Abstract:
Some dynamoelectric machines develop shaft E.M.F.&#39;s during operation which lead to circulation currents in the shaft of the machine. The shaft current will attempt to flow through the bearings of the machine, unless some action is taken to prevent or limit said current flow. This invention minimizes the circulation of rotor shaft currents by providing a return path for the shaft current through an insulated conductor located in the center of the shaft of the machine. The insulated conductor is electrically attached to the shaft at the ends of the conductor. The conductor may extend the length of the shaft.

Description:
BACKGROUND OF THE INVENTION 
     The presence of shaft currents in dynamoelectric machines (DEM&#39;s) has long been known, but is difficult to measure. Designers have postulated on methods of eliminating the troublesome circulating shaft currents which in some instances can rise to such levels that damage to the bearings, which journal the rotating shaft, often occurs. 
     Because of the difficulty in assessing the cause of bearing failure in operating DEM&#39;s, many bearings have been replaced in DEM&#39;s, the demise of which has often been attributed to bearing misalignment or lubrication failure when in reality, the cause of the bearing failure was caused by current circulation through the bearings of the machine and into the DEM pedestals where it could find a convenient return path. 
     In some troublesome machines, the useful life of a bearing set has been reduced to the order of hours or days by the presence of large circulating currents through the machine. The exact cause of the generation of shaft EMF&#39;s is not well understood, but is believed that the production of unbalanced magnetic fields in the DEM&#39;s result in generation of sizable zero sequence shaft voltages being induced in the DEM rotor shafts. The magnitude of the resulting circulatory currents in the shaft of the DEM is largely determined by the path resistance through the bearings and the ground return systems and not by the resistance of the shaft itself. 
     Many solutions have been attempted to ameliorate the circulating current problem and usually the most direct solution lay in increasing the resistance of the ground return paths. 
     The simplest solution utilized in the past, involved mounting part of the bearing structure in such a manner that the bearings themselves were insulated from their pedestals, or the pedestals were insulated from ground. 
     SUMMARY OF THE INVENTION 
     The machine of the invention is modified in its construction to minimize the circulating currents flowing in the shaft of the DEM. The modification consists of providing a hollow shaft for the DEM and passing an insulated conductor through the passageway in the hollow shaft which is rigidly connected to the ends of the rotor shaft. The insulated conductor inside the DEM rotor shaft rotates with the shaft and inhibits any circulating currents from entering the bearings of the machine by providing a low resistance return path. In the end, the insulating conductor functions as a transformer secondary in which any induced current in the insulated conductor opposes the magnetic flux which would induce a circulating current. 
     RELEVANT PRIOR ART 
     PUBLICATIONS 
     WALKER, P. “Preventing Motor Shaft-Current Bearing Failures” Plant Engineering, Oct. 4, 1990. 
     The Walker reference above provides a lucid explanation of the generation of shaft EMF&#39;s and provides various solutions to prevent large circulating shaft currents from being developed. Amongst the solutions are: insulating the bearing pedestals of a DEM; insulating the components of a bearing to insert large resistance in the flow path through the bearings of the DEM; providing an alternative path for current around the machine. The last solution requires the use of brushes mounted on the DEM at each location where the shaft leaves the DEM housing, so that a conductor may be connected to the brushes to “short-circuit” the brushes together and prevent any significant current through the bearings of the DEM. COSTELLO, M. J. “Shaft Voltages &amp; Rotating Machinery” IEEE Paper No. PCIC -91-13, 1991. 
     This paper lists a number of causes of circulating shaft currents in both electrical and non-electrical machinery. Besides describing the physical damage phenomena resulting from the troublesome circulating shaft currents, the reference teaches the various stimuli which result in circulating shaft currents. 
     The two types of stimuli that are discussed which are of interest to this application are “sources of residual magnetism” and “magnetic dissymmetry” of the ac machine. Both stimuli result in circulating currents in the rotor shaft and various techniques of inserting insulation into the current circulation loop between the shaft and ground are described. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective of a DEM showing the invention of this application. 
     FIG. 2 is an equivalent circuit of the current circulation path of the machine of FIG.  1 . 
     FIG. 3 is an alternative form of this invention. 
     FIG. 4 is yet another alternative of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a typical DEM  10  is shown which is a three phase induction motor. DEM  10  has a stator  12  and a rotor  14  mounted on shaft  16 . 
     The stator of DEM  10  comprises packets of magnetic laminations  18  which are mounted within the stator frame, a portion of which is shown at  20 . The lamination packets  18  are separated by spacers  21  at predetermined locations to form ventilation ducts  22  for the passage of cooling air therethrough. 
     The stator is provided with windings  24  which are fitted in slots formed in the lamination packets of DEM  10 . The windings  24  are energized from a multi-phase source (in this instance, three phase) to set up a conventional rotating magnetic field in DEM  10 . 
     Rotor  14  follows the conventional construction for a multiphase induction motor rotor. Rotor  14  comprises groups of magnetic laminations arranged in packets  26  which are separated by ventilation spacers  28  to form ventilation ducts  30  through which cooling air may flow. 
     The rotor ventilation ducts  30  are generally arranged to be aligned with the ventilation ducts  22  in the stator  12  to permit air to readily flow from the rotor  14  into stator  12 . The rotor  14  is supplied with conventional windings  32  which usually are bars formed of aluminum or copper or alloys thereof which pass through lamination packets  26 . Rotor lamination packets  26  are held secure by suitable fastening means (not shown) which pass through rotor  14 . 
     Rotor  14  will be provided with a pair of shorting rings at each end of rotor  14  (not shown) to which each rotor bar  32  is integrally attached to permit the necessary rotor currents to flow. 
     The entire stator-rotor structure is housed in housing  36  to provide containment for the DEM  10  and provide air passages for the ventilation air. 
     Shaft  16  is hollow having a bore  38  passing completely through the shaft. An insulated conductor  40  is located within bore  38  and is integrally fastened to each shaft end  42 . Conductor  40  is insulated with insulation  44 . 
     The windings  24  are responsible for the generation of the rotating magnetic field in the DEM  10 . In a conventional three phase machine, the windings are each connected in the stator in groups to form the required winding configuration to generate the rotating field. In an ideal situation, the windings in all three phases are identically located, and equally energized and the three magnetic fields are ideally balanced so that the net magnetic fields developed in the stator sum to zero. 
     But the windings and magnetic structure are never perfect and the imbalance in the stator structure causes a net magnetic field (stray) to induce a current in the shaft  16  of rotor  14 . 
     The rotor  14  also has its own idiosyncrasies and the magnetic fields induced in the rotor bars by the rotating magnetic field of the stator are not entirely balanced. It is believed that the resultant stray magnetic field produced by the rotor  14  and stator  12  causes an induced EMF in shaft  16 . 
     Without conductor  40  in shaft  16 , circulating currents will exist where a return path exists for the shaft  16 , this path generally includes the bearings of the machine. 
     In the DEM  10  of this invention, any current induced in shaft  16  is shorted by conductor  40  which is integrally attached to shaft ends  42 . It is believed that the conductor  40  acts as a shorted secondary of a transformer and any current circulating in the conductor  40  tends to oppose the magnetic flux, hence there is only a very small resultant current. 
     FIG. 2 is a schematic diagram of the DEM of FIG.  1 . 
     A generator  100  is shown producing an EMF which is believed to be due to magnetic dissymmetries in the existing machine. The generator  100  causes current to flow through the shaft where the latter is represented by the resistor  102 . The potential generated by the generator causes current to flow through the bearings of the DEM  10  as represented by resistors  104  and  106 . Resistors  104  and  106  include any insulation inserted in the bearing-ground return circuit which may have been inserted into the current loop to reduce the magnitude of the circulating current. The resistor  108  represents the resistance of the lead passing through the center of the shaft  16 . This value may include the resistance of any devices inserted in the shaft lead for purposes of monitoring the shaft current. 
     It will be found that with the presence of the shaft return lead, the magnitude of the current passing through the bearings is reduced to an insignificant magnitude, so that the integrity of the bearings is preserved and the service life of the DEM is significantly increased. 
     It will be found that the shaft lead functions to magnetically oppose magnetic fluxes which have induced troublesome shaft and bearing currents in prior art machines. 
     The shaft lead will also permit the insertion of suitable instrumentation apparatus into the shaft lead to permit monitoring of the currents and voltages present in the shaft lead. 
     Referring to FIG. 3 where an alternative arrangement of the applicant&#39;s invention is shown, wherein like reference numerals are used for like parts of the DEM  100 . 
     In FIG. 3, DEM  100  utilizes a shaft  16  which has been provided with a bore  50 . Bore  50  does not traverse the complete shaft as in FIG. 1 but passes through that portion of shaft  16  of DEM  100  which is subject to the active flux linkage causing the induced EMF in the shaft  16 . Bore  50  terminates in this instance in a threaded bore  52  which is co-axial with bore  50 . 
     An insulated conductor  54  is shown passing through bore  50 . A threaded end  56  of conductor  54  is threaded into threaded bore  52  of shaft  16  so as to form an excellent electrical joint. It will be noted that conductor  54  is provided with a suitable insulative coating  58 . The threaded end  52  of insulated conductor  54  forms an electrical connection with the shaft  16  at the location of threaded bore  52  and a bridging member  60  at the right hand end of shaft  16  completes the circuit with conductor  54 . It will be found that the conductor  54  may extend throughout the axial length of the stator or rotor and prevent the circulation of destructive currents through the shaft and bearings of the machine. At times it may not be necessary to pass conductor  54  the entire distance that the rotor extends along the shaft  16 , depending on the severity of the shaft circulation currents. This method of controlling the magnitude of the circulating current in the shaft and bearings is especially important as a retrofit. 
     FIG. 4 shows another alternative form of the invention. Here, a DEM  200  is shown having a shaft  16  on which rotor  14  is mounted as in previous illustrations. An insulated conductor  70  is mounted adjacent shaft  16 . Conductor  70  is connected to a pair of terminals  72  and  74  which are bolted securely to shaft  16  by screws or bolts  76  and  78 . The insulated conductor  70  must be closely bound to shaft  16  to preserve the rotor balance and to prevent the generation of EMF&#39;s in the insulated conductor  70 . 
     It is possible to use more than one insulated conductor  70  to assist in reducing the level of circulating current in the rotor shaft. Multiple conductors may also improve the unbalance produced by the presence of only one conductor on the shaft.