abb bearing currents abb technical guide 5

Bearing Currents inModern AC Drive Systems

Technical Guide No. 5 Technical Guide No. 5

Technical Guide No.5 - Bearing currents in modern AC drive systems2

3Technical Guide No.5 - Bearing currents in modern AC drive systems

1 Introduction ...............................................................General ........................................................................Avoiding bearing currents ............................................

2 Generating Bearing Currents ....................................High frequency current pulses .....................................Faster switching ..........................................................How are HF bearing currents generated? ....................

Circulating current ...................................................Shaft grounding current ...........................................Capacitive discharge current ...................................

Common mode circuit .................................................Stray capacitances .....................................................How does the current flow through the system? .......Voltage drops ............................................................Common mode transformer .......................................Capacitive voltage divider .........................................

3 Preventing high frequency bearing currentdamage .....................................................................Three approaches......................................................

Multicore motor cables ..........................................Short impedance path ...........................................High frequency bonding connections .....................

Follow product specific instructions ..........................Additional solutions ...............................................

Measuring high frequency bearing currents ...............Leave the measurements to the specialists ..............

4 References ...............................................................

5 Index .........................................................................

Contents

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Some new drive installations can have their bearings fail onlya few months after start-up. Failure can be caused by highfrequency currents, which flow through the motor bearings.

While bearing currents have been around since the advent ofelectric motors, the incidence of damage they cause hasincreased during the last few years. This is because modernvariable speed drives with their fast rising voltage pulses andhigh switching frequencies can cause current pulses throughthe bearings whose repeated discharging can gradually erodethe bearing races.

To avoid damage occurring, it is essential to provide properearthing paths and allow stray currents to return to the inverterframe without passing through the bearings. The magnitude ofthe currents can be reduced by using symmetrical motor cablesor inverter output filtering. Proper insulation of the motor bearingconstruction breaks the bearing current paths.

Chapter 1 - Introduction

General

Avoidingbearingcurrents


Bearing currents come in several different guises. However,while modern motor design and manufacturing practices havenearly eliminated the low frequency bearing currents inducedby the asymmetry of the motor, the rapid switching in modernAC drive systems may generate high frequency current pulsesthrough the bearings. If the energy of these pulses issufficiently high, metal transfers from the ball and the racesto the lubricant. This is known as electrical dischargemachining or EDM. The effect of a single pulse is insignificant,but a tiny EDM pit is an incontinuity that will collect morepulses and expand into a typical EDM crater. The switchingfrequency of modern AC drives is very high and the vastnumber of pulses causes the erosion to quickly accumulate.As a result, the bearing may need replacing after only a shorttime in service.

High frequency bearing currents have been investigated byABB since 1987. The importance of system design has beenhighlighted in the last few years. Each individual item involved,such as the motor, the gearbox or the drive controller, is theproduct of sophisticated manufacturing techniques andnormally carries a favourable Mean Time Between Failure(MTBF) rate. It is when these components are combined andthe installed system is looked upon as a whole, that itbecomes clear that certain installation practices are required.

Figure 1: Bearing currents can cause “bearing fluting”, a rhythmicpattern on the bearing’s races.

Current AC drive technology, incorporating Insulated GateBipolar Transistors (IGBT), creates switching events 20 timesfaster than those considered typical ten years ago. Recent yearshave seen a rising number of EDM-type bearing failures in ACdrive systems relatively soon after start up, within one to sixmonths. The extent to which this occurs depends on the ACdrive system architecture and the installation techniques used.

Chapter 2 - Generating Bearing Currents

High frequencycurrent pulses

Faster switching


The source of bearing currents is the voltage that is inducedover the bearing. In the case of high frequency bearing currents,this voltage can be generated in three different ways. The mostimportant factors that define which mechanism is prominent,are the size of the motor and how the motor frame and shaftare grounded. The electrical installation, meaning a suitablecable type and proper bonding of the protective conductorsand the electrical shield, plays an important role. Du/dt of theAC drive power stage components and the DC-link voltagelevel affect the level of bearing currents.

In large motors, high frequency voltage is induced between theends of the motor shaft by the high frequency flux circulatingaround the stator. This flux is caused by a net asymmetry ofcapacitive current leaking from the winding into the stator framealong the stator circumference. The voltage between the shaftends affects the bearings. If it is high enough to overcome theimpedance of the bearings’ oil film, a current that tries tocompensate the net flux in the stator starts to flow in the loopformed by the shaft, the bearings and the stator frame. Thiscurrent is a circulating type of high frequency bearing current.

The current leaking into the stator frame needs to flow back tothe inverter, which is the source of this current. Any routeback contains impedance, and therefore the voltage of themotor frame increases in comparison to the source groundlevel. If the motor shaft is earthed via the driven machinery,the increase of the motor frame voltage is seen over thebearings. If the voltage rises high enough to overcome theimpedance of the drive-end bearing oil film, part of the currentmay flow via the drive-end bearing, the shaft and the drivenmachine back to the inverter. This current is a shaft groundingtype of high frequency bearing current.

In small motors, the internal voltage division of the commonmode voltage over the internal stray capacitances of the motormay cause shaft voltages high enough to create high frequencybearing current pulses. This can happen if the shaft is notearthed via the driven machinery while the motor frame isearthed in the standard way for protection.

High frequency bearing currents are a consequence of the currentflow in the common mode circuit of the AC drive system.

A typical three-phase sinusoidal power supply is balanced andsymmetrical under normal conditions. That is, the vector sumof the three phases always equals zero. Thus, it is normal thatthe neutral is at zero volts. However, this is not the case witha PWM switched three-phase power supply, where a dc voltageis converted into three phase voltages. Even though the

How are HFbearing currentsgenerated?

Generating Bearing Currents

Circulatingcurrent

Shaft groundingcurrent

Capacitivedischarge current

Common modecircuit


fundamental frequency components of the output voltages aresymmetrical and balanced, it is impossible to make the sumof three output voltages instantaneously equal to zero withtwo possible output levels available. The resulting neutral pointvoltage is not zero. This voltage may be defined as a commonmode voltage source. It is measurable at the zero point of anyload, eg. the star point of the motor winding.

Figure 2: This schematic shows the phase voltages of a typical threephase PWM power supply and the average of the three, or neutralpoint voltage, in a modern AC drive system. The neutral voltage isclearly not zero and its presence can be defined as a common modevoltage source. The voltage is proportional to the DC bus voltage,and has a frequency equal to the inverter switching frequency.

Any time one of the three inverter outputs is changed fromone of the possible potentials to another, a current proportionalto this voltage change is forced to flow to earth via the earthcapacitances of all the components of the output circuit. Thecurrent flows back to the source via the earth conductor andstray capacitances of the inverter, which are external to thethree phase system. This type of current, which flows throughthe system in a loop that is closed externally to the system, iscalled common mode current.



Figure 3: An example of the common mode current at the inverteroutput. The pulse is a superposition of several frequencies due tothe different natural frequencies of the parallel routes of commonmode current.

A capacitance is created any time two conductive componentsare separated by an insulator. For instance, the cable phasewire has capacitance to the PE-wire separated by PVCinsulation, for example, and the motor winding turn is insulatedfrom the frame by enamel coating and slot insulation, and sohas a value of capacitance to the motor frame. Thecapacitances within a cable and especially inside the motorare very small. A small capacitance means high impedancefor low frequencies, thus blocking the low frequency straycurrents. However, fast rising pulses produced by modernpower supplies contain frequencies so high that even smallcapacitances inside the motor provide a low impedance pathfor current to flow.

Figure 4: Simplified loop of the common mode current of a PWMinverter and induction motor. The inverter power supply acts as a


Straycapacitances


common mode voltage source (V cm). Common mode current (CMC)flows through the common mode cable and motor inductances, L cLm and through the stray capacitances between the motor windingsand motor frame, combined to be C m. From the motor frame, thecurrent proceeds through the factory earth circuit which has theinductance L g. Lg is also fed common mode current from the straycable capacitance C c. The inverter frame is connected to the factoryearth and couples the common mode current/earth currents throughstray inverter to frame capacitances, combined as C in, back to thecommon mode voltage source.

The return path of the leakage current from the motor frameback to the inverter frame consists of the motor frame, cableshielding or PE-conductors and possibly steel or aluminiumparts of the factory building structure. All these elementscontain inductance. The flow of common mode current throughsuch inductance will cause a voltage drop that raises the motorframe potential above the source ground potential at theinverter frame. This motor frame voltage is a portion of theinverter’s common mode voltage. The common mode currentwill seek the path of least impedance. If a high amount ofimpedance is present in the intended paths, like the PE-connection of the motor frame, the motor frame voltage willcause some of the common mode current to be diverted intoan unintended path, through the building. In practicalinstallations a number of parallel paths exist. Most have aminor effect on the value of common mode current or bearingcurrents, but may be significant in coping with EMC-requirements.

If the value of this inductance is high enough, the reactanceat the upper range of typical common mode currentfrequencies, 50 kHz to 1 MHz, can support voltage drops ofover 100 volts between the motor frame and the inverter frame.If, in such a case, the motor shaft is connected through ametallic coupling to a gearbox or other driven machinery thatis solidly earthed and near the same earth potential as theinverter frame, then it is possible, that part of the invertercommon mode current flows via the motor bearings, the shaftand the driven machinery back to the inverter.

How does thecurrent flowthrough thesystem?


Voltage drops


i+∆ i

i-∆ i

∆ i

∆ iD N

Figure 5: A schematic presentation showing the circulatingcurrent and shaft grounding current, the latter resulting from highmotor frame voltage with superior machine earthing.

If the shaft of the machinery has no direct contact to the groundlevel, current may flow via the gearbox or machine bearings.These bearings may be damaged before the motor bearings.

Figure 6: Source of circulating high frequency bearing current. Currentleakage through distributed stator capacitances gives a non-zerocurrent sum over the stator circumference. This leads to a netmagnetising effect and flux around the motor shaft.

The largest share of the motor’s stray capacitance, is formedbetween the stator windings and the motor frame. Thiscapacitance is distributed around the circumference and lengthof the stator. As the current leaks into the stator along the coil,the high frequency content of the current entering the statorcoil is greater than the current leaving.


Commonmodetransformer


This net current produces a high frequency magnetic flux thatwill circulate in the stator laminations, inducing an axial voltagein the shaft ends. If the voltage becomes large enough, a highfrequency circulating current can flow, internal to the motor,through the shaft and both bearings. The motor can, in thiscase, be thought of as a transformer, where the common modecurrent flowing in the stator frame acts as a primary and inducesthe circulating current into the rotor circuit or secondary. Thisbearing current is considered to be the most damaging withtypical peak values of 3 to 20 amps depending on the ratedpower of the motor, du/dt of the AC drive power stagecomponents and DC-link voltage level.

Figure 7: The high frequency axial shaft voltage can be thought ofas the resultant of a transformer effect, in which the common modecurrent flowing in the stator frame acts as a primary, and inducesthe circulating current into the rotor circuit or secondary.

Another version of circulating bearing current occurs when,the current, instead of circulating completely inside the motor,flows via the shaft and the bearings of the gearbox or drivenmachinery and in a structural element that is both external andcommon to the motor and the driven machine. The origin of thecurrent is the same as in the current circulating inside themotor. An example of this “vagabond” circulating bearing currentis shown in figure 8.



Figure 8: “Vagabond” circulating bearing current, where the currentloop is external to the motor.

Other stray capacitances are also present in the motor, suchas the capacitance between the stator windings and the rotor,or that existing in the motor’s airgap between the stator ironand the rotor. The bearings themselves may even have straycapacitance.

The existence of capacitance between the stator windings andthe rotor effectively couples the stator windings to the rotoriron, which is also connected to the shaft and the bearing’sinner races. Fast changes in the common mode current fromthe inverter can not only result in currents in the capacitancearound the circumference and length of the motor, but alsobetween the stator windings and the rotor into the bearings.

Figure 9: Common mode loop of variable speed drive, showingstator, rotor and bearing stray capacitances.

Capacitivevoltagedivider



The current flow into the bearings can change rapidly, as thisdepends on the physical state of the bearing at any one time.For instance, the presence of stray capacitance in the bearingsis only sustained for as long as the balls of the bearings arecovered in oil or grease and are non-conducting. Thiscapacitance, where the induced shaft voltage builds up, canbe short-circuited if the bearing voltage exceeds the thresholdof its breakover value or if a “high spot” on a ball breaks throughthe oil film and makes contact with both bearing races. At verylow speed, the bearings have metallic contact since the ballshave not risen on an oil film.

Generally, the bearing impedance governs the voltage level atwhich the bearings start to conduct. This impedance is a non-linear function of bearing load, temperature, speed of rotationand lubricant used, and the impedance varies from case tocase.



There are three approaches used to affect high frequencybearing currents: a proper cabling and earthing system; breakingthe bearing current loops; and damping the high frequencycommon mode current. All these aim to decrease the bearingvoltage to values that do not induce high frequency bearingcurrent pulses at all, or damp the value of the pulses to a levelthat has no effect on bearing life. For different types of highfrequency bearing currents, different measures need to be taken.

The basis of all high frequency current mastering is the properearthing system. Standard equipment earthing practices aremainly designed to provide a sufficiently low impedanceconnection to protect people and equipment against systemfrequency faults. A variable speed drive can be effectivelyearthed at the high common mode current frequencies, if theinstallation follows three practices:

Use only symmetrical multicore motor cables. The earth(protective earth, PE) connector arrangement in the motor cablemust be symmetrical to avoid bearing currents at fundamentalfrequency. The symmetricity of the PE- conductor is achievedby a conductor surrounding all the phase leads or a cable thatcontains a symmetrical arrangement of three phase leads andthree earth conductors.

Figure 10: Recommended motor cable with symmetrical coreconfiguration.

Chapter 3 - Preventing high frequencybearing current damage

Multicore motorcables

Threeapproaches


Define a short, low impedance path for common mode currentto return to the inverter. The best and easiest way to do this isto use shielded motor cables. The shield must be continuousand of good conducting material, i.e. copper or aluminium andthe connections at both ends need to be made with 360°termination.

Figures 11a and 11b show 360° terminations for European andAmerican cabling practices.

Figure 11 a: Proper 360° termination with European cablingpractice. The shield is connected with as short a pigtail aspossible to the PE terminal. To make a 360° high frequencyconnection between the EMC sleeve and the cable shield, theouter insulation of the cable is stripped away.

Figure 11 b: Proper 360° termination with American cablingpractice. An earthing bushing should be used on both ends of themotor cable to effectively connect the earth wires to the armour orconduit.

Preventing high frequency bearing current damage

Short impedancepath



Add high frequency bonding connections between theinstallation and known earth reference points to equalise thepotential of affected items, using braided straps of copper 50 -100mm wide; flat conductors will provide a lower inductancepath than round wires. This must be made at the points wherediscontinuity between the earth level of the inverter and thatof the motor is suspected. Additionally it may be necessary toequalise the potential between the frames of the motor and thedriven machinery to short the current path through the motorand the driven machine bearings.

Figure 12: HF Bonding Strap.

Although the basic principles of installations are the same, fordifferent products suitable installation practices may differ.Therefore, it is essential to carefully follow the installationinstructions given in product specific manuals.

Breaking the bearing current loops is achieved by insulatingthe bearing construction. The high frequency common modecurrent can be damped by using dedicated filters. As amanufacturer of both inverters and motors, ABB can offer themost appropriate solution in each case as well as detailedinstructions on proper earthing and cabling practices.

High frequencybonding

connections

Follow productspecificinstructions

Additionalsolutions


Monitoring the bearing condition must be conducted withestablished vibration measurements.

It is impossible to measure bearing currents directly from astandard motor. But if high frequency bearing currents aresuspected, field measurements can be taken to verify theexistence of suspected current loops. Measuring equipmentneeds to have wide bandwidth (minimum 10kHz to 2 MHz)capable of detecting peak values of at least 150 to 200A andRMS values at least down to 10mA. The crest factor ofmeasured signals is seldom less than 20. The current mayflow in unusual places, such as rotating shafts. Thus, specialequipment and experienced personnel are needed.

ABB uses a specially designed, flexible, air-cored, Rogowski-type current sensor with dedicated accessories and has vastexperience of over one thousand measured drives in differentapplications worldwide.

The most important measurement points are within the motor.During measurements, the motor speed needs to be at least10% of the nominal for the bearings to rise on the oil film. Asan example, basic measurements are shown in figure 13. Figure14 shows examples of measured current waveforms. GTOinverters were used mainly in the 1980s and IGBT invertersare used today. Note the different scale in the various graphs.

Figure 13: Basic measurements: A) circulating current measuredwith a jumper, B) shaft grounding current.

Measuringhighfrequencybearingcurrents



A) Circulating current

GTO-inverter, 5 mm mm m s/div, 2A/div IGBT-inverter, 5 mm mm m s/div, 2A/div

B) Shaft grounding current

GTO-inverter, 2 mm mm m s/div, 10A/div IGBT-inverter, 5 mm mm m s/div, 500mA/div

Figure 14: Examples of current waveforms at the measuringpoints shown in Figure 13.

Since suitable commercial measurement equipment is notavailable on the market and specialised experience is neededto make the measurements and interpret the results, it isadvisable that bearing current measurements are made bydedicated personnel only.


Leave themeasurementsto thespecialists


Chapter 4 - References

1. Grounding and Cabling of the Drive System,ABB Industry Oy, 3AFY 61201998 R0125

2. A New Reason for Bearing Current Damage in VariableSpeed AC Drivesby J. Ollila, T. Hammar, J. Iisakkala, H. Tuusa. EPE 97, 7thEuropean Conference on Power Electronics andApplications, 8-10 September 1997. Trondheim, Norway.

3. On the Bearing Currents in Medium Power Variable SpeedAC Drivesby J. Ollila, T. Hammar, J. Iisakkala, H. Tuusa. proceedingsof the IEEE IEDMC in Milwaukee, May 1997.

4. Minimizing Electrical Bearing Currents in Adjustable SpeedDrive Systemsby Patrick Link. IEEE IAS Pulp & Paper ConferencePortland, ME, USA. June 1998.

5. Instruction on Measuring Bearing Currents with a RogowskiCoil, ABB Industry Oy, 3BFA 61363602.EN.

6. Laakerivir ta ja sen minimoiminen säädettyjenvaihtovirtakäyttöjen moottoreissa,I. Erkkilä, Automaatio 1999, 16.9.1999, Helsinki, Finland.(In Finnish).

7. High Frequency Bearing Currents in Low VoltageAsyncronous Motors,ABB Motors Oy and ABB Industry Oy, 00018323.doc.

8. Bearing Currents in AC Drivesby ABB Industry Oy and ABB Motors Oy. Set of overheadsin LN database “Document Directory Intranet” onABB_FI01_SPK08/FI01/ABB

9. The Motor GuideGB 98-12.

See also product specific installation manuals.


Chapter 5 - Index

360° termination 16AABB 17, 18AC drive 6, 7, 8armour 16axial shaft voltage 12, 13axial voltage 12Bball 14bearing current loops 15, 17bearing current paths 5bearing currents 5, 6, 7, 12, 15,18, 19bearing fluting 6bearing races 5bearing voltage 14bearings 5, 6, 7, 12, 13, 14bonding connections 17braided straps 17Ccable 15cable capacitance 10cable shield 16circulating current 12common mode cable 10common mode circuit 7common mode current 8, 9, 10, 11, 12, 13, 15, 16, 17Common Mode Loop 9, 13common mode voltage 7, 8, 10conduit 16crest factor 18current pulses 5DDC bus voltage 8dedicated filters 17drive controller 6driven machine 7, 17driven machinery 7, 10, 12Eearthing paths 5EDM crater 6electric motors 5electrical discharge machining(EDM) 6electrical shield 7Ffield measurements 18flat conductors 17frame 17Ggearbox 6, 10, 12GTO inverters 18

Hhigh frequency bearingcurrents 6, 7High frequency bearingvoltage 7high frequency circulatingcurrent 12high frequency currentmastering 15high frequency flux 7high switching frequencies 5IIGBT inverters 18induced shaft voltage 14Insulated Gate BipolarTransistors (IGBT) 6internal voltage division 7inverter 7, 8, 9, 10, 13, 16, 17inverter frame 5, 10inverter output filtering 5inverter power supply 9inverter switching frequency 8Llow frequency bearingcurrents 6Mmagnetic flux 12Mean Time Between Failure(MTBF) 6metallic coupling 10motor 6, 7, 9, 10, 11, 12, 13, 15,17, 18motor bearings 5motor cable 15, 16motor frame 7, 9, 10, 11motor shaft 5, 7, 10motor windings 10Nneutral point voltage 8Ooil film 7, 18Pprimary 12PWM 7, 9Rraces 6, 14Rogowski-type currentsensor 18rotor 12, 13rotor circuit 12Ssecondary 12shaft 7, 12, 13


shaft ends 12shaft voltages 7shield 16stator 7, 11, 13stator frame 7, 11, 12stator laminations 12stator windings 11, 13stray capacitance 7, 8, 10, 11,13, 14stray currents 5symmetrical motor cables 5, 15Tthree phase power supply 7, 8transformer 12Vvariable speed drive 5, 13, 15voltage drop 10voltage pulses 5Wwinding 7, 8, 9, 10, 11, 13

ABB Industry OyDrivesP.O. Box 184FIN-00381 HelsinkiFINLANDTel: +358 10 222 000Fax: +358 10 222 2681Internet: http://www.abb.com/automation

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abb bearing currents abb technical guide 5

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