By John M. Vance Alan B. Palazzolo and Fouad Y. Zeidan ABSTRACT

ELECTRIC SHAFT CURRENTS IN TURBOMACHINERY by John M. Vance Professor Alan B. Palazzolo Assistant Professor and Fouad Y. Zeidan Research Assistant Department of Mechanical Engineering Texas A&M University College Station, Texas john M. Vance is a Professor of Alan B. Palazzolo, Assistant Professor Mechanical Engineering at Texas A&M of Mechanical Engineering at Texas University. He received his B.S.M.E. A&M, received his B.S. (1976) from the (1960), M. S.M.E. (1964), and Ph.D. University of Toledo, and his M. S.M. E. (1967) degrees from the University of (1977), and Ph. D.M. E. (1981) degrees Texas. from the University of Virginia. He Dr. Vance came to Texas A&M from worked for Bently Nevada (1977-78), the University of Florida, where he de University of Virginia (1980-81), Allis veloped a Rotordynamics and Vibrations Chalmers (1981), and Southwest Re Laboratory and conducted research on search Institute (1981-85) beforejo ining squeeze film bearing dampers, helicopter Texas A&M, in 1985. He researched dur propulsion systems and other topics related to rotordynamics ing the summers of1986 and 1987 at NASA Lewis, as a Faculty of turbomachinery. Fellow. Prior to joining the Mechanical Engineering fa culty at Dr. Palazzolo's expertise is in vibrations, rotordynamics, fi Florida, he worked fo r five years as a Mechanical Engineer in nite and boundary elements. He has also been extensively in industry, holding positions at Armco Steel, Texaco Research, volved with field troubleshooting of mechanical malfunction in and Tracor, Incorporated. rotating and reciprocating machinery. Dr. Palazzolo has pre Dr. Vance is currently conducting research on rotor sented papers at ASME Gas Turbineand Vibration Conferences dynamics, electromagnetic shaft currents, and bearing dampers and has published many papers in various engineering journals. in the Tu rbomachinery Laboratory. He has published more Dr. Palazzolo is a newly elected member of the Turbomachin than fiftytec hnical articles and reports on rotordynamic insta ery Symposium Advisory Committee. bility, squeeze film bearing dampers, vibration isolaters, and related subjects. He is an active consultant to industry and government laboratories, and has held eleven summer appoint Fouad Y. Zeidan is currently a Re ments at Pratt & Whitney Aircraft, USATL (Helicopter Prop search Assistant with the Turbomachin ulsion Lab, Ft. Eustis), Southwest Research Institute, and Shell ery Laboratory at Texas A&M Univer Development Company. He organized a short course fo r indus sity. He received his B.S.M.E. (1978), try at Texas A&M on "Rotordynamics of Turbomachinery," and M.S.M.E. (1979) degrees from Texas and co-organized the biennial "Workshop on Rotordynamics A&M University. He is presently working Instability Problems in High Performance Turbomachinery." on his Doctorate in Mechanical Engineer Dr. Vance was named the Dresser Industries Associate Pro ing. He worked fo r five years as a Rotat fe ssor at Texas A&M in 1979-80, was Associate Head of the ing Machinery Field Engineer and Senior Department of Mechanical Engineering in 1980-81, and was Mechanical Engineer at Qatar Fertiliser named a Halliburton Professor fo r 1986-87. He is a member Company, Qatar. His current research of ASME and ASEE, and is a registered professional engineer projects include squeeze film damper bearings and shaft in the State of Texas. currents. ABSTRACT Electrical damage to turbomachinery parts has caused a number of machinery failures and many hours of costly down time. The problem of electrical voltages and currents being generated in non-electrical machines has puzzled both users and manufacturers of these equipment, and has prompted on- 51 52 PROCEEDINGS OF THE SIXTEENTH TURBOMACHINERY SYMPOSIUM going research at Texas A&M to help identify and better classify currents is shown in Figure 1, which is a photograph of the the different sources of these voltages and the mechanisms by steel backing of a tilt pad that was welded to the bearing shell. which they cause damage to bearings, seals, and other critical The weld spots illustrate the high current densities that must machinery parts. exist to cause such damage. This is supported by the discolora Electrostatic and electromagnetic type voltages are often mis tion due to the high thermal effects. identified, a situation that might lead to a wrong or costly remedy. The distinctions between these two major sources of shaft voltages and currents are clearly draw11 herein. Lubricat ing oil characteristics and their influence on the buildup of shaft voltage potential have also been carefully scrutinized. Breakdown voltages were generated and measured at different operating conditions to help highlight the influence of the dif ferent variables such as bearing clearance and dielectric strength of the oil. INTRODUCTION The phenomenon of electrical damage to machinery compo nents is not new and has been addressed in several articles and technical publications. Sohre and Nippes were the firstto iden tify the different types of shaft currents generation and to pre sent a theory that explains some aspects of electromagnetic shaft induetion [1]. A research effort at Texas A&M University has been funded by the Turbomachinery Research Consortim� since 1981. It was originally directed at verifying the theory introduced by Sohre and Nippes [1] in a laboratory controlled environment. These efforts have since been expanded to in Figure 1. Welding Spots on a Radial Tilt Pad Bearing. clude studies of other mechanisms of shaft voltage generation, as well as field measurements. This research will be referred Another important aspect of shaft currents which is of prime to hereafter as "the TAM U project." importance, but which has not been adequately addressed by Research to date has suggested the f(>Jlowing classification of previous publications on the snbjed, is the oil dielectric the sources of shaft voltages/currents: strength or breakdown voltage. This topic has been under study • Electrostatic. in the TAMU project and is included under a later section • Electromagnetic. heading Breahlown Volt-age. AC induced by rotating or alternating residual magnetic · Chronological Perspective poles, usually localized and possibly self�exciting as described by Sohre and Nippes [1, 14]. The origin of shaft currents research began in the study of electrical machinery. Merrick [3] published extensive results · DC Homopolar ([l]) induced by magnetic flelds fixed in on shaft current causes and effects in generators. His results the stator interacting with eddy currents in the rotor, as in a suggested that dissymmetry in the magnetic circuits was the Faraday disk. principal cause of induced voltage in the shaft. This dissym DC self�exciting, as demonstrated experimentally hy · metry could result fi·om mechanical joints in the stator, a differ Schier [2] in Germany. This phenomenon can produce ence in the permeability of the magnetic material constituting thousands of amperes with no measured voltage exceeding one two flux paths, or by differencein the effective lengths of these volt. paths. ln addition, Merrick showed that shaft currents could The principle of operation of electric generators centers exist in both AC and DC machinery, and that it was strongly around the fact that a conductor travelling through a magnetic dependent on bearing clearance. He suggested two fixes for field will generate a voltage at its two ends, one end being the shaft current problem: positive, the other negative. If the ends are electrically con nected, a current will flow. This current will be proportional • Brushes are placed on both ends of the shaft and are either to the dimensions of the conductor, the strength of the magnetic grounded on the bearing pedestals or connected together by field, and the velocity with which the conductor is travelling a heavy copper conductor. �Ierrick felt that this may not be through the magnetic fields. If the above prerequisites are met, the best fix, because the objective of reducing the amount of any machine will generate internal currents. In a turbine com current passing between the shaft and bearings •vould not be pressor unit, these currents are short-circuited, with the elec met, if the oil film resistance is less than the contact resistance tric energy transformed into heat in the rotor and stator. \Vhere of the brushes. the currents travel across the interface between parts, for exam • Insulation is inserted between one or more of the bearing ple, the very thin oil film at bearings and seals, or at any other pedestals and the machine base. In addition, it would be neces contact point offering an electrical resistance, there will be sary to insulate all piping, stairs, etc., which would otherwise sparking and smface damage. If currents are high, and they complete the electrical circuit. Merrick felt that this method, can reach the very high values of several thousand amperes, if properly applied, would provide absolute protection to jour there can be massive arc-welding. As contacting points are nals and bearings. consumed by arcing, clearances will increase. This raises the Buchanan [4] made similar observations to those of Merrick electrical resistance at this given location until another location noting that shaft currents could be caused by: offers a lower resistance, and the current \Viii change to another • Unequal reluctances of the main field circuit, due to un location path. This process continues until vibration or leakage equal air gaps, pole joints or permeability of poles. becomes excessive, or a failure occurs in the thrust bearings, • Unequal air gaps, due to eccentricit) of the rotor relative journal bearings, or seals. A typical damage from this type of to the stator. ELECTRIC SHAFT CURRENTS IN TURBOMACHINERY 53 • Joints in the stator core. [13, 14]. Their joint report contains an interesting description His final conclusions were: of the effectiveness and limitations of shaft brushes [ 14]. • If the number of stator joints are equal to the number of FIELD MEASUREMENTS BY THE TAMU PROJECT field poles, and all joints have the same reluctance, no shaft electro motive force (EMF) will occur.

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