electrical fluting - csi

DoctorKnow Application PaperTitle: Proactive Maintenance Strategy for Electrically Induced Bearing DamageSource/Author:David KowalProduct: GeneralTechnology: Motor, VibrationClassification:

Proactive Maintenance Strategy for Electrically Induced Bearing Damageby David Kowal

Application Development Engineer for Computational Systems, Inc. (CSI)

Abstract

Unknown to many, a small percentage of machinery component damage can be traced to the passage ofelectrical current through the component (i.e. bearings, couplings, gears, seals). The damage, referred to asEDM (Electrical Discharge Machining), originates from either an electromagnetic source, an electrostaticsource, an external voltage source, or a combination of the sources. EDM damage occurs more often invariable frequency drive AC motors and DC motors then other types of machinery, but this kind of damageisn't limited to only these types of equipment.

This paper describes checks you can perform to 1) assist in detecting for the existence EDM damage, 2)check shaft grounding systems to determine their effectiveness, and 3) track down the origin of the voltagesource. It is not the intention of this paper to discuss in any detail the different voltage sources which couldresult in or the corrective actions which can be taken to prevent EDM damage.

Introduction

Under the best of conditions every bearing has, like any component, a point at which it will eventually fail. Abearing's life, depending on its operation (i.e. number hours in service, number of starts and stops, load, speed,etc.) can vary from 5,000 -100,000 operating hours. Table 1 lists some typical bearing operating hoursprovided by one bearing manufacturer [1].

Machine Operating Hours

Machines not fully utilized 8 hours per day: gear drives forgeneral purpose, electric motors for industrial use, rotarycrushers

10,000 - 25,000

Machines fully utilized 8 hours per day: ventilator fans,conveyor belts, printing equipment, separators andcentrifuges

20,000 - 30,000

Machines for continuous use 24 hours per day: mediumsized electrical machinery, compressors, pumps, textilemachinery

40,000 - 50,000

Water works machinery, rotary furnaces, propulsionmachinery for ocean-going vessels

60,000 - 100,000

Large electric machinery, mine pumps and mine ventilatorfans, tunnel bearings for ocean-going vessels

~ 100,000

Table 1. Typical bearing operating hours.

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The maximum life of the bearing is reduced when additional factors (e.g. misalignment, imbalance,overloaded conditions, EDM, etc.) are present.

EDM Damage

Some machines have been plagued with chronic bearing failures, due to EDM, in less than 250 hours ofoperation. Other machines with EDM damaged bearings may achieve greater hours of operation, but in allcases the life of the bearing was cut short. One source mentions that one bearing manufacturer suggests that8% of all electric motor bearing failures were electrically induced [2]. Another source mentions that 25% ofall motor bearing failures are due to EDM; more specifically, high-frequencies switching [3]. The differencein the two papers could be contributed to a difference in motor population considered.

Before EDM damage can occur, a voltage potential and a path for current to flow must exist. Shaft voltages,to some degree, exist on all machines. EDM problems occur either from a decrease in lubrication dielectric orfrom an increase in the shaft voltages resulting from [4]:

Electromagnetic voltages (i.e. broken rotor bars, turn-to-turn shorts, eccentric rotors, etc.).Electrostatic voltages (i.e. accumula-tion of charged ions from stem turbines and conveyor belts, AC

variable frequency drive systems, DC drive systems, etc.).External voltages (i.e. welders, voltages applied to the process, etc.).Combinations of the above voltages.

A path, for current to flow, is the second necessary ingredient for EDM damage to occur. In most cases thepath is usually through the bearing. The bearing can be thought of as being two capacitors in series. Like anycapacitor, if enough voltage is applied to one plate (i.e. shaft or bearing inner race) the dielectric (i.e. bearinglubrication) will breakdown resulting in a path to the second plate (i.e. bearing ball or roller). This in turnresults in a path to the third plate (i.e. shaft or bearing outer race). When the path is created current flowsthrough it. The resulting current flow creates an electric arc across the plates, thus damaging the bearing'ssurfaces.

Proactive Maintenance Strategy

What do you do when you suspect EDM damage? If you suspect bearing damage from EDM, there arequestions you can ask and checks you can perform assist you in determining whether EDM could be aproblem or not and if so, how to locate the source of the problem.

1. Does the machine have a history of either undiagnosed failures or diagnosed failures due to EDM? Somemachines are plagued with repeated bearing failures due to EDM. How long has the machine been service? Ifthe machine has been in service for 15 years, and the problem just started, something may have changed forthe EDM problem to occur.

2. Look at the past history of other machines of the same type and application. If one machine has problemsdue to EDM (depending on the machine, the machine's operation, the machine's environment, etc.) it is likelyother machines of the same type and operation will be susceptible to EDM damage.

3. Is the machine in the highest risk group? Machines in the high risk group have been determined to be moresusceptible to EDM damage then other machines [4].

Variable frequency variable speed AC and DC motorsLarge frame motors (over 1000 HP)Motors with a history of unexplained chronic bearing failure or failure due to EDMNewly installed and rebuilt motors placed into serviceMotors with shaft grounding systems installed


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Vertical motors.

4. Look at the machine's construction. Does the machine have a shaft grounding system and if so, is the shaftgrounding system sealed or open to environmental contamination? Are any of the bearings or the couplinginsulated? Is the bearing lubrication conductive? Are the bearing balls ceramic? Is the motor equipped with aFaraday Shield? Are filters installed in either the power supply or line to reduce problems due to EDM?Depending on where the failure occurred, knowing these things can help determine the origin of and thefailure responsible for the EDM damage.

5. What is the machine driving or being driven by? If the problem is in an AC variable frequency drive motoror DC motor driving a gearbox, the odds are the EDM problem is from the motor's power supply and not thedriven unit. If the problem is in the inboard bearing of a center hung fan being driven by a AC constant speedmotor and the fan is moving air saturated with particulate, then the odds are the problem is from the a staticcharge build up on the fan. Note: Even though the odds indicate the possible origin of EDM problem, othertypes of EDM damage can result in the same failures.

6. Is the problem the result of a decrease in dielectric insulation? Is the correct bearing lubrication being used?Is the bearing lubrication contaminated? Has the bearing lubrication dielectric break-down threshold beenreduced due to misalignment, excessive load, lack of lubrication, etc.? Each bearing lubrication has its owndielectric break-down threshold. The breakdown threshold is dependent on the dielectric of the lubricationand distance between the plates. If the wrong lubrication is used, the lubrication becomes contaminated, orthe distance between the plates is reduced then the amount of voltage required to cross the gap is alsoreduced.

7. What was the last thing done to the machine? In one case, a plant replaced the rusted shims from under alltheir gearboxes, with new stainless steel shims. After doing this, they started losing the outboard bearings onthe input shaft of all gearboxes to EDM damage. The gearboxes were being driven by DC motors with bothbearings insulated. The rusted shims acted like an insulator preventing the flow of current. In some cases thesource of the problem can be traced back to the last thing done to the machine (i.e. x-raying parts on amachine and not degaussing when finished or using the wrong bearing lubrication).

8. One thing you can and should do if the machine's damaged bearing has already been removed, is to visuallyinspect it. Visual inspection of the bearing damage is one of the best ways to identify the type of damage.EDM damage has four characteristic appearances: 1) fluting, 2) frosting, 3) electric pitting, and 4) electric arctracks. The washboard looking damage, referred to as flutes, shown in Figure 1 and the symmetrical frostingdamage, shown in Figure 2, are probably the easiest to visually identify. I say this only because I don't knowof another fault that will result in damage with this kind of appearance. The best way to confirm EDMdamage, for all four characteristics, is through magnification of the damaged area.


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Figure 1. Bearing outer race with EDM fluting damage.

Figure 2. Bearing outer race with EDM frosting damage

9. Check to see if the machine's temperature and noise levels have increased. Increases in these are notnecessarily indications of EDM damage, but EDM damage can result in increased levels. Note: There havebeen reported cases where catastrophic bearing failure occurred before temperature and noise levelsincreased to noticeable levels.

10. Acquire vibration spectrum data, using at least 800 lines of resolution, at all bearing locations. Thisincludes insulated bearings. One of the worst fluted damaged bearings I have seen was a bearing with itsinsulation compromised. Typically, in a bearing with fluting damage (see Figure 1) and frosting damage (seeFigure 2), you are looking for is a mound of energy with BPFO and/or BPFI sideband spacing. In a previouspaper, I stated that the mound of energy appears between 2000 - 4000 Hz (see Figure 3) [4].


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Figure 3. Spectrum plot with mound of energy due to EDM

While continued research has substantiated that many machines with this type of EDM damage do exhibitthis mound of energy in this range, we have found a few machines which show this characteristic patternoutside this range. On two variable frequency drive AC motors with EDM damage I have seen the energymound center frequency around 900 Hz on one motor running at 723 RPM and 5600 Hz on the other motorrunning at 4500 RPM. In both cases this is about 80 times turning speed. On DC motors, which so far most ofmy work has been on, the energy mound center frequency was around 1000 Hz on a 457 RPM motor andbetween 2000 Hz and 4000 Hz on motors running between 900 RPM and 1350 RPM. In these cases this isabout 130 times turning speed. At this time all of the variables involved in calculating the location of theenergy mound center frequency have not been identified yet.

In addition to the vibration spectrum data, acquire and look at the vibration waveform in G's. If you have aCSI Model 2120 analyzer, acquire and look at PeakVue® data using a filter setting above the energy moundsuspected of being due to EDM. Like temperature and noise levels, these are not necessarily indications ofEDM damage. Bearing damage due to EDM can result in increased waveform amplitudes and bearing faultfrequencies in the PeakVue® data.

11. Acquire shaft-to-ground voltage and current measurements using a shaft riding probe. You will want toacquire AC RMS voltage and current readings, DC voltage and current readings, and peak voltage readings.The shaft riding probe manufactured by Computational Systems, Inc. or an oscilloscope attached to a shaftriding probe can be used to acquire all of these readings. Note: The CSI shaft riding probe works inconjunction with one their analyzers. A multimeter can be used to acquire the AC RMS voltage and currentreadings and DC voltage and current readings. It can not be used to acquire the peak voltage readings, sincethe peak voltage spikes you need to detect can have very narrow pulse duration's. Peak voltage spiking,


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which is responsible for most AC variable frequency drive and DC drive bearing failures, occurs too fast to beread by a multimeter. For example, bearing damage may be occurring from 30 V (peak) spikes, but amultimeter may only read 100 millivolts AC RMS. The higher voltage reading should be a concern. Note: Theminimum peak voltage spike duration that can be measured will vary from instrument to instrument.

WARNING!!! Extreme caution should be used when placing anything near or against a rotating shaft.Exercise extreme care to keep all body parts, clothing, cables, etc. away from the shaft. It is recommendedthat before you place anything near or against the rotating shaft that you use a strobe light and look at theshaft surface to check for any possible obstructions or hazards, such as a key or key way.

The question most often asked with this type of data is, "What voltage and/or current level is damaging?"Depending on whether the voltage source is electromagnetic, electrostatic, or externally supplied, the answercan vary depending on who you ask. NEMA MG 1-1993, Section IV, Part 31 states that bearing failure due toelectrical arcing, on motors with frame sizes less than the 500 frame series, can occur if shaft voltages higherthan 300 millivolts (peak) are present [5]. One source suggests that peak voltages greater than 3 volts willresult in EDM bearing damage [3]. Some have reported peak voltage levels of 50 - 60 volts on motors withEDM damage. Others have reported levels greater than 100 volts (peak) on motors with EDM damage. I haveseen levels as small as 2 volts (peak) on motors with and without bearing damage. The amount of voltagerequired to breakdown the bearing lubrication will vary from machine-to-machine. It has been my experience,the initial levels established in Table 2 [4] are a good starting point for determining voltage and current levelsfor most machines.

RELATIVESEVERITYGUIDLINES

Measurement Amplitudes

Low Questionable High

RMS or DC <1 1 - 3 >3

Voltage (volts)

RMS or DC TBD TBD TBD

Current (mAmps)

Peak Voltage <3 3 - 10 >10

(volts)

Table 2. Amplitude severity guidelines for shaft voltages and currents.TBD - these values are to be determined.Low - little likelihood of damage sustained.Questionable - if levels are measured and the machine has a history of chronic problems which could be

related, then corrective action should be considered.High - these levels should be considered unusual and that on-going damage due to EDM between the shaft

and ground is probable.

On machines with electromagnetic induced voltages you should see increased AC RMS voltage readings. Onmachines with electrostatic and externally applied voltages you should see higher peak voltage levels.Increased current levels on machines with constant voltage levels could be the result of a low resistance path(e.g. bearing). Shaft-to-ground voltage and current measurements are be useful in

1. measuring shaft voltage and current levels which could potentially result in damage,2. locating the origin of the voltage source resulting in the damage,


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3. checking shaft grounding systems to determine their effectiveness,4. looking for changes in voltage and current levels which may result from EDM damage or other mechanicalfaults (i.e. rubs, eccentricity, misalignment, etc.), and5. establishing baseline levels which can be used in making corrective action decisions on machines, hopefullylong before damage occurs.

ConclusionBearing damage resulting from EDM doesn't have to be chronic or remain unexplained. Understanding whatvoltage sources result in and which machines are more susceptible to EDM damage, knowing what questionsto ask, knowing how to identify EDM damage through visual inspection and vibration data, and acquiringshaft-to-ground voltage and current readings can assist you in combating this phenomena.

PeakVue® is a registered trademark of Computational Systems, Inc.

References1. SKF USA Inc., Mounted Products catalog, publication 610-711, 1997, pg. 11.2. D. Busse, J. Erdman, R. Kerkman, D. Schlegel, and G. Skibinski, "Characteristics of Shaft Voltage andBearing Currents," IEEE Industrial Applications Magazine, November/December 1997, pp. 21-32.3. Annette von Jouanne, and Haoran Zhang, "Bearing Currents: A Major Source of Mechanical Failure forMotors in Adjustable Speed Drive Applications," Turning Point, September 1998, pg. 3 & 7.4. David Kowal, "Bearing Damage Resulting from Shaft Voltages and Currents,"Reliability Magazine, April1998, pp. 29-38 or CSI 1997 User Conference, Nashville, TN, October 13 - 17, 1997, pp. 373 - 390.5. NEMA Motors and Generators Standards Publication, MG 1-1993, Section IV, Part 31.

All contents copyright © 1998 - 2006, Computational Systems, Inc.All Rights Reserved.


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electrical fluting - csi

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