Patent Publication Number: US-8974190-B2

Title: Variable-speed scroll refrigeration compressor

Description:
The present invention relates to a variable-speed scroll refrigeration compressor. 
     Document FR 2 885 966 describes a variable-speed scroll refrigeration compressor, comprising a sealed enclosure containing a compression stage, an electric motor equipped with a stator and rotor, a drive shaft rotationally coupled to the rotor of the electric motor, the drive shaft comprising a first end arranged to drive the motion of a moving part of the compression stage, and a second end rotationally coupled to an oil pump arranged to supply, from oil contained in the sump situated in the lower portion of the enclosure, a lubrication duct formed in the central portion of the drive shaft. The lubrication duct includes lubrication ports at the various guide bearings of the drive shaft. 
     When such a compressor is stopped for a prolonged period of time, the refrigerant present inside the compressor may condense, in particular on the parts making up the compression stage of the guide bearings of the drive shaft, and thereby cause degreasing of those various parts. Such degreasing involves significant forces when the compressor is restarted, in particular between the parts making up the compression stage, and between the drive shaft and the guide bearings of the latter part, which causes significant and premature wear of those various parts, as well as vibration phenomena. Furthermore, a so-called “dry” start-up, which is very harmful for the compressor, cannot be avoided in the case of complete or nearly complete degreasing. 
     This wear is even more significant inasmuch as, during the start-up of such compressor, the rotor is rotated at a high speed, which creates significant forces at the previously mentioned parts. 
     Document U.S. Pat. No. 5,253,481 describes a solution to limit the wear of these various parts during restarting of the compressor after a prolonged stop thereof. The solution consists of providing a start-up phase of the compressor at a very low speed, before a normal operation phase of the compressor. 
     Thus, document U.S. Pat. No. 5,253,481 describes a variable-speed scroll refrigeration compressor in particular comprising control means arranged to control the operation of the electric motor according to at least one start-up mode in which the rotor of the electric motor is rotated at a first speed of rotation comprised in a first speed range, and a normal operating mode in which the rotor is rotated at a second speed of rotation comprised in a second speed range greater than the first speed range. 
     The first speed of rotation is approximately one revolution per second so as to ensure circulation of the refrigerant inside the compressor and discharge of the excess refrigerant outside the compressor on the one hand, and a supply of oil for the lubrication duct of the drive shaft on the other hand, without creating significant forces on the parts making up the compression stage and on the guide bearings. During the circulation of the refrigerant in the compressor, said refrigerant, which is slightly charged with oil, participates in a slight lubrication of the parts of the compressor with which it comes into contact. Furthermore, the lubrication duct of the drive shaft participates in particular in lubricating the guide bearings. 
     The compressor described in document U.S. Pat. No. 5,253,481 thereby makes it possible to avoid any risk of a so-called “dry” start-up of the compressor, and to limit vibration phenomena. 
     However, due to the low-speed driving of the rotor, the oil pump does not allow a significant injection of oil into the lubrication duct formed inside the drive shaft. 
     As a result, during the start-up mode, the component pieces of the compression stage are not lubricated or are only very slightly lubricated, which necessarily leads to the creation of significant forces on those parts in the first phase of the normal operating mode. This results in premature wear of the parts making up the compression stage. 
     The present invention aims to resolve this drawback. 
     The technical problem at the base of the invention therefore consists of providing a variable-speed scroll refrigeration compressor that has a simple and cost-effective structure, while limiting the risks of premature wear of the compressor. 
     To that end, the present invention relates to a variable-speed scroll refrigeration compressor, comprising:
         a sealed enclosure containing a compression stage,   an oil sump housed in the lower portion of the sealed enclosure,   an electric motor having a stator and a rotor,   an oil pump rotationally coupled to the electric motor, the oil pump comprising an oil inlet port connected to the oil sump of the compressor and at least one first oil output port, and   control means arranged to control the operation of the electric motor according to at least one start-up mode in which the rotor of the electric motor is rotated at a first speed of rotation comprised in a first speed range, and a normal operating mode in which the rotor is rotated a second speed of rotation comprised in a second speed range, the second speed range being higher than the first speed range,       

     wherein the compressor comprises an oil injection device including at least one oil injection duct connected to the first oil outlet port of the oil pump and arranged to supply the compression stage of the compressor with oil, and the control means include monitoring means arranged to vary a value that is representative of the output torque of the electric motor so as to keep the first speed of rotation substantially constant during the start-up mode, the control means being arranged to control the operation of the electric motor in the start-up mode until the value representative of the output torque becomes lower than a predetermined value. 
     The presence of such an oil injection device ensures, during the start-up phase of the compressor, satisfactory lubrication of the parts of the compression stage, despite a low speed of rotation of the rotor, and therefore the oil pump. As a result, the injection device makes it possible to limit the forces applied on the parts making up the compression stage during the first phase of the normal operating mode of the compressor. 
     Furthermore, such a configuration of the control means makes it possible to ensure maintenance of the start-up mode until the component parts of the compression stage are sufficiently lubricated. 
     The control and monitoring means may for example be made up of program elements or software elements run by one or more processors, or for example by a dedicated electronic circuit designed to implement the desired control logic. 
     The monitoring and control means may in particular be made up of elements of the same computer program run by one or more processors, in particular by the same processor. 
     The control means may also be formed by an electronic control unit. 
     It should be noted that the start-up mode is used irrespective of the surrounding conditions of the compressor, and is not limited to low temperature conditions, for example. 
     The injection device thus greatly limits the risks of premature wear of the compressor. 
     According to one embodiment of the invention, the first speed of rotation is comprised between 2 and 10% of the maximum continuous speed of rotation of the electric motor. 
     The bearings and the body supporting the compression stage have a certain capacity to operate without oil. That capacity depends on their size, their material, and the forces they must bear. Knowing the maximum forces, it is therefore easy to deduce the speed after which oil must be provided. This intrinsic capacity of the bearings and the body makes it possible to set the lower value of the first speed range (2%). 
     According to one embodiment of the invention, the second speed of rotation is comprised between 12.5 and 100% of the maximum continuous speed of rotation of the electric motor, and advantageously between 15 and 100% of the maximum continuous speed of rotation of the electric motor. 
     Preferably, the second speed of rotation varies in the second speed range. According to one embodiment, the second speed of rotation varies from a minimum value to a maximum value. The second speed of rotation can vary from the minimum value to the maximum value, for example, continuously or by level. 
     Advantageously, the monitoring means are arranged to vary the feed current of the electric motor so as to keep the first speed of rotation substantially constant during the start-up mode, the control means being arranged to command operation of the electric motor in the start-up mode until the value of the feed current of the electric motor becomes lower than a predetermined current value. 
     Advantageously, the compressor comprises a drive shaft rotationally coupled to the rotor of the electric motor and arranged to rotate the oil pump, the oil pump comprising a second oil outlet port connected to a lubrication duct formed in the central portion of the drive shaft. 
     According to one embodiment of the invention, the drive shaft comprises a first end arranged to drive a moving part of the compression stage, and a second end rotationally coupled to the oil pump. The drive shaft preferably comprises lubrication ports respectively emerging on the one hand in the lubrication duct and on the other hand in the outer surface of the drive shaft. Each lubrication port advantageously emerges at a guide bearing of the drive shaft. 
     According to one embodiment of the invention, the sealed enclosure has a suction volume and a compression volume respectively arranged on either side of the body contained in the sealed enclosure, the suction volume including the oil sump and the compression volume including the compression stage, an end of each oil injection duct opposite the oil pump emerging in the compression volume. Advantageously, the compression stage comprises a stationary volute and a moving volute driven in an orbital movement, the stationary volute being equipped with a scroll engaged in a scroll of the moving volute, the moving volute bearing against the body separating the compression and suction volumes. 
     Preferably, the end portion of each oil injection duct opposite the oil pump is inserted into a through bore formed in the body separating the compression and suction volumes. 
     Advantageously, each oil injection duct comprises a choke member, such as an injection nozzle, mounted at the end of the oil injection duct opposite the oil pump. 
     Preferably, the oil injection device includes a plurality of oil injection ducts. 
     Advantageously, each oil injection duct has a substantially constant transverse section. Preferably, each oil injection duct is a flexible or rigid tubing. Each injection duct advantageously extends inside the enclosure of the compressor. 
     Preferably, the oil pump is a displacement pump, for example with gears. 
     According to a first alternative embodiment of the invention, the oil injection device also comprises an oil return duct connected to the first oil output port of the oil pump and designed to return the oil into the oil sump of the compressor, and each oil injection duct in the oil return duct is configured such that the pressure losses in each oil, injection duct are primarily singular pressure losses proportional to the square of the oil flow rate passing through said oil injection duct, and the pressure losses in the oil return duct are primarily pressure losses due to friction proportional to the oil flow rate passing through the oil return duct. 
     According to one embodiment, each oil injection duct in the oil return duct is configured such that the pressure losses in each oil injection duct may for example be lower than the pressure losses in the oil return duct when the speed of rotation of the rotor is below a first predetermined value belonging to the second speed range, and such that the pressure losses in each oil injection duct are greater than the pressure losses in the oil return duct when the speed of rotation of the rotor is above a second predetermined value belonging to the second speed range, the second predetermined value being greater than or identical to the first predetermined value. 
     The oil injection device preferably comprises a connector including at least one oil inlet port supplied with oil by a supply duct connected to the first output port of the oil pump, a first oil outlet port connected to the at least one oil injection duct, and a second oil outlet port connected to the oil return duct. The connector may for example be housed in the sealed enclosure of the compressor. 
     According to a second alternative embodiment of the invention, the oil injection device comprises a solenoid valve having a body mounted on the sealed enclosure and a core housed in the body of the solenoid valve, the body of the solenoid valve having at least one oil inlet port supplied with oil by a supply duct connected to the first outlet port of the oil pump, a first oil outlet port connected to the at least one oil injection duct emerging in the compression stage, and a second oil outlet port emerging in the sealed enclosure, the core being able to move, under the effect of a magnetic field, between the closing position of the second oil outlet port, in which all of the oil entering the solenoid valve through the oil inlet port is oriented toward the first oil outlet port, and an open position of the second oil outlet port, in which all or nearly all of the oil entering the solenoid valve through the oil inlet port is oriented toward the second oil outlet port. 
     The compressor advantageously comprises monitoring means arranged to move the core of the solenoid valve between its open and closed positions as a function of the speed of rotation of the rotor of the electric motor. The monitoring means are preferably arranged to move the core of the solenoid valve into its open position when the speed of the rotor is above a predetermined value belonging to the second speed range. 
    
    
     
       In any event, the invention will be well understood using the following description in reference to the appended diagrammatic drawing showing, as non-limiting examples, two embodiments of the compressor. 
         FIG. 1  is a longitudinal cross-sectional view of a compressor according to a first embodiment of the invention. 
         FIG. 2  is an enlarged view of a detail of  FIG. 1 . 
         FIG. 3  is an enlarged cross-sectional view of the displacement pump of the injection device of  FIG. 1 . 
         FIG. 4  is a diagram showing the speed of rotation of the motor of the compressor of  FIG. 1  as a function of time. 
         FIG. 5  is a cross-sectional view of a solenoid valve belonging to a compressor according to a second embodiment of the invention. 
         FIG. 6  is a partial cross-sectional view of a compressor according to a third embodiment of the invention. 
     
    
    
       FIG. 1  describes a scroll refrigeration compressor in a vertical position. However, the compressor according to the invention may be in a tilted position or a horizontal position, without its structure being significantly altered. 
     The compressor shown in  FIG. 1  comprises a sealed enclosure delimited by a shroud  2 , the upper and lower ends of which are respectively closed by a cover  3  and the base  4 . The assembly of this enclosure may in particular be done using weld seams. 
     The intermediate part of the compressor is occupied by a body  5  that delimits two volumes, i.e. a suction volume situated below the body  5  and a compression volume arranged above the body. The shroud  2  comprises a refrigerant inlet  6  emerging in the suction volume to bring refrigerant into the compressor. 
     The body  5  is used to mount a compression stage  7  for the refrigerant. Said compression stage  7  comprises a stationary volute  8  including a plate  9  from which a stationary scroll  10  extends turned downward, and moving volute  11  including a plate  12  bearing against the body  5  and from which a scroll  13  extends turned upward. The two scrolls  10  and  13  of the two volutes penetrate one another to form variable-volume compression chambers  14 . 
     The compressor also comprises a discharge duct  15  formed in the central portion of the stationary volute  8 . The discharge duct  15  comprises a first end emerging in the central compression chamber and a second end intended to be put in communication with a high-pressure discharge chamber  16  formed in the enclosure of the compressor. The discharge chamber  16  is delimited partially by a separating plate  17  mounted on the plate  9  of the stationary volute  8  so as to surround the discharge duct  15 . 
     The compressor also comprises a refrigerant outlet  18  emerging in the discharge chamber  16 . 
     The compressor also comprises a non-return device  19  mounted on the plate  9  of the stationary volute  8  at the second end of the discharge duct  15 , and in particular having a discharge valve that can move between a closing position, preventing the discharge duct  15  and the discharge chamber  16  from being put in communication, and a released position, allowing the discharge duct  15  and the discharge chamber  16  to be put in communication. The discharge valve is designed to be moved into its released position when the pressure in the discharge duct  15  exceeds the pressure in the discharge chamber  16  by a first predetermined value substantially corresponding to the adjustment pressure of the discharge valve. 
     The compressor comprises a three-phase electric motor arranged in the suction volume. The electric motor comprises a stator  21 , at the center of which a rotor  22  is arranged. The rotor  22  is secured with a drive shaft  23 , the upper end of which is off-centered like a crankshaft. This upper portion is engaged in a sleeve or bush  24  of the moving volute  11 . When it is rotated by the motor, the drive shaft  23  drives the moving volute  11  following an orbital movement. The drive shaft  23  comprises a lubrication duct  25  formed in the central portion thereof. The supply duct  25  is off-centered and preferably extends over the entire length of the drive shaft  23 . The drive shaft  23  also comprises lubrication ports respectively emerging on the one hand in the lubrication duct  25  and on the other hand in the outer surface of the drive shaft. Preferably, the drive shaft  23  comprises a lubrication port at each guide bearing of the drive shaft. 
     The compressor also comprises an intermediate jacket  26  surrounding the stator  21 . The upper end of the intermediate jacket  26  is secured on the body  5  separating the suction and compression volumes, such that the intermediate jacket  26  is used to fasten the electric motor. The intermediate jacket  26  on the one hand delimits an annular outer volume  27  with the sealed enclosure, and on the other hand an inner volume  28  containing the electric motor. 
     The compressor also comprises a centering piece  29 , fastened on the sealed enclosure using a fastening piece  31 , provided with a guide bearing  32  arranged to guide the lower end portion of the drive shaft  23 . The lower end of the intermediate jacket  26  rests on the centering piece  29  such that the centering piece substantially closes all of the lower end of the intermediate jacket. 
     The compressor also includes an oil separator device mounted on the outer wall of the intermediate jacket  26 . The oil separator device includes at least one refrigerant circulation channel  33 , and for example two refrigerant circulation channels  33 . Each refrigerant circulation channel  33  has a refrigerant inlet opening  34  emerging in the annular outer volume  27  and a refrigerant outlet opening emerging in the inner volume  28 . 
     According to one embodiment of the invention, the refrigerant outlet opening emerges at a window  35  formed in the intermediate jacket  26  so as to put the refrigerant circulation channel  33  and the inner volume  28  delimited by the intermediate enclosure  26  in communication. 
     Advantageously, the refrigerant inlet opening  34  is axially offset relative to the refrigerant inlet  6 , and is situated near the end of the electric motor turned toward the compression stage  7 . 
     The compressor is configured such that under usage conditions, a flow of refrigerant circulates through the refrigerant inlet  6 , the annular outer volume  27 , the refrigerant circulation channel  33 , the window  35 , the inner volume  28 , the compression stage  7 , the discharge duct  15 , the non-return device  19 , the discharge chamber  16  and the refrigerant outlet  18 . 
     The compressor also comprises an oil pump  36  housed in the lower portion of the sealed enclosure. The oil pump  36  is rotationally coupled to the lower end of the drive shaft  23 . The oil pump  36  is advantageously a displacement pump, for example with gears. 
     The oil pump  36  comprises an oil inlet port  37  emerging in an oil sump  38  delimited partially by the base  4  and the shroud  2 , a first oil outlet port  39  and a second oil outlet port  40 . 
     The second oil outlet port  40  is connected to the lubrication duct  25  formed in the central portion of the drive shaft  23 . The oil pump  36  is thus arranged to supply the lubrication duct  25  with oil from the oil contained in the oil sump  38 . 
     The compressor comprises an oil injection device having a connector  41  housed in the sealed enclosure of the compressor. The connector  41  includes, as shown more particularly in  FIG. 2 , an oil inlet port  42  supplied with oil through a supply duct  43  connected to the first oil outlet port  39  of the oil pump  36 , a first oil outlet port  43  connected to the oil injection duct  44  designed to supply the compression stage  7  with oil, and a second oil outlet port  45  connected to an oil return duct  46  designed to return oil into the oil sump  38 . The oil pump  36  is thus also arranged to supply the compression stage  7  with oil via the supply duct  43  and the oil injection duct  44 . 
     The oil inlet port  42  is connected to the oil outlet ports  43 ,  45  by a connecting chamber  47  formed in the connector  41 . 
     Advantageously, the oil injection device includes a second oil injection duct  44 . According to one embodiment of the invention, the connector  41  has a second oil outlet port  43  emerging in the connecting chamber  47  and connected to the second injection duct  44 . According to another embodiment of the invention, the two oil injection ducts  44  are connected to the same outlet port  43  by means of a duct portion. 
     The end portion of each oil injection duct  44  opposite the oil pump  36  is inserted into a through bore  50  formed in the body  5  separating the compression and suction volumes. 
     Each oil injection duct  44  includes an injection tubing having a substantially constant transverse section. 
     The oil injection ducts  44  are configured such that the pressure losses in each oil injection duct  44  are primarily singular pressure losses proportional to the square of the oil flow rate in the oil injection duct  44 . In this way, each oil injection duct  44  also comprises a choke member, such as an injection nozzle, mounted at the end of the respective injection tubing opposite the oil pump  36 . 
     Advantageously, the oil return duct  46  is formed by a tubing having a substantially constant transverse section. The pressure losses in the oil return duct  46  are primarily pressure losses due to friction proportional to the oil flow rate in the oil return duct  46 . 
     The compressor also has a control unit  48  arranged to control the operation of the electric motor according to at least one start-up mode in which the rotor of the electric motor is rotated at a first speed of rotation V 1  comprised in a first speed range, and a normal operating mode in which the rotor is rotated at a second speed of rotation V 2  comprised in a second speed range higher than the first speed range. 
     The first speed of rotation V 1  is substantially constant, and advantageously comprised between 2 and 10% of the maximum continuous speed of rotation of the electric motor. 
     The second speed of rotation V 2  is preferably variable, and advantageously varies in the second speed range. The second speed of rotation can vary between a minimum value and a maximum value, for example continuously or by level. 
     The control unit  48  includes monitoring means  49  arranged to vary a value representative of the output torque of the electric motor so as to keep the first speed of rotation V 1  substantially constant during the start-up mode, and the control unit  48  is arranged to command the operation of the electric motor in the start-up mode until the value representative of the output torque from the electric motor becomes lower than a predetermined value. Advantageously, the monitoring means  49  are arranged to vary the value of the feed current of the electric motor so as to keep the first speed of rotation V 1  substantially constant during the start-up mode, and the control unit  48  is arranged to command operation of the electric motor in the start-up mode until the value of the feed current of the electric motor becomes lower than a predetermined current value. 
     As shown in  FIG. 4 , the control unit  48  is arranged to command the operation of the electric motor in the start-up mode for a variable period of time P corresponding to the necessary period of time, from the command of the start-up mode, for the value of the feed current of the electric motor to become lower than a predetermined current value. When the feed current value becomes lower than the predetermined current value, the control unit  48  is arranged to command the operation of the electric motor in the normal operating mode. 
     The operation of the scroll compressor will now be described. 
     When the scroll compressor according to the invention is started, the control unit  48  commands the electric motor in the start-up mode such that the rotor  22  is rotated at the first speed of rotation V 1 , i.e. at a low speed. The rotor  22  then rotates the drive shaft  23  such that the oil pump  36  supplies the supply duct  43  and the lubrication duct  25  from oil contained in the sump  38 . The oil circulating in the lubrication duct  25  then penetrates the lubrication ports formed in the drive shaft  23  so as to lubricate the guide bearings of the drive shaft. The oil circulating in the supply duct  43  then penetrates the oil inlet port  42  of the connector  41 . The rotor  22  being run in the start-up mode, the speed of rotation of the rotor, and therefore the oil pump  36 , is low. Thus, the pressure losses in each oil injection duct  44  are relatively low. As a result, a significant proportion of the oil having penetrated the connector  41  is oriented toward the first and second injection ducts  44  via the connecting chamber  47  and the first output port  43 . Lastly, the oil is injected into the compression stage  7  by means of the injection nozzles mounted at the ends of the injection ducts  44 . It should be noted that the end of at least one of the oil injection ducts  44  opposite the oil pump  36  is covered by the moving the volute  11  during at least part of the orbital movement of the latter part. As a result, the oil injected into the compression stage  7  ensures lubrication of the interface between the body  5  and the moving volute  11 . 
     In this way, when the electric motor is operating in the start-up mode, the oil injection device and the lubrication duct ensure complete lubrication of the parts of the compression stage and the guide bearings. 
     Furthermore, given that the first speed of rotation V 1  is very low relative to a normal operating speed of the motor, the forces exerted in particular on the stationary and moving volutes of the compression stage are not very high during operation of the motor in the start-up mode. 
     As a result, the combination of the control unit and the injection device ensures, during start-up of the compressor, complete lubrication of the parts of the compression stage and guide bearings, while limiting the risks of wear of those parts. 
     When the compressor is started up, the parts making up the compression stage  7  and guide bearings of the drive shaft  23  are slightly lubricated, with the result that the forces applied on those parts, and therefore the resistant torque applied on the rotor  22 , are not very high. The feed current of the electric motor must thus be relatively low such that the output torque from the motor can counter that resistant torque, and ensure that the first speed of rotation is kept at the desired value. As previously indicated, during the rotation of the rotor  22 , the injection device supplies the compression stage with oil, which results in improving the lubrication of the parts making up the compression stage, and therefore reducing the forces applied on those parts on the one hand, and the resistant torque exerted on the rotor  22  on the other hand. As a result, the monitoring means  49  can decrease the value of the feed current of the electric motor so as to ensure that the first speed of rotation V 1  is kept at the desired value. 
     Once the value of the feed current becomes lower than the predetermined value, which is predetermined to ensure sufficient lubrication of the parts making up the compression stage in the guide bearings, the control unit  48  commands the transition to the normal operating mode, such that the rotor  22  is rotated at the second speed of rotation V 2 , i.e. a high speed. At such a speed of rotation of the rotor, the forces exerted on the parts of the compression stage are significant. However, due to the proper lubrication of those parts during the start-up phase of the compressor, the wear of those parts is greatly limited. 
     As the speed of the compressor, and therefore of the oil pump, increases, the proportion of oil entering the connector  41  through the oil inlet port  42  and oriented toward the oil injection ducts  44  decreases, while the proportion of oil feeding the oil return duct  46  and returned into the oil sump  38  of the compressor increases, in light of the fact that the pressure losses in each injection duct  44  increase much more quickly with the flow rate passing through each injection duct  44  than the pressure losses in the oil return duct  46 . 
     At a high speed of the rotor, and therefore the oil pump, the majority of the oil entering the connector  41  through the oil inlet port  42  is oriented toward the oil return duct  44  via the second oil outlet port  45 , and falls by gravity into the oil sump  38 . 
     Consequently, the injection device makes it possible to limit the quantity of oil injected into the compression stage during normal operation of the compressor, and therefore to limit the level of oil in the refrigerant at a high speed of the compressor. As a result, the performance of the compressor is improved at low speeds without harming the effectiveness thereof at high speeds. 
       FIG. 5  shows a partial view of a compressor according to a second embodiment of the invention that differs from that shown in  FIG. 1  essentially in that the oil injection device comprises a solenoid valve  51  in place of the connector  41 . 
     The solenoid valve  51  includes a body  52  mounted on the sealed enclosure  2  of the compressor and a core  53  housed in the body  52 . The body  52  of the solenoid valve includes an oil inlet port  54  supplied with oil by the supply duct  43  connected to the first outlet port  39  of the oil pump  36 , a first oil outlet port  55  connected to the oil injection ducts  44 , and a second oil outlet port  56  emerging in the sealed enclosure. The core can move, under the effect of the magnetic field, between a position closing the second oil outlet port  55 , in which all of the oil entering the solenoid valve through the oil inlet port  54  is oriented toward the first oil outlet port  55 , and a position opening the second oil outlet port  56 , in which all or nearly all of the oil entering the solenoid valve through the oil inlet port  54  is oriented toward the second oil outlet port  56 . The oil in the port  54  is connected to the oil outlet ports  55 ,  56  by a connecting chamber  57  formed in the body of the solenoid valve  51 . 
     According to the second embodiment, the compressor comprises monitoring means  58  arranged to move the core  53  of the solenoid valve between its open and closed positions as a function of the speed of rotation of the rotor of the electric motor. Monitoring means  58  are preferably arranged to move the core  53  of the solenoid valve  51  into its open position when the speed of the rotor  22  is above a predetermined value belonging to the second speed range. 
     In this way, as long as the speed of rotation of the rotor  22  is below the predetermined value, the core  53  is kept in its closed position and all of the oil entering a solenoid valve  51  through the oil inlet port  54  is oriented toward the compression stage  7  via the first oil outlet port  55  in the injection ducts  44 . When the speed of rotation of the rotor  22  exceeds the predetermined value, the monitoring means  58  move the core  53  into its second position and all or nearly all of the oil entering the solenoid valve  51  through the oil inlet port  54  is oriented toward the second oil outlet port  56 , due to the fact that at high speeds, the pressure losses formed in the first oil outlet port  55  and each injection duct  44  are substantially greater than those formed in the second oil outlet port  56 . 
     As a result, the injection device including the solenoid valve  51  ensures a supply of oil for the compression stage  7  and a return of oil toward the oil sump  38  similarly to the injection device shown in  FIG. 1 . 
       FIG. 6  shows a partial view of a compressor according to a third embodiment of the invention that differs from that shown in  FIG. 1  essentially in that the end of each through bore  50  formed the body  5  separating the compression and suction volumes is covered by the moving volume  11  during all of the orbital movement thereof. According to this embodiment, the injection nozzle  51  of each oil injection duct is situated at the end of the corresponding through bore  50  opposite the moving volute  11 . 
     The invention is of course not limited solely to the embodiments of this compressor described above as examples, but on the contrary encompasses all alternative embodiments.