�

dealing with shaft and bearing currents

By Tom Bishop Technical Support Specialist Electrical Apparatus Service Association St. Louis, MO

introductionThis paper addresses key issues related to shaft

and bearing currents in electric motors and generators. Critical topics that will be covered include recogniz-ing symptoms of shaft and bearing currents, and de-termining if damaging current levels are present. Also, possible causes of the damaging current, such as ma-chine dissymmetry and operation on variable frequency drives (VFDs) will be dealt with. Methods of testing to confirm the presence of shaft or bearing currents will be described, as well as how to assess the magnitude of the damaging currents. Further, solutions to eliminate or control shaft and bearing currents, such as insula-tors, isolators, and ceramic bearings, will be offered.

While shaft and bearing currents are not a new prob-lem (papers on the subject date back prior to �930), what is “new” is the increased understanding of how to identify and solve the problem. Shaft and bearing cur-rents have been described as shaft voltages, circulat-ing voltages, circulating currents and bearing currents. Shaft voltage only becomes a problem when it leads to bearing current and consequential damage to the motor bearings. If this voltage, referred to as “common mode voltage” or “shaft voltage,” builds up to a sufficient lev-el, it can discharge to ground through the lubricant film on the bearings. Current that finds its way to ground through the motor bearings in this manner is called “bearing current.”

This paper will primarily refer to the damage phe-nomenon from shaft or bearing currents as bearing current(s) because it is the current through the bearings (not the shaft) that causes the damage. In cases where the distinction between shaft and bearing currents need to be made, the specific term shaft current(s) or bearing current(s) will be used.

recognizing symptoms of bearing current

All too frequently the first symptom of bearing current is audible noise from the bearing, indicating it is in ad-vanced stages of failure. Inspection of the bearing after failure may reveal fluting of the races (Figure 1), balls or rollers “frosted” (Figure 2), or an overall dull grey or dark “smoky” finish (Figure 3) on both balls/rollers and race-ways. The lubricant may also be dark in appearance.

current damageThe appearance of damaged surfaces is related to

three major types of current. The first type of electric current damage is electric pitting (Figure 4). It is mostly

figure 1: fluting

Fluting of the bearing races due to electrical current.

figure 2: frosting

Frosting of the balls/rollers due to electrical current.

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related to single crater damage and typically seen in DC applications such as railway traction motors. The size of the crater is from 0.004-0.020” (0.�- 0.5 mm) in diameter and is visible to the naked eye. Such craters are usually produced by a very high voltage source.

The next type of damage is fluting, which is a pat-tern of multiple lines across the inner and outer races (Figure 1). The reason for this fluting is mechanical res-onance vibration caused by the dynamic effect of the rolling elements as they roll over smaller craters in the races. Strictly speaking, fluting is not a primary mode of failure produced by the current flow through the bear-ing itself. Rather, it is secondary bearing damage that becomes visible only after a period of time, and it has the craters as its initial point.

The third type of current damage, micro-cratering is the most common type of current damage when the motor is powered by a VFD. The damaged surface ap-pears dull and is characterized by molten pit marks (Fig-ure 5). Multiple micro-craters cover the rolling elements

and races. Crater sizes are small, mostly from 0.0002-0.0003” (5 - 8 μm) in diameter, regardless of whether the crater is on an inner ring, outer ring or a rolling ele-ment. The true shape of these craters can only be seen under a microscope using very high magnification.

other symptomsIf a bearing is noisy it is important to remove it from

service and dismantle and inspect it prior to complete bearing failure. If the bearing has been destroyed by failure, the evidence of bearing current will also be de-stroyed and the root cause of the failure will not be de-termined.

Visual indications of shaft voltages include fluting or a “picket-fence” pattern on the races of the bearing (Figure 6). The spacing of the fluting marks depends on the speed (rpm), bearing diameter, radial load and magnitude of the bearing current. The balls or rollers

figure 3: darkened finish

The darkened finish of the ball on the right was caused by electrical current.

Microscopic view of electrical pitting.

figure 4: pitting

figure 5: dull finish

Dull finish and electrical craters in the inner race of a roller bearing.

figure 6: “picket fence”

Note the “picket fence” pattern of the bearing outer race. This is often a visual clue to shaft voltages.

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may have a dull grey or dark “smoky” finish (Figure 3). If the motor speed was varying, the races may also have a frosted pattern. The grease may be black in appear-ance, due to the burning of the metal that leaves ferrous oxide.

Bearing current damage usually initially appears vi-sually in the areas of the bearing that are most heav-ily loaded. The reason for this is that the lubricant film will be thinnest in the areas subjected to the heaviest load. For example, in a belt drive application the dam-age to the bearing will be most pronounced in line with the direction of the belt tension. Regarding lubricant film thickness, studies have found that a bearing usually has a lubricant film thickness of 0.2-2.0 μm (0.000008-0.00008”) at normal operating speeds. Given this film thickness, damaging bearing currents can be caused by 60-Hz shaft voltages as low as 0.2–2 V peak.

Bearing currents can also cause the lubricant in the bearing to change its composition and degrade rapidly. The locally high temperature causes lubricant additives and the base oil to react, often causing burning or char-ring of the base oil. Additives will then be used up more quickly and the lubricant becomes hard and blackened. A rapid breakdown of the grease is a typical failure mode that results from bearing current.

determining if damaging current levels are present

At present there is no known method to measure bear-ing currents and no practical way to directly measure shaft currents. Mea-suring shaft current would require placing a current transformer coil around the shaft inside the motor. In rare cases, with the internal area of the motor accessible, a “Rogowski” coil (Fig-ure 7) can be wrapped around the shaft and used to measure shaft current. The usual way to detect the presence of potentially detrimental shaft and bearing current is to measure the voltage from shaft to ground, that is, from the shaft to the motor frame.

shaft to frame voltageAmong the challenges in measuring the shaft to frame

voltage is that the bearings change from an insulating mode to a conducting mode in a somewhat random manner. Consequently it is necessary to use the high-est voltage measured when assessing the possibility of

the presence of damaging bearing currents.If the shaft to frame voltage exceeds �00 millivolts

AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough the degrade the bearings. Another test method, based on NEMA MG�, is to measure the shaft voltage from end to end of the shaft. If the voltage exceeds 300 millivolts AC, bearing damage may occur. The NEMA method uses the same 300 millivolts AC limit for any type bearing.

The magnitude of the shaft current can also be an indicator of the presence of damaging shaft current. If a welding cable is touched to both ends of the shaft and a spark is generated, that is an indicator of shaft current, though the magnitude is unknown. If the current can be measured, a value in excess of 20 A is considered an indicator that damaging levels of bearing current are present.

VFDs can generate a “common mode voltage” which raises the three phase winding neutral potential signifi-cantly above ground potential. For 3-phase motors, the sum of the 3 phases equals zero for sinusoidal power. For a VFD, where each phase is rectified, the common mode voltage is the instantaneous sum of the 3 phases. When the 3- phase output from the drive is rectified, DC is either positive or negative; and the common mode voltage is approximately equal to the RMS voltage. The inherent magnetic dissymmetry can result in shaft and

figure 7: rogowski-style coil

A Rogowski-style coil that can be wrapped around a shaft to measure shaft current.

Shaft

Rotor

FaradayShield

Stator

Win

ding

s

Inner race Outer race

Ball bearingGrease

Rotor toground

Rotor towinding

VFD in

figure 8: capacitive coupling

Capacitive coupling between the stator and the rotor sup-plied from the VFD.

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bearing currents.This common mode voltage oscillates at high frequen-

cy and is capacitively coupled to the rotor (Figure 8). The result is pulses as high as 25 volts from shaft to ground, with the current path being through either or both bearings to ground. If a motor is supplied from a VFD, a good practice would be to check the frame to shaft voltage and determine if damaging bearing cur-rent may be present.

bearing currentIf the shaft voltage caused by a VFD is large enough,

current can flow through the shaft and both bearings (Figure 9) and, in some cases, through the shaft and bearings of the load machine. This circulating current may damage the bearings if the magnitude exceeds 3-20 A, depending on the size of the bearings, the rate of rise of the voltage at the motor terminals and the in-verter DC bus voltage level.

The reason that the magnitude of current varies with the size of a bearing is that it is more directly related to current density. The current density is the current divid-ed by the rolling element contact area, and can cause damage when it exceeds 1 A/mm2 (645A/in2). The dif-ficulty with this tolerance is that the current density can-not be measured in an actual application because the contact area is normally not known, and can vary.

Interesting results were found in experimental studies where the shaft current caused by capacitive coupling between stator and rotor of motors supplied by VFDs

could be measured. It was found that a current of 5- 200 mA, depending on bearing size, did not cause damage to the bearings. Further, with one bearing insulated the current level was reduced to less than 40% of the value with uninsulated bearings; and in the case of both bear-ings insulated, the currents were reduced to less than 20%. It was also found that applying filters or reactors between the drive and motor reduced shaft current by 30-50%.

residual magnetismBearing currents can result if a shaft has residual mag-

netism. Residual magnetic field levels are best mea-sured with the machinery not operating. A gaussmeter can be used in the DC mode to measure the residual magnetic field level of a shaft. Magnetic field strength can change if the motor is disassembled because of the alteration in the flux paths. For example, when a coupling is removed, the adjacent shaft magnetic field level may increase or decrease. Therefore it is best not to disturb any shaft components so that the measured magnetic field levels are representative of the actual operating conditions.

The following table is based on test results compiled by one investigator from more than 200 machines over a �5 year period.

Maximum allowable residual magnetic field levels (mea-sured in free air)2 gauss Bearing components, bearing journals; seals,

gears and coupling teeth.4 gauss Bearing housings.6 gauss Mid-shaft and rotor core area.

An important factor is that the measured magnetic field levels are only fringing values and may be consid-erably lower than the actual levels inside a component.

possible causes of the damaging current

There are three main sources of circulating currents that may pass through the shaft and consequently the bearings. One source is magnetic dissymmetry, anoth-er is electrostatic discharges, and the third is capacitive coupling between the stator windings and rotor. Any of these sources may be present independently or simul-taneously, and thus result in bearing currents.

Other sources of shaft voltage include supply voltage unbalance, circulating currents in unbalanced parallel circuits of a 3-phase winding, and transient voltage con-ditions. Less common causes of shaft and bearing cur-rents include non-insulated through-bolts in the rotor or armature, eccentric air gap, and sections of shorted iron in the rotor or stator cores. Any of these sources may

The addition of a grounding brush creates a parallelcircuit, sharing the current flow through the bearing.The flow of current through each path of the parallelcircuit depends upon the relative resistance. Currenton the drive end remains unchanged.

Electrically isolating one bearing can be done byinsulating the housing or the bracket at the frame, orby use of special factory-insulated bearings. Whenthis method is used, the flow of current through bothbearings is halted. No current means no damage.

Bearing is electricallyisolated to interrupt flowof current.

figure 9: path of circulating current

Circulating curent path through the shaft, bearings and frame.

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be present independently or simultaneously, and thus result in bearing currents.

magnetic dissymmetryMagnetic dissymmetry is due to any differences in the

magnetic circuit between stator and rotor. It is often as-sociated with larger motors that have segmented lami-nations and are supplied from sinusoidal (as opposed to VFD) power sources. The resulting asymmetric flux in the motor results in a low frequency circulating cur-rent through the bearings.

Although dissymmetry in the core laminations is not the only source of magnetic dissymmetry, it is the pre-dominant cause. Other causes of magnetic dissymme-try include an uneven air gap between stator and rotor, and damage to the stator core laminations such as from a winding ground fault.

Magnetic dissymmetry is also common in DC ma-chines, where the individual field poles and interpoles are likely to have variations in individual air gap. An in-formal study found shaft currents in most DC machines larger than �0 hp (7.5 kW.)

In any motor, including those with one piece rather than segmented laminations, the magnetic core steel is not completely uniform in composition. Because of this, flux paths through the core are not completely symmet-rical, resulting in time-varying flux lines that enclose the shaft. These induce a current flow through the shaft, through a bearing, through the frame, and back to the shaft through the other bearing (Figure 9).

Slight differences in the spaces between the pieces of segmented lamination cores (Figure �0), and how they stack layer by layer, lead to a lack of symmetry in the stator core. To visualize this, think of one layer of the core laminations as a circle made of many piec-

es abutting each other (like slices of a pizza pie). The space between each adjacent lamination segment var-ies slightly, and each layer of laminations adds to the distortion. The finished core thus amplifies the minute distortions since they effectively add up as they distort the magnetic circuit.

Smaller cores that use single piece laminations are much less likely to cause magnetic distortion, but it is still possible. A potential reason is that one of the main qualities of magnetic steel is a property termed perme-ability, which can vary from one part of the circular lami-nation to another part. The permeability is associated with the flux density at which the core steel will mag-netically saturate. Thus, if the permeability of a lamina-tion varies, some parts of it may become saturated and become hotter, leading to thermally induced distortion such as bowing. For this reason, some motor manufac-turers shift laminations circumferentially when stacking 2-pole cores, because with the low number of poles, these are the most likely to be affected by changes in permeability.

electrostatic dischargesThe second major cause of damaging bearing current

is somewhat surprising, namely, static electricity. We don’t usually consider static electricity as a source of harmful currents. However, damaging levels of bearing current caused by electrostatic discharges (Figure ��) can occur in applications such as belt drives, paper roll winders and fans or blowers. The larger the machine is (e.g., wind generators, some with 330 ft. (�00 meter) blades), the greater the potential bearing current.

capacitive couplingCapacitive coupling between stator and rotor often

occurs when the motor is supplied from a VFD. Voltage associated with the extremely fast switching of insulat-

figure 10: segmented laminations

Segmented laminations, each make up 1/8 of a layer of the stator core.

figure 11: dull finish

Dull finish and electrical craters in the inner race of a roller bearing.

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ed gate bipolar transistors (IGBTs) in the pulse-width modulated (PWM) output waveform (Figure �2) of the VFD is induced in the rotor and shaft via the air gap be-tween stator and rotor. The IGBT switching frequency, also termed the carrier frequency, is the rate at which it “chops” direct current into simulated blocks of AC in the DC bus of the inverter section of the drive.

The induced voltage caused by switching builds up in potential on the rotor until it exceeds the dielectric breakdown capability of the bearing lubricant, result-ing in a brief discharge pulse through the bearing to ground. This process continually repeats itself, thus increasing the magnitude of the damage to the bear-ing. The discharge pulses are a form of electrical dis-charge machining (EDM) that literally removes material from the surfaces of the affected parts of the bearings (Figure ��). In a similar manner, the voltage associated with the “chopping” of AC into DC power by means of transistors and silicon controller rectifiers (SCRs) in a DC drive can cause damaging bearing currents in a DC motor.

The higher the switching frequency, the higher the rate of the current discharge pulses and the faster the damage will occur. VFDs are typically audibly quieter at higher switching frequencies; however, the trade-off is that the higher frequencies are more destructive than lower frequencies. When possible, the switching fre-quency should be adjusted as low as possible without creating unacceptable audible noise levels, preferably avoiding frequencies above 6 kHz. It is usually good practice to use a VFD with a carrier frequency that can be adjusted in increments of less than � kHz. That al-lows fine-tuning of the carrier frequency to the lowest level that provides acceptable operation.

The experience of some users and failure analysts has led to speculation that constant speed operation increases the probability of bearing current damage. Al-

though this does not appear to have been proven, it has been found that bearings operating at constant speed are more susceptible to damage from the EDM action associated with electrical current discharges.

If the motor and drive are not effectively grounded to each other and to the electrical system ground, the voltage magnitude can increase. That is because it is a common mode voltage, meaning the voltage poten-tial from the motor winding neutral is raised above the normal ground potential level. Also, this common mode voltage oscillates at a very high frequency and capaci-tively couples the stator to the rotor.

The higher the common mode voltage and VFD switching frequency, the greater is the possibility of damaging bearing currents. It is recommended that stranded, low-impedance, ground cable (Figure �3) be used to establish a dedicated common ground path be-tween motor and drive as skin effect is a factor. The higher the frequency, the more alternating current (AC) “travels” on or near the surface (“skin”) of the conduc-tor, so the resistance-to-voltage flow is affected by the surface area of the conductor. Stranded cable has more surface area than a solid conductor, thus provides a lower resistance (impedance) path.

methods of testing to confirm and assess the presence of shaft or bearing currents

The most common methods of testing to confirm the presence of shaft and bearing currents are vibration analysis and voltage to frame (ground) analysis. Other techniques that can be used include lubricant analysis and microscopic analysis, though the microscopic anal-

figure 12: pwm waveforms

Voltage (top) and current (bottom) waveforms of a PWM drive.

figure 13: stranded cable

Stranded low-impedance cable made specifically for VFD applications.

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ysis can only be applied after a bearing has failed.Although diagnostic testing may reveal the presence

of potentially damaging bearing currents, a better ap-proach is condition monitoring. Condition monitoring begins with establishing baseline values for the test measurements and then trending these measurements over time. An increased level, that is, an upward trend, indicates probable need for action. The early detec-tion of an upward trend also normally allows time for a planned outage or other course of action. That is, action is taken on a planned rather than a crisis basis.

vibration analysisVibration analysis can be used to determine if bear-

ings have fluting damage, which may have been caused by discharge currents. A high resolution spectrum of the 2-4kHz range (Figure �4) can indicate an abnormally high band of energy, that is, a hill-like shape as the amplitude gradually increases to a rounded peak and then decreases. As the fault condition progresses over time, vibration levels at the bearing fault frequencies will become apparent. Some vibration analyzers have the ability to measure “spike energy” or g’s of acceleration; each is a means of quantifying impact energy to diag-nose impending bearing failure.

shaft to frame (ground) analysisAs mentioned previously, there is no known method

to measure bearing currents and no practical way to di-rectly measure shaft currents. Measuring shaft current would require placing a current transformer coil around the shaft inside the motor. The usual way to detect the presence of potentially detrimental shaft and bearing current is to measure the voltage from shaft to ground, i.e., the motor frame.

shaft voltageAmong the challenges in measuring the shaft to frame

voltage is that the bearings change from an insulating mode to a conducting mode in a somewhat random manner. Consequently it is necessary to use the high-est voltage measured when assessing the possibility of the presence of damaging bearing currents.

Shaft voltage can be measured using a digital multi-meter (DMM) set to the voltage scale. The impedance of the meter will affect the resulting measurements; higher impedance meters are more accurate. Connect one lead to the frame of the machine. Take a #2 pen-cil and remove the wood at the eraser end, to expose the carbon “lead”. Use an ohmmeter to verify that the graphite “lead” in the pencil is not broken, by checking the resistance from the former eraser end to the writ-ing tip (point). Attach the other lead of the DMM to the pencil lead, and allow the pencil point to contact the revolving shaft. The contact point should be as close as possible to the bearing (Figure �5).

If the voltage exceeds �00 millivolts AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough the degrade the bearings. Another test method, based on NEMA MG�, is to measure the shaft voltage from end to end of the shaft (using two pencil points and the DMM). If the voltage exceeds 300 millivolts AC, bearing damage may occur. The NEMA method uses the same 300 mil-livolts AC limit for any type bearing.

Note: When a motor is operating from a VFD, the duration of the voltage discharges may be only a few nano-seconds. Conventional DMM’s may consequently indicate much lower voltages than the oscilloscope test described later in this paper.

figure 14: vibration spectrum

High resolution vibration spectrum.

figure 15: measuring shaft voltage

Shaft voltage can be measured with a DMM. Note the use of a pencil to contact the shaft.

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shaft currentAn “old-timers” method of checking for damaging lev-

els of bearing current was to use a welding cable and touch each end of it to each end of the motor shaft while in operation. If a spark was generated when the sec-ond end of the cable was applied to or removed from the shaft it was assumed that damaging bearing current was present. A modern variation of that test is to apply the welding cable as a shorting bridge from shaft end to shaft end and to measure the current through the cable. A current in excess of 20 A is considered an indicator that damaging levels of bearing current are present.

ieee 112 methodsThe following test procedures are based on three al-

ternative methods of measuring shaft potential for shaft circulating currents described in IEEE standard ��2. In motors that have all or all but one bearing insulated, the tests can be used to detect the presence of shaft potential while the motor is operating.

First, a shaft riding brush (Figure �6) is used to short out the uninsulated bearing (or one bearing, if all are insulated). The brush is applied to the shaft near the bearing and connected to the frame with a short piece

of copper cable. The test is completed by measuring the shaft potential to the frame at each of the other bearings.

A high impedance oscilloscope is then connected with one lead grounded to the frame and the other lead at-tached to the shaft brush. It is preferable to use a low-im-pedance shielded conductor for the oscilloscope leads to minimize electromagnetic interference. This shield should be grounded at one end only. After connecting the leads, the peak voltages are then measured.

The second method is an alternative to the oscillo-scope, using a high-impedance voltmeter. Both AC and DC voltages should be measured at each bearing. The peak voltage can be estimated by adding the DC level and �.4 times the AC rms level. This estimated peak voltage, however, may be considerably below the ac-tual peak value.

Another alternate method involves measuring the AC voltage with shaft riding brushes contacting opposite ends of the shaft, with the motor operating at rated volt-age and speed. The procedure is identical to that of the first method, with the exception being that a low-resis-tance ammeter is used in place of the oscilloscope. In this test arrangement, the ammeter is used as a low-impedance, uncalibrated voltmeter. The meter readings may not be a true indication of the current that might flow when there is a breakdown of the lubrication film in the bearing(s). This method is useful if a history of similar tests is available for comparison.

The IEEE standard does not provide any tolerance or threshold levels for determining if damaging bear-ing current levels are present. However it suggests that trending of these tests can be useful for detecting in-creased levels and therefore the increased probability of damaging bearing currents.

lubricant analysisLubricant analysis utilizes a testing laboratory to an-

alyze the bearing lubricant. Grease sampling is often impractical as there is usually no direct means of ob-taining a fresh sample unless the motor is partially dis-assembled. Grease that is extracted from the relief port is unreliable as the length of time since it last lubricated the bearing cannot be determined.

Oil used to lubricate bearings is normally available for sampling by simply accessing the oil ring area of a sleeve bearing machine, or draining some oil from a reservoir of an idle oil-lubricated motor, either rolling element bearing or sleeve bearing. It is not advisable to attempt to draw a sample on an operating motor be-cause of the risk of a rapid loss of oil.

If the lubricant analysis indicates the presence of met-als from the bearings, electrical discharge machining (EDM) associated with bearing current discharges are a possible cause. There is however, no means of distin-

figure 16: shaft grounding brush

“Toothbrush” style shaft grounding brush shorts out bearing current path.

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guishing between particles from the passage of electri-cal current and particles of mechanical wear origin.

microscopic analysisMicroscopic analysis can be performed by a testing

laboratory with that capability, often a bearing manufac-turer. If you procure a shop-type microscope you may be able to perform your own analysis. The shop-type microscope can be placed directly on the bearing to magnify the area to be assessed. Electrical current will usually result in molten particles. Damage of mechani-cal origin often appears as smearing marks with evi-dence of heating due to mechanical friction.

solutions to eliminate or control shaft and bearing currents

The first step in addressing solutions to shaft and bearing current issues is to determine the type of source that is the probable cause. We have previously identi-fied these sources, and it is worth repeating them, to make clear what they are and then proceed to address eliminating or controlling them.

There are three main sources of circulating currents that may pass through the shaft and consequently the bearings. One source is magnetic dissymmetry, anoth-er is electrostatic discharges, and the third is capacitive coupling between the stator windings and rotor. Any of these sources may be present independently or simul-taneously, and thus result in bearing currents.

magnetic dissymmetry Magnetic dissymmetry can result in a circulating cur-

rent from the frame through a bearing, along the shaft, through the other bearing and back to the frame. The circulating current in this case can be eliminated by in-terrupting the circuit, usually by insulating the opposite drive end bearing (Figure �7).

Bearing current due to dissymmetry can be reduced, but not eliminated, by installing a shaft grounding brush. That is, if the motor can be modified to accept such an installation; and if it is economically viable. The brush circuit from frame to shaft provides a low resistance par-allel (bypass) circuit (Figure 9) to divert current from the much higher resistance bearing circuit. The lower the resistance of the brush circuit compared to the resis-tance through the bearing, the more current is diverted from the bearing. To be most effective, the brush should be located inboard of the bearing, that is, between the bearings. If it is located outboard of a bearing it will be outside of the circulating current “loop” of the motor.

Note that the brush resistance is critical to this so-lution. The special grounding brushes supplied for this purpose are extremely low in resistance (Figure �8). Conventional carbon brushes lack the required low resistance, and so brush manufacturers use brushes that are impregnated with silver. Another type of shaft grounding brush is more like a brush in that it has bris-tles that contact the shaft surface (Figure �6). Manufac-turers of this type of grounding brush usually use silver or gold impregnation in the bristles, for low resistance and to maintain a low resistance film on the shaft.

If the brush path diverting current from the bearing increases in resistance, such as due to contamination or a change in the film, more potentially damaging cur-

The addition of a grounding brush creates a parallelcircuit, sharing the current flow through the bearing.The flow of current through each path of the parallelcircuit depends upon the relative resistance. Currenton the drive end remains unchanged.

Grounding brush installedto create parallel circuit.

Electrically isolating one bearing can be done byinsulating the housing or the bracket at the frame, orby use of special factory-insulated bearings. Whenthis method is used, the flow of current through bothbearings is halted. No current means no damage.

figure 17: insulated opposite drive end bearing

Circulating current due to magnetic dissymmetry can be eliminated by insulating one bearing.

figure 18: shaft grounding carbon-silver brush

Low-resistance silver-impregnated brushes are suitable for use as shaft grounding brushes.

�0

rent will flow through the bearing. The part of the brush circuit most at risk for increased resistance is the brush to shaft contact surface. As the shaft surface becomes dirty or corroded the resistance of the film formed in the brush path increases. And higher resistance in the brush circuit results in less current through it, and cor-respondingly higher current through the bearing.

To most effectively protect both motor bearings a grounding brush should be placed on the inboard side of each bearing. Therefore, two brush installations are needed. However, if there is a possibility of circulating current between the motor and the driven equipment, and only one brush is used, it should be on the drive end.

electrostatic dischargesThe second major cause of damaging bearing current

is static electricity. Damaging levels of bearing current caused by electrostatic discharges can occur in applica-tions such as belt drives, paper roll winders and fans or blowers. The solutions previously mentioned for mag-netic dissymmetry can be used to address electrostatic discharges. In some applications, such as rolls of pro-cess product that develop static electricity, a grounded garland-like material contacts the roll to “bleed off” the static electricity.

capacitive couplingCapacitive coupling between stator and rotor

(Figure 8) can occur when the motor is supplied from a VFD. These drives can generate a “common mode volt-age” which raises the three phase winding neutral po-tential significantly above ground potential. The result is discharges from shaft to ground, with the current path being through either or both bearings to ground.

According to the International Electrotechnical Com-mission (IEC) standard IEC 60034-25, “Guidance for the design and performance of a.c. motors specifically designed for converter supply” some of the physical fac-tors and other features of a motor that can lead to shaft voltages and consequently bearing currents include:

• Large physical size or high output power of the ma-chine tends to increase the induced shaft voltage.

• The physical shape of the motor also has an effect on the induced shaft voltage: short and fat shape is generally better than long and thin motor design.

• High pole numbers tend to reduce the induced shaft voltage.

• Low running speed and high bearing temperature as well as high bearing load increase the bearing current risk due to thinner lubricant film.

capacitive coupling solutionsSolutions when dealing with capacitive coupling in-

clude insulating both bearings (Figure �9), and using

shaft grounding brushes for both bearings. The bear-ings can be insulated by a number of different methods including, insulated housings, insulated bearing jour-nals, insulated bearing outer race, insulated bearing in-ner race, and ceramic rolling elements.

It is worth noting that insulating only one bearing, such as the non-drive end is not effective as the current still has a path through the drive end bearing. Conduc-tive bearing greases are also available, using metallic particles suspended in the grease to make it conduc-tive. However there has not been strong evidence in-dicating that this method is effective. One laboratory study found indications that the conductive material in the grease accelerated mechanical wear and would therefore shorten the life of a bearing.

Another key point to put the capacitive coupling is-sue in perspective is that VFDs are the source. If future drive technology can make advances in this area it may be possible through drive design to eliminate bearing currents due to capacitive coupling.

insulated bearingsThe key word in the term capacitive coupling is “ca-

pacitive”. What is desired is to have a low capacitance if the circuit is insulated. To achieve this with an insulated bearing it is better to insulate the inner race (journal face) rather than the outer race (housing face) as the inner race being of smaller diameter it is likewise small-er in area. This is an advantage of the hybrid ceramic bearing: The capacitance is based on the small contact area between the rolling elements and the races, and the thickness of the insulation is equal to the diameter

The addition of a grounding brush creates a parallelcircuit, sharing the current flow through the bearing.The flow of current through each path of the parallelcircuit depends upon the relative resistance. Currenton the drive end remains unchanged.

Grounding brush installedto create parallel circuit.

Electrically isolating one bearing can be done byinsulating the housing or the bracket at the frame, orby use of special factory-insulated bearings. Whenthis method is used, the flow of current through bothbearings is halted. No current means no damage.

Bearing is electricallyisolated to interrupt flowof current.

figure 19: insulating both bearings

Both bearings insulated to prevent bearing current.

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of one rolling element.Whenever insulation is added, heat transfer is re-

duced compared to when there is metal to metal con-tact. An inherent trade off with bearing insulation is to use the thickest coating possible without significantly affecting heat transfer, and consequently causing the bearing operating temperature to increase.

Some bearing manufacturers offer bearings with ce-ramic or aluminum oxide coatings on the outer race. When the outer race rather than the inner race is in-sulated, a thicker ceramic or aluminum oxide coating, sometimes as thick as 0.012” (300 μm) is used. Be-cause these coatings cover a wider area than on the inner race, they are not as effective as most of the other bearing insulating options.

If the bearing or bearings in a motor are found to have an insulated coating on the outside of the outer race, and inspection does not reveal evidence of bear-ing currents, it is reasonable to consider replacing with the same type of bearing. Conversely, if evidence of damaging bearing current is detected, consider using a more effective insulation method. If inspection of an in-sulated bearing reveals damage from bearing currents, inspect the bearing cap and bracket as an assembly. Undesirable contact between the bearing and the bear-ing cap or bearing resistance detectors (RTDs) may be bypassing the insulation, rendering it ineffective.

A drawback to the ceramic or aluminum oxide coat-ing is that it chips easily. This can occur when a bear-ing is removed, or if the coating is accidentally struck. Ceramic or aluminum oxide coating of a bearing jour-nal (Figure 20) makes dynamic balancing more difficult

as the rotor can not be placed on the coated journals in the balancing stand. An adjacent shaft area that is concentric to the bearing journal should be used. The line-loading of the balance stand rollers combined with the rotor weight would probably fracture the ceramic or aluminum oxide coating.

If an epoxy is used it must be able to withstand the bearing compression and load forces. Wear-resistant and high-strength epoxy products such as Belzona Su-permetal or Devcon Titanium putty are often suitable, but other less durable epoxies should be avoided.

Sleeve bearings may be ceramic or aluminum oxide coated on the outer shell to mitigate bearing current. The coating can be applied by the original manufactur-er, or as part of a retrofit. The oil film in the bearing also acts as an insulator to prevent or reduce bearing cur-rent. Thus coating of the outer race of a sleeve bearing is more effective against bearing currents than coating a rolling element bearing. Care is needed in handling the bearing as the ceramic or aluminum oxide coating cracks easily.

Other methods of insulating sleeve bearings include adding Micarta or Glastic (fiberglass) between the outer shell and the housing (Figure 2�), applying Scotchply to the outer shell of the bearing, and applying armature type banding material to the outer shell of the bearing. Except for the last method, these are all usually effec-tive. The banding material tends to delaminate and de-grade due to the lubricating oil coming in contact with it; therefore this method is not recommended.

insulated housingsInsulating the housing (Figure 22) is frequently done

by the original manufacturer and by rebuilders as a ret-

figure 20: insulated bearing journal

A potential drawback to ceramic coatings is that they chip easily.

figure 21: insulated sleeve bearing

Sleeve bearing outside diameter insulated with Glastic G-9.

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rofit method of providing bearing insulation. The thicker the housing insulation, the lower the capacitance, and consequently, the potential bearing current is lower. However, thicker insulation inhibits heat transfer, so this method always requires a judgment in thickness to be used.

hybrid ceramic bearingsThe use of ceramic rolling element bearing offers a

simple, though relatively expensive, way to insulate the bearings. These bearings are termed “hybrid ceramic bearings” (Figure 23) because although the rolling el-ements are ceramic, the inner and outer races are of hardened bearing steel composition. Completely ce-ramic bearings, with ceramic races as well as rolling elements, are made but are not normally readily avail-

able and are expensive even when compared to hybrid ceramic bearings.

The insulation thickness of the hybrid bearing is the diameter of the rolling element (i.e., the ceramic ball or roller). The surface area is limited to the small contact area between the balls and the races. The ceramic roll-ing element therefore results in an insulation thickness much greater than that of coated race options, and with a much lower capacitance.

At one time the load rating of hybrid ceramic bearings was typically less than that of comparable conventional all-steel bearings. However, many manufacturers now offer hybrid bearings with dynamic load capacity equal to or in some cases greater than conventional steel bearings. It is good practice to verify the dynamic load rating of a ceramic bearing that is to be used in place of a conventional steel bearing. Another advantage of ce-ramic rolling elements is that they are lighter than steel. Consequently, the ceramic bearing is capable of higher operating speeds than comparable steel rolling element bearings.

other solutions

Where it is possible, other solutions include insulat-ed couplings (between motor and driven equipment or accessory such as tachometer and motor), insulating the bearings of the driven equipment, or adding shaft brushes to the driven equipment. In belt drive applica-tions, the belts may or may not effectively diminish the capacitive shaft voltage of the motor.

precautions with insulating bearingsInsulating both motor bearings is normally effective

in eliminating bearing currents in the motor. However, doing so can create a current path between the mo-tor shaft and the connected equipment, e.g., fan, pump, gearbox. If that is the case, damage due to bearing cur-rent can occur in the driven equipment and accessories such as tachometers. Solutions in such a case include an insulated coupling or adding a shaft grounding brush to the drive end bearing of the motor. The coupling in-terrupts the shaft current path and the grounding brush creates a bypass circuit for shaft and bearing current, and significantly reduces the shaft voltage.

When the bearing housing is insulated, provision must be made to insulate the end faces of the bearing. Axial changes in bearing location could cause the bearing to contact a bearing cap, which if not insulated would by-pass the bearing insulation path. Common methods of insulating the bearing face are to apply an insulation material such as Micarta or Glastic (fiberglass) to the face of the bearing caps on each side of the bearing.

shaft grounding brushesAdding grounding brushes to motors with insulated

figure 22: insulated housing

Insulated rolling bearing housing.

figure 23: ceramic bearing

Hybrid ceramic roller and ball bearings

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bearings is not the primary application for shaft ground-ing brushes. They are more commonly used “stand-alone” to divert shaft current away from the bearings. Although they can be effective in VFD applications, insulating the bearing circuit is usually the preferred method. That is because the shaft grounding brush, as a bypass or parallel circuit, will always allow some cur-rent to pass through the bearings.

To be effective, shaft grounding brushes must main-tain a very low resistance (actually low impedance) cir-cuit between motor frame and shaft. The magnitude of the shaft current in most cases does not create a very high current density in the grounding brush. That in turn can lead to excess film formation, with the film being of increased resistance compared to the bare shaft sur-face. As the film resistance increases, the grounding brush circuit carries less current, and the bearing more current. The result is more rapid degradation of the bearing due to the increased rate of electrical discharge machining (EDM), and premature bearing failure.

Conventional carbon-silver (Figure �8) or bristle-type (Figure �6) shaft grounding brushes also represent a maintenance issue as they wear over time and must be replaced, and should be inspected periodically. There is a recent development in shaft grounding brush tech-nology that uses an atmosphere of microscopic carbon fibers to create a conductive path from frame to shaft. The device is similar in appearance to a shaft lip seal, and is known as the Aegis “shaft grounding ring™” (Fig-ure 24).

faraday shieldOne other method of dealing with capacitive coupling

is to install a Faraday shield in the stator. This method typically uses a grounded copper foil material on top of the stator windings. It creates an electrostatic shield that reduces the magnitude of currents caused by the capacitive coupling between stator and rotor. The Fara-day shield is of complex and fragile construction, mak-ing it impractical and uneconomical for mass-produced motors.

groundingThe quality of the ground between motor and drive

is also a critical factor. The higher the common mode voltage and VFD switching frequency is, the greater the possibility of damaging bearing currents. One alterna-tive is to reduce the switching frequency of the VFD. In many applications the switching frequency can be reduced without negatively affecting drive or motor per-formance.

A partial solution that can reduce bearing currents is to use shielded and stranded ground cable (Figure �3) to establish a dedicated common ground path between motor and drive as skin effect is a factor. The higher the frequency, the more alternating current (AC) “trav-els” on or near the surface (“skin”) of the conductor, so the resistance-to-voltage flow is affected by the surface area of the conductor. Stranded cable has more surface area than a solid conductor, thus provides a lower resis-tance (impedance) path, and the shielding helps reduce the high frequency effects.

filters and chokesInstalling filters between the VFD and motor also helps

reduce the magnitude of bearing currents, but does not eliminate the current. The filters modify the VFD output waveform so as to minimize the common mode voltage. The main drawbacks to filters are their cost and the complexities associated with their installation. Adding filters, or chokes (modify the current waveform), also helps reduce transient voltages that could overstress the motor windings and increase audible noise.

figure 24: shaft grounding ring

Carbon-micro-fiber shaft grounding brush.