Abstract:
It is possible to monitor the performance of a lubrication system performance of a Dynamoelectric Machine (DEM) by observing the electric current flowing in the DEM shaft. Suitable Rogowski coils are mounted around the shaft of the rotor of a DEM just inboard the bearings. The Rogowski coil may be mounted on the shaft to rotate with the shaft, or it may be mounted on said DEM so as to surround the shaft and remain stationary. Shaft current magnitudes may be established for start-up and steady state operating conditions. 
     Any deviation from the steady state shaft current magnitude may be taken as a warning that the DEM bearing lubrication system is malfunctioning.

Description:
FIELD OF INVENTION 
     This invention is useful in determining the performance of hydrodynamically lubricated self contained bearings in a dynamoelectric machine (DEM) during operation of the machine. The invention is applicable to the evaluation of the thickness of an oil film and the condition of the oil forming the film, which film is developed by the pumping action of a rotating oil ring mounted on the rotating shaft of a self contained bearing in a DEM. 
     BACKGROUND OF THE INVENTION 
     The evaluation of the performance of self contained hydrodynamically lubricated bearings in DEM&#39;s has hitherto been a difficult task to perform. Oil rings mounted on a shaft in a bearing assembly are not readily accessible during operation of the DEM for close examination to determine how effectively the lubricating oil is being transferred to the bearing from the oil reservoir located in the bearing housing. 
     Because the rotation of the oil ring is reliant on frictional engagement between the shaft and oil ring mating surfaces, it is apparent that any disturbance which alters the coefficient of friction between the oil ring and the shaft will lead to increase slippage of the oil ring on the shaft with a resultant consequent decrease in the volume of lubricating oil delivered to the bearing. 
     It will be readily apparent that as the shaft speeds of DEM&#39;s are increased, the oil ring speed must increase with increasing shaft speed to the point where the oil film between the oil ring and the shaft causes the oil ring to begin to slip on the shaft because the drag force on the immersed portion of the oil ring constantly increases as the oil ring moves through the oil reservoir of the bearing. The shearing forces acting on the oil ring continue to increase while the driving force causing the ring to rotate remains relatively constant. At some point the driving force, tending to turn the oil ring, is largely dissipated in overcoming the shear forces of the oil ring moving through the oil in the bearing reservoir and the ring begins to slip. 
     Attempts to evaluate the effectiveness of this method of lubricating the bearings of DEM&#39;s have usually resorted to the visual observation of the rotation of an oil ring which has been suitably marked to enable the observer to count the rotations of the oil ring during operation of the DEM. The correlation between oil ring rotation and the amount of oil delivered to the bearing in a relationship which is relatively easy to establish by those skilled in the tribological art. 
     SUMMARY OF THE INVENTION 
     This invention seeks to overcome the deficiencies of the above prior art methods of evaluation of the oil ring performance by measuring the shaft currents existing in the shaft of a DEM. Shaft currents are a natural occurrence in most DEM&#39;s and this invention makes use of the occurrence of shaft currents to monitor the lubrication performance of the oil ring lubrication system. 
     In a first embodiment of this invention, a pair of Rogowski coils are tightly wound around the DEM shaft at the remote ends of the DEM rotor shaft just inside the location of the shaft bearing. The coil ends are fed to a monitoring station through the shaft. The coils are calibrated and the DEM is started. The monitoring apparatus must preferably be capable of measuring the complete spectrum of voltages induced in the Rogowski coils by the shaft flux in order to obtain meaningful data. The shaft currents must pass through the shaft, the oil film on the bearing surfaces, the bearing structure and the machine frame. In the pathway defined above, the only component which is subject to significant change during operation of the DEM is the oil film present in the bearing. The value of the shaft current may be calculated from the signal produced by the Rogowski coils and a steady state value of shaft current may be established when the shaft speed is slow enough to assure that the oil ring is delivering the required design amount of oil to the bearing. At start up of the DEM when an oil film is not present in the shaft bearing interface, the shaft current will usually be at its maximum value. As the speed of the rotor shaft is increased, the shaft current will drop significantly as an oil film is formed in the shaft bearing interface. This condition will continue to exist within the “design” range for the bearing. If the shaft speed is increased beyond the design range, the oil ring will begin to slip on the rotor shaft at some point. At this time, the bearing oil film will be diminished and the resistance to the flow of shaft current through the bearings will drop significantly due to the decrease in oil film thickness. Consequently, the current circulating through the rotor shaft win dramatically increase at this time (as evidenced by the Rogowski coil output) indicating a potential bearing failure is imminent. 
     The Rogowski apparatus of this invention may be used to study the performance of the lubrication system of the bearings of a DEM whether the oil is pumped into the shaft-bearing interface by an oil ring, or some other kind of oil pump, the above apparatus will provide a meaningful indication of the presence of a bearing oil film in the DEM. 
     A second embodiment of this invention will disclose the operation of a Rogowski coil apparatus wherein at least one stationary Rogowski coil is mounted on the DEM (preferably on a bearing housing) so as to be concentric with and closely envelop the rotating shaft of the DEM and supply an output signal to monitoring equipment which is indicative of the current flowing in the rotor shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a dynamoelectric induction machine. 
     FIG. 2 is a sectional view of a bearing for the machine of FIG.  1 . 
     FIG. 3 is a circuit which is representative of the resistive elements through which motor shaft current flows. 
     FIG. 4 is a sectional view of a bearing of a DEM on which a stationary Rogowski coil is mounted. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the DEM of FIG. 1, it will be seen that a housing  12  is provided with a stator  14  and a rotor  16  mounted therein. The rotor  16  is mounted on a shaft  18  which is journalled in suitable bearings  20  and  22 . 
     In this illustration, the stator  14  is composed of suitable packets of magnetic laminations  24  through which stator windings such as  26  pass through slots (not shown) in the stator lamination packets  24 . 
     The rotor  16  is provided with a spider  28  on which the rotor lamination packets  30  are mounted. The rotor packets are provided with rotor winding slots (not shown) in which rotor windings  32  are mounted. The rotor windings are traditionally copper or aluminum bars (uninsulated) which are connected to shorting rings  34  and  36  at each end of the rotor. 
     The rotor shaft is provided with a pair of Rogowski coils  38  and  40  wound tightly on shaft  18  just in board of bearings  20  and  22 . 
     FIG. 2 shows an enlarged view of the bearing  22  having Rogowski coil  40  mounted on shaft  18 . Bearing  22  comprises a lower support portion  50  and an upper cap  52 . The bearing support  50  is solidly mounted into the frame (not shown) of the DEM to provide the vertical force necessary to carry the rotor  16  and supply the required horizontal stabilizing forces necessary to maintain the rotor  16  in alignment with stator  14 . 
     Bearing  22  is supplied with a lower bearing shoe  54  which encircles the lower half of shaft  18 . Shoe  54  is supported on webs  56  and  58  which encircle the half shoe  54 . 
     A bearing cap  60  encircles the top half of shaft  18  and is supported by webs  62  and  63  of bearing cap  52 . Bearing shoes  54  and  60  are mated to each other so as to form complimentary bearing components. A pair of complimentary thrust bearings  66  and  68  which are incorporated into the bearing shoes  54  and  60  to keep shaft  18  located in the desired horizontal location. 
     Because shaft  18  has a depressed section of smaller diameter at  64 , the two rings  66  and  68  which are held firmly in place by shoes  54  and  60 , are able to provide the required stabilization of the shaft to withstand severe side thrust forces on the shaft  18 . 
     In this illustration, a bearing oil ring  70  is located in the center of shoes  54  and  60  of bearing  22  and the ring  70  is located in recess  72  of upper shoe  60 . Ring  70  rests on shaft  18  and is free to rotate in recess  72 . Bearing oil  74  is contained in the reservoir  76  formed in the lower portion  50  of bearing  22 . 
     Bearing  22  has a plate  78  to seal one side of the bearing  22 . On the drive end of shaft  18 , a pair of sealing rings  80 ,  82  provide the necessary barrier to keep oil in the bearing, whilst keeping foreign material out of the interior of the bearing  22 . 
     Mounted on shaft  18  adjacent bearing  22  is Rogowski Coil  40 . The coil is wound on shaft  18  so as to be frictionally secured to the surface of shaft  18 . Plastic clamping devices may be used to secure the coil  84  in place if needed. 
     A pair of insulated leads  84  and  86  are connected to the two ends of coil  40  and are fed through a pair of passageways  88  and  90  in shaft  18  to slip ring device  92  mounted on the end of shaft  18 . The leads  84  and  86  emanating from slip ring device  92  are taken to suitable monitoring equipment (not shown). 
     In operation, rotor  16  is rotated by the field produced in stator  14 . At the same time, as the flux from stator  14  produces a rotating field to rotate, a homopolar flux is produced in the DEM which attempts to establish a flow of shaft current in the DEM. 
     In most DEM&#39;s, the shaft current is relatively small but does exist. Shaft current flows through a circuit comprising the shaft, bearings, (and bearing pedestals if present) through the DEM frame or ground return. 
     FIG. 3 shows the electrical equivalent circuit  100  in which the shaft circulating current of the DEM such as shown in FIG. 1 flows. The circuit comprises generator  102 , a shaft resistance  104 , Rogowski coil  40 , a resistance  108  representing the oil film in the bearing such as  22 ; and a resistance  1   10  representing insulation which may be present between the bearing  22  and the machine frame, or ground. Resistance  112  represents the oil film resistance of bearing  20 ; and element  38  represents the second shaft mounted Rogowski coil. 
     As soon as the stator is energized, the flux causing the shaft current to flow is produced by the stator. Shaft current begins to flow through the shaft and bearings according to the circuit shown in FIG.  3 . At the time of start up, the shaft rotation has not been sufficient to produce any flow of lubricant to the bearings  20  and  22 . At this time, the oil film resistance of bearings  20  and  22  is a minimum and the magnitude of the circulating current in shaft  18  is a maximum. This magnitude of shaft current will be evident from the Rogowski coil outputs. 
     As the shaft speed increases and oil rings such as  70  begin to transfer oil from the reservoir  74  to shaft  18 , an oil film builds up in bearings  20  and  22 , the Rogowski coils will signal a decreasing shaft current in the machine. The shaft current decreases to a steady state value when the oil film stabilizes in bearings  20  and  22 . 
     The Rogowski coils produce outputs which may be used to monitor bearing performance, and oil ring performance within the bearing. The Rogowski coil outputs may be sampled to produce a characteristic chart of shaft current versus oil film present in the bearings. 
     Any operating deficiency which decreases the flow of lubricating oil to bearings of a DEM will be evident from the output of the Rogowski Coils. 
     This system may be used to measure the shaft current in a DEM at standstill, before rotation of the shaft has begun, and at various shaft speeds as the shaft is accelerated. It is well known by those skilled in the art that the oil ring will begin to slip on the shaft when a critical shaft speed has been reached. This method of measurement will allow a skilled operator to measure the point at which slippage of the oil ring on the motor shaft begins. 
     FIG. 4 shows a bearing  122  slightly modified from the bearing  22  shown in FIG. 2 but where the elements common to both figures bear the same reference numerals. 
     Bearing  122  journals shaft  18  therein and oil ring  70  provides the necessary lubricant to the shaft-bearing interface as described heretofore. 
     A Rogowski coil  124  is mounted on seal  80  so that coil  124  is concentric with the shaft  18  but is spaced a predetermined distance say 0.05-0.5 inches, this distance is not critical; there must be sufficient clearance to permit shaft  18  to rotate without contact with coil  124 . Coil  124  is an air core induction device and the coupling with shaft  18  is not critical until the clearance distance from the shaft  18  becomes excessively large. 
     The Rogowski coil  124  may be mounted on bearing  122  in any convenient manner, in this instance, the body of seal  80  has been chosen for convenience of installation and easy access to the leads  126  and  128 . 
     The Rogowski coil installation of FIG. 4 requires no ducted or hollow shaft to achieve the shaft current signals from the coil surrounding the shaft, thus in certain instances may offer advantages for installation on DEM&#39;s already installed in which it is desired to determine the magnitude of shaft current flowing in the machine. It is especially adaptable to installations where it is desired to measure the impedance of a lubrication system in a vertical shaft machine such as a waterwheel generator. 
     It will be obvious to those skilled in the art that the condition of the oil in the bearings may be evaluated over a period of time by periodic sampling of the Rogowski coil output; contamination of bearing oil tends to render the oil more conductive for passage of the shaft current. 
     Other variations will be apparent to those skilled in the art after reading this specification, however, the ambit of applicant&#39;s invention is limited in scope only by the following claims.