Abstract:
A refrigerant motor/compressor employs both a serviceable shaft-grounding device and a ceramic coating to protect a rolling element bearing that could otherwise be damaged by high frequency induced common mode voltage and current originating from an inverter that includes a plurality of IGBTs (insulate gate bipolar transistors). The shaft-grounding device includes a stranded copper wire brush that rides against an axial end of the shaft and a high frequency stranded grounding wire that conducts the induced current away from the shaft. The shaft-grounding device is sized and positioned so that it can be momentarily removed for inspection without having to evacuate the refrigerant. The ceramic coating provides an electrical insulating surface on a bearing bracket and other parts that support the bearing. The coating comprises titanium dioxide and aluminum oxide to provide a surface that is sufficiently hard and tough to resist damage during assembly, thereby maintaining the coating&#39;s integrity.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The subject invention generally pertains to a hermetically sealed compressor with an inverter-driven motor and more specifically to a means for protecting the motor&#39;s bearings against certain induced voltages and currents.  
         [0003]     2. Description of Related Art  
         [0004]     For years, it has been known that shaft induced current, driven by shaft induced voltage, can damage motor bearings. In some cases, the energy generated by shaft induced current causes deterioration of the lubricant, which can ultimately damage the bearings. If the induced voltage is sufficiently high, electrical arching across the bearing can erode the bearing&#39;s surfaces directly.  
         [0005]     Shaft induced voltage can come from different sources. It can be electrostatically generated within the motor, or the voltage can arise from imbalanced ampere-turns in the stator, or from stator or rotor asymmetries. In cases where the motor is driven by an inverter or variable speed drive, the induced voltage is known as common mode voltage, which can be caused by the switching frequency of the inverter&#39;s SCRs (silicone controlled rectifiers), BJTs (bipolar junction transistors), GTOs (gate turn off tyristors), or, more recently, IGBTs (insulate gate bipolar transistors). Additional background on shaft induced voltage and related information can be found in U.S. Pat. Nos. 5,313,129; 5,914,547; 6,030,128; 5,735,615; 5,139,425; 5,059,041; 4,109,978; 4,378,138; 4,220,879 and 6,555,943.  
         [0006]     Although electrically grounding the shaft or electrically isolating the bearing can reduce the effects of shaft induced voltage, such measures are usually not necessary due to improvements in the design and manufacture of modem day motors and their variable speed drives. More recently, however, the SCRs, BJTs and GTOs of inverters have been replaced by much faster IGBTs. While SCR&#39;s, BJTs and GTOs operate at relatively low frequencies, IGBTs operate at switching frequencies of 2-4 kHz and higher. At these higher switching frequencies, IGBT&#39;s appear to generate common mode current in the range of 2 to 10 MHz, which can be very difficult to limit to a conductive path that bypasses the bearing.  
         [0007]     The increasing popularity of IGBTs for variable speed drives has not only resurrected the problem of induced common mode voltage, it has raised the problem to a new level where conventional methods of correction no longer work. Consequently, a need exists for a way to protect the bearings of a motor driven by an inverter with IGBTs. A better method is particularly needed for hermetically sealed motors whose bearings are relatively inaccessible for repair.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the invention to help prevent certain induced currents from damaging a rolling element bearing of a refrigerant compressor system driven by an inverter.  
         [0009]     Another object of some embodiments is support a motor/compressor shaft with two different style bearings, a rolling element bearing and a journal bearing, where only the rolling element bearing needs a shaft-grounding device, which is generally accessible for servicing.  
         [0010]     Another object of some embodiments is to use both shaft-grounding and electrical insulation to protect a bearing against induced common mode voltage originating from an inverter&#39;s IGBTs.  
         [0011]     Another object of some embodiments is to provide a way of servicing a shaft-grounding device without adversely affecting the refrigerant charge of a hermetically sealed compressor system.  
         [0012]     Another object of some embodiments is to contact the end of a shaft with a shaft-grounding device that applies an ideal magnitude of contact force.  
         [0013]     Another object of some embodiments is to electrically insulate a bearing from an adjacent supporting member by coating the member with a ceramic layer that is harder than the outer periphery of the bearing and harder than the material of the supporting member, wherein the hardness of the coating is by virtue of the ceramic layer having certain proportions of titanium dioxide and aluminum oxide.  
         [0014]     Another object of some embodiments is to provide an outboard with an adjacent labyrinth seal that inhibits excessive gas flow when the shaft-grounding device is momentarily removed.  
         [0015]     Another object of some embodiments is to screw a shaft-grounding device into a threaded hole that is sufficiently small to minimize any gas exchange between the compressor and the atmosphere when the shaft-grounding device is temporarily removed.  
         [0016]     Another object of some embodiments is to ground the end of a shaft using a stranded copper wire brush rather than using a carbon block, as the wire brush is more effective at conducting induced common mode current.  
         [0017]     Another object of some embodiments is to align a shaft-grounding device with a rotational axis of a shaft to minimize wear between the shaft and the shaft-grounding device.  
         [0018]     Another object of some embodiments is to provide the brush of a shaft-grounding device with some axial movement to ensure contact between the brush and the end of the shaft even after the brush experiences some wear.  
         [0019]     Another object of some embodiments is to restrict relative rotation between a brush and an outer housing of a shaft-grounding device to prevent the shaft from rotating the brush and creating wear within the shaft-grounding device.  
         [0020]     Another object of some embodiments is to use a stranded grounding wire to effectively convey high frequency common mode current from a shaft.  
         [0021]     One or more of these and/or other objects of the invention are provided by a compressor system that employs both a serviceable shaft-grounding device and a ceramic coating to protect a rolling element bearing that could otherwise be damaged by high frequency induced common mode voltage and current originating from an inverter that includes a plurality of IGBTs.  
         [0022]     The present invention provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing extending from the compressor housing; a bearing bracket extending from the motor housing; a bearing having an outer periphery supported by the bearing bracket; and a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and rotatable relative to the compressor housing to force the refrigerant from the suction inlet to the discharge outlet. The compressor element is closer to the inboard end than to the outboard end. The system includes a minimally conductive coating disposed on the bearing bracket. The coating is between the bearing bracket and the bearing to provide electrical resistance therebetween, and the coating is harder than the bearing bracket and the outer periphery of the bearing.  
         [0023]     The present invention also provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing attached to the compressor housing; a bearing bracket attached to the motor housing; a bearing supported by the bearing bracket; a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet. The compressor element is closer to the inboard end than to the outboard end. The system further includes an endplate spaced apart from the shaft, spaced apart from the bearing, and electrically coupled to the compressor housing, a ceramic coating disposed on the bearing bracket and a shaft grounding device. The ceramic coating is between the bearing bracket and the bearing. The shaft-grounding device includes a wire brush, a brush housing, and a spring. The brush housing is attached to the endplate. The wire brush is movable along a longitudinal centerline of the brush housing. The spring urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft. The wire brush is electrically coupled to the brush housing. The brush housing is electrically coupled to the endplate, thereby establishing electrical continuity between the compressor housing and the outboard end of the shaft while the ceramic coating provides electrical resistance directly between the bearing and the bearing bracket.  
         [0024]     The present invention further provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing adjacent to the compressor housing; a bearing supported within the motor housing; and a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet and a shaft-grounding device. The compressor element is closer to the inboard end than to the outboard end. The shaft grounding device includes a wire brush, a brush housing, a spring, and a grounding wire. The brush housing is electrically coupled to the motor housing. The grounding wire electrically couples the wire brush to the brush housing. The wire brush is movable along a substantially linear path. The spring is contained within the brush housing and urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft.  
         [0025]     The present invention additionally provides a compressor system. The system includes a motor housing having an interior containing a refrigerant and an exterior exposed to a surrounding atmosphere; a compressor housing hermetically sealed to the motor housing; an endplate extending from the motor housing; a refrigerant disposed within the compressor housing and the motor housing; a rolling element bearing inside the motor housing; a journal bearing inside at least one of the motor housing and the compressor housing; and a shaft having an inboard end, an outboard end, and an intermediate section therebetween. The rolling element bearing supports the outboard end, and the journal bearing supports the intermediate section. The system also includes a compressor element mounted to the inboard end of the shaft and being rotatable for compressing the refrigerant; and a shaft-grounding device extending into the opening of the endplate such that the shaft-grounding device is in electrical contact with the outboard end of the shaft and is exposed to the refrigerant and the surrounding atmosphere.  
         [0026]     The present invention yet further provides a method of servicing a hermetically sealed compressor system that includes a motor housing exposed to a surrounding atmosphere, a shaft rotatable within the motor housing, a refrigerant disposed within the motor housing, and a shaft-grounding device that when properly installed is exposed to the refrigerant and the surrounding atmosphere and completes an electrical path between the shaft and the motor housing. The method includes the steps of adjusting the temperature of the refrigerant until the refrigerant in the motor housing is at a pressure substantially equal to that of the surrounding atmosphere; and removing the shaft-grounding device from within the motor housing while the pressure of the refrigerant is substantially equal to that of the surrounding atmosphere, thereby providing an opportunity to inspect the shaft-grounding device without having to evacuate the refrigerant from within the motor housing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a cross-sectional view of a compressor system connected to a schematically illustrated refrigerant circuit and inverter.  
         [0028]      FIG. 2  is cross-sectional view of the compressor system of  FIG. 1  showing one end of the compressor&#39;s motor.  
         [0029]      FIG. 3  is a cross-sectional view of a shaft-grounding device attached to an endplate and engaging a shaft.  
         [0030]      FIG. 4  is a cross-sectional view similar to  FIG. 3  but showing the shaft-grounding device separated from the endplate and the shaft.  
         [0031]      FIG. 5  is a cross-sectional view similar to  FIG. 1  but showing how the shaft-grounding device can be temporarily removed for servicing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]      FIG. 1  illustrates a hermetically sealed compressor system  10  comprising a compressor  12  and a motor  14 . Compressor system  10  also includes a novel shaft-grounding device  16  for electrically grounding a shaft  18  and an electrically insulating material, such as a minimally conductive coating  20  ( FIG. 2 ), for electrically isolating a conductive bearing  22  that supports shaft  18 . An inverter  24  controls the speed of motor  12 , which in turn drives compressor  14 . The term, “hermetically sealed” refers to a non-sliding, substantially airtight transition  26  between a motor housing  28  of motor  12  and a compressor housing  30  of compressor  14  such that transition  26  does not provide a significant leak path for gas or refrigerant within system  10  to escape to an atmosphere  32  surrounding housings  28  and  30 . Hermetic sealing of system  10  can be accomplished by a solid, airtight joint between housings  28  and  30 , as shown, or by making housings  28  and  30  as a unitary piece.  
         [0033]     In a currently preferred embodiment, compressor system  10  contains a refrigerant that compressor  14  forces from a suction inlet  34  to a discharge outlet  36 , both of which are defined by compressor housing  30 . Compressor system  10  can be used for powering a refrigerant circuit  38  comprising the basic components of a condenser  40 , an expansion device  42 , and an evaporator  44 . In some embodiments of the invention, condenser  40  is water-cooled and evaporator  44  absorbs heat from a circulating water system  46 . Pumps  48  and  50  can be used for controlling the flow rate of the water through condenser  40  and evaporator  44 . The structure and function of refrigerant circuit  38  and its many variations are well known to those of ordinary skill in the art.  
         [0034]     Although the actual structure of compressor system  10  may vary, the illustrated embodiment has shaft  18  supporting both a rotor  52  of motor  12  and at least one compressor element  54 . The term, “compressor element” refers to any component that can be driven to compress a gas. Examples of compressor element  54  include, but are not limited to, a centrifugal impeller, an axial impeller, a multi-lobed screw compressor rotor, an involute scroll compressor rotor, a reciprocating piston, and the like. Although a single shaft  18  is shown supporting both rotor  52  and impeller  54 , it is well within the scope of the invention to have rotor  52  and impeller  54  supported by two separate shafts that are coupled to each other by way of gears or some other appropriate coupling.  
         [0035]     For this direct drive example, bearing  22  supports an outboard end  56  of shaft  18 , and another bearing  58  closer to an inboard end  60  supports shaft  18  at an intermediate section  62  of shaft  18  such that shaft  18  supports compressor element  54  in a cantilevered manner. Bearing  22  is a rolling element duplex bearing for providing shaft  18  with both axial and radial support, and bearing  58  is preferably a journal bearing for pure radial support of the shaft intermediate section  62 . Bearings  22 ,  58  are lubricated with a thin film of refrigerant/lubricant mixture. Rotor  52  is situated between bearings  22  and  58 , and a stator  64  supported by motor housing  28  encircles the rotor.  
         [0036]     Referring further to  FIG. 2 , motor housing  28  is shown supporting bearing  22  by way of a bearing bracket  66 , a seal ring  68  and a clamp ring  70 . Bearing bracket  66  is bolted to a cylindrical shell  72  of housing  28 , and an inner bore of bracket  66  provides bearing  22  with radial support. In an axial direction, parallel to shaft  18 , the outer races of bearing  22  are captured between clamp ring  70  and seal ring  68 , which are both bolted to bearing bracket  66 . A shoulder  74  and an internally threaded ring  76  axially clamp the inner races of bearing  22  to shaft  18 .  
         [0037]     To help prevent the lubricant for bearing  22  from freely draining into the main chamber of motor housing  28  and eventually becoming lost within refrigerant circuit  38 , seal ring  68  includes a labyrinth seal  78  that is spaced just a slight radial distance away from shaft  18 .  
         [0038]     To drive compressor system  10  at various speeds, electrical cables  80  connect inverter  24  to the windings of stator  64  (stator  64  includes its windings and its core). One example of inverter  24  is a “LiquiFlo 2.0 AC Drive” manufactured by Reliance Electric, which is part of Rockwell Automation of Milwaukee, Wis. with further headquarters in Greenville, S.C. Inverter  24  includes a converter section  82  with a plurality of insulate gate bipolar transistors  84  for converting an incoming 3-phase AC supply voltage  86  to a DC voltage  88 , and an inverter section  90  electrically coupled to converter section  82  and comprising a plurality of insulate gate bipolar transistors  92  for converting DC voltage  88  to a variable frequency 3-phase output voltage  94  that cables  80  feed to stator  64 . In addition to their intended purpose, the plurality of insulate gate bipolar transistors  84  and  92  induce a potentially detrimental common mode current in shaft  18 . The common mode current can exceed one megahertz (e.g., 2-3 MHz range) and has been observed to have a frequency as high as 10 MHz.  
         [0039]     To inhibit bearing  22  from conveying the common mode current to an electrical ground  96 , a non-conductive or minimally conductive coating  20  is disposed on several bearing-contact surfaces including a surface  98  of clamp ring  70 , a surface  100  of seal ring  68 , and the inner bore of bearing bracket  66 . Coating  20  is preferably harder and less electrically conductive than the base material to which it is applied and harder and less conductive than an outer periphery  102  of bearing  22 . Preferably, coating  20  is a ceramic coating but other insulative coatings are contemplated such as silicon oxides or metal oxides. For purposes of this application, a minimally conductive coating conducts at less than the dielectric strength of the elastohydrodynamic thickness of the film on the bearing. This will vary depending on the refrigerants and lubricants being used in a particular system.  
         [0040]     In a currently preferred embodiment, coating  20  is a METCO 130 Alumima-Titania Composite Powder (METCO is a registered trademark of Sulzer Metco of Winterthur, Switzerland). The METCO coating is comprised of about 13% titanium dioxide and about 87% aluminum oxide. Coating  20  can be sprayed on selected surfaces of parts  66 ,  68  and  70  and subsequently machined or ground to size with a final layer thickness ranging from a few thousandth of an inch to 0.020-inches. The thickness of ceramic coating  20  has been exaggerated in the drawing figures so that the coating is clearly visible. With a hardness of 60 Rc, coating  20  is not readily scratched by bearing  22  or the other components of compressor system  10  during assembly.  
         [0041]     Since coating  20  alone does not adequately solve the problem of induced common mode voltage, shaft-grounding device  16  is used for grounding shaft  18 . In some cases, shaft  18  may include a bolt head  18 ′ or some other suitably conductive member that can be engaged by shaft-grounding device  16 .  
         [0042]     Referring further to  FIG. 3 , to successfully ground common mode voltage whose frequency is above 2-MHz, it has been found that shaft-grounding device  16  should have a stranded wire brush  104  made of copper and a stranded high-frequency grounding wire  106  that can effectively draw the current away from brush  104 . Moreover, a spring  108  is needed to urge brush  104  against shaft  18  with an axial force  110  that is neither too great (to avoid excessive wear) or too light (to ensure continuous electrical contact). Force  110  should be 4-20 ounces and preferably 8-14 ounces.  
         [0043]     In some embodiments, shaft-grounding device  16  comprises a brush housing  112  within which a spring-loaded plunger  114  can slide along a generally linear path  116 . Housing  112  can be an electrically conductive tubular body having a longitudinal centerline  118 . Plunger  114  includes a copper tube  120  with one end  122  that crimps the copper strands of brush  104  to grounding wire  106 . A pin  124  fastens tube  120  to a brass sleeve  126  to complete the assembly of plunger  114 . Another pin  128  fixed to housing  112  protrudes into a slot  130  in sleeve  126  to provide an anti-rotation element that not only restricts the rotation of plunger  114  (inhibits shaft  18  from spinning brush  104 ) but also limits the axial extension of plunger  114  relative to housing  112 . A nut  132  with an internal shoulder  134  screws onto to housing  112  to clamp an electrically conductive plug  136  between shoulder  134  and one end  138  of housing  112 . An electrical terminal  140  connects grounding wire  106  to plug  136 .  
         [0044]     When brush housing  112  is screwed into a threaded hole  142  in endplate  144  of motor housing  28 , spring  108  is compressed a certain degree between sleeve  126  and plug  136 . The characteristics of spring  108  and the amount it is compressed determines the force that brush  104  exerts against bolt head  18 ′ or against some other axial surface of shaft  18 . To minimize rubbing between shaft  18  and brush  104 , a rotational axis  146  of shaft  18 , the longitudinal centerline  118  of housing  112 , and the linear path  116  along which brush  104  and plunger  114  can move are generally collinear with each other.  
         [0045]     When properly installed, shaft-grounding device  16  completes an electrical path between shaft  18  and motor housing  28 . More specifically, induced common mode current in shaft  18  can travel in series through shaft  18 , wire brush  104 , grounding wire  106 , terminal  140 , plug  136 , brush housing  112 , endplate  144 , bearing bracket  66 , shell  72  of motor housing  28 , and ground  96 . Other electrical paths are also possible such as, for example, series flow through shaft  18 , brush  104 , tube  120 , sleeve  126 , brush housing  112 , endplate  144 , bearing bracket  66 , compressor shell  72 , and ground  96 .  
         [0046]     During normal operation, an O-ring  148  between housing  112  and endplate  144  plus another O-ring  150  between plug  136  and housing  112  helps maintain the hermetic integrity of compressor system  10 . With certain refrigerants and temperature conditions, however, it may still be possible to inspect, replace, repair or otherwise service shaft-grounding device  16  without losing a significant amount of refrigerant charge or introducing non-condensable air into compressor system  10 .  
         [0047]     In some embodiments, for example, the refrigerant in system  10  is R123, which begins boiling at atmospheric pressure (14.7 psig) when its temperature is about 81.7° F. So, if the temperature of the refrigerant in motor housing ooo is adjusted to about 81.7° F., or slightly less, the refrigerant pressure within motor housing  28  will be about the same as the surrounding atmospheric pressure. Under these conditions, shaft-grounding device  16  can be momentarily unscrewed from within hole  142  and inspected without an excessive exchange of gas between system  10  and the surround atmosphere  32 , provided opening  142  is not too large. Preferably, opening  142  should have a cross-sectional area that is less than 4 in 2 .  
         [0048]     The refrigerant pressure within system  10  can be adjusted to atmospheric pressure by adjusting the temperature of the refrigerant, which can be done in various ways. The water flow rate through evaporator  40 , for instance, could be adjusted while compressor system  10  is de-energized. It is also conceivable to heat or cool the refrigerant by adjusting the temperature and flow rate of the water flowing through condenser  40 .  
         [0049]      FIG. 5  illustrates the steps of adjusting the temperature of the refrigerant until the refrigerant in motor housing  28  is at a pressure substantially equal to that of the surrounding atmosphere  32 , and removing shaft-grounding device  16  from within motor housing  28  while the pressure of the refrigerant is substantially equal to that of the surrounding atmosphere, thereby providing an opportunity to inspect shaft-grounding device  16  without having to evacuate the refrigerant from within motor housing  28 . This is possible because compressor system  10  has only one duplex rolling element bearing  22  that needs protection from induced common mode voltage. The shaft&#39;s other bearing  58 , which is installed at a less accessible location deep within compressor system  10 , is a journal bearing which is much more tolerant of induced common mode voltage, thus bearing  58  does not need the same protection as bearing  22 .  
         [0050]     Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art. In some embodiments, for instance, motor housing  28  can be considered to comprise cylindrical shell  72 , bearing bracket  66  and endplate  144 . And in some embodiments, shaft  18  can be considered to include bolt head  18 ′ and/or other items extending from or attached to shaft  18 . Although refrigerant circuit  38  is shown comprising a water-cooled condenser and an evaporator providing chilled water, condenser  40  could be air-cooled and the cooling effect of evaporator  4  could be used for absorbing heat from something other than water. Therefore, the scope of the invention is to be determined by reference to the following claims.