Patent Publication Number: US-2022220960-A1

Title: Scroll compressor including end-plate side stepped portions of each of the scrolls corresponding to wall-portion side stepped portions of each of the scrolls

Description:
This application is a Divisional of copending application Ser. No. 15/551,621, filed on Aug. 17, 2017, which is the National Phase under 35 U.S.C. § 371 of International Application No. PCT/JP2016/058314, filed on Mar. 16, 2016, which claims the benefit under 35 U.S.C. § 119(a) to Patent Application No. 2015-053693, filed in Japan on Mar. 17, 2015, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to three-dimensional compression-type scroll compressors. 
     BACKGROUND ART 
     A scroll compressor is provided with a pair of a stationary scroll and an orbiting scroll. The scrolls each include an end plate with a spiral wrap disposed in an upright manner thereon. The spiral wraps (spiral wall portions) of the pair of the stationary scroll and the orbiting scroll are opposed and engaged with each other with a 180 degree phase difference, thus forming a sealed compression chamber between the scrolls. As a result, the scroll compressor is configured to compress fluid. The above-discussed scroll compressor generally has a two-dimensional compression structure in which the wrap heights of the spiral wraps of the stationary scroll and the orbiting scroll are set to be constant over the entire circumference in the spiral direction, a compression chamber is made to move from the outer circumferential side to the inner circumferential side while having its capacity gradually reduced, and the fluid having been sucked into the compression chamber is compressed in the circumferential direction of the spiral wraps. 
     Meanwhile, in order to improve efficiency of the scroll compressor and to achieve downsizing and weight-reduction thereof, a three-dimensional compression-type scroll compressor has been provided. Such a three-dimensional compression-type scroll compressor has a structure in which a stepped portion is provided at a predetermined position, along the spiral direction, on each of the tooth crest and the tooth base of the spiral wraps of the stationary scroll and the orbiting scroll, such that the stepped portion forms a boundary at which the wrap height of the spiral wraps shifts from higher on the outer circumferential side to lower on the inner circumferential side. By causing the height of the compression chamber in the axial direction to be higher on the outer circumferential side of the spiral wraps than on the inner circumferential side thereof, the fluid is compressed both in the circumferential direction and in the height direction of the spiral wraps. 
     As such a three-dimensional compression-type scroll compressor, for example, a scroll compressor in which an end-plate side stepped portion is formed on an end plate of each of a stationary scroll and an orbiting scroll, and a wrap side stepped portion corresponding to the end-plate side stepped portion is provided on a spiral wrap of each of the stationary scroll and the orbiting scroll is well-known, as described in Patent Literature 1. 
     Further, as described in Patent Literature 2, a scroll compressor in which an end-plate side stepped portion is provided on an end plate of one of a stationary scroll and an orbiting scroll, and a wrap side stepped portion corresponding to the end-plate side stepped portion is formed on a spiral wrap of the other of the scrolls is well-known. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2002-5052A 
         Patent Literature 2: Japanese Patent Publication No. 1985-17956B (See FIG. 8) 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     As described in Patent Literature 1, in the case where the stepped portions are provided in both the stationary scroll and the orbiting scroll and these stepped portions have the same height, the stationary and orbiting scrolls are formed in the same shape. As such, because capacities of a pair of compression chambers facing each other on either side of the center of the stationary scroll are theoretically equal to each other at every swivel angle, the pressures in these compression chambers become the same. 
     However, in the case where the heights of the stepped portions of the stationary scroll and the orbiting scroll are different from each other, both the scrolls are not formed in the same shape. Accordingly, because the capacities of the pair of compression chambers facing each other on either side of the center of the stationary scroll are not always equal to each other at every swivel angle, the pressures in the compression chambers differ from each other. 
     Likewise, as described in Patent Literature 2, also in the case where an end-plate side stepped portion is provided on an end plate of one of the stationary scroll and the orbiting scroll, and a wrap side stepped portion corresponding to the end-plate side stepped portion is provided on a spiral wrap of the other of the scrolls, the stationary and orbiting scrolls are not formed in the same shape. Accordingly, because the capacities of the pair of compression chambers facing each other on either side of the center of the stationary scroll are not always equal to each other at every swivel angle, the pressures in the compression chambers differ from each other. 
     As discussed above, in the case where the pressures in the pair of compression chambers facing each other on either side of the center of the stationary scroll are different, one of the compression chambers is excessively compressed in some case, which causes a reduction in compression efficiency. 
     In particular, in an intermediate period like the spring when a low pressure ratio is required, overcompression noticeably occurs in one of the compression chambers. 
     Having been conceived in light of such circumstances, an object of the present invention is to provide a scroll compressor capable of preventing overcompression. 
     Solution to Problem 
     A scroll compressor of the present invention employs the following methods to solve the problems described above. 
     The scroll compressor according to the present invention is provided with a stationary scroll including a spiral wall portion erected on one side surface of an end plate, an orbiting scroll that includes a spiral wall portion erected on one side surface of an end plate and is supported so as to be capable of orbital revolution movement while being prevented from self-rotation by the wall portions being engaged with each other, and a discharge port through which a fluid that has been compressed by both the scrolls is discharged. On the one side of the end plate of one of the scrolls, there is provided an end-plate side stepped portion formed in such a way that, along a spiral of the wall portion, the height thereof increases toward a central side of the spiral and decreases toward an outer end side thereof; and on the other wall portion of the scrolls, there is provided a wall-portion side stepped portion formed corresponding to the end-plate side stepped portion in such a way that the height thereof decreases toward the central side of the spiral and increases toward the outer end side thereof. In the stated scroll compressor, of a pair of compression chambers facing each other on either side of the center of the stationary scroll, the compression chamber in which the pressure is higher communicates with the discharge port before the compression chamber in which the pressure is lower communicates with the discharge port. 
     In the case where the end-plate side stepped portion is provided in one of the stationary scroll and the orbiting scroll while the wall-portion side stepped portion is provided in the other of the scrolls, both the scrolls are not formed in the same shape. 
     Accordingly, the pressures in the pair of compression chambers facing each other on either side of the center of the stationary scroll are not the same. In the present invention, of the pair of compression chambers, the compression chamber in which the pressure is higher is made to communicate with the discharge port before the compression chamber in which the pressure is lower communicates with the discharge port. This makes it possible to avoid the overcompression. 
     For example, in the case where the end-plate side stepped portion is provided in the orbiting scroll and the wall-portion side stepped portion is provided in the stationary scroll, of the compression chambers facing each other against the wall portion of the stationary scroll, the compression chamber on a ventral side (inner circumferential side) is made to communicate with the discharge port earlier than the other one. 
     The scroll compressor according to the present invention is provided with a stationary scroll including a spiral wall portion erected on one side surface of an end plate, an orbiting scroll that includes a spiral wall portion erected on one side surface of an end plate and is supported so as to be capable of orbital revolution movement while being prevented from self-rotation by the wall portions being engaged with each other, and a discharge port through which a fluid that has been compressed by both the scrolls is discharged. On the one side surface of the end plate of each of the scrolls, there is provided an end-plate side stepped portion formed in such a way that, along a spiral of the wall portion, the height thereof increases toward a central side of the spiral and decreases toward an outer end side thereof; on the wall portion of each of the scrolls, there is provided a wall-portion side stepped portion formed corresponding to the end-plate side stepped portion in such a way that the height thereof decreases toward the central side of the spiral and increases toward the outer end side thereof; and the heights of the end-plate side stepped portion and the wall-portion side stepped portion corresponding to each other are different. In the stated scroll compressor, of a pair of compression chambers facing each other on either side of the center of the stationary scroll, the compression chamber in which the pressure is higher communicates with the discharge port before the compression chamber in which the pressure is lower communicates with the discharge port. 
     In the case where the end-plate side stepped portion is formed in each of the stationary scroll and the orbiting scroll, the wall-portion side stepped portion corresponding to the end-plate side stepped portion is formed on the wall portion of each of the stationary scroll and the orbiting scroll, and the heights of the end-plate side stepped portion and the wall-portion side stepped portion corresponding to each other are different, both the scrolls are not formed in the same shape. 
     Accordingly, the pressures in the pair of compression chambers facing each other on either side of the center of the stationary scroll are not the same. In the present invention, of the pair of compression chambers, the compression chamber in which the pressure is higher is made to communicate with the discharge port before the compression chamber in which the pressure is lower communicates with the discharge port. This makes it possible to avoid the overcompression. 
     For example, in the case where the end-plate side stepped portion of the orbiting scroll is larger in height than the wall-portion side stepped portion of the stationary scroll, of the compression chambers facing each other against the wall portion of the stationary scroll, the compression chamber on the ventral side (inner circumferential side) is made to communicate with the discharge port earlier than the other one. 
     The scroll compressor according to the present invention is provided with a stationary scroll including a spiral wall portion erected on one side surface of an end plate, an orbiting scroll that includes a spiral wall portion erected on one side surface of an end plate and is supported so as to be capable of orbital revolution movement while being prevented from self-rotation by the wall portions being engaged with each other, a discharge port through which a fluid that has been compressed by both the scrolls is discharged, and an extraction port for discharging a fluid with a pressure equal to or greater than a predetermined pressure before the fluid being discharged through the discharge port. On the one side surface of the end plate of one of the scrolls, there is provided an end-plate side stepped portion formed in such a way that, along a spiral of the wall portion, the height thereof increases toward a central side of the spiral and decreases toward an outer end side thereof; and on the wall portion of the other of the scrolls, there is provided a wall-portion side stepped portion formed corresponding to the end-plate side stepped portion in such a way that the height thereof decreases toward the central side of the spiral and increases toward the outer end side thereof. In the stated scroll compressor, of a pair of compression chambers facing each other on either side of the center of the stationary scroll, the compression chamber in which the pressure is higher communicates with the extraction port before the compression chamber in which the pressure is lower communicates with the extraction port. 
     In the case where the end-plate side stepped portion is provided in one of the stationary scroll and the orbiting scroll while the wall-portion side stepped portion is provided in the other of the scrolls, both the scrolls are not formed in the same shape. 
     Accordingly, the pressures in the pair of compression chambers facing each other on either side of the center of the stationary scroll are not the same. In the present invention, of the pair of compression chambers, the compression chamber in which the pressure is higher is made to communicate with the extraction port (what is called a bypass port) before the compression chamber in which the pressure is lower communicates with the extraction port. This makes it possible to avoid the overcompression. 
     For example, in the case where the end-plate side stepped portion is provided in the orbiting scroll and the wall-portion side stepped portion is provided in the stationary scroll, of the compression chambers facing each other against the wall portion of the stationary scroll, the compression chamber on the ventral side (inner circumferential side) is made to communicate with the extraction port earlier than the other one. 
     The scroll compressor according to the present invention is provided with a stationary scroll including a spiral wall portion erected on one side surface of an end plate, an orbiting scroll that includes a spiral wall portion erected on one side surface of an end plate and is supported so as to be capable of orbital revolution movement while being prevented from self-rotation by the wall portions being engaged with each other, a discharge port through which a fluid that has been compressed by both the scrolls is discharged, and an extraction port for discharging a fluid with a pressure equal to or greater than a predetermined pressure before the fluid being discharged through the discharge port. On the one side surface of the end plate of each of the scrolls, there is provided an end-plate side stepped portion formed in such a way that, along a spiral of the wall portion, the height thereof increases toward a central side of the spiral and decreases toward an outer end side thereof; on the wall portion of each of the scrolls, there is provided a wall-portion side stepped portion formed corresponding to the end-plate side stepped portion in such a way that the height thereof decreases toward the central side of the spiral and increases toward the outer end side thereof; and the height of the end-plate side stepped portion and the height of the wall-portion side stepped portion are different. In the stated scroll compressor, of a pair of compression chambers facing each other on either side of the center of the stationary scroll, the compression chamber in which the pressure is higher communicates with the extraction port before the compression chamber in which the pressure is lower communicates with the extraction port. 
     In the case where the end-plate side stepped portion is formed in each of the stationary scroll and the orbiting scroll, the wall-portion side stepped portion corresponding to the end-plate side stepped portion is formed on the wall portion of each of the stationary scroll and the orbiting scroll, and the heights of the end-plate side stepped portion and the wall-portion side stepped portion corresponding to each other are different, both the scrolls are not formed in the same shape. 
     Accordingly, the pressures in the pair of compression chambers facing each other on either side of the center of the stationary scroll are not the same. In the present invention, of the pair of compression chambers, the compression chamber in which the pressure is higher is made to communicate with the extraction port (what is called the bypass port) before the compression chamber in which the pressure is lower communicates with the extraction port. This makes it possible to avoid the overcompression. 
     For example, in the case where the end-plate side stepped portion of the orbiting scroll is larger in height than the wall-portion side stepped portion of the stationary scroll, of the compression chambers facing each other against the wall portion of the stationary scroll, the compression chamber on the ventral side (inner circumferential side) is made to communicate with the discharge port earlier than the other one. 
     Advantageous Effects of Invention 
     The overcompression can be prevented because the compression chamber in which the pressure is higher is made to communicate with the discharge port or the extraction port earlier than the other one. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical cross-sectional view of a scroll compressor according to a first embodiment of the present invention. 
         FIG. 2  is a horizontal cross-sectional view illustrating an engaged state of a stationary scroll and an orbiting scroll. 
         FIG. 3  is a graph showing changes in capacity of a ventral side compression chamber and a dorsal side compression chamber. 
         FIG. 4A  is a horizontal cross-sectional view illustrating an engaged state of central portions of the stationary scroll and the orbiting scroll in an enlarged manner,  FIG. 4B  is a horizontal cross-sectional view illustrating a position adjustment of a discharge port, and  FIG. 4C  is a horizontal cross-sectional view illustrating a position adjustment of a discharge port as a variation. 
         FIG. 5  is a graph showing changes in capacity of the ventral side compression chamber and the dorsal side compression chamber according to the first embodiment. 
         FIG. 6A  and  FIG. 6B  are horizontal cross-sectional views each illustrating an engaged state of a stationary scroll and an orbiting scroll according to a second embodiment. 
         FIG. 7A  and  FIG. 7B  are horizontal cross-sectional views each illustrating an engaged state of a stationary scroll and an orbiting scroll as a comparative example. 
         FIG. 8  is a graph showing changes in capacity of a ventral side compression chamber and a dorsal side compression chamber according to the second embodiment. 
         FIG. 9  illustrates a scroll compressor according to a first embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. 
     First Embodiment 
     A first embodiment of the present invention will be described below, using  FIGS. 1 to 5 and 9 . 
     As illustrated in  FIG. 1 , a scroll compressor  1  includes a housing  2  constituting an outline. This housing  2  is a cylinder with an open front end side (left side in the drawing) and a sealed rear end side. By fastening and fixing a front housing  3  into the opening on the front end side using bolts  4 , a sealed space is formed in the interior of the housing  2 , and a scroll compression mechanism  5  and a drive shaft  6  are incorporated in the sealed space. 
     The drive shaft  6  is rotatably supported by the front housing  3  via a main bearing  7  and an auxiliary bearing  8 . A pulley  11 , which is rotatably provided on an outer circumferential portion of the front housing  3  via a bearing  10 , is connected, via an electromagnetic clutch  12 , to a front end portion of the drive shaft  6 , which protrudes to the outside from the front housing  3  via a mechanical seal  9 , such that motive power from outside can be transmitted. A crank pin  13 , which is eccentric by a predetermined dimension, is integrally provided on the rear end of the drive shaft  6 , and is connected to an orbiting scroll  16  of the scroll compression mechanism  5  described below, via a known slave crank mechanism  14  that includes a drive bushing having a variable turn radius and a drive bearing. 
     In the scroll compression mechanism  5 , a pair of compression chambers  17 , facing each other on either side of the center of a stationary scroll  15 , are formed between the stationary scroll  15  and the orbiting scroll  16 , as a result of a pair of the stationary and orbiting scrolls  15  and  16  being engaged with each other with a 180 degrees phase difference. The scroll compression mechanism  5  is configured to compress a fluid (a refrigerant gas) by moving each of the compression chambers  17  from an outer circumferential position to a center position while gradually reducing the capacity thereof. 
     A discharge port  18 , which discharges compressed gas, is provided in a center section of the stationary scroll  15 , and the stationary scroll  15  is fixedly provided on a bottom wall surface of the housing  2  via bolts  19 . Further, the orbiting scroll  16  is connected to the crank pin  13  of the drive shaft  6  via the slave crank mechanism  14 , and is supported by a thrust bearing surface of the front housing  3 , via a known self-rotation prevention mechanism  20 , such that the orbiting scroll  16  is freely capable of orbital revolution drive. 
     An O-ring  21  is provided around the outer circumference of an end plate  15 A of the stationary scroll  15 . As a result of the O-ring  21  making close contact with the inner circumferential surface of the housing  2 , the internal space of the housing  2  is partitioned into a discharge chamber  22  and an intake chamber  23 . The discharge port  18  opens into the discharge chamber  22 . The compressed gas from the compression chambers  17  is discharged through the discharge port  18 , and then discharged to a refrigeration cycle side therefrom. 
     Further, an intake port  24 , which is provided in the housing  2 , opens into the intake chamber  23 . A low-pressure gas, which has circulated through the refrigeration cycle, is taken into the intake port  24 , and then, the refrigerant gas is taken into the interior of the compression chambers  17  via the intake chamber  23 . 
     Further, the pair of the stationary scroll  15  and the orbiting scroll  16  includes spiral wraps  15 B and  16 B disposed as wall portions in an upright manner on the end plate  15 A and an end plate  16 A, respectively. A tooth crest  15 C of the stationary scroll  15  makes contact with a tooth base  16 D of the orbiting scroll  16 , and a tooth crest  16 C of the orbiting scroll  16  makes contact with a tooth base  15 D of the stationary scroll  15 . 
     On the end plate  16 A of the orbiting scroll  16 , there is provided an end-plate side stepped portion  16 E formed in such a way that, along a spiral of the spiral wrap  16 B, the height thereof increases toward a central side of the spiral and decreases toward an outer end side thereof. To be specific, as illustrated in  FIG. 2 , the end-plate side stepped portion  16 E is provided at a position of 180 degrees apart from a wrapping end position of the spiral wrap  16 B of the orbiting scroll  16 . 
     On the spiral wrap  15 B of the stationary scroll  15 , there is provided a wrap side stepped portion  15 E corresponding to the end-plate side stepped portion  16 E of the orbiting scroll  16  in such a way that the height thereof decreases toward the central side of the spiral and increases toward the outer end side thereof. To be specific, as illustrated in  FIG. 2 , the wrap side stepped portion  15 E is provided at a position of 360 degrees apart from the wrapping end position of the spiral wrap  15 B of the stationary scroll  15 . 
     In other words, the end-plate side stepped portion  16 E is provided only on the end plate  16 A of the orbiting scroll  16 , and the wrap side stepped portion  15 E is provided only on the spiral wrap  15 B of the stationary scroll  15 . Accordingly, no stepped portion is provided on the spiral wrap  16 B of the orbiting scroll  16 , and a tip end of the spiral wrap  16 B is leveled in height. Further, no stepped portion is provided on the end plate  15 A of the stationary scroll  15  so as for the end plate  15 A thereof to have a flat surface. 
       FIG. 9  includes the stationary scroll  15  provided with an end-plate side stepped portion having a height lower than the end-plate side stepped portion  16 E of the orbiting scroll  16 , with respect to  FIG. 1 .  FIG. 9  further includes an end plate side stepped portion  15 G provided on the stationary scroll  15 , and a wrap side stepped portion  16 G provided on the orbiting scroll  16 . 
     As illustrated in  FIG. 2 , the compression chambers  17  are formed of at least a pair of compression chambers  17 A and  17 B facing each other on either side of the center of the stationary scroll  15 . In  FIG. 2 , in order to distinguish the pair of compression chambers  17 A and  17 B, the compression chamber formed on a ventral side (inner circumferential side) of the spiral wrap  15 B of the stationary scroll  15  is defined as a ventral side compression chamber  17 A while the compression chamber formed on a dorsal side (outer circumferential side) of the spiral wrap  15 B of the stationary scroll  15  is defined as a dorsal side compression chamber  17 B. 
       FIG. 3  shows changes in capacity of the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B. In the graph, the horizontal axis represents a swivel angle θ*, and the vertical axis represents the capacity of the compression chambers  17 A and  17 B. 
     As can be understood from  FIG. 3 , after a pair of compression chambers is formed on the outermost circumferential side when the intake is ended at a swivel angle α 1 , the compression is performed from the above swivel angle, with the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B having different capacity, up to a swivel angle α 2 , which is a swivel angle at which the ventral side and dorsal side compression chambers  17 A and  17 B have the same capacity and the fluid is discharged. Because a change rate (slant) of the capacity of the ventral side compression chamber  17 A is larger than that of the dorsal side compression chamber  17 B, the pressure in the ventral side compression chamber  17 A becomes higher than that in the dorsal side compression chamber  17 B, which raises a risk that an excessive discharge pressure may be brought about in the ventral side compression chamber  17 A. 
     As such, in the present embodiment, as illustrated in  FIGS. 4A and 4B , a shape of the discharge port  18  is adjusted so that the ventral side compression chamber  17 A communicates with the discharge port  18  earlier than the dorsal side compression chamber  17 B. As a method for adjusting the shape of the discharge port  18 , it is sufficient that the discharge port  18  has a larger diameter than a diameter of a discharge port  18 ′ adjusted so that the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B open at the same time. 
     Positions a and b illustrated in the drawings indicate communication start points of the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B, respectively, in a case of using the discharge port  18 ′ adjusted so that the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B open at the same time. As can be understood from the drawings, with the discharge port  18  having a larger diameter than the diameter of the discharge port  18 ′ adjusted so that the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B open at the same time, the ventral side compression chamber  17 A communicates with the discharge port  18  earlier than the dorsal side compression chamber  17 B. 
     As another method for adjusting the shape of the discharge port  18 , as illustrated in  FIG. 4C , the discharge port  18  may have the same diameter as that of the discharge port  18 ′ adjusted so that the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B open at the same time, and a center position thereof may be moved toward the ventral side compression chamber  17 A side, that is, toward an outer side (left side in the drawing) of the wrapping of the spiral wrap  15 B of the stationary scroll  15 . Alternatively, a cross section of the discharge port  18  may not have a circular shape but have a shape such as an elliptical shape or a keyhole shape, so that the discharge port  18  may communicate earlier with the ventral side compression chamber  17 A. 
     According to the scroll compressor  1  of the present embodiment, it is possible to obtain the following effects. 
     Of the pair of the compression chambers  17 A and  17 B facing each other on either side of the center of the stationary scroll  15 , the ventral side compression chamber  17 A in which the pressure is higher is made to communicate with the discharge port earlier than the dorsal side compression chamber  17 B in which the pressure is lower. 
     With this, even if the scroll compressor  1  is configured such that the stepped portion  16 E is provided on the end plate  16 A of the orbiting scroll  16 , the stepped portion  15 E corresponding to the stepped portion  16 E is provided on the spiral wrap  15 B of the other scroll, that is, the stationary scroll  15 , and the pressures in the pair of the compression chambers  17 A and  17 B facing each other on either side of the center of the stationary scroll  15  are not the same, thus, the overcompression of the ventral side compression chamber  17 A can be avoided. 
     To be specific, as shown in  FIG. 5 , because the ventral side compression chamber  17 A communicates with the discharge port  18  at a swivel angle α 3  before a swivel angle α 4  at which the dorsal side compression chamber  17 B communicates with the discharge port  18 , the ventral side compression chamber  17 A is not further compressed after the swivel angle α 3 . With this, it can be avoided that energy corresponding to a substantially triangular region A 1  shown in  FIG. 5  becomes motive power loss and reduces the compression efficiency. 
     The description of the present embodiment is given using the configuration in which the end-plate side stepped portion  16 E is provided only on the end plate  16 A of the orbiting scroll  16 , and the wrap side stepped portion  15 E is provided only on the spiral wrap  15 B of the stationary scroll  15 . However, a configuration in which the above constituent elements are provided in a reversed manner may be used. 
     In other words, the present invention can be also applied to the configuration in which the end-plate side stepped portion is provided only on the end plate  15 A of the stationary scroll  15 , and the wrap side stepped portion is provided only on the spiral wrap  16 B of the orbiting scroll  16 . 
     In this case, because the pressure in the dorsal side compression chamber  17 B becomes higher than that in the ventral side compression chamber  17 A, the configuration should be such that the dorsal side compression chamber  17 B communicates with the discharge port  18  earlier than the ventral side compression chamber  17 A. For example, in  FIG. 4A , a notch, a groove, or the like is provided on the ventral side of the spiral wrap  16 B of the orbiting scroll  16  so that a gap is generated earlier at the position b. 
     The present invention can be also applied to a scroll compressor in which end-plate side stepped portions are provided on end plates of both a stationary scroll and an orbiting scroll as explained using Patent Literature 1. 
     That is, in the case where the height of the end-plate side stepped portion provided on the end plate of the orbiting scroll is larger than that of the end-plate side stepped portion provided on the end plate of the stationary scroll, because, like in the present embodiment, the pressure in the ventral side compression chamber  17 A becomes higher than that in the dorsal side compression chamber  17 B, adjusting the shape of the discharge port makes it possible to avoid the overcompression of the ventral side compression chamber  17 A. 
     On the other hand, in the case where the height of the end-plate side stepped portion provided on the end plate of the stationary scroll is larger than that of the end-plate side stepped portion provided on the end plate of the orbiting scroll, because the pressure in the dorsal side compression chamber  17 B becomes higher than that in the ventral side compression chamber  17 A, providing a notch, a groove, or the like on the ventral side of the spiral wrap  16 B of the orbiting scroll  16  makes it possible to avoid the overcompression of the dorsal side compression chamber  17 B. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 6A  to  FIG. 8 . 
     The present embodiment differs from the first embodiment in a point that a bypass port is provided in addition to the configuration of the first embodiment. As such, same configurations as those in the first embodiment are given the same reference signs, and explanations thereof are omitted. 
     A scroll compressor  1  of the present embodiment has a vertical cross-sectional shape as illustrated in  FIG. 1 . In addition, in the scroll compressor  1  of the present embodiment, as illustrated in  FIGS. 6A and 6B , bypass ports (extraction ports)  30 A and  30 B are formed in the end plate  15 A of the stationary scroll  15 . The bypass ports  30 A and  30 B each include a check valve or the like, where the valve opens when the pressure becomes equal to or greater than a predetermined one. A fluid with a pressure equal to or greater than the predetermined one is discharged through the bypass ports before the fluid is discharged through the discharge port  18 , thereby avoiding the overcompression. In  FIGS. 6A and 6B , one bypass port  30 A corresponds to the ventral side compression chamber  17 A, and the other bypass port, that is, the bypass port  30 B corresponds to the dorsal side compression chamber  17 B. 
     In the present embodiment, as illustrated in  FIG. 6A , at a swivel angle β 1 , the ventral side compression chamber  17 A communicates with the bypass port  30 A while the dorsal side compression chamber  17 B does not communicate with the bypass port  30 B. Accordingly, at the swivel angle β 1 , an amount of fluid corresponding to an excessive pressure is extracted only from the ventral side compression chamber  17 A. Then, as illustrated in  FIG. 6B , when having advanced to a swivel angle β 2 , the dorsal side compression chamber  17 B communicates with the bypass port  30 B. At the swivel angle β 2 , the ventral side compression chamber  17 A has already communicated with the bypass port  30 A. 
       FIGS. 7A and 7B  illustrate communication start timings of the bypass ports as a comparative example. The configuration of this comparative example corresponds to a case in which a pressure differential between the ventral side compression chamber  17 A and the dorsal side compression chamber  17 B is substantially zero, or is small so as not to affect the performance. As illustrated in  FIG. 7A , none of the bypass ports  30 A and  30 B communicate with the compression chambers  17 A and  17 B at the swivel angle β 1 ; as illustrated in  FIG. 7B , at the swivel angle β 2 , the compression chambers  17 A and  17 B communicate with the bypass ports  30 A and  30 B at the same time. 
       FIG. 8  shows pressure changes due to the bypass ports  30 A and  30 B of the present embodiment illustrated in  FIGS. 6A and 6B . In the graph, the horizontal axis represents the swivel angle, and the vertical axis represents the pressure. As can be understood from the graph, the pressure in the ventral side compression chamber  17 A becomes higher than that in the dorsal side compression chamber  17 B from around a swivel angle β 0 . 
     Then, as illustrated in  FIG. 6A , at the swivel angle β 1 , the ventral side compression chamber  17 A starts communicating with the bypass port  30 A, and is not excessively compressed to a pressure equal to or greater than a requested discharge pressure. Thereafter, as illustrated in  FIG. 6B , at the swivel angle β 2 , the dorsal side compression chamber  17 B starts communicating with the bypass port  30 B, and is adjusted to the requested discharge pressure until at a swivel angle β 3  at which the compression chamber communicates with the discharge port  18 . 
     In contrast, in the case where both the compression chambers  17 A and  17 B start communicating with the bypass ports  30 A and  30 B at the same time at the swivel angle β 2 , as illustrated in  FIGS. 7A and 7B , the ventral side compression chamber  17 A is excessively compressed to a pressure equal to or greater than the requested discharge pressure as shown in  FIG. 8 . Accordingly, energy corresponding to a substantially triangular region A 2  shown in  FIG. 8  becomes motive power loss and reduces the compression efficiency. 
     According to the scroll compressor  1  of the present embodiment, it is possible to obtain the following effects. 
     Of the pair of the compression chambers  17 A and  17 B facing each other on either side of the center of the stationary scroll  15 , the ventral side compression chamber  17 A in which the pressure is higher is made to communicate with the bypass port  30 A earlier than the dorsal side compression chamber  17 B in which the pressure is lower. 
     With this, even if the scroll compressor  1  is configured such that the stepped portion  16 E is provided on the end plate  16 A of the orbiting scroll  16 , the spiral wrap  15 B of the other scroll, that is, the stationary scroll  15  includes a shape of the stepped portion  15 E corresponding to the stepped portion  16 E, and the pressures in the pair of the compression chambers  17 A and  17 B facing each other on either side of the center of the stationary scroll  15  are not the same, the overcompression of the ventral side compression chamber  17 A can be avoided. 
     In the present embodiment, such a configuration is assumed that the end-plate side stepped portion  16 E is provided only on the end plate  16 A of the orbiting scroll  16 , and the wrap side stepped portion  15 E is provided only on the spiral wrap  15 B of the stationary scroll  15 . However, a configuration in which the above constituent elements are provided in a reversed manner may be employed. 
     In other words, the present invention can be also applied to the configuration in which the end-plate side stepped portion is provided only on the end plate  15 A of the stationary scroll  15 , and the wrap side stepped portion is provided only on the spiral wrap  16 B of the orbiting scroll  16 . 
     In this case, because the pressure in the dorsal side compression chamber  17 B becomes higher than that in the ventral side compression chamber  17 A, the position of the bypass port  30 B is adjusted so that the dorsal side compression chamber  17 B communicates with the bypass port  30 B earlier than the ventral side compression chamber  17 A. 
     The present invention can be also applied to a scroll compressor in which end-plate side stepped portions are provided on end plates of both a stationary scroll and an orbiting scroll as explained using Patent Literature 1. 
     That is, in the case where the height of the end-plate side stepped portion provided on the end plate of the orbiting scroll is larger than that of the end-plate side stepped portion provided on the end plate of the stationary scroll, because, like in the present embodiment, the pressure in the ventral side compression chamber  17 A becomes higher than that in the dorsal side compression chamber  17 B, adjusting the position of the bypass port  30 A makes it possible to avoid the overcompression of the ventral side compression chamber  17 A. 
     On the other hand, in the case where the height of the end-plate side stepped portion provided on the end plate of the stationary scroll is larger than that of the end-plate side stepped portion provided on the end plate of the orbiting scroll, because the pressure in the dorsal side compression chamber  17 B becomes higher than that in the ventral side compression chamber  17 A, adjusting the position of the bypass port  30 B makes it possible to avoid the overcompression of the dorsal side compression chamber  17 B. 
     REFERENCE SIGNS LIST 
     
         
           1  Scroll compressor 
           15  Stationary scroll 
           16  Orbiting scroll 
           15 A,  16 A End plate 
           15 B,  16 B Spiral wrap 
           15 C,  16 C Tooth crest 
           15 D,  16 D Tooth base 
           15 E Wrap side stepped portion (Wall-portion side stepped portion) 
           16 E End-plate side stepped portion 
           17  Compression chamber 
           17 A Ventral side compression chamber 
           17 B Dorsal side compression chamber 
           30 A,  30 B Bypass port (Extraction port)