Patent Publication Number: US-8994237-B2

Title: Method for on-line detection of liquid and potential for the occurrence of resistance to ground faults in active magnetic bearing systems

Description:
BACKGROUND 
     This application claims priority to U.S. patent application Ser. No. 61/428,443, which was filed Dec. 30, 2010. The priority application is hereby incorporated by reference in its entirety into the present application. 
     A motor can be combined with a compressor in a single housing to provide a motor-compressor system. Using a shared shaft, or two or more shafts coupled together, the motor drives the compressor in order to generate a flow of compressed process gas. In hermetically-sealed motor-compressors, the shaft is typically supported by two or more radial magnetic journal bearings and often includes at least one axial magnetic bearing for thrust compensation. The magnetic bearings may be passive magnetic bearing systems using permanent magnets, or they may include active magnetic bearing systems having one or more electromagnets actively controlled by an external power source adapted to centralize or otherwise levitate the shaft. 
     Magnetic bearings installed within a hermetically-sealed motor-compressor are typically pressurized to a level close to the process inlet pressure, and cooled by process gas derived from the compressor and circulated via a cooling loop. Although the cooling process gas is ordinarily first treated in a gas conditioning skid to remove contaminants and free liquids, there is still potential for the generation and accumulation of liquids within the cooling loop. For example, liquids such as water, hydrocarbon condensate, or other wellstream fluids can often form, and magnetic bearings are particularly susceptible to damage if they come into contact with these liquids or “dirty” cooling process gas. In such cases, the resistance to ground of the electrical windings of the bearings may be reduced which, if not reversed or at least stopped, could eventually lead to the complete failure of the bearing. 
     One way to protect the electrical windings from liquid penetration is the application of vacuum-pressure impregnation (VPI) to the windings which provides a protective coating that insulates the windings. Magnetic bearings, however, are subjected to repeated pressurization-depressurization cycles which increase the risk of liquids penetrating the VPI coating over time. Once the VPI coating is penetrated, the bearing coil resistance to ground gradually diminishes, and if the liquid penetration is not reversed or stopped, the coil resistance to ground will eventually become zero, thereby causing the bearing to short out and fail. 
     What is needed, therefore, is a method and system for monitoring the bearing coil resistance to ground as an indicator of the accumulation of liquids in the cooling loop of a motor-compressor, and more particularly within the magnetic bearings. 
     SUMMARY OF THE INVENTION 
     Embodiments of the disclosure may provide a magnetic bearing monitoring system. The system may include one or more radial bearings arranged within a bearing cavity defined within a casing, each radial bearing having radial bearing coils configured to levitate a rotatable shaft, and a cooling loop configured to circulate a cooling gas to the bearing cavities. The system may further include a dummy bearing arranged in the cooling loop and having at least one dummy bearing coil made of an electrical winding, and a sensing device communicably coupled to the electrical winding to monitor a resistance to ground for the at least one dummy bearing coil, wherein the resistance to ground of the at least one dummy bearing coil is indicative of a resistance to ground of the radial bearing coils. 
     Embodiments of the disclosure may further provide a method of monitoring a magnetic bearing. The method may include circulating a cooling gas through a magnetic bearing arranged in a cooling loop, the magnetic bearing having a plurality of bearing coils, and circulating the cooling gas through a dummy bearing arranged in the cooling loop, the dummy bearing having at least one dummy bearing coil made of an electrical winding. The method may further include detecting a resistance to ground of the at least one dummy bearing coil with a sensing device communicably coupled to the electrical winding, wherein the resistance to ground of the at least one dummy bearing coil is indicative of a resistance to ground of the plurality of bearing coils. 
     Embodiments of the disclosure may further provide a dummy bearing. The dummy bearing may include a bearing stator having one or more protrusions extending radially inward therefrom, and an electrical winding wrapped around the one or more protrusions to form one or more dummy bearing coils. The dummy bearing may further include a sensing device communicable with the electrical winding to monitor the resistance to ground of the one or more dummy bearing coils, wherein the dummy bearing is arranged within a cooling loop having at least one magnetic bearing, and the resistance to ground of the one or more dummy bearing coils is indicative of a resistance to ground for bearing coils of the at least one magnetic bearing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying Figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  depicts an illustrative radial magnetic bearing, according to one or more embodiments disclosed. 
         FIG. 2  depicts another illustrative radial magnetic bearing configured to monitor the resistance to ground, according to one or more embodiments disclosed. 
         FIG. 3  depicts a schematic of a method of monitoring the resistance to ground of magnetic bearing coils, according to one or more embodiments disclosed. 
         FIG. 4  depicts an illustrative system for monitoring the resistance to ground of magnetic bearing stator coils, according to one or more embodiments disclosed. 
         FIG. 5  depicts a schematic of another method of monitoring the resistance to ground of magnetic bearing coils, according to one or more embodiments disclosed. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the exemplary embodiments presented below may be combined in any combination of ways, i.e., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure. 
     Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Additionally, in the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. Furthermore, as it is used in the claims or specification, the term “or” is intended to encompass both exclusive and inclusive cases, i.e., “A or B” is intended to be synonymous with “at least one of A and B,” unless otherwise expressly specified herein. 
       FIG. 1  illustrates an exemplary radial magnetic bearing  100 , according to one or more embodiments disclosed. In one embodiment, the radial magnetic bearing  100  may be an active magnetic bearing having a circular stator  102  with a plurality of protrusions  104   a - 104   h  extending radially-inward therefrom. The protrusions  104   a - h  may be spaced about the circumference of the stator  102  and be configured to surround a centrally-disposed shaft or rotor  106 . In an embodiment, the rotor  106  may be arranged within a hermetically-sealed motor-compressor and the magnetic bearing  100  may be configured to levitate the rotor  106  during operation. As will be appreciated, however, the rotor  106  may be arranged in various types of machinery, including turbomachinery, without departing from the scope of the disclosure. 
     As illustrated, the protrusions  104   a - h  may be equidistantly-spaced about the stator  102 . It is also contemplated, however, to have protrusions  104   a - h  spaced at predetermined intervals but not necessarily equidistant from each other. While  FIG. 1  depicts a total of eight protrusions  104   a - h , it will be appreciated that the number of protrusions  104   a - h  may vary to suit any particular application, without departing from the scope of the disclosure. 
     Each protrusion  104   a - h  may include a plurality of electrical windings  108  wound thereabout multiple times in order to create a corresponding plurality of coils  110   a - 110   h  or magnetic “poles.” Accordingly, the depicted magnetic bearing  100  includes a total of eight magnetic poles. Similar to the number of protrusions  104   a - h , it will be appreciated that the number of magnetic poles may vary to suit any particular application, without departing from the scope of the disclosure. 
     The electrical windings  108  may be made of an electrically-conductive material insulated on its exterior surface with, for example, a plastic or Teflon® coating. Via the windings  108 , the coils  110   a - h  are placed in electrical communication with a controller  112 . The controller  112  receives signals from one or more rotor position sensors  114  and processes the signals to calculate how to redistribute electromagnetic currents in each coil  110   a - h  in order to keep the rotor  106  centered within a clearance gap  116  defined between the stator  102  and the rotor  106 . In operation, the windings  108  generate an electromagnetic field between the stator  102  and the rotor  106  that levitates and stabilizes the rotor  106 . 
     In one embodiment, the coils  110   a - h  may be generally insulated against the ingress of liquids, chemicals, and other contaminants. In one embodiment, the coils  110   a - h  are protected via an insulative coating applied using vacuum pressure impregnation (VPI). The VPI process entails submerging the stator  102 , including the windings  108  and coils  110   a - h , in a resin, such as a solvent-less epoxy resin. Through a combination of vacuum/pressure cycles and thermal processing, the impregnated windings  108  become a void-free and homogeneous structure having higher dielectric strength and increased thermal conductivity. As will be appreciated by those skilled in the art, however, the windings  108  may be insulated or otherwise protected using various other techniques besides VPI, without departing from the scope of the disclosure. 
     Over time, the VPI coating (or a similar insulative coating) on the coils  110   a - h  may degrade, thereby exposing the coils  110   a - h  to liquids or other contaminants. This is especially true in magnetic bearings where the environment is pressurized and depressurized routinely, and liquids are therefore intermittently forced into miniscule pores or cracks propagated in the VPI coating. Once the coils  110   a - h  become wet, the resistance to ground (e.g., the stator  102  or surrounding machinery) of the windings  108  gradually diminishes and if not remedied will eventually go to zero, thereby resulting in the failure of the magnetic bearing  100 . As can be appreciated, failure of a magnetic bearing during operation can severely damage the rotor  106  and potentially have catastrophic effects on the motor-compressor as a whole. 
     Referring now to  FIG. 2 , illustrated is another exemplary radial magnetic bearing  200 , similar in some respects to the radial magnetic bearing  100  of  FIG. 1 . Accordingly, the radial magnetic bearing  200  may be best understood with reference to  FIG. 1 , where like numerals correspond to like components and therefore will not be described again in detail. As illustrated in  FIG. 2 , one or more sensing wires  202  may be coupled to or otherwise wrapped around the coils  110   a - h . Briefly, and as will be described in more detail below, the sensing wire  202  may be adapted to sense/detect the presence of liquids within the magnetic bearing  200 , thereby indicating an equivalent resistance to ground for each coil  110   a - h  as the VPI coating (or similar insulative coating) degrades over time. Accordingly, the sensing wires  202  may facilitate or initiate proactive management of the bearing  200  before any damage or failure to the magnetic bearing  200  transpires. 
     Similar to the windings  108  described above, the sensing wire  202  may also be made of an electrically-conductive material that is insulated on its external surface. The sensing wire  202  may be communicably or otherwise electrically coupled to a sensing device  204  configured to measure the resistance to ground of each sensing wire  202  and thereby provide a current or real-time reading of the resistance to ground of the coils  110   a - h , as will be discussed below. In an embodiment, the sensing device  204  may be located on a control panel (not shown) for the motor-compressor and may provide an output using any conventional peripheral including, and without limitation to, a printer, a monitor, a graphical user interface, audio speakers, etc. Accordingly, the sensing wire  202  and sensing device  204  may be adapted to sense/detect liquid around the bearing coils  110   a - h  (e.g., a common cause of diminished resistance to ground in magnetic bearings) without having to disconnect case penetration connectors as part of an external conditioning monitoring system. 
     To secure or otherwise attach the sensing wire  202  to one or more of the coils  110   a - h , the sensing wire  202  may be wrapped or otherwise wound around the coil  110   a - h , generally similar to how the windings  108  are wrapped about each protrusion  104   a - h . As shown in  FIG. 2 , it is contemplated to wrap the sensing wire  202  around each coil  110   a - h  at least two times or revolutions. However, it will be appreciated that the sensing wire  202  may be wrapped around the coil  110   a - h  more or less than two revolutions, and may directly depend on available space within the bearing  200  for additional turns and braze joints related to the sensing wire  202 . Moreover, it will be appreciated that the sensing wire  202  may be simply coupled to the coils  110   a - h  in any suitable manner known to those skilled in the art. 
     In one embodiment, as illustrated, a single sensing wire  202  may be applied to adjacent magnetic poles of the bearing  200 . For instance, a single sensing wire  202  may be wrapped around both the first coil  110   a  and the second coil  110   b  (e.g., the first magnetic pole pair), having each end of the sensing wire  202  coupled or otherwise leading to the sensing device  204 . If desired, the same process may be repeated for the third coil  110   c  and the fourth coil  110   d  (e.g., the second magnetic pole pair), the fifth coil  110   e  and the sixth coil  110   f  (e.g., the third magnetic pole pair), and the seventh coil  110   g  and the eighth coil  110   h  (e.g., the fourth magnetic pole pair). In other embodiments, a single sensing wire  202  may be applied to each of the coils  110   a - h  in tandem, where the sensing wire  202  is wrapped around the first coil  110   a,  the second coil  110   b,  and continuing in clockwise fashion around the stator  102  until finally being wrapped around the last or eighth coil  110   h . In such an embodiment, the sensing device  204  would receive a signal representative of the collective resistance to ground of the coils  110   a - h . In yet other embodiments, single sensing wires  202  may be wrapped around each coil  110   a - h  individually, thereby providing the sensing device  204  with signals derived from each coil  110   a - h  individually. 
     In at least one embodiment, the sensing wire  202  may be wrapped around the coils  110   a - h  at a location where liquids and contaminants are more prone to or known to accumulate inside the magnetic bearing  200 . For example, gravity will tend to force generated liquids to amass in the bottom of the magnetic bearing  200 , such as, in the area of the bearing  200  below a horizontal line A. Accordingly, in one embodiment the sensing wire  202  may be applied to only the second magnetic pole pair (i.e., the third coil  110   c  and the fourth coil  110   d ) or only the third magnetic pole pair (i.e., the fifth coil  110   e  and the sixth coil  110   f ), or both, which are generally located near the bottom of the magnetic bearing  200  and therefore more susceptible to the ingress of liquids or other contaminants. In other embodiments, the sensing wire  202  may be singly coupled to either the fourth coil  110   d  or the fifth coil  110   e , or both, since they are also located near the bottom of the magnetic bearing  200 . It will be appreciated, however, that the “bottom” of the magnetic bearing  200  once installed may be relative to its general disposition in view of the horizontal line A, and may therefore be altered during maintenance or turnaround operations if the bearing  200  is re-installed in a different circumferential disposition. 
     In one embodiment, the sensing wire  202  is arranged on the magnetic bearing  200  prior to the application of the VPI coating (or similar insulative coating) about the coils  110   a - h . Consequently, the sensing device  204  may be used to measure resistance to ground of the sensing wire  202  which is partly indicative of the degradation of the insulative coating (i.e., the VPI coating) about the coils  110   a - h , and also the concomitant degradation of the insulative layer disposed about the windings  108  and the sensing wire  202  individually. For example, as the insulative coating on the coils  110   a - h  degrades over time, the windings  108  and the sensing wire  202  are equally exposed to liquids and other contaminants. Once exposed to the liquids and contaminants, the sensing wire  202  is configured to transmit a signal to the sensing device  204  corresponding to the resistance to ground of the sensing wire  202 , which may be equally indicative of the resistance to ground of the respective coils  110   a - h.    
     The resistance to ground of the sensing wire  202  may be monitored continuously or periodically during operation of the motor-compressor. By trending over time the diminishing resistance value of the sensing wire  202  (and thus the coils  110   a - h  vicariously through the sensing wire  202 ), the magnetic bearing  200  may be proactively managed in order to avoid potential damage or failure. For instance, once the resistance to ground of the sensing wire  202  reaches or approaches a predetermined level, such as indicating a general inability of the magnetic bearing  200  to adequately levitate the rotor  106  without adversely affecting rotordynamics or overall performance, appropriate actions may then be taken to avoid the complete failure of the magnetic bearing  200  and damage to the rotor  106 . At least one appropriate action may be shutting down the motor-compressor to drain the accumulated liquids from the magnetic bearings  200  and to dry the bearings  200  for continued use. 
     Referring now to  FIG. 3 , depicted is a schematic of a method  300  of operating a magnetic bearing. The method  300  may include circulating a cooling gas through a magnetic bearing, as at  302 . The magnetic bearing may be disposed within a bearing cavity defined within a casing, such as a casing for a turbomachine. The magnetic bearing may include a circular stator with a plurality of coils extending radially-inward therefrom, and the plurality coils may have an insulative coating disposed thereon to protect against the ingress of liquids. In at least one embodiment, the insulative coating may be a protective layer of an epoxy resin applied using VPI techniques. 
     The method  300  may also include sensing the resistance to ground of a coil among the plurality of coils using a sensing wire coupled thereto or otherwise wrapped thereabout, as at  304 . The sensing wire may also be embedded or otherwise disposed within the insulative coating. As the insulative coating degrades over time, the sensing wire is adapted to sense and transmit a signal indicative of the resistance to ground for the coil, as at  306 . Being communicably coupled to the sensing wire, a sensing device detects or otherwise monitors the signal indicative of the resistance to ground for the coil, as at  308 . 
     Referring now to  FIG. 4 , illustrated is a system  400  configured to monitor the resistance to ground of one or more magnetic bearing stator coils. The system  400  may include a compressor  402  having a housing or casing  404 . While not shown, the compressor  402  may include a rotor assembly enclosed by the casing  404  and configured to rotate therein. The compressor  402  may be a motor-compressor or other fluid compression apparatus, and the casing  404  may be configured to hermetically-seal the components of the compressor  402  therein. A rotatable shaft  406  may be arranged within the casing  406  and adapted to rotate about a central axis X. In one embodiment, the ends of the shaft  406  may penetrate the casing  404  at one or both ends of the casing  404  to allow the shaft  406  to be potentially coupled to an external driver (not shown) or additional driven equipment (not shown). As will become more evident below, it will be appreciated that the compressor  402  may be any rotatable machinery or device, generally rotatable about the central axis X. 
     The shaft  406  may be supported at each end by one or more radial bearings  408  arranged within respective bearing cavities  410 . The radial bearings  408  may be directly or indirectly supported by the casing  404 , and in turn provide support to the shaft  406  and any accompanying rotor assembly during operation of the compressor  402 . In one embodiment, the bearings  408  may be magnetic bearings, such as active or passive magnetic bearings. In at least one embodiment, and for the purposes of this disclosure, the bearings  408  may be substantially similar to the magnetic bearing  100  discussed above with reference to  FIG. 1 . 
     The system  400  may further include a cooling loop  412  configured to circulate a cooling gas through the compressor  402 , and particularly through the bearing cavities  410  in order to regulate the temperature of the bearings  408 . The cooling loop  412  may include a blower device  414  configured to pressurize the cooling gas in the cooling loop  412 . In one embodiment, the cooling loop  412  and the blower device  414  are substantially similar to the cooling loop(s) and blower device(s) described in co-owned U.S. patent application Ser. No. 13/233,436,059 entitled “Method and System for Cooling a Motor-Compressor with a Closed-Loop Cooling Circuit,” filed on Sep. 15, 2011, the contents of which are herein incorporated by reference to the extent not inconsistent with the present disclosure. 
     The cooling loop  412  may further include a gas conditioning skid  416  adapted to filter the cooling gas and reduce its temperature before injecting the cooling gas into the bearing cavities  410  arranged within the compressor  402 . The gas conditioning skid  416  may include a heat exchanger (not shown) or any device adapted to reduce the temperature of a fluid. For example, the heat exchanger may include, but is not limited to, a direct contact heat exchanger, a trim cooler, a mechanical refrigeration unit, and/or any combination thereof. In one embodiment, the gas conditioning skid  416  may also include a density-based separator (not shown), or the like, configured to remove any condensation generated by reducing the temperature of the cooling gas. Accumulated liquids or contaminants within the gas conditioning skid  416  may be discharged via line  418 . 
     Notwithstanding the collection and removal of liquids from the cooling gas via the gas conditioning skid  416 , liquids or other contaminants may nonetheless accumulate throughout the cooling loop  412 , thereby threatening the integrity of the radial bearings  408 . Accordingly, the system  400  may further include one or more dummy bearings  420  that can be monitored to detect liquid within the system  400  and thereby indirectly monitor an equivalent resistance to ground of the radial bearings  408  arranged about the shaft  406 . Since the dummy bearing  420  is arranged within the same cooling loop  412  as the radial bearings  408 , the environmental conditions affecting the dummy bearing  420  (e.g., the presence of liquids or other contaminants) are indicative of the environmental conditions affecting the radial bearings  408 . Consequently, monitoring the resistance to ground of the dummy bearing  420  may be indicative of the resistance to ground of the radial bearings  408 . Thus, once the resistance to ground of the coils  422   a ,  422   b  of the dummy bearing  420  reaches a predetermined level, appropriate preventative measures may be undertaken with the radial bearings  408  to avoid damage or eventual failure. 
     The dummy bearing(s)  420  may be substantially similar in construction to the radial bearings  408  (or the radial magnetic bearing  100  as described above). For example, the dummy bearing  420  may include one or more magnetic “poles” or bearing coils, such as coils  422   a  and  422   b , formed by multiple revolutions of electrical windings  426  about corresponding protrusions  427   a  and  427   b . The coils  422   a, b  may be coupled to a bearing stator  424 , or at least a section of a bearing stator annulus, and may combine to form a dummy bearing magnetic pole pair, as defined above. In other words, the dummy bearing  420  may be made or otherwise manufactured with only two protrusions  427   a, b , which save on manufacturing costs. It will be appreciated, however, that the dummy bearing  420  may equally function with only a single magnetic pole or coil (i.e.,  422   a  or  422   b ). 
     The coils  422   a, b , stator  424 , and windings  426  may be substantially similar to the coils  110   a - h , stator  102 , and windings  108 , respectively, described above with reference to  FIG. 1 , and therefore will not be described again in detail. Moreover, similar to the coils  110   a - h  described above, the coils  422   a, b  of  FIG. 4  may have a VPI coating (or similar insulative or protective coating) applied thereto in order to prevent the general ingress of liquids or other contaminants to the coils  422   a, b.    
     Much like the sensing wire  202  of  FIG. 2 , the windings  426  may act as a sensing wire for the dummy bearing  420  by monitoring the resistance to ground for each coil  422   a, b  as the VPI coating (or similar insulative coating) degrades over time. Also, similar to the sensing wire  202  of  FIG. 2 , the windings  426  may be communicably or otherwise electrically coupled to a sensing device  428  not unlike the sensing device  204  described above. Accordingly, the sensing device  428  may be configured to continuously or periodically monitor the resistance to ground of the windings  426  and thereby provide a current or real-time reading of the resistance to ground of each coil  422   a, b.    
     In operation, the dummy bearing  420  is not used to support the shaft  406  like the radial bearings  408  are. Instead, the dummy bearing  420  serves as a representative bearing arranged within the same cooling loop  412  as the radial bearings  408 , and therefore is affected by the same environmental conditions affecting the radial bearings  408 . Accordingly, monitoring the resistance to ground of the coils  422   a, b  arranged within the dummy bearing  420  may be substantially if not equally indicative of the resistance to ground of the coils arranged within the radial bearings  408 . Consequently, appropriate corrective action may be undertaken in the radial bearings  408  once the resistance to ground measured by the coils  422   a, b  of the dummy bearing  410  reaches a predetermined threat level. 
     In one or more embodiments, the dummy bearing(s)  420  may be arranged at one or more “low points” in the cooling loop  412 , in other words, locations in the cooling loop  412  where liquids are more prone to accrue. For example, one or more dummy bearings  420  may be arranged within the casing  404  vertically below each radial bearing  408 , such as at a first location  430  and a second location  432 , as shown in  FIG. 4 . In another embodiment, the dummy bearing  420  may be disposed outside of the casing  404  and arranged within the cooling loop  412  after the gas conditioning skid  428 , such as at a third location  434 . In at least one embodiment, the third location  434  is also located physically below the vertical height of the radial bearings  408 , thereby being arranged at another “low point” in the cooling loop  412 . It will be appreciated, however, that dummy bearings  420  may be placed in various locations inside and outside of the casing  404 , without departing from the scope of the disclosure. For instance, it is also contemplated herein to arrange the dummy bearing  420  about the shaft  406 , axially-adjacent at least one of the radial bearings  408 . 
     Moreover, the coils  422   a, b  may be arranged within the dummy bearing  420  itself where liquids and contaminants are more prone to accumulate. For example, liquids will tend to amass in the bottom section of the dummy bearing  420 , such as near the area where the stator  424  or bearing  420  are located below the dummy bearing  420  horizontal line B. Accordingly, the coils  422   a, b  may be generally located near the bottom of the dummy bearing  420  and therefore more susceptible to the ingress of liquids or other contaminants. 
     Referring now to  FIG. 5 , illustrated is a schematic method  500  of monitoring the resistance to ground of a magnetic bearing. The method  500  may include circulating a cooling gas through the magnetic bearing, as at  502 . The magnetic bearing may be arranged in a cooling loop and have a plurality of bearing coils. The magnetic bearing may also be disposed or otherwise arranged within a bearing cavity defined within a casing. In one embodiment, the magnetic bearing has an insulative coating disposed thereon to protect against the ingress of liquids. The cooling gas may also be circulated through a dummy bearing arranged in the cooling loop, as at  504 . The dummy bearing may have at least one dummy bearing coil made of, at least in part, an electrical winding, wherein the electrical winding has insulative coating disposed thereon for protection. In at least one embodiment, the insulative coating disposed on the electrical windings is the same type of material disposed on the bearing coils of the magnetic bearing. 
     The method  500  may also include sensing a resistance to ground for the dummy bearing coil using the electrical winding, as at  506 . Once sensed, a signal of the resistance to ground for the dummy bearing coil may be transmitted to a sensing device communicably coupled to the electrical winding, as at  508 . In one embodiment, the signal of the resistance to ground for the dummy bearing coil is indicative of a signal of the resistance to ground for the radial bearing coils. Accordingly, monitoring or otherwise detecting the resistance to ground of the dummy bearing coils with the sensing device, may provide insight as to the internal condition and resistance to ground of the radial bearing coils. 
     In the preceding description of the representative embodiments of the disclosure, the directional terms “bottom” and “below” are used for convenience in referring to the accompanying Figures. In general, “below” refers to a direction away from the horizontal with respect to the stator of the magnetic bearing, regardless of the radial or circumferential disposition of the magnetic bearing. Moreover, the term “bottom” is not to be limiting of the actual device or system or use of the device or system. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.