Download EASA-Shaft-Bearing

dealing with shaft and bearing currents
By Tom Bishop
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
Figure 1: fluting
Introduction
This paper addresses key issues related to shaft
and bearing currents in electric motors and generators.
Critical topics that will be covered include recognizing symptoms of shaft and bearing currents, and determining if damaging current levels are present. Also,
possible causes of the damaging current, such as machine 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 insulators, isolators, and ceramic bearings, will be offered.
While shaft and bearing currents are not a new problem (papers on the subject date back prior to 1930),
what is “new” is the increased understanding of how to
identify and solve the problem. Shaft and bearing currents have been described as shaft voltages, circulating 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 level, 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 phenomenon 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.
Fluting of the bearing races due to electrical current.
Figure 2: frosting
Recognizing symptoms of bearing
current
All too frequently the first symptom of bearing current
is audible noise from the bearing, indicating it is in advanced 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 raceways. The lubricant may also be dark in appearance.
Frosting of the balls/rollers due to electrical current.
Current damage
The 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 3: darkened finish
Figure 5: dull finish
The darkened finish of the ball on the right was caused by
electrical current.
Dull finish and electrical craters in the inner race of a roller
bearing.
Figure 4: pitting
and races. Crater sizes are small, mostly from 0.00020.0003” (5 - 8 μm) in diameter, regardless of whether
the crater is on an inner ring, outer ring or a rolling element. The true shape of these craters can only be seen
under a microscope using very high magnification.
Other symptoms
If 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 destroyed and the root cause of the failure will not be determined.
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
Microscopic view of electrical pitting.
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” (0.1- 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 pattern of multiple lines across the inner and outer races
(Figure 1). The reason for this fluting is mechanical resonance 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 bearing 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 appears dull and is characterized by molten pit marks (Figure 5). Multiple micro-craters cover the rolling elements
Figure 6: “picket fence”
Note the “picket fence” pattern of the bearing outer race.
This is often a visual clue to shaft voltages.
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 appearance, due to the burning of the metal that leaves ferrous
oxide.
Bearing current damage usually initially appears visually in the areas of the bearing that are most heavily 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 damage 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 (0.0000080.00008”) 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 charring 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.
the presence of damaging bearing currents.
If the shaft to frame voltage exceeds 100 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 MG1, 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 significantly 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
Determining if damaging current levels
are present
At present there is no known method to measure bearing currents and no practical way to directly measure
shaft currents. MeaFigure 7: rogowskisuring shaft current
style coil
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 (Figure 7) can be wrapped
around the shaft and A Rogowski-style coil that can
used to measure shaft be wrapped around a shaft to
current. The usual way measure shaft current.
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.
Figure 8: capacitive coupling
Grease
Inner race
Ball bearing
Outer race
Shaft
Rotor
Faraday
Shield
Rotor to
winding
Windings
Rotor to
ground
Shaft to frame voltage
Among 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 highest voltage measured when assessing the possibility of
Stator
VFD in
Capacitive coupling between the stator and the rotor supplied from the VFD.
bearing currents.
This common mode voltage oscillates at high frequency 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 current may be present.
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 bearings 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%.
Bearing current
If 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 320 A, depending on the size of the bearings, the rate
of rise of the voltage at the motor terminals and the inverter DC bus voltage level.
Residual magnetism
Bearing currents can result if a shaft has residual magnetism. Residual magnetic field levels are best measured 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 15 year period.
Figure 9: Path of circulating current
Bearing is electrically
Maximum allowable residual
magnetic
field levels
isolated
to interrupt
flow (measured in free air)
of current.
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 considerably lower than the actual levels inside a component.
The addition
of path
a grounding
creates
a parallel
Circulating
curent
throughbrush
the shaft,
bearings
and
frame.
circuit, sharing the current flow through the bearing.
The flow of current through each path of the parallel
circuit depends upon the relative resistance. Current
on the
drive end
unchanged.
The
reason
thatremains
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 divided by the rolling element contact area, and can cause
damage when it exceeds 1 A/mm2 (645A/in2). The difficulty with this tolerance is that the current density cannot 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
Possible causes of the damaging
Electrically
isolating one bearing can be done by
current
insulating
the
the bracket
at the
frame, or
There arehousing
three or
main
sources
of circulating
currents
by use of special factory-insulated bearings. When
that may pass through the shaft and consequently the
this method is used, the flow of current through both
bearings.
One No
source
is means
magnetic
dissymmetry, anothbearings
is halted.
current
no damage.
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 simultaneously, 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 conditions. Less common causes of shaft and bearing currents 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
be present independently or simultaneously, and thus
result in bearing currents.
es abutting each other (like slices of a pizza pie). The
space between each adjacent lamination segment varies 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 permeability, which can vary from one part of the circular lamination to another part. The permeability is associated
with the flux density at which the core steel will magnetically saturate. Thus, if the permeability of a lamination varies, some parts of it may become saturated and
become hotter, leading to thermally induced distortion
such as bowing. For this reason, some motor manufacturers 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.
Magnetic dissymmetry
Magnetic dissymmetry is due to any differences in the
magnetic circuit between stator and rotor. It is often associated with larger motors that have segmented laminations and are supplied from sinusoidal (as opposed
to VFD) power sources. The resulting asymmetric flux
in the motor results in a low frequency circulating current through the bearings.
Although dissymmetry in the core laminations is not
the only source of magnetic dissymmetry, it is the predominant cause. Other causes of magnetic dissymmetry 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 machines, where the individual field poles and interpoles
are likely to have variations in individual air gap. An informal study found shaft currents in most DC machines
larger than 10 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 symmetrical, 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 10), 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-
Electrostatic discharges
The 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 11)
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. (100 meter)
blades), the greater the potential bearing current.
Figure 11: dull finish
Figure 10: segmented laminations
Dull finish and electrical craters in the inner race of a roller
bearing.
Capacitive coupling
Capacitive coupling between stator and rotor often
occurs when the motor is supplied from a VFD. Voltage
associated with the extremely fast switching of insulat-
Segmented laminations, each make up 1/8 of a layer of the
stator core.
Figure 12: pwm waveforms
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 potential 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 capacitively 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 13) be
used 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)
“travels” 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 resistance (impedance) path.
Voltage (top) and current (bottom) waveforms of a PWM
drive.
ed gate bipolar transistors (IGBTs) in the pulse-width
modulated (PWM) output waveform (Figure 12) of the
VFD is induced in the rotor and shaft via the air gap between 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, resulting in a brief discharge pulse through the bearing to
ground. This process continually repeats itself, thus
increasing the magnitude of the damage to the bearing. The discharge pulses are a form of electrical discharge machining (EDM) that literally removes material
from the surfaces of the affected parts of the bearings
(Figure 11). 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 frequency 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 1 kHz. That allows 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-
Figure 13: stranded cable
Stranded low-impedance cable made specifically for VFD
applications.
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-
ysis can only be applied after a bearing has failed.
Although diagnostic testing may reveal the presence
of potentially damaging bearing currents, a better approach 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 detection 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.
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 highest voltage measured when assessing the possibility of
the presence of damaging bearing currents.
Shaft voltage can be measured using a digital multimeter (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 pencil 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 writing 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 15).
Vibration analysis
Vibration analysis can be used to determine if bearings have fluting damage, which may have been caused
by discharge currents. A high resolution spectrum of the
2-4kHz range (Figure 14) 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 diagnose impending bearing failure.
Figure 15: measuring shaft voltage
Figure 14: vibration spectrum
Shaft voltage can be measured with a DMM. Note the use
of a pencil to contact the shaft.
If the voltage exceeds 100 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
MG1, 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 millivolts 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.
High resolution vibration spectrum.
Shaft to frame (ground) analysis
As mentioned previously, there is no known method
to measure bearing currents and no practical way to directly 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 voltage
Among the challenges in measuring the shaft to frame
Shaft current
An “old-timers” method of checking for damaging levels 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 second 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.
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 attached to the shaft brush. It is preferable to use a low-impedance 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 oscilloscope, 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 1.4 times the AC rms level. This estimated peak
voltage, however, may be considerably below the actual 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 voltage and speed. The procedure is identical to that of the
first method, with the exception being that a low-resistance ammeter is used in place of the oscilloscope. In
this test arrangement, the ammeter is used as a lowimpedance, 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 bearing current levels are present. However it suggests that
trending of these tests can be useful for detecting increased levels and therefore the increased probability
of damaging bearing currents.
IEEE 112 methods
The following test procedures are based on three alternative methods of measuring shaft potential for shaft
circulating currents described in IEEE standard 112. 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 16) 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
Figure 16: shaft grounding brush
Lubricant analysis
Lubricant analysis utilizes a testing laboratory to analyze the bearing lubricant. Grease sampling is often
impractical as there is usually no direct means of obtaining a fresh sample unless the motor is partially disassembled. 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 because of the risk of a rapid loss of oil.
If the lubricant analysis indicates the presence of metals from the bearings, electrical discharge machining
(EDM) associated with bearing current discharges are
a possible cause. There is however, no means of distin-
“Toothbrush” style shaft grounding brush shorts out bearing
current path.
Magnetic dissymmetry
Magnetic dissymmetry can result in a circulating current 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 interrupting the circuit, usually by insulating the opposite
drive end bearing (Figure 17).
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 parallel (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 resistance 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 solution. The special grounding brushes supplied for this
purpose are extremely low in resistance (Figure 18).
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 bristles that contact the shaft surface (Figure 16). Manufacturers 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-
guishing between particles from the passage of electrical current and particles of mechanical wear origin.
Microscopic analysis
Microscopic analysis can be performed by a testing
laboratory with that capability, often a bearing manufacturer. 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 mechanical origin often appears as smearing marks with evidence 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 identified 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, another is electrostatic discharges, and the third is capacitive
coupling between the stator windings and rotor. Any of
these sources may be present independently or simultaneously, and thus result in bearing currents.
Figure 17: Insulated opposite drive end
bearing
Figure 18: shaft grounding
carbon-silver brush
h installed
l circuit.
parallel
bearing.
e parallel
. Current
Electrically
isolating
bearingdissymmetry
can be done by
Circulating
current
due toone
magnetic
can be
insulating
the housing
orbearing.
the bracket at the frame, or
eliminated
by insulating
one
by use of special factory-insulated bearings. When
this method is used, the flow of current through both
bearings is halted. No current means no damage.
Low-resistance silver-impregnated brushes are suitable for
use as shaft grounding brushes.
Figure 19: insulating both bearings
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 correspondingly higher current through the bearing.
To most effectively protect both motor bearings a
Grounding
brush installed
grounding brush should be placed
on the inboard
side
to create
parallel circuit.
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.
Bearing is electrically
isolated to interrupt flow
of current.
Electrostatic discharges
The second major cause of damaging bearing current
is static electricity. Damaging levels of bearing current
caused by electrostatic discharges can occur in applications such as belt drives, paper roll winders and fans or
blowers. The
solutions
mentioned
magThe addition
of a previously
grounding brush
creates afor
parallel
netic dissymmetry
can
be
used
to
address
electrostatic
circuit, sharing the current flow through the bearing.
discharges.
some
applications,
such
asofrolls
of proTheIn
flow
of current
through each
path
the parallel
cess product
that
develop
electricity,
a grounded
circuit
depends
uponstatic
the relative
resistance.
Current
on the
drive end
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isolating to
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can be done
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Both Electrically
bearings insulated
prevent
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insulating the housing or the bracket at the frame, or
by use of special factory-insulated bearings. When
shaft grounding brushes for both bearings. The bearthis method is used, the flow of current through both
ings can
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meansofno
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including, insulated housings, insulated bearing journals, insulated bearing outer race, insulated bearing inner 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. Conductive bearing greases are also available, using metallic
particles suspended in the grease to make it conductive. However there has not been strong evidence indicating 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 issue 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.
Capacitive coupling
Capacitive coupling between stator and rotor
(Figure 8) can occur when the motor is supplied from a
VFD. These drives can generate a “common mode voltage” which raises the three phase winding neutral potential 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 Commission (IEC) standard IEC 60034-25, “Guidance for
the design and performance of a.c. motors specifically
designed for converter supply” some of the physical factors 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 machine 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.
Insulated bearings
The key word in the term capacitive coupling is “capacitive”. 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 smaller 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
Capacitive coupling solutions
Solutions when dealing with capacitive coupling include insulating both bearings (Figure 19), and using
10
of one rolling element.
Whenever insulation is added, heat transfer is reduced compared to when there is metal to metal contact. 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 ceramic or aluminum oxide coatings on the outer race.
When the outer race rather than the inner race is insulated, a thicker ceramic or aluminum oxide coating,
sometimes as thick as 0.012” (300 μm) is used. Because 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 bearing 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 insulated bearing reveals damage from bearing currents,
inspect the bearing cap and bracket as an assembly.
Undesirable contact between the bearing and the bearing cap or bearing resistance detectors (RTDs) may be
bypassing the insulation, rendering it ineffective.
A drawback to the ceramic or aluminum oxide coating is that it chips easily. This can occur when a bearing is removed, or if the coating is accidentally struck.
Ceramic or aluminum oxide coating of a bearing journal (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 Supermetal 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 manufacturer, or as part of a retrofit. The oil film in the bearing also
acts as an insulator to prevent or reduce bearing current. 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 21), 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 effective. The banding material tends to delaminate and degrade due to the lubricating oil coming in contact with it;
therefore this method is not recommended.
Figure 21: insulated sleeve bearing
Figure 20: insulated bearing journal
Sleeve bearing outside diameter insulated with Glastic G-9.
Insulated housings
Insulating the housing (Figure 22) is frequently done
by the original manufacturer and by rebuilders as a ret-
A potential drawback to ceramic coatings is that they chip
easily.
11
Figure 22: insulated housing
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 rolling 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 ceramic 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.
Insulated rolling bearing housing.
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.
Other solutions
Where it is possible, other solutions include insulated 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 applications, the belts may or may not effectively diminish the
capacitive shaft voltage of the motor.
Hybrid ceramic bearings
The 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 elements are ceramic, the inner and outer races are of
hardened bearing steel composition. Completely ceramic bearings, with ceramic races as well as rolling
elements, are made but are not normally readily avail-
Precautions with insulating bearings
Insulating both motor bearings is normally effective
in eliminating bearing currents in the motor. However,
doing so can create a current path between the motor shaft and the connected equipment, e.g., fan, pump,
gearbox. If that is the case, damage due to bearing current 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 interrupts 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 bypass 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.
Figure 23: ceramic bearing
Shaft grounding brushes
Adding grounding brushes to motors with insulated
Hybrid ceramic roller and ball bearings
12
Faraday shield
One 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 Faraday shield is of complex and fragile construction, making it impractical and uneconomical for mass-produced
motors.
bearings is not the primary application for shaft grounding brushes. They are more commonly used “standalone” 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 current to pass through the bearings.
To be effective, shaft grounding brushes must maintain a very low resistance (actually low impedance) circuit 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 surface. 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 18) or bristle-type
(Figure 16) 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 technology 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™” (Figure 24).
Grounding
The 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 alternative is to reduce the switching frequency of the VFD.
In many applications the switching frequency can be
reduced without negatively affecting drive or motor performance.
A partial solution that can reduce bearing currents is
to use shielded and stranded ground cable (Figure 13)
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) “travels” 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 resistance (impedance) path, and the shielding helps reduce
the high frequency effects.
Filters and chokes
Installing 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.
13