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EDITOR IN CHARGE Prof. Gheorghe V. LEPĂDATU Ph.D
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METALURGIA INTERNATIONAL
NO. 2 - 2013
ISSN 1582 - 2214
Romanian Metallurgical Foundation
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F.M.R.
EDITOR IN CHARGE Prof. Gheorghe V. LEPĂDATU Ph.D
PRESIDENT HONOURARY STAFF ŞTEFĂNESCU DORU Ph. D., Ohio State University Columbus, S.U.A.
(in alphabetical order)
Chenguan BAI - College of Materials science and engineering Chongqing University, CHINA Constantin DUMITRESCU - Member of Romanian Academy of Technical Sciences, ROMANIA I. IMRIŠ - Technical University of Košice, SLOVAK REPUBLIC Jingshe LI - University of Science and Technology Beijing, CHINA W.M. NICOLA - Tri. State University Angola, Indiana, S.U.A. Ion STANCU - Bucharest Academy of Economic Studies
Qingguo XUE - University of Science and Technology Beijing, CHINA Zhengliang XUE - Wuhan University of Science and Technology, CHINA Gencang YANG - Chongqing University, CHINA Zhenkui YIN - University of Science and Technology Beijing, CHINA Xianqing YOU - Wuhan University of Science and Technology, Wuhan, CHINA Liangying WEN - Chongqing University, CHINA
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PRESIDENT: Prof. Maria NICOLAE Ph. D., Bucharest Politechnical University, Romania Prof. Marian BORDEI Ph. D., Dunărea de Jos University Galați, Romania Prof. Voicu BRABIE Dalarna Univerbity Suedia Prof. Ilie BUTNARIU Ph. D., Bucharest Politechnical University, Romania Prof. Horia COLAN Ph. D., Deputy member of Romanian Academy Prof. Nicolae CONSTANTIN Ph. D., Bucharest Politechnical University, Romania Prof. Adrian DIMA Ph. D., Member of Romanian Academy of Technical Sciences Prof. Corina Adriana DUMITRESCU Ph. D., Dimitrie Cantemir University, Romania Prof. Arpad FAY Ph. D., University of Miskolcz, Hungary Prof. FRENCH DAVID Ph. D., CSIRO Energy Technology, Australia Prof. Mira Ricardo GARCIA Ph. D., University of Coruna, Spain Prof. Teodor HEPUT Ph. D., Hunedoara Engineering Faculty, Romania Prof. Gheorghe LEPADATU PH.D. Dimitrie Cantemir University, Romania Prof. Tiberiu MĂNESCU Ph. D., Eftimie Murgu University Reşiţa, Romania Prof. Ioan MILOŞAN Ph. D., Transylvania University Brasov, Romania Prof. Valentin NEDEF Ph. D., Engineering Faculty of Bacău University, România Prof. Avram NICOLAE Ph. D., Bucharest Politechnical University, Romania Prof. doc. Florea OPREA Ph. D., Member of Romanian Academy of Technical Sciences Prof. Florentina POTECAŞU Ph. D., Dunărea de Jos University of Galaţi, Romania Prof. Radu TAMARA Ph. D., Dunărea de Jos University Galati, Romania Prof. Doina RĂDUCANU Ph. D., Bucharest Politechnical University, Romania Prof. Iulian RIPOSAN Ph. D., Bucharest Politechnical University, Romania Prof. Tiberiu RUSU Ph. D., Technical University Cluj-Napoca, Romania Prof. Jean-Marc SAITER Ph. D., Rouen University, France Prof. V.I. SAFTA Ph. D., Member of Romanian Academy of Technical U.T.Timisoara,Romania Prof. Luc SALVO Ph. D., Grenoble National Politechnical Institute, France Prof. Cornel SAMOILĂ Ph., Transilvania University, Deputy member of Romanian Academy of Technical Science Prof. Laurenţie SOFRONI Ph. D., Member of Romanian Academy of Technical Sciences Prof. Marin TRUŞCULESCU Ph. D., Member of Romanian Academy of Technical Sciences Prof. Petrica VIZUREANU Ph.D ,Technical University Gh. Asachi, Iasi, Romania Prof. Viorel ZAINEA Ph. D., University of Medicine and Pharmacy “CAROL DAVILA”, Bucharest, Institute of Phonoaudiology and Functional ENT Surgery Prof. Dr. D. Hociota, Bucharest, Romania
ADVISORY BOARD :
PRESIDENT: Prof. Augustin SEMENESCU Ph. D. Bucharest Politechnical University, Romania VICE-PRESIDENT: Prof. Alina Adriana MINEA Ph. D., Technical University Gh. Asachi Iasi, Romania Prof. Marin ANDREICA Ph. D - Academy of Economic Studies, Bucharest, Romania Prof. Erika ARDELEAN Ph.D - Politehnica University of Timişoara, Romania Prof. Mircea BEJAN Ph. D - Technical University Cluj-Napoca, Romania Prof. Ionel BOSTAN Ph. D - “Stefan cel Mare” University of Suceava, Romania Prof. Florina BRAN Ph.D - The Bucharest Academy of Economic Studies, Romania Assoc. Prof. Denis CHAUMONT Ph.D - Bourgogne University of Dijon France Prof. Anisoara CIOCAN Ph.D - “Dunărea de Jos” University of Galaţi, Romania Lecturer Floarea GEORGESCU Ph.D - Spiru Haret Universitaty, Romania Prof. Brandusa GHIBAN Ph. D - Politehnica University Bucharest, Romania Prof. Nicolae GHIBAN Ph.D - Politehnica University Bucharest, Romania Assoc. Prof. Guibao QIU Ph. D.- Chongqing University, China Assoc. Prof. Xuewei LÜ Ph. D.- Chongqing University, China Prof. Dumitru V. LEPADATU Ph. D - Tehnical University “ Gh. Assachi” Iasi, Romania Assoc. Prof. Nicu MARCU Ph. D - University of Craiova, Romania Assist. Bogdan MASTALIER M.D., University of Medicine and Pharmacy “Carol Davila” Bucharest, Romania Narcisa Roxana MOȘTEANU PhD.- The Bucharest Academy of Economic Studies, Romania Prof. Ion PARGARU Ph. D - Politehnica University Bucharest, Romania Prof. Rodica POPESCU Ph.D. - Transylvania University Brasov, Romania Prof. Ana SOCALICI Ph.D - Politehnica University of Timişoara, Romania Prof. Ion STANCU Ph. D.- Bucharest Academy of Economic Studies Prof. Aurelia Felicia STĂNCIOIU Ph. D. - Academy of Economic Studies, Bucharest, Romania Prof. Rami ŞABAN Ph. D. - Politehnica University Bucharest, Romania Prof. Daniela TARATA Ph.D - University of Craiova, Romania Prof. Bela VARGA Ph. D - Transylvania University Brasov, Romania Assoc. Prof. Marius VASILESCU Ph.D - Politehnica University Bucharest, Romania Prof. Ana VETELEANU Ph. D - Transylvania University Brasov, Romania Prof. Maria VLAD Ph .D - “Dunarea de Jos” University Galati, Romania Assoc. Prof. Chen YUANQING - Chongqing University, China Assoc. Prof. Shengfu ZHANG Ph. D. - Chongqing University, China
METALURGIA INTERNATIONAL VOL. XVIII (2013), NO. 2 ISSN 1582-2214
I
CONTENTS
MATERIALS SCIENCE Claudiu CONSTANTIN, Nicolae CONSTANTIN: ANALYTICAL METHODS FOR FAST DETERMINATION OF CONSUMPTION OF COKE IN BLAST FURNACES, USING VARIOUS FUELS AUXILIARY .............................................................................................................5 Claudiu CONSTANTIN, Nicolae CONSTANTIN: THE APPLICATION OF STATISTICAL AND MATHEMATICAL METHODS OF ANALYSIS TO REAL DATA PROCESSING OPERATION OF A BLAST FURNACE .............................................................................................................8 Valentin MINCU, Sînziana ITTU: REFINING STEELS PRODUCED IN ELECTRIC ARC FURNACES ...........................................................................................................12 Ioan Constantin DIMA, Mariana MAN, Vladimir MODRAK: USING THE SIMULATION TECHNIQUE FOR PLANNING THE INDUSTRIAL PRODUCTION IN METALLURGY ...........................................................................................................17 Ioan Constantin DIMA, Mariana MAN, Janusz GRABARA: USING THE EXPERT SYSTEMS IN THE MANAGEMENT OF THE TECHNOLOGICAL PROCESS WITHIN THE FOUNDING DEPARTMENTS ...........................................................................................................21 Mirela TEODORESCU, Nabyl KHENOUSSI, Laurence SCHACHER, Dominique C. ADOLPHE, Maricel AGOP, Ştefan TOMA: SURFACE MORPHOLOGY INFLUENCE IN STRUCTURAL COLOURS DISPLAYED BY SPIN-COATED THIN FILM ...........................................................................................................27 Mihai BERNEVIG: CHEMICAL INFLUENCE OF FLUIDIZED BED SAND ON AlMg2 SURFACE ALLOY DURING SOLUTION ANNEALING HEAT TREATMENT ...........................................................................................................30 Ilija BELIĆ, Zagorka AĆIMOVIĆ-PAVLOVIĆ: METHODS OF INVESTIGATION OF FILLERS FOR CERAMIC COATING IN FOUNDRY
...........................................................................................................35 Robert CIOCOIU, Octavian TRANTE: CARBON FIBER AND STAINLESS STEEL REINFORCED EPOXY COMPOSITE MECHANICAL PROPERTIES ...........................................................................................................40 Gheorghe IACOB: TECHNOLOGICAL CHARACTERISTICS OF Al/Al2O3/Gr POWDER COMPOSITES OBTAINED BY MECHANICAL ALLOYING ...........................................................................................................43 Hui-ning ZHANG, An-jun XU, Dongfeng HE, Jian CUI: SELECTIVE EXTRACTION BEHAVIOR OF STEELMAKING SLAG IN CH3COOH SOLUTION AT ENVIRONMENTAL PRESSURE ...........................................................................................................49 Damnjan RADOSAVLJEVIĆ, Slaviša TRAJKOVIĆ, Srećko MILAČIĆ: 3D MODELING OF BALL BEARING ...........................................................................................................57
Claudiu NICOLICESCU, Victor Horia NICOARA, Constantin PREDESCU: TRIBOLOGICAL BEHAVIOUR OF W/Cu NANOSTRUCTURED MATERIALS USED FOR ELECTRICAL CONTACTS ...........................................................................................................61 Constantin PREDESCU, Claudiu NICOLICESCU, Victor Horia NICOARA: STUDIES REGARDING THE ELABORATION OF TUNGSTEN NANOPOWDERS BY MECHANICAL MILLING PROCESS ...........................................................................................................65 Ionela BIRLOAGA: RECOVERY OF BASE METALS FROM WASTE CPU BY HYDROMETALLURGICAL PROCESSES ...........................................................................................................69 Sinziana ITTU, Nicolae CONSTANTIN: SOME CHARACTERISTICS OF VERMICULITE MINERAL
...........................................................................................................73 Jasmina PEKEZ, Miroslav LAMBIĆ, Slobodan STOJADINOVIĆ: MATERIALS FOR FLAT PLAT SOLAR COLLECTORS ...........................................................................................................77 Svetislav Lj. MARKOVIĆ, Ljubica MILOVIĆ, Miran VRHOVAC, Tatjana LAZOVIĆ, Aleksandar MARINKOVIĆ, Vujadin ALEKSIĆ: LIFE EXTENSION OF GEARS FLANK SURFACES REGENERATED BY HARD FACING ...........................................................................................................81 Yan-hui SUN, Lei LUO, Zhuo-ying YU, Shi-wei LI, Han GAO: STUDY ON MODIFICATIVE INCLUSIONS BY CALCIUM TREATMENT ON HIGH QUALITY PIPE LINE STEEL ...........................................................................................................86 Miroslav BJEKIĆ, Danilo STOJANOVIĆ, Marko ROSIĆ, Miloš BOŽIĆ: ANALYSIS OF MITIGATION TECHNIQUES FOR BEARING CURRENTS IN PWM INVERTER DRIVES ...........................................................................................................91 Zhijiang XIE Gang LEI Junjie WANG: STRUCTURE DESIGN AND PRECISION ANALYSIS FOR 6-URS MEASUREMENT PLATFORM ...........................................................................................................98 Raluca Ioana GUICĂ, Cristian DRAGOMIRESCU, Laura Elena TRIFAN, Constantin Anton MICU: A NEW MODEL OF A HARD PROBE USED IN ORTHOPEDIC MEASUREMENTS .........................................................................................................104 Radu D. STANCIU, Ion PÂRGARU: AN ANALYSIS OF THE HR PRACTICES IN THE SMALL AND MEDIUM-SIZED ENTERPRISES FROM THE DEVELOPMENT REGION BUCHAREST-ILFOV .........................................................................................................111 Mariana TRANDAFIR: MONETARY POLICY STRATEGIES FROM THE PERSPECTIVE OF THE SHIFTS OF THE PARADIGM: INTERNATIONAL EXPERIENCES .........................................................................................................114 Vali Ifigenia IORDĂCHESCU: FATIGUE CALCULATION OF MECHNICAL STRUCTURES FROM MATERIALS WITH LINEAR OR NONLINEAR BEHAVIOUR .........................................................................................................120
METALURGIA INTERNATIONAL VOL. XVIII (2013), NO. 2 ISSN 1582-2214
II
Dragan GRAHOVAC, Dijana KARUOVIC, Branislav EGIC: MANAGING THE PROCESS OF INFORMAL LEARNING .........................................................................................................129 Miroslav RADOJICIC, Zoran NESIC, Ivana BULUT: APPLICATION OF INTELLIGENT DECISION SUPPORT SYSTEMS IN RESOLUTION PROCESS OF MANAGERIAL DECISION-MAKING .........................................................................................................134 Daniel ŞTEFAN: STUDY ON THE ACTUAL EUROPEAN COMPETITIVENESS .........................................................................................................139 Nicoleta STANCIU: IMPORTANCE OF EVENT LOG MANAGEMENT TO ENSURE INFORMATION SYSTEM SECURITY .........................................................................................................144 Slobodan BABIĆ, Zoran ANĐELKOVIĆ, Dušan BARAĆ, Zorica BOGDANOVIĆ, Marijana DESPOTOVIĆ-ZRAKIĆ: MODEL OF INTEROPERABLE E-BUSINESS OF PAYMENT SYSTEMS BASED ON ONTOLOGIES .........................................................................................................150 Olimpia NEAGU: THE HUMAN CAPITAL IN THE ROMANIAN INDUSTRY: AN EMPIRICAL ASSESSMENT .........................................................................................................156 Biljana STOSIC, Sonja ISLJAMOVIC, Marko M. MIHIC: IMPROVEMENT OF INNOVATION PROJECT RISK IDENTIFICATION BY APPLYING RBS METHOD .........................................................................................................161 Miroslav RADOJICIC, Zoran NESIC, Ivana BULUT: INTEGRATING OF PROGRAMMING TOOLS IN FUNCTION OF IMPROVING MANAGERIAL DECISION MAKING .........................................................................................................167 Nebojsa LAPCEVIC, Mirjana MISITA, Marija MILANOVIC, Vesna SPASOJEVIC BRKIC, Danijela TADIC: RESEARCH INTO FACTORS THAT INFLUENCE THE SUCCESS OF ERP IMPLEMENTATION IN A SERBIAN PRODUCTION COMPANY .........................................................................................................172 Oana-Ramona LOBONŢ: A QUESTION ABOUT SIGNIFICANCE OF FISCAL DECISIONS IN THE DESIGN OF ROMANIAN ECONOMIC PERFORMANCES .........................................................................................................175 Mihaela VARTOLOMEI: THE MANAGEMENT OF CONVERGENCE PLAY OPTIMISATION .........................................................................................................178 Dragana VOJTEŠKI KLJENAK, Radojko LUKIĆ, Goran KVRGIĆ: THE INVENTORY MANAGEMENT EFFICIENCY IMPACT ON THE PERFORMANCE OF TRADE IN SERBIA .........................................................................................................183 Gheorghe POPESCU: ACHIEVING COMPETITIVE ADVANTAGE THROUGH SHAPING SHARP BRAINS: TOWARDS STRATEGIC DYNAMICS .........................................................................................................190
Elvira NICA: WORKING PAPER UPON MODERN EDUCATION STRATEGIC PATTERN OUTPOST TOWARDS HUMAN CAPITAL PROFESSIONAL IMPROVEMENTS .........................................................................................................193 Silviu BECIU, Georgiana Raluca LÃDARU: THE ROMANIAN TOBACCO MARKET: SHARE AND TRENDS IN THE WORLD TOBACCO MARKETS .........................................................................................................196 Elena STOIAN: STUDY ON THE INTERNATIONAL FLOWER TRADE EVOLUTION IN ROMANIA FOR THE PERIOD 2002-2011 .........................................................................................................200 Luise MLADEN: THE PERFORMANCE OF THE PENSION SYSTEM IN THE EUROPEAN UNION MEMBER STATES: A COMPARATIVE APPROACH .........................................................................................................204 Mirela HALMI: THE INVESTMENT DECISION AND THE ECONOMIC GROWTH .........................................................................................................210 Constantin ANGHELACHE, Gabriela Victoria ANGHELACHE, Florin Paul Costel LILEA: THE USE OF ECONOMETRIC MODELS FOR ANALYZING THE EVOLUTION OF PROPERTY BETWEEN 1997-2010 IN ROMANIA .........................................................................................................214 Gabriela-Victoria ANGHELACHE, Elena BUGUDUI, Alexandru MANOLE, Florin Paul Costel LILEA, Cătălina Claudia SAVA: USING THE LINEAR SIMPLE REGRESSION MODEL IN THE ANALYSIS OF TRADE ACTIVITY .........................................................................................................220 Florin Paul Costel LILEA: STATISTICS ANALYSIS MODEL USED IN TOURISM ACTIVITY .........................................................................................................223 Alecxandrina DEACONU, Valentin BICHIR: USING THE LINEAR SIMPLE REGRESSION MODEL FOR THE ANALYSIS OF THE CORRELATION ON THE INFORMATIONAL BASIS OF ECONOMIC INDICATORS .........................................................................................................226 Dragoş Gabriel MECU: ANALYTICAL MODELS AND EMPIRICAL RESULTS IN NEGOTIATION .........................................................................................................230 Dragoş Gabriel MECU: NEGOTIATION OF COMPANY ACQUISITION AND MERGING .........................................................................................................236 Viorica STEFAN: THE DOCTORAL SCHOOLS IN SUPPORT OF THE DURABLE DEVELOPMENT CONCERNING NATIONAL ECONOMY .........................................................................................................238 Leposava GRUBIC-NESIC, Valentin KONJA, Danijela LALIC: LEADERSHIP IN LEARNING ORGANIZATIONS .........................................................................................................241
ANALYSIS OF MITIGATION TECHNIQUES FOR BEARING CURRENTS
IN PWM INVERTER DRIVES
Miroslav BJEKIĆ, Danilo STOJANOVIĆ, Marko ROSIĆ, and Miloš BOŽIĆ
Technical faculty in Čačak – University of Kragujevac, Serbia
Key words: Bearing, common-mode voltage, EDM, filter. PWM inverter, power drive
Assoc. prof. Ph.D. EE
Miroslav Bjekić
Prof. Ph.D. EE
Danilo Stojanović
Assistant, Graduated EE
Marko Rosić
Assistant, Graduated EE
Miloš Božić
Abstract: The purpose of this paper is to provide the reader with an introductory knowledge of the phenomenon of bearing currents,
in which situations it occurs and how to mitigate the problem. This study presents different techniques and topologies of equipment
for mitigation of bearing current in PWM inverter drives, available on the market. We present the techniques that can be applied to
the existing drives. Many other methods which require special drive hardware or a motor solution are not discussed here. Since
there are numerous industry papers available that discuss the details of problems caused by PWM VFD, this paper will focus on a
more general discussion of the causes, analysis and possible solutions.
1. INTRODUCTION
One of the fastest growing applications of pulse
with modulation (PWM) variable frequency drive
(VFD) inverters is in commercial and industrial
applications. VFD´s allow system designers to realize
substantial energy savings and motor control capability
provided by PWM drives. As the use of VFD´s to
control motors in air handlers, heaters, fans, blowers,
pumps, air conditioning units, chillers and in other
industrial application has increased over years, there
has also been an increase in motor failure from bearing
currents as the VFD´s induce voltage onto shaft of the
driven motor which ultimately may cause pitting,
fluting and finally bearing and motor failure.
This cause of motor failure has become an
increasingly important reliability issue and the
prevention of bearing failure in VFD driven motors an
important design consideration to ensure motor
reliability and to reduce operating costs. Technical
articles and actual testing results show the severity of
bearing damage that may occur in motors when drives
are used-often in a relatively short period of time.
2. THE CAUSE OF BEARING CURRENTS IN
INVERTER DRIVES
The inverter output voltage is generated as a pulse
string, typically as shown in Fig. 1. The resultant
current is modified substantially by motor inductance
and consists basically of a sinewave at the fundamental
frequency.
When a voltage is synthesized by an inverter there
are a limited number of potential switch positions, and
the switching pattern of the semiconductors is
governed by a need to maintain acceptable switch
positions. This leads to a non-zero neutral voltage at
the inverter output, this instantaneous voltage sum is
called the common mode voltage.
Fig. 1. Typical PWM voltage and current motor waveform
Fig. 2 shows the inputs at the windings of a three
phase motor relative to earth (VU, VV, VW). The
common mode voltage is proportional to the d.c. link
voltage and has a frequency equal to the inverter carrier
frequency.
When a PWM inverter is connected to a motor
through a cable, the fast-changing voltage waveform
causes pulses of current to flow in all of the stray
capacitances at every voltage transition. The
capacitance of the motor cable phase conductors to the
earth conductor common mode voltage causes high
frequency currents to flow from the windings through
the stator lamination, the air gap, the rotor, the shaft
and the bearings to the frame.
Fig. 2. Typical line voltages for three phases and common
mode voltage
The most important capacitances in terms of bearing
current are those within the motor:
Csf between stator winding and motor frame
Csr between stator winding and rotor
Crf between rotor and frame.
The capacitive coupling can give rise to bearing
current through two different mechanisms, which are
generally referred to as capacitive discharge current
and shaft grounding current. These are illustrated by
the equivalent circuit in Fig. 3.
Fig. 3. Capacitive discharge current equivalent circuit
The inverter generated bearing currents may destroy
motor bearings within a short time of operation. The
bearing currents do not always flow through the
bearings of a motor, they can also destroy the bearing
of a load or a sensor connected to the shaft of a motor.
According to [3], motors controlled by frequency
converters may have bearing problem after only a
month of operation, and bearing currents are classified
as a serious problem among motor manufacturers [4].
The main reasons for inverter induced bearing
currents are the common mode voltage caused by the
influence of the fast voltage rise and fall times ( high
dv/dt) at the motor terminals [2].There is always a
nonzero common mode voltage present at the motor
terminals if a typical two-level frequency converter is
used to drive a motor. In certain multilevel converter
topologies, it is possible to choose modulation and
switching combination to obtain a zero common mode
voltage [5].However, this is not always practical or
possible, because it would have a negative effect on the
dynamic performance of a motor.
The parasitic capacitances are not important at low
frequencies, such as the normal operating frequency of
10-100 Hz in most applications. The carrier frequency
of an inverter can be up a few kilohertz, which can also
be considered to be a relatively low frequency. The
most important reason for common mode currents is
the fast switching of the IGBT (Insulated-Gate Bipolar
Transistor). The frequencies present caused by a high
dv/dt can reach up to several megahertz [6]. These fast
voltage rise and fall times generate high-frequency
currents that flow through parasitic capacitances. This
can lead to the emergence of different types of bearing
currents that can be further categorized into small
capacitive currents, EDM currents, circulating bearing
currents and rotor ground currents [7].
2.1. Small capacitive currents
The high dv/dt of the common GBT switches on or
off [9]. When bearing rollers are running at higher
motor speeds, there is a thin insulation layer between
the bearing rollers and rings. This insulating oil/grease
layer forms the bearing capacitance Cb in the bearing,
between the shaft and the frame. In this case, a
capacitive current ib is formed according next equation:
ib = Cb dvb/dt (1)
These capacitive currents are typically in the range
of 5 – 10 mA and they are usually considered to be
harmless because of the much smaller amplitude when
compared with the other types of bearing currents.
2.2. EDM bearing currents
Parasitic capacitances in the motor and the high
frequency of the common mode voltage can also cause
another type of bearing currents. The requirement is
that the motor is running at a sufficient speed for the
bearing rollers to roll on top of a lubrication film so
that the bearing capacitance is formed. Because of a
capacitive coupling of the winding to the rotor, the
bearing capacitance and other parasitic capacitances are
charged and the voltage at the shaft of the motor rises
to a certain value. If the charged voltage at the shaft
exceeds the breakdown voltage of the lubrication film,
the energy in the capacitances discharges through the
bearing and causes the flow of the EDM (Electric
Discharge Machining) current [2].
The bearing capacitance and the breakdown voltage
of the lubrication film are not constant, because the
lubricant film thickness depends on many factors, such
as fluid velocity, fluid viscosity, load acting on the
bearing, temperature, dielectric strength of the
lubricant, additives in the lubricant and surface of the
roughness of the bearing. For these reasons discharges
can occur randomly and are not directly linked to the
switching moments of the IGBT. The maximum EDM
bearing currents occur slightly below or at the rated
speed of the motor [10]. The lubricant thickness ranges
from 0,2 to 2,0 µm in the bearing and is estimated that
the dielectric strength of the lubricant is 15 V/ µm. This
leads to an assumption that shaft voltages between 3
and 30 V can be large enough to cause discharges in
the bearing.
The magnitude of the shaft voltage (in some articles
shaft voltage is referred to as bearing voltage) depends
on the common mode voltage, because it is determined
via a capacitance voltage divider from the common
mode voltage vcm, Fig. 3. This capacitance voltage
divider has been termed as BVR (Bearing Voltage
Ratio). The BVR is typically in the order of several %,
with an upper bound of 10% for typical machines.
2.3. High-frequency circulating bearing currents
High-frequency circulating bearing currents are
somewhat more complicated than the current types
introduced above, and they can be briefly explained as
follows: A high dv/dt of the voltage at the motor
terminals together with the parasitic capacitances
between the motor winding and the stator laminations
Cws cause an additional high-frequency common mode
current with frequencies up to several megahertz. The
current enters the motor through the windings and
leaves via the lamination and the frame. This current
excites a high frequency circular magnetic flux around
the motor shaft, which in turn induces a shaft voltage
along the shaft of a motor. If this induced shaft voltage
is high enough, it can discharge through the bearings
and generate a circulating current through the bearings,
the shaft and the frame of a motor, Fig. 4 [8].
Fig. 4.The path of the circulating bearing current
This phenomenon was first presented by Chen et al.
[2, 12] but later [13] and [14] have explained this kind
of a bearing in more detail.
2.4. Rotor ground currents
If the frame of an electric motor is poorly grounded,
rotor ground currents (sometimes referred to as shaft
grounding currents) can occur in the system. This may
place when the rotor ground impedance is significantly
lower then the grounding impedance of the motor
frame. Typically this means that the rotor is well
grounded through the driven load.
Fig. 5. The path of the rotor ground current
The generation mechanism of rotor ground currents
is as follows: The high dv/dt at the motor terminals
creates a high frequency common mode current to the
motor frame. If the rotor is better grounded than the
stator, this currents flow through the bearing of the
motor, the shaft and the load machine back to the
frequency converter, Fig. 5 [8].
3. BEARING DAMAGES CAUSED BY
BEARING CURRENTS
Damages caused by electric currents in bearings can
be divided into two categories, [15]. The first type of
damage is related to currents through a bearing without
sparking. The second type of damage is caused by
discharge currents.
The first type of damage is possible if a low-
resistivity lubricant is used, or a motor is running at
such low speed that there is no sufficient insulating
lubrication film in the bearings. In this case, bearing
currents may cause surface heating and decompose the
lubricant. These can accelerate bearing damages that
are mainly due to the dynamic effect of the rollers and
not due to electrical pitting.
In the second type of damage, the requirements are
that a high-resistivity lubricant is used and the motor is
running at a sufficient high speed so that there is a thin
insulating lubrication film between the rollers and the
races of the bearing. In this case, bearing currents cause
discharges (sparking) in the bearings if the shaft
voltage exceeds a critical threshold voltage of the
lubricant.
The damage of bearing race surface can be
described in general as follow: The puncturing of the
lubrication film is caused by short arcs, leaving small
craters on the smooth polished metallic surface. These
craters (pittings) are formed by the local melting of the
metal due to the arc current. These pittings can be seen
in Fig. 6, showing an average diameter of about 0,5
µm. As the balls roll over these craters, they flattened
again and vanish. In the meantime new craters are
created by further bearing current flow. So, bearing
current flow has to be understood as a current flow in
local contact points with very small diameters, this
yielding a very high local bearing current density. This
very high current density is reason for locally very high
temperature causing melting of metal and thus forming
the above noted craters.
For the eye, these small pittings are not visible, but
they lead to a grey, now reflecting trace, which is an
early indication of current flow through the bearings.
This grey trace may also be still visible after 1000 h
and more, but often a dramatic change in surface
deterioration occurs, the so-called fluting, [7]. Fluting
is a pattern of multiple grey line on bearing raceways
and is caused by the dynamic effect of the rolling
elements when they constantly rollover small craters,
Fig. 6 [23].
Fig. 6. Damages of the bearing
The threshold level at which the discharge occurs is
determined by the breakdown voltage of the lubrication
film. However, the breakdown voltage of the
lubrication film in the bearing is not constant, as the
thickness of the lubrication film is changing when the
motor is running. The reasons are for example
vibration, load variation, temperature changes, and the
surface finish of the bearing. For instance, the surface
roughness of the bearing races may cause random
puncturing of the lubrication film.
There are no exact limits for the shaft voltage or the
bearing current which, if exceeded, can cause bearing
damage. It is stated that shaft voltages above 200 mV
can increase discharges in the bearing of a motor [15].
The others in turn gives the following values: < 0,3 V
is safe, 0,5 – 1,0 V may be harmful and a shaft voltage
over 2 V may destroy the bearing [18]. There is a
recommendation for safe bearing current density on
contact area. Bearing current density below 0,1 A/mm²
do not have an effect on the bearing life, whereas
bearing current density over 0,7 A/mm² may shorten
the bearing life, [7].
Bearing sparking can also cause the lubricant in the
bearing to change its composition and degrade rapidly.
The local high temperature can cause burning or
charring of the base oil. Further, metal particles are
transferred to the lubricant because of electric
discharging. These issues can accelerate the
development of a bearing damage and finally lead to a
bearing failure.
It can be difficult to distinguish between electric
current damage and vibration damage. A feature of the
fluting caused by electric current is the dark bottom of
the corrugations, as opposed to the bright or rusty
appearance at the bottom of the vibration induced
fluting. Another distinguishing feature is the lack of
damage to the rolling elements of bearings with
raceway fluting caused by vibrations.
The only way of avoiding damage of this nature is
to prevent any electric current from passing through the
bearing.
4. MITIGATION TECHNIQUES OF BEARING
CURRENTS
Multiple techniques have been proposed to mitigate
harmful bearing currents [6,18,19]. Unfortunately,
most of them have certain drawbacks. They may be
difficult or impossible on existing systems and
typically always raise the cost of the system.
Applicability and effectiveness of these techniques also
depend upon the type and source of the bearing current
present in the system, [20]. Mitigation techniques can
be divided into tow classes depending on whether they
are applied on the inverter side or the motor. Inverter
output filters, such as dv/dt -reactors, dv/dt-filters,
sinusoidal filters and common mode chokes, special
cables, carrier frequency reduction are examples of
inverter-side mitigation techniques. Some of the
techniques used at the motor end are insulated
bearings, ceramic roller, slip-rings, brushes,
electrostatic shielding between the rotor and the stator
and conductive grease.
As mentioned formerly, electrical damage to the
bearings of VFD controlled motors can build over time.
In these cases the bearings eventually deteriorate to the
point of failure. To prevent such damage in the first
place, the induced shaft current must be kept from
discharging through the bearings by insulation or
providing and an alternate path to ground.
Insulating motor bearings may be accomplished by
either adding an insulating material to the bearing race
or bearing journal such as an insulating sleeve or
ceramic coating or using ceramic ball bearings. This
solution tends to shift the problem elsewhere as shaft
current looks for another path to ground. Then, the
other equipment often winds up with bearing damage
of its own. Finally, insulation and other bearing-
isolation strategies may be costly to implement, may
result in special motor modification and may be only
partially effective.
Alternate discharge paths when properly
implemented, a conductive link is established between
the rotor and stator, usually in the form of a brush.
These strategies are preferable to insulation because
they provide an alternate discharge path for the shaft
voltage and prevent bearing current. The ideal solution
would provide a very low resistance path from shaft to
frame, would be low cost, and could be broadly applied
across all VFD/AC motor applications, affording the
greatest degree of bearing protection and maximum
return on investment.
4.1. Inverter output filters
dv/dt –reactors
The principal design of dv/dt – reactors comprise
three coils that are inserted into the motor phase, Fig. 7.
The function of these reactors is to increase the voltage
rise time and to reduce thereby the voltage change rate
dv/dt. The rise time increases with inductivity Lph of
reactor, therefore the rated voltage drop at the reactor
vk, determined at rated motor frequency, has to be
relatively large, typically between vk = (2…8) % rated
motor voltage.
Fig. 7. Principal design of dv/dt–reactor
These dv/dt – reactors have originally been designed
to reduce capacitive currents in the motor cable. As the
phase capacitance increases with cable length, these
reactors are recommended especially for long motor
cables. In this case the reactor reduces the rise time of
the voltage pulse, resulting in a smaller value for the
capacitive leakage current.
This is a typical solution when cable lengths are
some hundred meters long. If the total sum of cable
lengths exceeds 500 meters (as in the case of
multimotor applications fed by a single inverter), a
sinusoidal filter is recommended.
Sinusoidal filters
Sinusoidal filters consist of three phase reactors
(with phase inductivity Lph) and condensers (with line-
to-line capacity CLL) forming a low pass filter, Fig. 8.
Voltages with frequencies below the resonance
frequency f0 of filter pass almost without reduction of
amplitude, whereas voltage signals with f > f0 are
diminished with 40 dB/decade.
Fig. 8. Principal design of a sinusoidal filter
Usually, these filters are designed in the following
way: The resonance frequency f0 must be well above
the fundamental frequency of the inverter that operates
the motor and well below the inverter switching
frequency to reduce high frequency current ripples,
which may cause additional motor losses and noise.
A sinusoidal filter acts mainly on the line-to-line
voltage which is nearly ideally sinusoidal at the motor
terminals whereas the common mode voltage is not
suppressed.
Sinusoidal filters can be used in step-up the
applications. Here an output transformer is fed from a
inverter to step-up the into the medium voltage range.
The filter reduces the high frequency components that
would otherwise saturate the transformers magnetic
circuit while also generating voltage reflections at the
transformers high voltage secondary and the motor
cables. The filter, therefore, protects the medium
voltage motors. Inherently, motor noise levels decrease
because of the sinusoidal voltage. Large reactors in the
sinusoidal filter cause typically some 5 – 7 % voltage
drop which means that normally the voltage of the
inverter needs to be boosted to correctly flux the motor.
Also in the field weakening range the pull-out torque
drops faster then without the filter. Basically, field-
weakening operation should be avoided. Sinusoidal
filters are usually very large and expensive, as the full
apparent power of the voltage harmonics determines
the voltage rating.
dv/dt – filters
These filters are RLC components with resistances
added to attenuate the high frequency components, Fig.
9.
The dv/dt – filters are often complex circuits, some
including connections to the dc-link of the inverter,
diodes, and/or capacitances to the grounding
connection. The main difference in comparison with
sinusoidal filters is, that the resonance frequency is
much higher than the inverter switching frequency due
to usually small values of L and C.
Fig. 9. Principal design of a dv/dt – filter and a commercially
available on the market
The dv/dt – filters are the passive devices that
reduce transmissions line effects of motor cables. It
accomplishes this by dampening the rate of voltage
increase and minimizes the peak voltage that occurs at
the motor terminals, Fig. 10. These filters usually have
small insertion impedance which ensures motor torque
is not affected by added voltage drops from the filter.
An advantage of dv/dt – filters is that, due to the small
L and C, the filter itself is small and rather cheep.
Fig. 10. Output voltage without (left) and with (right) dv/dt –
filter
Common mode filters
These filters are designed to eliminate the high
frequency components of the common mode voltage.
These filters are complex circuits and many different
designs have been proposed. The principal design of a
common mode filter is shown at Fig. 11, and waveform
of output voltage at inverter and filter at Fig. 12, [24].
Tipical applications these filters are with extremely
long cables, without causing radiation problems, with
unshielded cables and multiple motors in parallel. The
benefit of these filters is conversion of the PWM output
signal of motors drives into a smooth sine wave with
low residual ripple. They, also, reduce the required
EMI suppression efforts on the line side.
Fig. 11. Principal design of a common mode filter
Fig. 12. Waveform of output voltage at inverter and at
common mode filter [24]
4.2. Cable selection
The fast switching speeds of the solid-state devices
in VFD increase the potential for EMI (electromagnetic
interference). The electronics of the drives generate RF
energy, which can be radiated by the inverter-to-motor
cable and picked up by nearby circuits. This
interference can cause malfunctions in sensitive
electronics. As a result, VFD manufacturers usually
recommended the use of a shielded power cable
between the inverter and the motor to contain this RF
energy.
According to some authors [7, 23], the bearing
current can be minimized by using an inverter-to motor
shielded power cable with three symmetrically
positioned grounding conductors in combination with a
low impedance shield. However, it is shown by
measurements that shielded cable increases circulating
currents, because the lower grounding impedance of a
shielded cable increases stator grounding currents,
which results in a higher magnetic flux and hence a
higher shaft voltage.
The cable selection have not effect on the EDM
bearing currents, because they are not related to the
stator ground currents. However, the high frequencies
generated by the VFD, coupled with long cable runs
between the VFD and the motor, can result in reflected
waves in the cable. Reflected voltages of up to 2,5
times the normal system voltage have been measured
on the VFD power cable. Because of these high voltage
pulses and the radiated EMI as the cable for VFD
application is recommended a three-conductor,
impervious corrugated aluminium armor cable with
three symmetrically positioned grounding conductors
as illustrated below, Fig. 13.
Fig. 13. The cable for VFD application
4.3. Multilevel inverters
The concept of a multilevel inverter is to use a series
of power semiconductors switches with several lower
voltage dc sources to perform the power conversion by
synthesizing a staircase voltage waveform. A
multilevel inverter has several advantages over a
conventional two-level inverter that uses high
switching frequency pulse width modulation. The
advantages are that a multilevel inverter produce
smaller common mode voltage which can be
eliminated by using advanced modulation strategies
These inverters can reduce the dv/dt stresses, therefore
EMI problems can be reduced. Unfortunately,
multilevel inverter do have some disadvantages. One
particular disadvantage is that the overall system is
very complex and expensive.
4.4. Insulated bearing
Electrical insulating of the bearings blocks the
bearing current path, thus preventing damage of the
bearings (Fig. 14). The insulating material, usually a
nonconductive resin or ceramic layer, isolates the
bearings and prevents shaft current from discharging
through them to the frame. Sometimes, high frequency,
VFD induced currents may be capacitively coupled
through the insulating layer and cause bearing damage
inside the bearing anyway. Because the insulation
thickness has to be adequate to properly insulate high
frequency bearing currents. It is proposed that the
insulating coat has to be at least 250 µm to sufficiently
reduce the high frequency circulating current.
Fig. 14. Insulating bearing showing the insulation layer
However, insulating the bearing may shift the
problem elsewhere, for example to the bearings of
pump or tachometer or even to the load. This may
occur, for example, when there are EDM currents
present and the bearing of a motor are insulated. If only
one end is insulated, the noninsulated bearing may fail
twice as fast. If both bearings are insulated and there is
a galvanic connection through shaft to the load
machine, the bearings of the load machine may fail
instead. In the case of circulating currents, it suffices
if only one end of the motor is properly insulated.
Another drawback is the potential for contaminated
insulation, which can, over time, establish a current
path around the insulation, allowing current flow
through the bearing. A disadvantage of insulated
bearing can be 2 -10 times the price of a normal one.
The insulating of bearing journals, due to high cost, is
generally limited to lager sized IEC motors.
4.5. Ceramic bearings
Ceramic bearings are rotary bearings constructed of
ceramic. Ceramic hybrid bearings, the most common
type of ceramic bearing, have rings of bearing steel and
rolling elements of bearing grade silicon nitride
(Si3N4), Fig. 15 [25]. Because silicon nitride has high
resistivity, hybrid bearings provide insulation from
electric currents in both AC and DC motors.
Fig. 15. Hybrid bearing
These bearings have several advantages over all
steel bearings, such as higher speed and acceleration
capability, increased stiffness, higher accuracy, lower
friction and heat generation, reduced lubrication
requirements, low thermal expansion and extended
operation life. The use of nonconductive ceramic balls
prevents the discharge of shaft current through the
bearings. As with other isolation measures, shaft
current will seek an alternate path to ground possibly
through equipment connected to the motor.
Such bearings are very costly and, in most cases,
motors with ceramic bearings must be special ordered
and so have long lead times.
4.6. Brushes or slip-rings
The well-known technique for the shaft voltage
mitigation is with brushes or slip-rings. In this method,
a shaft brush, which connects the shaft to the frame of
motor can be added to a motor bearing to provide a
preferred path for bearing currents around the bearing.
If the brush impedance is low enough, all shaft to
ground currents will flow through this brush and will
avoid an alternate path through the motor bearing
without the brush. Fig. 16 shows the current flow paths
with a brush on one bearing [8].
Fig. 16. Current flow paths with a brush on drive end
bearing
Ideally, brushes work perfectly against EDM and
circulating currents. However, field experience has
shown that it is hard to maintain low-resistance contact
to the ground because of brush wear and the build-up
of an oxide layer. This leads to a need for regular
maintenance. A solution for these problems is proposed
in [3], where a conductive microfiber ring is presented,
Fig. 17 [26]. This is a brush made of millions of
conductive microfibers. According to [3], this
technique involves hardly any direct frictional wear
and can be considered as maintenance free and robust
against contamination.
Fig. 17. Microfiber ring on drive end bearing
5. CONCLUSIONS
It is important to understand that modern PWM type
VFD are being utilized the dynamics of the power
system change from low frequency conditions (50 Hz).
The lack of understanding of high frequency behavior,
in what has traditionally been a low frequency circuit,
is a major contributor to many of the field problems
involving PWM type VFD.
The common source of all inverter-induced bearing
currents is the common mode voltage of the inverter.
As the causal chain of different bearing current
phenomena is different, mitigation techniques have to
be chosen according to the type of bearing current that
shall be reduced or eliminated.
a) It is important to note that not all output filters
perform as well in minimizing common mode voltages
as others. Some of the more common VFD output
filters utilized are intended to primary address voltage
spike issues and may not be sufficient to address
bearing current problems. In some severe installations,
a dv/dt filter may need to be used in addition to a
common mode filter. Voltage levels delivered to the
motor terminals need to be checked since the VFD and
the filters can both drop the output voltage to un
acceptable levels.
b) Insulated bearings can be useful in sine wave and
VFD installations where circulating currents are
flowing and damaging the bearings or connected
equipment. Insulated bearings block the path that
currents want to take to flow back to the source ground,
but they do not eliminate the voltage from seeking
other path to ground. Current flow through the
remaining paths to ground may increase. Insulating one
or both bearing of a motor may require a shaft
grounding system be added to the system and/or an
insulated equipment coupling to avoid other potential
failure modes.
c) Shaft grounding systems, when used as a stand
alone option, provide an additional path for shaft
voltage to flow back to the ground of the power source.
The hope is this path will have lower impedance so the
majority of current will flow through the grounding
system instead of the other paths. A single shaft
grounding system will not protect both bearing from
circulating currents, but will tend to protect both
bearings from EDM or static discharge. Proper
selection of the shaft grounding material, the
establishing a proper grounding of the motor frame are
important variables in using this possible solution.
d) Cabling with PWM type power cable is intended to
minimize the common mode voltage by providing a
better high frequency return path to the VFD power
source. In addition, it can reduce cable capacitance to
earth ground, help limit EMI and provide a longer
cable insulation life.
Grounding practices and products better suited to
high frequency application are important. Flat cables
for bonding equipment, multiple smaller diameter
conductors to provide circuit ground, 360 degree
terminations, aluminium jacketed power cables with
shields are just some installation considerations when
PWM type power units are utilized
6. ACKNOWLEDGMENT
These results are part of a project financed by the
Ministry of Science and Technological Development,
Republic of Serbia. (TR33016).
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Correspondence to
Miroslav BJEKIĆ
[email protected], Technical faculty in Čačak -
University of Kragujevac, Serbia
Danilo STOJANOVIĆ
[email protected], Technical faculty in
Čačak - University of Kragujevac, Serbia
Marko ROSIĆ
[email protected], Technical faculty in Čačak - University
of Kragujevac, Serbia
Miloš BOŽIĆ
[email protected],Technical faculty in Čačak - University
of Kragujevac, Serbia
