A Practical Guide to Understanding Bearing Damage Related to Pwm Drives_cnf

8/3/2019 A Practical Guide to Understanding Bearing Damage Related to Pwm Drives_cnf

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A PRAC TICAL GU IDE TO UNDERSTANDING IBEARING DAMA GERELATED TO PWM DRIVES

Don MacdonaldIEEE Member

Toshiba International C orporationAbstract - The performance and reliability of AC AdjustableSpeed Drives (ASDs) is continually improving. One of thekey reasons for improvement has been the advent,development and use of Pulse Width Modulated (PWM)drives utilizing faster switching devices, primarily InsulatedGate Bipolar Transistors (IGBTs). As with many otherdevelopments, improvements in some areas may causeproblems in others. An increased bearing failure rate inmotors is one of the negative effects of these types of drives.

To mitigate bearing current damage in motors, as wellas in loads and other auxiliary equipment attached to themotor shaft, it is important to understand how these currentsare generated.

In addition to theoretical explanations, actual fieldcases and solutions will be reviewed.

I . INTRODUCTION

The phenomenon of motor shaft voltages producingcirculating shaft currents has been recognized since the1920s. When a motor is operated by sinusoidal power,shaft voltages are caused by alternating flux linkages withthe shaft. The linkages are associated with flux unbalancecaused by:

rotor static or dynamic eccentricityrotor and stator slottingaxial cooling holes in the stator and/or rotorlaminationsshaft keywaysrotor core support armsjoints between segmental laminationsdirectional properties of magnetic materialssupply unbalancetransient conditions.

Shaft voltages exceeding 300mV require one bearingof the motor to be insulated to prevent circulating currentdamage to the bearings (see Fig. 1). Typically t h sphenomenon only occurs on 500 frames and largermachines.2 Normally the Opposite Drive End (ODE)bearing is chosen. If the Drive End (DE) is insulated, thedriven load can provide an electrical path that completesthe loop to allow current to flow.

PWM drives can cause increased circulating currentsto flow due to a high-frequency flux produced bycommon-mode currents which link the stator, rotor andbearing loop. This is an inductive rather than capacitivee f f e ~ t . ~ otors become more asymmetrical at highfrequencies because the hgh-frequency capacitivelycoupled currents depend heavily on the location of the first

Will GrayIEEE Member

Toshiba International Corporation

Statorf-

BaseFig. 1 Inductive circulating currents

few turns withm the slot. Since placement of the turns inrandom-wound motors is not well controlled by anymanufacturer, even a motor which is symmetrical at lowfrequencies becomes asymmetrical at high fre q~e ncie s.~

In addition to the preceding, PWM drives utilizingBipolar Junction Transistors (BJTs) or IGBTs can causeElectric Discharge Machining (EDM) current^.^ PWMinverters excite capacitive coupling between the statorwindings, the rotor and the stator frame. This commonmode current does not circulate but rather travels to groundsee Fig. 2). The path to ground can be through both motor

bearings andor load or auxiliary equipment bearings.This paper will investigate the phenomenon of

induced shaft voltages caused by PWM, AC variable-speed drives and will discuss methods of mitigating theirharmful effects.

I Stator IRotor

Bearing Bearing

-Base.-

Fig, 2 Capacitively coupled current flow

0-7803-4785-4198/$10.00 @ 1998 IEEE 159


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11 mC0GNITION OF SHAFT-CURRENT DAMAGE Theoretically, a periodic square wave is composed of

Ideally, when a bearing fails, the cause of the failureis investigated. Often, however, bearings are replacedduring normal maintenance procedure and the root causeof the problem is not always immediately discovered. Thismakes elimination of the failure source much moredifficult since the equipment is back in service.

Electrical damage to anti-friction bearings primarilyappears as fluting. Initially, EDM currents causepermanent microscopic marks in the bearing race. Themarking interval is evenly spaced according to the ballspacing. The initial microscopic marks cause slightvibration wh ch is too small to be picked up by vibration-analysis equipment. Bearing balls or rollers fall into thesemicroscopic pits in the races load zone, displacing a smallportion of lubricant. Slight removal of some of thelubricant reduces the dielectric value, allowing a voltagetransient to cause a larger current to flow. This damagecauses the bearing threshold voltage to be lowered,

allowing lower-level transients to further mark the bearing.Continued deterioration usually occurs at the bottom of theoriginal race markings. This is why fluting marks occur inthe same place on the bearing-mce load zone and whymany bearing fluting failures appear the same.6 Anexample of fluting damage is shown in Fig. 3. (Currentdamage often initially appears as frosting which looks llkea satin finish on the raceways and balls or rollers).

One study7 investigated 1150 ASD powered ACmotors in similar clean-room applications. The resultsshowed that 25% of motors in operation for less than 18months had bearing damage caused by electricaldischarge. It also showed that for motors which had beenin operation longer than 18 months, with an average age oftwo years, 65% had bearings damaged due to electricaldischarge. Note that clean-room applications generally usedrives running with hg h carrier frequencies (typically12KHz or higher) to minimize motor audible noise and runat the same speed continuously (24 hours per day, 7 daysper week).

a fundamental frequency plus infinite harmonicfrequencies. Because of the very fast rise times of IGBTs,the PWM pulses produced are almost perfect squarewaves, rich in high frequencies Since a capacitorapproximates a short circuit at high frequencies, IGBTs

induce more capacitively coupled current than slowerdevices ( I ,N C x dv/dr).

Higher carrier frequencies induce more capacitiveenergy to the rotor and the stator frame since there aremore pulses in a given period.

Ou r field experience has shown that IBGT driveswith high carrier frequencies (e.g., above 8 KHz) lead tosigmficantly faster bearing deterioration than those withlower carrier frequencies.

An example of problems related to ASDs occurred ata large semiconductor manufacturer in the US PacificNorthwest. After 18 months of operation a pattern ofbearing noise and vibration was noticed in a population of

approximately 100 motors. The 15 HP, 460V, 900RPM,open drip proof (ODP) motors were powered by high-carrier-frequency PWM IGBT inverters. The applicationwas plug fan drives in a clean-room ch p fabricationfacility. Even though the noise and vibration were atrelatively low levels and the motors continued to operate,the application was still considered a failure. Initialinspection verified that bearing damage was a result ofshaft current caused by the IGBT inverters.

The next step was to repair the motors and applycountermeasures to prevent further problems In an effortto minimize downtime and cost, a single insulated bearingbracket was fitted. As discussed earlier, this was the mostcommon solution used on large machines for many years.Insulating one bearing proved to be unsatisfactory becausethe uninsulated bearing failed at a faster rate. The nextmeasure was to install shaft-grounding brushes. This wasalso not completely successful, due to improperinstallation. In addrtion, brushes posed maintenanceproblems and contamination concerns due to the carbonbrush material. As a retrofit, insulating one beanng andadding a ground brush was the most practical choice;however, in hindsight, it would have been better to insulateboth bearings. At the time, the phenomenon was not aswell understood and t h ~ s reventative measure was notconsidered.

III. BEARING AMAGE ECHANISMThe degree of damage caused by EDM currents

depends on many factors. The contact area consistsprimarily of irregularities on the surfaces of the balls orrollers and races touching each other. This determines howmuch current can flow without causing localizedoverheating to the bearing assembly. At standstill, there is

Fig. 3 Typical fluting damage to anti-friction bearings.

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significant contact area and low resistance. As the motorspeeds up (typically above 10% of rated speed), thebearings float on a film of oil. The effective resistanceof this film is a function of the film thickness and the typeof lubricant.

When a hgh-resistivity grease is used and thebearings are ra floating'' on the oil film, the equivalent-circuit characteristic changes from a resistor to a capacitor.Imperfections on the bearing surfaces occasionallypuncture the oil film and discharge the rotor. (Dischargesdue to metal-to-metal contact occur at low voltage levelsand do very little hrect damage to the bearing surface.)

The better quality the bearing, the less often theselow level discharges occur, allowing he rotor to charge forlonger periods of time and hence attain higher voltagelevels. (Typically, high-quality bearings charge as much as80 % of the time due to a uniform oil film. Low-qualitybearings charge as little as 5% of the time due to frequentmetal-to-metal contact.) If the rotor voltage exceeds thethreshold voltage (V,) of the oil film between the balls or

rollers and the races of the bearing, the oil films dielectricstrength is exceeded. At this point, destructive EDMcurrents and arcing occur. It is interesting to note thatcontact time between the balls and outer race is longer thancontact time between the balls and inner race, hence thebearing wear from EDM and dv/dt currents is greater inthe outer race.*

The existence of EDM currents with PWM voltagesource inverter dnves depends on the presence of all of thefollowing conditions:1. Excitation, which is provided by the source voltage to

ground (Vsa2. A capacitive coupling mechamsm, between stator and

rotor (C,,)3. Sufficient rotor voltage build-up whch is dependenton the existence of bearing capacitance (Cb)

There are two basic groups of variables which affectcapacitive bearing current:

1. Mechanical Variables - Shaft voltages an d bearingcurrents depend on the existence of C b . Bearingimpedance becomes capacitive only when alubricating film occurs in the contact regions betweenthe balls or rollers and the raceways. The capacitanceis dependent on film thickness, which is a function ofradial load, velocity, temperature, lubricant dielectricstrength and lubricant viscosity. The contact area

increases proportional to the bearing load raised toapproximately the % power.System Impedance - The system impedance (see Figs. 4 and 5) is composed of the stator winding to framecapacitance (C,& the stator winding zero sequenceimpedance (Lo and %), rotor to frame capacitance(Crf), C,, and Cb. ZI accounts for the mechanical andelectrical abnormalities and randomness of thebearing.

2.

-1

Fig. 4 Pictorial Diagram of Motor Capacitances

Fig. 5 Common mode equivalent model

R, & Lo are stator winding zero sequence impedanceV,, is the Source Voltage to GroundVsng s Stator Neutral t o Ground VoltageV is Rotor to Ground VoltageQ i s Stator Winding to Frame CapacitanceC,, is Stator Winding to Rotor Capacitance(&is Rotor to Frame CapacitanceCb is Bearing CapacitanceRb is Bearing ResistanceZ, is the variable impedance of the bearing, often represented as a switch

PE Gnd is Protective Earth Groundwhich randomly closes due to quasi-metallic surface contact

IV. bfUIGATION O F BEARING AMAGE UE TOELECTRICAL ISCHARGE

As mentioned previously, low-quality bearings havemore contact between balls or rollers and the racewaysbecause of surface irregularities. This increased contactprovides less opportunity for damagmg ED M currents andarcing to occur. The implication is that a low qualitybearing may, given a certain set of circumstances, providelonger life than its higher-quality counterpart. This paper isnot suggesting use of low-quality bearings as arecommended solution.

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EDM discharge can be virtually eliminated byproviding electrostatic shielding between the rotor andstator. The same concept has been used for many years intransformers and is referred to as a Faraday shield. Onemethod of accomplishing this is to install a groundedmetallic foil tape so that it covers the stator slots and the

end turns of the windings. Experiments on an unloadedmotor have confirmed that t h ~ s ramatically reduces therotor voltage and the dv/dt currents produced through thebearings.' T h s solution may not be practical since specialmotors and spares are required.

Another solution to reduce rotor to ground voltage(VJ buildup is to use a low-conductivity grease. Theassociated drawback is that experience with conductivegrease shows that bearing life can decrease by a factor ofthree as compared to conventional gr e a ~ e . ~ ne bearingmanufacturer suggests that damage due to bearing currentscan be considerably reduced by using a low-viscositygrease of about 7 centistokes with the addition of graphiteto reduce current density. The size of the graphte particlesshould be approximately the same as the thxkness of thelubricating film between contact surfaces so that theconducting area is increased, but not so small that theparticles are suspended in the oil film." (A point ofinterest is that as bearings age, contaminants from EDMand mechanical wear can sigmficantly change thelubricant's electrical characteristics.)

The most common way to eliminate bearing currentsis to insulate the bearings. Please note that both bearingsmust be insulated because capacitively induced currentsflow to ground. If only one bearing is insulated, all thecurrent will flow through the uninsulated bearing, causingrapid failure. If the insulated bearing solution is chosen,

any connected mechanical load must be insulated or thecurrent will flow from the motor shaft through the loadbearing(s) to ground or through other connectedcomponents such as tachometer bearings. If it is notpossible to isolate the mechanical load and connectedcomponents, a shaft-grounding brush should be added toprovide a low-impedance path to ground.

A common solution, which has been used with DCmachines for many years, is to simply add ashaft-grounding brush without insulating the bearings.When adding a grounding brush, it is important that themotor frame be adequately grounded for h g h frequencies.If not, there is still a potential current path through the loadbearing(s).

Induced voltage is created by the steep wave front ofeach PWM pulse. The faster the rise times, the higher theorders of harmonic currents present. The spectrum rangesfrom almost DC to as high as 30 MHz for IGBT drives,which switch at 0.1 ps.

High frequency currents only travel on the surface ofthe grounding conductor. If the ground conductor is longit is very possible that a portion of the current will stillflow from the motor frame through the load bearings to

100000

"p loo00C.-

1000Cm

.3.-2 100

Fig. 6 Relative Motor Capacitance Values5

ground.

Effective h g h frequency grounding can often beaccomplished by simply making sure that the motormounting base is welded to the mechanical load's base. Abraided copper grounding strap bonding the motor frameto the mechanical load's base will serve the same purpose.(Sometimes the load is connected to the framework of thebuilding through piping, etc. which, to the high frequencycomponents, is a lower impedance than a conventionalground wire from the motor frame to the electricalground.)

As seen in Fig 6, the calculated capacitance betweenthe windings and the stator franie C,* is typically in theorder of 30 - 100 times higher than the capacitancebetween the windings and the rotor Csr. This means thatstator-coupling currents, though not normally taken intoaccount, are considerable. If the motor is not adequatelygrounded for high frequencies or not grounded at all, thesecurrents can flow through the shaft brush and then throughthe load bearings. In this case, adding a shaft brush canactually increase bearing currents in the driven equipment(see Fig. 7).

Motor Load

Bearing

Fig. 7 Paths fo r common-mode currents to ground

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Peak Voltage atMotor Terminal

Differential modeI dv/dt at motor I ' I ' I ' I

Without Conventional ProposedFilter Filter Filter1260V 750V 730V

3037 V/us 1267 V/us 120 v/usterminalsInduced shaft I 864mV I 442mV I 234 mVVoltage to GroundLeakage Current toGroundTotal Filter PowerLoss

(RMS) (RMS) (RMS)430 mA 252 mA 132 mA(RMS) (RMS) (RMS)

0 53.5 125(Watts) (Watts) (Watts)

I " II I 1

Fig. 8a Common-mode current flow in a conventional ASD and motor"

Fig. 8b Proposed output filter to reduce differential-mode and common-mode dv/dt at the terminals oft he motor."

inductance arid capacitance such that a near sinusoidalline-to-line voltage is provided, common mode dv/dt isstill high."

Figure 8a shows the path of common-mode currentflow in a typical installation. Figure 8b shows the additionof a common-mode filter which connects the wye point ofthe filter to a "neutral" point on the DC bus. This filterarrangement provides a low-impedance path from theoutput of the ASD back to a neutral point on the DC businstead of through the motor. (Note that further researchhas shown that the wye point of the filter can be connectedto the negative DC bus with similar results.")

An easily implemented mitigation is to keep thecarrier frequency as low as practical. A value of between1500 and 3000Hz minimizes the amount of energytransferred to the rotor while maintaining good dnveperformance. The tradeoff is increased audible noise.

Output reactors have an effect on the common modenoise generated which can increase the possibility ofelevating the transfer impedance voltage drop (UL). Thisadds to the probability of Ct, charging, thereby causing

EDM discharges. RLC-type output filters should beconsidered instead.Cable length, cable type and groundmg arrangement

can impact thie likelihood of discharge through the motor,load or auxihary equipment bearings. Refer to Fig. 9afor a simplified physical arrangement drawing.

NEC Grounding

Cubicle Lead Plate

Inverter

IL---- ..---J

PF -e Connection

12Fig. 9a Cable Configuration Diagram

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G I ,2,3 'GI ,2,3

Inverter PE Bus

'CGlt

Connected Load

t'S

ZT *-Shield~

I 'PEG = i C G M + i B R G I P E G S 'MFZPEC

Fig. 9b Cable Equivalent Circui tl'*

Z G ~ , ~ , ~s the common mode impedance in each of phases

Z B R ~s the equivalent bearing impedance2s is the shield or sheath impedanceZm is the common mode impedance to ground through

ZT s the cable-shield or sheath-transfer impedanceZ ~ E Fs impedance of the Protective Earth GroundiG1.2.3 is the common mode current in each ofphas es 1 ,2 & 3is is the shield or sheath currentipE0 is the Protective Earth Ground currenticGM IS the common mode current through the motor frame to groundiBRG is the bearing current to groundipEGs is the Protective Earth Ground System current flowing from othex

ULis the Transfer Impedance voltage drop

1 , 2 & 3

the motor frame

parts of the system

Fig. 9b is the equivalent schematic of a typical cable.Due to the existence of stray capacitance, cables causecommon-mode currents to flow. If the motor frame is notgrounded (Zm is open-circuited) or if Zm has a h g himpedance to high frequencies, the return path forcommon-mode current is the cable she ld (or armor).

Shield currents produce a resultant cable-transferimpedance (2,) oltage drop, shown as U, in Fig. 9b. Thehigher the value of the cable-shield or sheath-transferimpedance (2,) and of common-mode impedance toground through the motor frame (Zm), the more likelydischarge will occur through the alternative current returnpath, which is through the PE Ground via the motor andorload bearings.

Testing has shown that cables which have acontinuous shield or continuous armor provide the lowestcommon-mode current plus relatively low frame voltage. 2

The recommended cable for PW M AS D applicationhas six symmetrical conductors, 3 0 and 3 groundconductors) with a continuous corrugated-aluminumarmor-type sheath. To ensure that the cable characteristicsare fully exploited, proper connectors need to be utilized to

maintain low ohmic contact resistance to the armor whichessentially becomes a shield.I2

v. CONCLUSIONS

When a bearing fails, especially on a motor beingpowered by a PW M ASD, the bearing and lubricant shouldbe examined to determine the cause of failure. If thedamage is due to EDM, corrective measures should beconsidered.

As discussed, there are several possible practicalsolutions to mitigate bearing currents which include:

1.

2

3.

4 .

5 .

6 .

I.

Selecting a carrier frequency which is between 1500and 3000Hz if practical. This significantly reduces theenergy transferred to the rotor.Adding a common mode filter to mitigate commonmode noise,Insulating both motor bearings to prevent current flowplus isolating all mechanical load andor auxiliaryequipment bearings (such as tachometers).Adding a shaft grounding brush or brushes to shuntcommon mode currents (ideally with the ODE bearingbeing insulated).Making sure that the motor frame is suitably groundedfor high frequency currents. This prevents statorframe currents from flowing through the connectedmechanical load or auxiliary equipment bearings viathe motor bearings (or grounding brush).

Changing the cable to the recommended type tominimize the common mode current.As a temporary measure, using conductive grease.

New installations should be designed with thebearing current phenomenon in mind and take into accountthe issues discussed in this paper. This is particularlyimportant if high carrier frequencies are planned to beused.

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REFERENCES

[11 Pratt, J.W., Shaft Voltages Caused By AlternatingFlux Encircling The Shaft, Electrical Machines andDrives, 11-13 September 1995, Conference PublicationNo. 412 0 EE, 1995.

[2] NEMA MG1 Section 31.

[3] Shaotang Chen, Thomas A. Lipo, Donald W. Novotny,Circulating Type Motor Bearing Current in InverterDrives, IEEE - AS Conference Record, 1996, pp. 162- 167.

[4] S. Bhattacharya, L. Resta, D.M. Divan, D. W. Novotny,T. A. Lipo, Experimental Comparison of Motor BearingCurrents with PWM Hard and Soft Switched VoltageSource Inverters, IEEE - AS Conference Record, 1996,

[5] Doyle Busse, Jay Erdman, Russel J. Kerkman, DaveSchlegel and Gary Skibinski, Bearing Currents and TheirRelationship to PWM Drives, IECON 95, IEEE 21StAnnual Industrial Electronics Conference, Nov 6 - 10,1995, Vol. 1, pp. 698-705, or IEEE Transactions on PowerElectronics, Vol. 12, No. 2, March 1997, pp. 243-252.

[6] J. Alan Lawson, Motor Bearing Fluting, inConference Record of 1993 Pulp & Paper IndustryTechnical Conference, IEEE Industrial ApplicationSociety, pp. 32-35.

[7 ] Hugh Boyanton, Bearing Damage Due to ElectricalDischarge (ED), Publication by Shaft Groundmg SystemsInc., Albony, OR,

[SI Jay M. Erdman, Russel J. Kerkman, David W. Schlegeland Gary L. Skibinski, Effect of PWM Inverters on ACMotor Bearing Currents and Shaft Voltages, IEEE Trans.Ind. Application Vol. 2, Mar. Apr. 1996.

[9] T. Harris, Rolling Bearing Analysis. New York:Wiley, 1984.

[101 SK F publication, Passage of Electric CurrentThrough a Rotating Contact.

[111 D. Rendusara, P. Enjeti, New Inverter Output FilterConfiguration Reduces Common Mode and DifferentialMode dv/dt at the Motor Terminals in PWM DriveSystems, 2Sh IEEE Power Electronics Specialists

Conference, 1997 Vol. 2 pp. 1269-1275.[121 John M. Bentley, Patrick J. Link, Evaluation ofMotor Power Cables for PWM AC Drives, in ConferenceRecord of 1996 Pulp & Paper Industry TechnicalConference, IEEE Industrial Application Society, pp. 55-69.

pp. 1528-1534.

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A Practical Guide to Understanding Bearing Damage Related to Pwm Drives_cnf