Screw compressor with switched reluctance motor

Abstract

The invention relates to improvements in compressors and in particular to arrangements for driving screw compressors by reluctance drive motors. A screw compressor assembly is provided for compressed gas comprising a screw compressor having a casing ( 16 ) and at least one pair of intermeshing helically formed screw rotors ( 11,12 ) each of which is mounted on a rotor shaft rotatably mounted within the casing by means of bearings ( 15 ). Said assembly further comprises a switched reluctance motor comprising a stator ( 18 ) attached to the casing and a rotor ( 19 ) mounted on one of the motor shafts such that one of the screw rotors is driven directly by the motor. The motor rotor is displaced relative to the stator by a pre-determined axial distance

1 . A screw compressor assembly for providing compressed gas comprising a screw compressor ( 10 ) having a casing and at least one pair of intermeshing helically formed screw rotors ( 11 , 12 ) each of which is mounted on a rotor shaft ( 13 , 14 ) rotatably mounted within the casing ( 16 ) by means of bearings ( 15 ), said assembly further comprising a switched reluctance motor ( 17 ) comprising a stator ( 18 ) attached to the casing ( 16 ) and a rotor ( 19 ) mounted on one of the rotor shafts ( 13 ) such that one of the screw rotors ( 11 ) is driven directly by the motor ( 17 ), characterised in that the motor rotor ( 19 ) is displaced relative to the stator ( 18 ) by a predetermined axial distance such that a resultant magnetic force is provided in an axial direction.  
     
     
         2 . A screw compressor assembly as claimed in  claim 1  in which the motor rotor ( 19 ) is positioned relative to the stator ( 18 ) to direct the resultant magnetic force towards a delivery end of the compressor ( 10 ).  
     
     
         3 . A screw compressor assembly as claimed in  claim 1  in which the motor rotor ( 19 ) is positioned relative to the stator ( 18 ) to direct the resultant magnetic force to an inlet end of the compressor ( 10 ).  
     
     
         4 . A screw compressor assembly as claimed in any one of the preceding claims in which the motor rotor ( 19 ) is positioned relative to the stator ( 18 ) such that the resultant magnetic force is directed to change a load on the screw rotor bearings of the driven screw rotor ( 11 ).  
     
     
         5 . A screw compressor assembly as claimed in any one of the preceding claims in which the motor rotor ( 19 ) is positioned relative to the stator ( 18 ) such that the cyclic axial forces on the motor rotor ( 19 ) are in phase with cyclic axial loads produced in the screw rotors ( 11 , 12 ).  
     
     
         6 . A screw compressor assembly as claimed in any one of the preceding claims further comprising a cooling system, comprising means for injecting a liquid coolant into the compressor ( 10 ), during operation, to cool, seal and lubricate the compressor ( 10 ) and bearings ( 15 ), wherein the coolant is also used to cool the switched reluctance motor ( 17 ).  
     
     
         7 . A screw compressor assembly as claimed in  claim 6  in which the cooling system comprises a cooling jacket ( 26 ) surrounding the switched reluctance motor ( 17 ) for receiving a flow of coolant liquid.  
     
     
         8 . A screw compressor assembly as claimed in any one of the preceding claims comprising a controller for controlling the speed of the compressor ( 10 ), such that the volume and/or pressure of compressed gas delivered by the screw compressor assembly lies between predetermined limits.  
     
     
         9 . A screw compressor assembly as claimed in any one of the preceding claims in which the rotor ( 19 ) of the motor ( 17 ) is overhung on the bearings ( 15 ).  
     
     
         10 . A screw compressor assembly as claimed in any one of the preceding claims in which the speed at which the compressor ( 10 ) is driven lies in the range of 500 to 25,000 rpm.  
     
     
         11 . A screw compressor assembly as claimed in  claim 10  in which the speed at which the compressor ( 10 ) is driven lies in the range of 1000 to 10,000 rpm.