Abstract:
A system is provided for dissipating electrical charges that build on a rotatable shaft extending from an electrically-grounded motor housing or rotating shafts conductively linked to components that create and electrical charge, such as wind turbine blades, power generator turbines, and the like. The system includes a frame defining a central opening wherein the rotatable shaft is positioned with the central opening, a first grounding fiber secured within the frame, and a fiber clamp positioned with the frame. The fiber clamp acts to urge the first grounding fiber toward the central opening. The system may also include a mounting bracket having a frame mounting channel, wherein a portion of the frame is securely retained within the frame mounting channel, and an installation guide positioned on the mounting bracket. The installation guide may be configured to properly align the frame and the first grounding fiber relative to the rotatable motor shaft.

Description:
FIELD OF THE INVENTION  
       [0001]     Embodiments of the present invention generally relate to a grounding device for controlling shaft electrical current that is generated in AC motors controlled by variable speed drives or DC motors, and more particularly to a grounding ring apparatus for mitigating shaft electrical current. Additionally, embodiments of the present invention may be used in systems including rotating shafts conductively linked to components that create and electrical charge such as wind turbine blades, power generator turbines, and the like.  
       BACKGROUND OF THE INVENTION  
       [0002]     Shaft induced electrical current is experienced in electrical motors, and is commonly experienced with three-phase motors driven by variable speed drives.  
         [0003]     Variable speed drives utilize pulse width modulation technology to vary the speed of AC motors, thereby allowing use of less-expensive AC motors in applications where more expensive DC motors had been used previously. A drawback to the use of AC motors with variable speed drives is that higher common mode voltage (CMV) is generated by the variable speed drive that increases shaft induced currents.  
         [0004]     Voltage on the motor shaft induces current flow through the shaft bearings to the motor frame and then to ground. While the motor is running, the bearings become more resistive to current flow, causing a buildup of charge on the shaft surfaces. Over a short period of time, the CMV causes electrical charges to build to a high level. As the electrical charges pass the threshold level of the least electrically resistant path, usually through the ball bearings on the shaft, an instantaneous burst or discharge of electrical energy passes through the bearing. This discharge causes electric discharge machining (EDM), which can damage the surfaces of the bearing races and the balls in the bearing. The electrical energy burst creates fusion craters, and particulate from the crater formation remains inside the sealed bearing. Both the fusion crater and the particulate material in the bearing act to disturb the free flow rotation of the bearing, which can lead to physical damage and premature bearing failure.  
         [0005]     A number of mitigation technologies have been used in attempts to overcome this problem. Known attempts include using conductive bearing grease, insulating the bearings and using copper/phosphorus brushes and a Faraday shield. A common, somewhat cost-effective solution that has been used is to ground the shaft using spring-loaded copper brushes that provide a continuous flow of current to ground. Copper brushes, however, wear out quite rapidly and require frequent, periodic service and replacement. Additionally, oxide build-up on the shaft and other barriers between the brushes and the shaft reduce the current flow and cause a burst of electrical energy across the brush and shaft. Spring-loaded brushes also tend to vibrate due to alternating frictional stick/slip relationships between the brush and the shaft surface. Vibration of the brushes, from whatever cause, can result in undesirable sparking.  
         [0006]     U.S. patent application Publication No. 2004/0233592, entitled “Grounding Brush For Mitigating Electrical Current On Motor Shafts,” assigned to Illinois Tool Works, Inc., (the “&#39;592 publication”) discloses a grounding brush for mitigating static electric charge on a motor shaft. The grounding brush includes a plurality of filaments secured to an annular frame around the shaft, with the tips of the filaments disposed in a channel defined by the frame. The &#39;592 publication overcomes many of the drawbacks of previous attempts to mitigate shaft electrical current.  
         [0007]     The system disclosed in the &#39;592 publication includes numerous individual filaments. When the system is mounted onto a motor shaft, proper alignment of the ring to the motor shaft depends only on the fiber filaments supporting the ring evenly around the shaft. With the exception of the fiber filaments, there is nothing to ensure that the filaments are properly aligned with respect to the motor shaft. In short, the filaments may be misaligned with respect to the motor shaft, thereby decreasing charge mitigation.  
         [0008]     Thus, a need exists for an efficient grounding system that may be used effectively for a prolonged period of time, requiring minimal service or replacement.  
       SUMMARY OF THE INVENTION  
       [0009]     Embodiments of the present invention provide a system for dissipating charges that build on a rotatable shaft extending from an electrically grounded motor housing, or rotating shafts conductively linked to components that create and electrical charge, such as wind turbine blades, power generator turbines, and the like. The system includes a frame defining a central opening for the rotatable shaft, a plurality of grounding fibers secured within the frame, a plurality of fiber clamps positioned with the frame, a plurality of mounting brackets having frame mounting channels, and a plurality of installation guides positioned on the plurality of mounting brackets.  
         [0010]     The fiber clamps act to urge the plurality of grounding fibers toward the central opening. The mounting brackets are configured to securely mount to the electrically grounded motor housing, such as through screws and/or bolts, and portions of the frame are securely retained within the frame mounting channels.  
         [0011]     The installation guides are configured to properly align the frame and the grounding fibers relative to the rotatable motor shaft. The installation guides are removed after the mounting brackets are securely mounted to the electrically grounded motor housing.  
         [0012]     Each grounding fiber may be a contiguous piece of material wrapped within said frame to form a plurality of grounding rows. For example, each grounding fiber may be a loop of fiber or bundled fibers that is folded and wrapped within the frame, thereby forming multiple grounding rows of fiber material.  
         [0013]     The frame may be a single annular frame. Alternatively, the frame may be a split frame having two prongs joined together, wherein the two prongs have free ends separated by an opening. Also, alternatively, the frame may be a split frame having two separated distinct frame members, and the free ends may be joined together by frame joints.  
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0014]      FIG. 1  illustrates a plan view of a charge-dissipating assembly, according to an embodiment of the present invention.  
         [0015]      FIG. 2  illustrates a cross-sectional view of a charge-dissipating assembly through line  2 - 2  of  FIG. 1 .  
         [0016]      FIG. 3  illustrates a cross-sectional view of an annular frame positioned in a mounting bracket by an installation guide through line  3 - 3  of  FIG. 1 .  
         [0017]      FIG. 4  illustrates a cross-sectional view of an annular frame positioned within a mounting bracket after an installation guide has been removed through line  4 - 4  of  FIG. 1 .  
         [0018]      FIG. 5  illustrates a plan view of a charge-dissipating assembly, according to an embodiment of the present invention.  
         [0019]      FIG. 6  illustrates a plan view of a frame joint of a charge-dissipating assembly according to an embodiment of the present invention.  
         [0020]      FIG. 7  illustrates a cross-sectional view of a charge-dissipating assembly through line  7 - 7  of  FIG. 6 .  
         [0021]      FIG. 8  illustrates a plan view of a charge-dissipating assembly according to an embodiment of the present invention.  
         [0022]      FIG. 9  illustrates a cross-sectional view of a charge-dissipating assembly through line  9 - 9  of  FIG. 8 .  
         [0023]      FIG. 10  illustrates a cross-sectional view of a charge-dissipating assembly through line  10 - 10  of  FIG. 8 .  
         [0024]     Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1  illustrates a plan view of an electrical charge-dissipating assembly  10 , according to an embodiment of the present invention. The assembly  10  includes an annular frame  12  housing a plurality of grounding fibers, or filaments, (not shown in  FIG. 1 ), a plurality of mounting brackets  14  secured to the annular frame  12 , and a plurality of wedge-shaped fiber clamps  16  positioned within the annular frame  12 . The electrical charge-dissipating assembly  10  may be formed of metal, conductive plastic, or other electrically conductive materials. The electrical charge-dissipating assembly  10  is readily adaptable for use on motors of various sizes, having motor shafts of various diameters, and is operable to dissipate static and/or other such electrical charges that build on a motor shaft during operation of the motor.  
         [0026]     The annular frame  12  includes a central opening  18  that defines a rotatable shaft passage, such as a motor shaft passage. The annular frame  12  is configured to be positioned around a rotatable shaft (not shown in  FIG. 1 ) such that an inner diameter  21  of the annular frame  12  is proximate or contacts an outer diameter of the rotatable shaft, thereby allowing the rotatable shaft to rotate relative to the annular frame  12 .  
         [0027]     The mounting brackets  14  are configured to securely mount the electrical charge-dissipating assembly  10  to a portion of rotatable shaft and/or a motor housing, such as a motor faceplate (not shown in  FIG. 1 ). The mounting brackets  14  mount to the stationary portion of the motor, while the annular frame  12  is secured around, not necessarily to, a rotatable shaft of the motor. While three mounting brackets  14  are shown, more or less than the number shown in  FIG. 1  may be used. An installation guide  20  may be removably secured to each mounting bracket  14  in order to assist in properly aligning the grounding fibers around the rotatable shaft within the motor housing, as discussed below. While only one installation guide  20  is shown, the assembly  10  may use a separate installation guide  20  with respect to every mounting bracket  14 .  
         [0028]     The fiber clamps  16  include a clamping main body  22  movably secured within the annular frame  12  through a fastener  24 , such as a screw. The fiber clamps  16  are configured to urge rows of grounding fibers toward or into the motor shaft. For example, the fasteners  24  are tightened to urge the fiber clamps  16  toward the motor shaft, in the direction of the inner diameter  21  of the annular frame, thereby moving the rows of grounding fibers in the same direction.  
         [0029]      FIG. 2  illustrates a cross-sectional view of the electrical charge-dissipating assembly  10  through line  2 - 2  of  FIG. 1 . As shown in  FIG. 2 , the electrical charge-dissipating assembly  10  includes rows of grounding fibers  26  and  28 . The grounding fibers  26  and  28  may be coated with a thermoplastic material, such an electrical cable. The grounding fibers  26  and  28  may form a brush assembly, such as shown and described in U.S. patent application Publication No. 2004/0233592, assigned to Illinois Tool Works, Inc. (the “&#39;592 publication”), which is hereby incorporated by reference in its entirety. The grounding fibers  26  may be formed from carbon fibers, stainless steel, conductive plastics such as acrylic or nylon fibers, or any other conductive fiber-type filament that can be provided with diameters sufficiently small to induce ionization when in the presence of an electrical field.  
         [0030]     As shown in  FIG. 2 , the grounding fibers  26  are wound in a U-shape. The grounding fibers  26  and  28  are separate and distinct fibers wound and folded within the annular frame  12  to form four rows of grounding contact with respect to the rotatable shaft. That is, the grounding fiber  26  may be a contiguous piece or bundle of fiber material, such as a contiguous ring of fiber, that is wound within the annular frame  12  thereby forming two rows  30  and  32  of fiber. Similarly, the grounding fiber  28  may be a contiguous piece or bundle of fiber material that is wound within the annular frame  12  thereby forming two rows  34  and  36  of fiber. Thus, two pieces, or bundles, of fiber may be wound within the annular frame  12  to form four grounding rows  30 ,  32 ,  34 , and  36 . Alternatively, more or less fibers  26  and  28  may be used, such that more or less fiber rows are housed within the annular frame  12 .  
         [0031]     As shown in  FIGS. 1 and 2 , a first set of fiber clamps  16 ′ abuts the top rows of grounding fibers  26 , while a second set of fiber clamps  16 ″ abut the bottom rows of grounding fibers  28 . The clamping main bodies  22  of the fiber clamps  16  are urged inwardly by tightening the fasteners  24 , thereby inwardly urging the rows of grounding fibers  26  and  28 . The set of fiber clamps  16 ′ is disposed along the annular frame  12  ninety degrees with respect to the set of fiber clamps  16 ″, in order to provide uniform clamping of the grounding fibers  26  and  28  into the motor shaft (not shown in  FIG. 1  and  2 ). Optionally, more or less fiber clamps  16  may be positioned within the annular frame  12 .  
         [0032]      FIG. 3  illustrates a cross-sectional view of the annular frame  12  positioned in the mounting bracket  14  by the installation guide  20  through line  3 - 3  of  FIG. 1 . As shown in  FIG. 3 , the mounting bracket  14  includes a main body  40  having a frame channel  42 , a frame fastener through hole  44 , and a motor fastener through hole  46 . The annular frame  12  may secure to the mounting bracket  14  within the frame channel  42  through an interference engagement. Alternatively, the annular frame may be configured to snapably, threadably, or otherwise removably secure within the frame channel  42 . Optionally, the annular frame  12  may be integrally formed with the mounting bracket  14 , or it may be glued, bonded, welded, or the like within the frame channel  42 .  
         [0033]     The mounting bracket  14  is secured to a motor housing  48  through a fastener  50 , such as a screw, retained within the motor fastener through hole  46 . Alternatively, the mounting bracket  14  may be an integral component of the motor housing  48 .  
         [0034]     In order to properly position the annular ring  12  with respect to the rotatable shaft  38 , the installation guide  20  is used during an assembly process. The installation guide  20  includes a top plate  52  integrally formed with a spacer  54  having a tab  56  extending therefrom. The spacer  54  may be joined perpendicularly to the top plate  52 . A passage  58  is formed through the top plate  52 , and is configured to fit around a fastener head.  
         [0035]     During the assembly process, the installation guide  20  is positioned over the mounting bracket  14  such that the passage  58  is aligned with the frame fastener through hole  44 . A fastener  60  positioned within the frame fastener through hole  44  exerts a clamping force into top of the annular frame  12 , thereby clamping the annular frame  12  into the frame channel  42 . The passage  58  fits around the head of the fastener  60 .  
         [0036]     When the passage  58  is aligned with the frame fastener through hole  44 , the spacer  54  overlays an upper lateral surface  57  of the mounting bracket  14  such that the tab  56  abuts an edge  62  of an upper surface  64  of the annular frame  12 . The electrical charge-dissipating assembly  10  is then positioned around the rotatable shaft  38  such that the spacer  54  is sandwiched between the rotatable shaft  38  and the edge  62  of an upper surface  64  of the mounting bracket  14 . As such, the installation guide  20  acts to properly space or align the grounding fibers  26  and  28 , which are secured within the annular frame  12 , with respect to the rotatable shaft  38 .  
         [0037]      FIG. 4  illustrates a cross-sectional view of the annular frame  12  positioned within the mounting bracket  14  after the installation guide  20  (shown in  FIGS. 1 and 3 ) has been removed through line  4 - 4  of  FIG. 1 . The installation guide  20  is removed after the assembly process and before operation of the motor or rotating shaft. That is, the installation guides  20  remain on the mounting brackets  14  while mounting holes on the motor housing  48  corresponding to the mounting brackets  14  are located. After the mounting brackets  14  are secured to the motor housing  48 , the installation guides  20  may be removed.  
         [0038]     Conductive holders  65  are positioned with the annular frame  12  and secure the ground fibers  26  and  28  within the annular frame  12 . The conductive holders  65  may be formed of aluminum, for example. The grounding rows  30  and  32  are separated by a retainer  66  secured therebetween, while the grounding rows  34  and  36  are separated by a retainer  68  secured therebetween.  
         [0039]     The rows  30  and  32  of the grounding fiber  26  and the rows  34  and  36  of the grounding fiber  28  may be in direct contact with the rotatable shaft  38 , as a grounding brush, for direct transfer of electrical charge from the rotatable shaft  38  to the assembly  10 . Optionally, the rows  30 ,  32 ,  34 , and  36  may be minimally spaced from the rotatable shaft  38  such that, as an electrical field is generated by charges building on the rotatable shaft  38 , an ionized field is created, allowing indirect transfer of charges from the rotatable shaft  38  to the rows  30  and  32  of the grounding fiber  26  and the rows  34  and  36  of the grounding fiber  28 . Additionally, the rows  30 ,  32 ,  34 , and  36  may be in contact with the rotatable shaft  38  when the rotatable shaft  38  is at rest or rotating at low velocities.  
         [0040]     The grounding fibers  26  and  28  within the annular frame  12  are electrically connected to ground through the mounting brackets  14 , which are in turn secured to the rotatable shaft or grounded motor housing  48 . Charges that build on the rotatable shaft  38  are dissipated to ground through the electrical charge-dissipating assembly  10  allowing for the least resistant path to ground for the electrical charge to flow.  
         [0041]      FIG. 5  illustrates a plan view of a charge-dissipating assembly  80 , according to an embodiment of the present invention. The assembly  80  is a split-ring design comprising two semi-circular frame members  82  and  84  having a shaft opening  86  therebetween. Each frame member  82  and  84  includes mounting brackets  88 , installation guides  90 , and fiber clamps  92  similar to those described above with respect to  FIGS. 1-4 .  
         [0042]     The assembly  80  is split in order to accommodate shafts having large diameters and/or obstructions. Additionally, because of its split-ring design, the assembly  80  may be retrofit to existing motors. For example, because of the split-ring design, the assembly  80  can be installed without sliding it over the shaft.  
         [0043]     While the assembly  80  is shown having two frame members  82  and  84 , the assembly  80  may alternatively be a single piece having an opening separating two prongs. Also, alternatively, the assembly  80  may include more or less mounting brackets  88 , installation guides  90 , and fiber clamps  92  than those shown.  
         [0044]      FIG. 6  illustrates a plan view of a frame joint  100  of an electrical charge-dissipating assembly  101  according to an embodiment of the present invention. The frame joint  100  may be used to join free ends of a split ring assembly together, such as ends  102  and  104 . The frame joint  100  includes a mounting plate  106  that overlays a portion of the end  102  and a portion of the end  104 . Fasteners  108 , such as screws or bolts, securely affix the mounting plate  106  to the ends  102  and  104 . A dowel pin  110  extending from end  102  is positioned within a reciprocal opening  112  of the end  104 . The dowel pin  110  assists in properly aligning the ends  102  and  104  with respect to one another before the mounting plate  106  is secured to the ends  102  and  104 . Optionally, the end  104  may include the dowel pin  110 , while end  102  includes the reciprocal opening  112 .  
         [0045]      FIG. 7  illustrates a cross-sectional view of the frame joint  100  of the electrical charge-dissipating assembly  101  through line  7 - 7  of  FIG. 6 . Two mounting plates  106  are secured to both sides of the ends  102  and  104 . The fasteners  108  and dowel pins  109  secure the mounting plates  106  to the ends  102  and  104 , thereby fixing the end  102  to the end  104 .  
         [0046]      FIG. 8  illustrates a plan view of a charge-dissipating assembly  120  according to an embodiment of the present invention. The assembly  120  is a split-ring design including four semi-circular frame members  122 ,  124 ,  126 , and  128 . Frame member  122  is secured to frame member  124 , which is in turn secured to frame member  126 , which is in turn secured to frame member  128 , which is in turn secured to frame member  122 , thereby forming a general ring shape. A shaft opening  130  is defined between the frame members  122 ,  124 ,  126 , and  128 . While four frame members  122 ,  124 ,  126 , and  128  are shown, the assembly  120  may include more or less than four frame members. For example, the assembly  120  may include three, five, six, seven, . . . n members that form a ring-shaped assembly defining a shaft opening therebetween.  
         [0047]      FIG. 9  illustrates a cross-sectional view of the charge-dissipating assembly  120  through line  9 - 9  of  FIG. 8 .  FIG. 9  shows a frame joint in which one frame member, such as frame member  124 , is joined to another frame member, such as frame member  126 . A fastener  138 , such as a pin, screw or bolt, is secured within the assembly through a cup  132 , an inner washer  134 , and an outer washer  136 . A retainer half washer  140  mounts to the face of the outer washer  136  and the cup  132 .  
         [0048]      FIG. 10  illustrates a cross-sectional view of the charge-dissipating assembly  120  through line  10 - 10  of  FIG. 8 .  FIG. 10  shows a frame joint in which one frame member, such as frame member  124 , is joined to another frame member, such as frame member  126 . A fastener  142 , such as a pin, screw or bolt, is secured within the assembly through a cup  132 ′, an inner washer  134 ′, and an outer washer  136 ′. A retainer half washer  140 ′ mounts to the face of the outer washer  136 ′ and the cup  132 ′ 
         [0049]     As shown in  FIGS. 9 and 10 , an end portion of one frame member, such as frame member  124 , overlays a reciprocal mating end portion of an adjacent frame member, such as frame member  126 . The end portions of the frame members are securely fastened to one another through the fasteners  138  and  142 . Instead of separate fasteners, the end portions of the frame members may include integrally formed tabs and posts that are configured to snapably, latchably, or otherwise securely mate with integrally formed slots or channels formed in the end portions of adjacent frame members.  
         [0050]     The assembly  120  is separable in order to accommodate shafts having large diameters and obstructions. Additionally, because of the separable nature of the assembly  120 , it may be retrofit to existing shafts. For example, the assembly  120  may be installed to a shaft system without sliding it over the shaft.  
         [0051]     The embodiments of the present invention shown in  FIGS. 1-7  may be used with a variety of variable frequency drives, which control the speed of an AC electric motor by controlling the frequency of the voltage at the motor. Embodiments of the present invention may be used in various types of motor systems, such as those used with pumps, fans, machine tools and many others.  
         [0052]     While the embodiments above with respect to  FIGS. 1-7  show a single charge-dissipating assembly positioned around a motor shaft, additional single charge-dissipating assemblies may also be used. For example, two charge-dissipating assemblies may be positioned around a single motor shaft.  
         [0053]     Thus, embodiments of the present invention provide an efficient grounding system that may be used effectively for a prolonged period of time, requiring minimal service or replacement. Embodiments of the present invention provide a system that mitigates electrical current through the shaft bearings of a motor, thereby preventing bearing damage and failure.  
         [0054]     Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.  
         [0055]     Various features of the invention are set forth in the following claims.