Abstract:
A compressor system according to the present invention utilizes direct rotational input from a permanent magnet motor to generate compressed air. The permanent magnet motor is mounted directly to an air screw compressor. The rotational input is provided by the permanent magnet motor to the air screw compressor without a gear train. The permanent magnet motor and associated variable speed drive controls the rotational speed of the permanent magnet motor and hence the screw compressor. Differing motors may selectively mount, and provide rotational input to, the air screw compressor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a motor for an air screw compressor and, more particularly, to a permanent magnet motor rotor mounted directly to an air screw compressor rotor.  
         [0002]     An air screw compressor includes a male and female compressor rotor supported by bearings inside a housing which rotate relative to each other to produce compressed air. Conventional air screw compressors are typically driven by a gear train which receives rotational input from an induction motor. In some applications where a variable speed drive is utilized, the air compression output is adjusted by varying the rotational speed of the motor which adjusts the compressor rotor speed.  
         [0003]     Typically, two or four pole AC induction motors drive air screw compressors and are known for their competitive pricing, high reliability, and wide service channels. However, rising energy costs and associated government programs and rebates, have increased consumer interest in other, energy efficient options.  
         [0004]     One such option involves utilizing premium efficiency AC induction motors in conjunction with a variable speed drive. A drawback of this design is that induction motors reach their peak efficiency at their rated speed and their efficiency drops at lower speeds, thereby compromising the level of energy savings at a part load. In addition, the cooling requirements of AC induction motors limits the minimum operating speed which may compromise the capacity turn down.  
         [0005]     In conventional designs incorporating gear trains, the gear train communicates the rotational input from the induction motor to the air screw compressor. Operating the air screw compressor at a high efficiency requires the compressor rotors to rotate at or near an optimum tip speed. A desired tip speed is obtained by a specific selection of step-up gears or step-down gears in the gear train, thus optimizing the rotation input into the air screw.  
         [0006]     Although there are merits to the use of gear trains, there are several penalties associated with their use. One of these penalties is the parasitic losses associated with the gear train. These losses are continuously reflected in higher power consumption throughout the life of the compressor. Furthermore, gear trains require lubrication, maintenance, and may contribute to reduced reliability. In addition, gear trains emit noise and consist of several parts, which increases cost and take up more space.  
         [0007]     Current air screw compressor designs also typically rely on a flexible coupling, positioned between the motor and gear box, to dampen motor torque ripple and to compensate for any misalignment between the respective drive shafts of the motor and the gear box. Current flexible coupling designs include hubs, couplings and adaptors, all of which generally increase cost and size to the overall compressor package. The use of a flexible coupling between the motor and the gear box requires periodic alignment inspections and adjustments.  
         [0008]     Therefore, there exists a need to provide a more efficient drive mechanism for an air screw compressor system.  
       SUMMARY OF THE INVENTION  
       [0009]     A compressor system according to the present invention utilizes direct rotational input from a permanent magnet motor controlled by an inverter to generate compressed air. The permanent magnet motor is mounted directly to an air screw compressor, thus becoming an integral part of the system.  
         [0010]     The compressor system includes an air screw compressor male rotor having a shaft portion extending into the permanent magnet motor. The shaft portion is an integral portion of the air screw compressor which eliminates alignment inspections and maintenance of the shaft interface with the air screw compressor rotor.  
         [0011]     The shaft portion of the air screw compressor male rotor attaches to a permanent magnet motor rotor. Accordingly, rotation of the permanent magnet motor rotor rotates the air screw compressor male and female rotors. There is no need for a gear train, coupling, or other associated parts in the compressor system as the permanent magnet motor provides rotational control necessary to produce compressed air. The permanent magnet motor is an AC synchronous motor with no rotor slip leading to better speed control accuracy and higher efficiency than induction type motors. The higher efficiency nature of a permanent magnet motor translates into a cooler running motor, thus improving its speed turndown capability. The permanent magnet motor and the air screw compressor system thus maintain high efficiency throughout the speed range with significant speed turndown provided by the permanent magnet motor.  
         [0012]     Typically, a single locknut secures the first end of the shaft portion to the permanent magnet motor rotor, thus making it simple to service. If needed, the permanent magnet motor can be easily replaced by removing the end cover and unscrewing the lock nut.  
         [0013]     The stator portion of the permanent magnet motor is of the type that may be used with either an induction or permanent magnet rotor. Consequently, the permanent magnet motor stator may be repaired by a wide variety of existing motor repair shops.  
         [0014]     Bearings in the air screw compressor usually support the air screw compressor rotors. Because the male air screw compressor rotor attaches to the permanent magnet motor rotor, the compressor system may not include bearings in the permanent magnet motor. Instead, the air screw compressor bearings support the permanent magnet motor rotor, and the permanent magnet motor is preferably bearingless.  
         [0015]     The on board compressor lubricant is used to cool the motor, thus keeping the design simple. The coolant circulates through the compressor system, cooling the permanent magnet motor and lubricating the air screw compressor. Preferably, the coolant that enters the permanent magnet motor is channeled to a low pressure point in the air screw compressor system. Consequently, the coolant is re-circulated through the compressor package lubrication system, where it is filtered and cooled. Vertically orienting the permanent magnet motor relative to the air screw compressor aids in the coolant flow, through the motor.  
         [0016]     Internal seals in the motor assembly confine the coolant and aid in channeling it to certain areas. A seal is also placed at the interface between the motor stator housing and the compressor to prevent overboard leakage. The permanent magnet motor may be classified as a Totally Enclosed Liquid Cooled (TELC) motor, as the motor is hermetically sealed and isolated from the external environment. Since the motor is lubricant cooled and the male rotor extended shaft is housed in a sealed motor stator housing, a shaft seal will not be required between the motor and compressor.  
         [0017]     Accordingly, the present invention provides a more efficient and compact drive mechanism for an air screw compressor.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows.  
         [0019]      FIG. 1  is a schematic view of the compressor system depicting the flow of coolant through the present invention.  
         [0020]      FIG. 2  is a cross section view of the hermetic motor and compressor system of the present invention taken along a longitudinal axis.  
         [0021]      FIG. 3  is an exploded view of an air screw compressor system rotor.  
         [0022]      FIG. 4  is an exploded cross section view of the coolant path through a portion of the permanent magnet motor.  
         [0023]      FIG. 5  is a perspective view of a permanent magnet motor rotor for the motor and compressor system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]      FIG. 1  illustrates a general schematic block diagram of a compressor system  10 . The compressor system  10  includes a permanent magnet motor  14  mounted directly to an air screw compressor  18 . The air screw compressor  18  receives a direct rotational input from the permanent magnet motor  14 . The air screw compressor  18  utilizes the direct rotational input from the permanent magnet motor  14  to generate compressed air.  
         [0025]     During air compression, the compressor system  10  produces heat which is removed from the compressor system  10  by a coolant  22 . In addition, coolant  22  removes heat away from the permanent magnet motor  14  and lubricates the air screw compressor  18 . The pressure differential in the system circulates coolant  22  through the compressor system  10  within a coolant communication path  20 . The coolant communication path  20  circulates the coolant  22  through the air screw compressor  18  and a separator  26  which separates gas from within the coolant  22 . The coolant  22  could be any lubricant suitable for compressor operations. The coolant  22  is then communicated through a thermal valve  30 . In response to the temperature of the coolant  22 , and the requirements of the compressor system  10 , the thermal valve  30  regulates the coolant  22  temperature by selectively directing coolant  22  through a cooler  34  such as a liquid-to-air or liquid-to-water heat exchanger.  
         [0026]     The permanent magnet motor  14  is designed to operate with a coolant  22  inlet temperature that is similar to the compressor  18  injection temperature. During operation, the temperature of the coolant  22  is regulated by the thermal valve  30  which modulates the flow of the coolant  22  to the cooler  34  to extract heat from the coolant  22 . The coolant  22  communicates through a filter  38 , then to the permanent magnet motor  14  and the air screw compressor  18 .  
         [0027]     In this example, the permanent magnet motor  14  is a sensorless permanent magnet motor. Accordingly, the example permanent magnet motor  14  does not require positioning sensors.  
         [0028]     If temperature of the coolant  22  at the compressor  18  discharge is below the predetermined value, the thermal valve  30  bypasses the cooler  34  and directs all or part of the coolant  22  through a filter  38 , to the permanent magnet motor  14  and the air screw compressor  18 . If the temperature of the coolant  22  at the compressor  18  discharge is above the predetermined value, the thermal valve  30  directs all or part of the coolant  22  to the cooler  34 . The cooler  34  lowers the temperature of the coolant  22  below the predetermined value thus regulating the compressor  18  discharge temperature. The coolant  22  then communicates through the filter  38 , to the permanent magnet motor  14  and the air screw compressor  18 . It should be understood that various control systems, as well as temperature responsive valves, may be utilized with the present invention to define the predetermined temperature or temperatures in response to these or other conditions.  
         [0029]     Referring to  FIG. 2 , the compressor system  10  includes the permanent magnet motor  14  which is controlled by a variable speed drive. A permanent magnet motor  14  includes a motor housing  44 , a motor stator  48 , an end cover  15 , and a motor rotor  52  housing a multitude of permanent magnets  56 . The motor rotor  52  is mounted to a rotor shaft portion  60  for rotation therewith about an axis  64 . A fastener  54 , such as a lock nut, axially secures the motor rotor  52  to the rotor shaft portion  60 . The stator  48  includes coils selectively wound around laminations  50 , manufactured of multi-layered steel stampings.  
         [0030]     The air screw compressor  18  includes compressor housings  68 ,  78  and a compressor rotor system  72 , typically having a male rotor  72   m  and a female rotor  72   f,  mounted on a respective compressor bearing,  76   m,    76   f.  Rotating the compressor rotor system  72  produces compressed air. Inlet bearings  77   m,    77   f  also provide support to the male rotor  72   m  and the female rotor  72   f  respectively. The rotation speed of the compressor rotor system  72  affects compression parameters such as the volume of compressed air per unit of time.  
         [0031]     One of the compressor rotors  72   m  has an extended shaft portion  60 , that is a homogeneous part of the male rotor  72   m,  as shown in  FIG. 3 . Thus, rotation of the rotor shaft portion  60  directly rotates the compressor rotor system  72 . Adjusting the rotational speed of the rotor shaft portion  60  adjusts the rotation speed of the compressor rotors  72 . Having the rotor shaft  60  as a portion of one of the compressor rotors  72   m  within the compressor rotor system  72  enables relatively precise control of the compressor rotor system  72 . Because the motor rotor  52  directly connects to the rotor shaft portion  60 , one revolution of the motor rotor  52  causes one revolution of the rotor shaft portion  60 . Furthermore, because the compressor bearings  76  support the compressor rotor system  72 , the permanent magnet motor  14  need not be supported upon separate bearings to support the rotor shaft portion  60  and the motor rotor  52 . That is, the permanent magnet motor  14  is preferably a bearingless design since the compressor rotor system  72  supports the rotor shaft portion  60  and the motor rotor  52 . Although  FIG. 3  illustrates compressor rotor  72   m  as including the rotor shaft portion  60 , it should be understood that any rotor  72   m  or  72   f  may include the rotor shaft portion  60 .  
         [0032]      FIG. 4  illustrates the path of the coolant  22  through the permanent magnet motor  14 . The coolant communication path  20  circulates coolant  22  from the filter  38  to the permanent magnet motor  14 . A coolant spray manifold  84  communicates the coolant  22  to the permanent magnet motor  14 . The coolant spray manifold  84  directs and distributes the coolant  22  towards an annular chamber bound by the stator lamination  50  and the motor housing  44 . The coolant  22  thereby removes heat from the permanent magnet motor  14 . In addition, a smaller amount of coolant  22  is introduced into the air gap between the rotor  60  and the stator  56 .  
         [0033]     A multitude of seals  80 , typically O-ring seals, direct the coolant  22  within the permanent magnet motor  14 , and contain the majority of the coolant  22  about the perimeter of the permanent magnet motor  14 . A small amount of coolant  22  is directed through stator  48  coils and into the air gap between rotor  56  and stator  48 . The seals  80  in conjunction with predetermined orifices contain the majority of the circulating coolant  22  near the perimeter of the permanent magnet motor  14  and away from the rotor shaft portion  60 . Since the same coolant  22  is used throughout the compressor system  10 , a shaft seal between the motor  14  and the compressor  18  will not be required. As the coolant  22  is contained within the motor housing  44 , the permanent magnet motor  14  may be classified as a totally enclosed liquid cooled (TELC) motor.  
         [0034]     The coolant  22  circulates through the permanent magnet motor  14 , removing heat, and through air screw compressor  18 . The permanent magnet motor  14  defines a coolant flow passage  88  which forms a segment of the coolant communication path  20  for the coolant  22 . The coolant flow passage  88  directs the coolant  22  from the permanent magnet motor  14  to the air screw compressor  18  after cooling the permanent magnet motor  14 .  
         [0035]     Preferably, the permanent magnet motor  14  is mounted above the air screw compressor  18  when the compressor system  10  is mounted in a generally vertical orientation along axis  64 . As the coolant  22  flows from the permanent magnet motor  14  to the air screw compressor  18  through the coolant flow passages  88 , this orientation facilitates the coolant  22  flow from the permanent magnet motor  14  to the air screw compressor  18 . After lubricating and cooling the air screw compressor  18 , the coolant  22  is cooled, filtered and recirculated through the system  10  following the coolant communication path  20 .  
         [0036]     The permanent magnet motor  14  includes an adaptor plate  92  having a shaft opening  96  through which the rotor shaft portion  60  extends. Near the shaft opening  96 , adaptor plate  92  is increased in thickness relative to the outer perimeter portions of the adaptor plate  92 . Increasing the thickness of the adaptor plate  92  near the shaft opening  96  provides a favorable clearance between the cavities within the permanent magnet motor  14  and the air screw compressor  18 . Moreover, increasing the thickness of the adaptor plate  92  near the shaft opening  96  provides a favorable ratio between the thickness of the adaptor plate  92  and the diameter of the shaft opening  96 .  
         [0037]     Referring next to  FIG. 5 , the motor rotor  52  restrains the permanent magnets  56  and is preferably manufactured of multilayered steel stampings. The example motor rotor  52  has general cloverleaf cross-section and defines a rotor opening  100  that engages the rotor shaft portion  60 . Keyway features  104  on the motor rotor  52  and corresponding keyway features  104  on the rotor shaft portion  60  ensure that the two parts are locked together. The keyway features  104  prevent relative motion between the motor rotor  52  and shaft portion  60 .  
         [0038]     Furthermore, maintenance is readily facilitated in that, for example only, if the permanent magnet motor  14  requires replacement, it is readily removed from the air screw compressor  18  and replaced with a different permanent magnet motor  14 , without the heretofore necessity of disassembling the air screw compressor  18 . Typically, the fastener  54  ( FIG. 2 ) is a locknut, secured to the rotor shaft portion  60 , and rotatable about axis  64 . Removing the end cover  15  and motor rotor fastener  54  separates motor rotor  52  from the rotor shaft portion  60 . Subsequently removing the motor housing  44  and the adaptor plate  92  separates the permanent magnet motor  14  from the air screw compressor  18 . The replacement permanent magnet motor  14  then directly mounts to the air screw compressor  18 . The rotor opening  100  on the replacement permanent magnet motor  14  engages the rotor shaft portion  60  and is secured by the lock nut  54 .  
         [0039]     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.