on the direct current from the second power source if the second voltage is greater than the first voltage.