Abstract:
A method and apparatus for mitigating bearing failure in an AC induction motor includes installing a high frequency mechanical vibration-absorbing material between various components of the motor. With the vibration-absorbing material in place, the number of premature bearing failures caused by the presence of bearing current is greatly reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the filing benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/460,937, filed Jan. 11, 2011, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention pertains generally to bearings in AC induction motors, and more particularly to a method and apparatus for lessening bearing failure caused by high frequency mechanical vibrations. 
     BACKGROUND OF THE INVENTION 
     Published literature from manufacturers and users teaches that the bearings of AC induction motors powered from variable frequency drives are adversely affected by electrical current which circulates through the motor shaft and the bearings. In recent years, the incidence of premature bearing failures in AC induction motors powered from variable frequency drives has been increasing steadily, especially since the introduction of faster switching power electronics which allows better speed control by operating at higher operating frequencies utilized to generate sinusoidal waveforms. Because variable frequency drives use pulse switching techniques to provide a sinusoidal waveforms of variable frequency which is used to feed the motor stator field coils, the presence of faster switching waveforms allows more currents to be generated in the motor rotor, such current being available to circulate to ground by going through the bearings. The mechanism of failure of the bearings is identified as electrical arcing between the bearing races and its rotating balls or rollers. When electrical arcing occurs between the inner or outer race of a bearing, the energy in the electrical arc creates tiny pits in the bearing race, thereby initiating a self-sustaining mechanical destructive sequence where the pits generate more possibilities of arcing because of the surface deterioration of the metal. 
     In response to a continuously increasing number of electrical current related bearing failures in motors, the industry has developed a number of bearing current mitigating techniques associated with the utilization of variable frequency drive driven motors. 
     Stator coil design solutions involve reducing bearing current levels through coil design. The level of current made available to flow through the bearings of an AC motor is affected by the balancing of the magnetic field generated by each of the stator coils. Coil design solutions which are aimed at reducing the level of available bearing current have practical limitations. For AC induction motors, the limitations imposed on the design of field coils and their magnetic cores which generate very low levels of bearing current is the physical and electrical configuration of the field coils. Coil and core design options in motors are restricted by the need to provide electrical windings wound in physically opposite positions around the periphery of the motor frame. Winding and core design which would insure that no shaft current is generated in the rotor has been so far impossible to realize. Bearing electrical isolation solutions is another bearing current mitigating approach which has been developed. This involves coating the outer housing of the bearing, most often using plasma coatings to deposit a thin layer of ceramic type material displaying a high ohmic resistance. Unfortunately, the insulating coatings materials are brittle and thus are subject to loss of isolation due to the brittle ceramic coating added to bearing housings. The same may be said of bearings using ceramic coated steel segments. 
     Strategic equipment grounding techniques is yet a further solution to reduce the negative effects of bearing current. The goal of strategic grounding is to provide grounding paths which tend to minimize the level of available bearing current. The effectiveness of strategic equipment grounding techniques is subject to vary in time as electrical equipment is modified or added to new machinery and equipment in the electrical circuits attached to variable frequency drive of the motor. By providing new or different paths for the magnetic field to generate bearing currents, the current mitigating efficiency of strategic grounding locations is eventually nullified. 
     Shaft grounding techniques are yet another possibility for reducing the effect of bearing current is to provide a path for the current to flow to ground before reaching the bearing. This requires the installation of grounding brushes installed on the motor shaft. The use of grounding brushes has limitations regarding the level of shaft current it can carry to ground while preventing shaft voltage to rise significantly. The positioning of the grounding brushes is also critical in preventing a parallel current path through the bearing. Finally, the performance of the brushes diminishes as they wear and as dirt and other contaminants negatively affect the electrical resistance of the grounding brushes. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention addresses the growing concern related to premature failures of bearings in AC induction motors, more particularly when driven from variable frequency drives. Published literature clearly documents that the passage of electrical current through a bearing results in failures because electrical arcing occurs between the rotating balls or rollers and the inner and outer races of the bearing. However, the literature does not fully explain the technical mechanism which causes the arching. The inventors have developed a theory, which has been demonstrated in practice, as to the cause of the electrical arching in bearings. The inventors propose that the primary cause of the electrical arching is mechanical forces resulting from the high frequency mechanical vibrations. It is well known in the ultrasonic cleaning field that high frequency vibrations produces cavitation during the cleaning process. Because the magnetic attraction between the field coils of the motor stator and the rotor generates high frequency vibrations, cavitation occurs in the lubricant inside the bearing, causing the electrical conductivity between inner and outer races of the bearing to become discontinuous. In order to prove that the presence of high frequency mechanical vibrations could effectively alter the electrical characteristics of a bearing, the following laboratory experiment has been performed. Electrical wires were soldered to the inner and outer races of an ordinary  6201  bearing. As illustrated in  FIG. 1 , a 20 kHz electrical signal was applied to the bearing when it was submerged in an oil bath in the cavity of an ultrasonic cleaning device. The resulting waveform illustrates that when subjected to high frequency vibrations, the electrical contact between the inner and outer races of the bearing is broken under the influence of the high frequency shocks shown as spikes  10 .  FIG. 2  illustrates the effects on the waveform of the same level of high frequency vibrations with the same bearing resting in the ultrasonic cleaner cavity on a soft plastic cup, designed to absorb a large portion of the shock waves, thus reducing the level of high frequency shock waves, and eliminating the random interruption of electrical contact between the inner and outer races of the bearing. The experiment demonstrates that high frequency vibrations contribute to bearing failures under the presence of current flowing through the bearing. 
     As shown in  FIG. 1 , the inventors have demonstrated by lab experiments and by field measurements that the magnetic attraction between the stator and the rotor in AC induction motors results in mechanical excitation of the motor rotor at the frequency of the power supply. This frequency is normally between 3 kHz and 30 kHz for variable frequency driven AC induction motors. Even though the amplitude of the mechanical vibrations is very low, in the order of 7 mm E-6, the vibrations are of sufficient amplitude to cause cavitation in the medium used as lubricant for the bearing races. In turn, the cavitation causes the electrical contact between the inner and outer races of the bearing to perform rapid make and break actions, resulting in the instantaneous interruption of current flow which leads to electrical arcing and eventual pitting of the bearing races. 
     The present invention is directed to a method and apparatus for reducing premature bearing failures by significantly reducing the level of high frequency vibrations in an AC induction motor. The method and apparatus utilizes a vibration-absorbing material to prevent high frequency vibrations from reaching the bearing. The vibration-absorbing material can be placed at any or all of the following locations (1) between the motor frame and the stator, (2) between the bearing mount and the bearing outer race; and (3) between the bearing inner race and the motor shaft. 
     In a preferred embodiment, an improved AC induction motor is disclosed. The AC induction motor is of the type having a motor frame, a stator, a rotor having a shaft, an air gap between the stator and the rotor, a bearing mount, and a bearing having a outer race and an inner race. The improvement comprises: 
     a vibration-absorbing material disposed in at least one of the following locations;
         (1) between the motor frame and the stator;   (2) between the bearing mount and the outer race; and,   (3) between the inner race and the shaft.       

     In another embodiment, the vibration-absorbing material is disposed between the motor frame and the stator. 
     In another embodiment, the vibration-absorbing material is disposed between the bearing mount and the outer race. 
     In another embodiment, the vibration-absorbing material is disposed between the inner race and the shaft. 
     In another embodiment, the vibration-absorbing material is a silicone rubber. 
     In another embodiment, the vibration-absorbing material has a Shore A hardness of between about 30 and about 60. 
     In another embodiment, the vibration-absorbing material limits relative motion of the stator and the rotor so the air gap is always maintained. 
     Other embodiments, in addition to the embodiments enumerated above, will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the method and apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the high frequency voltage waveform when a bearing is subjected to high frequency vibrations and is firmly attached to a supporting frame; 
         FIG. 2  shows the high frequency voltage waveform when a bearing is subjected to high frequency vibrations and is attached to a supporting frame using a vibration-absorbing spacer; 
         FIG. 3  is an end elevation view of a prior art AC induction motor; 
         FIG. 4  is an end elevation view of an AC induction motor including a first embodiment of a method and apparatus for mitigating bearing failure in the AC induction motor; 
         FIG. 5  is an end elevation view of an AC induction motor including a second embodiment of the method and apparatus for mitigating bearing failure in the AC induction motor; and, 
         FIG. 6  is an end elevation view of an AC induction motor including a third embodiment of the method and apparatus for mitigating bearing failure in the AC induction motor; and, 
         FIG. 7  is an end elevation view showing another placement of vibration-absorbing material. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 3 , there is illustrated an end elevation view of a prior art AC induction motor, generally designated as  500 . Motor  500  has a motor frame  502 , a stator  504 , a rotor  506  having a shaft  508 , an air gap  505  between stator  504  and rotor  506 , a bearing mount  510 , and a bearing  512  having an outer race  514  and an inner race  516 . In the shown embodiment, bearing  512  is either a ball bearing or roller bearing. Stator  504  is rigidly connected to frame  502 . Rotor  506  is maintained in place by bearing  512 . Outside race  514  of bearing  512  is rigidly connected to bearing mount  510 , and inner race  516  of bearing  512  is rigidly connected to rotor shaft  508 . In the figure, the broken circle represents the path of rotation of inner race  516  with respect to outer race  514  of bearing  512 . Because of air gap  505 , rotor  506 , inner race  516 , and rotor shaft  508  are free to rotate with respect to fixed motor frame  502 , stator  504 , bearing mount  510 , and outer race  514 . In the figure arrows show which elements freely rotate. 
     Referring now to  FIG. 4 , there is shown an end elevation view of an AC induction motor  500  which includes a first embodiment of a method and apparatus for mitigating bearing failure in the motor, generally designated as  20 . To prevent high frequency vibrations from exciting bearing  512  and causing cavitation, a resilient vibration-absorbing material  22  in the form of a dampening spacer is circumferentially inserted between motor frame  502  and stator  504 . In an embodiment, the hardness of vibration-absorbing material  22  can vary between a Shore A hardness of about 30 and about 60, as is obtainable using for example a Dow Corning® silicone rubber RTV 3110. 
     The reference to a specific silicone material and Shore A harnesses are to be seen as examples only. Many other materials known to the industry are suitable to be used in the design of attenuators for high frequency vibrations. Additionally, the vibration-absorbing spacers  22  can consist of physically separated vibration-absorbing spacers  22  rather than a continuous spacer installed around stator  504  (refer to  FIG. 7 ). Actual physical shaft displacement data at 20 kHz was measured in a laboratory environment to be approximately 7 mm E-06. This value represents a very small displacement. Because the actual physical displacement of the motor shaft is very small in amplitude, the rigidity and thickness of high frequency vibration-absorbing material  22  can be chosen to limit the physical displacement of the stator  504  from being excessively large under the influence of the magnetic field, thus preventing it from appreciably reducing the air gap  505  between the stator  504  and rotor  506  from becoming too small or nonexistent (i.e. a sufficient air gap  505  is always maintained). In other words, the shape and thickness of vibration-absorbing material  22  is such that it prevents stator  504  from directly contacting motor rotor  506 . Put another ways, vibration-absorbing material  22  is chosen so that it limits the relative motion of stator  504  and rotor  506  so that air gap  505  is always maintained. In an embodiment, a 1/16 inch thickness has been found useful. 
       FIG. 5  is an end elevation view of an AC induction motor  500  which includes a second embodiment of the method and apparatus for mitigating bearing failure in the motor. In this embodiment vibration-absorbing material  22  is placed between bearing mount  510  and outer race  514  of bearing  512 . 
       FIG. 6  is an end elevation view of an AC induction motor  500  which includes a third embodiment of the method and apparatus for mitigating bearing failure in the motor. In this embodiment vibration-absorbing material  22  is placed between inner race  516  of bearing  512  and shaft  508  of rotor  506 . 
       FIG. 7  is an end elevation view showing another placement of vibration-absorbing material  22 . In this embodiment vibration-absorbing spacers  22  consist of a plurality of physically separated vibration-absorbing spacers  22  rather than the continuous spacers shown in  FIGS. 4-6 . 
     In summary, a method for mitigating bearing failure in an AC induction motor  500  having a motor frame  502 , a stator  504 , a rotor  506  having a shaft  508 , an air gap  505  between stator  504  and rotor  506 , a bearing mount  510 , and a bearing  512  having an outer race  514  and an inner race  516 , includes: 
     placing a vibration-absorbing material  22  in at least one of the following locations;
         (1) between motor frame  502  and stator  504 ;   (2) between bearing mount  510  and outer race  514 ; and,   (3) between inner race  516  and shaft  508 .       

     The method further including: 
     placing vibration-absorbing material  22  between motor frame  502  and stator  504 . 
     The method further including: 
     placing vibration-absorbing material  22  between bearing mount  510  and outer race  514 . 
     The method further including: 
     placing vibration-absorbing material  22  between inner race  516  and shaft  508 . 
     The method further including: 
     vibration-absorbing material  22  being a silicone rubber. 
     The method further including: 
     vibration-absorbing material  22  having a Shore A hardness of between about 30 and about 60. 
     The method further including: 
     vibration-absorbing material  22  limiting relative motion of stator  504  and rotor  506  so air gap  505  is always maintained. 
     The embodiments of the method and apparatus described herein are exemplary and numerous modifications, combinations, variations, and rearrangements can be readily envisioned to achieve an equivalent result, all of which are intended to be embraced within the scope of the appended claims. Further, nothing in the above-provided discussions of the method and apparatus should be construed as limiting the invention to a particular embodiment or combination of embodiments. The scope of the invention is best defined by the appended claims.