Different Types of
Bearing Currents –
The Fundamentals
(plus)
Mike Melfi
Different Types of Bearing Currents
– The Fundamentals
• Background
• Bearing Damage / Failure Mechanisms
• Causes
– Line-Fed
– Inverter-Fed
• Solutions
– Line-Fed
– Inverter-Fed
• Summary & Conclusions
Background
Q: Since bearing currents in rotating
machinery have been documented for at
least 75 years, why is this a
contemporary issue?
Background
Q: Since bearing currents in rotating machinery have
been documented for at least 75 years, why is this a
contemporary issue?
A: Modern PWM inverters create both
common mode voltages (CMV) and common
mode currents (CMC) which provide new
opportunities for current to flow through
rotating bearings (along with couplings,
gears, etc)
Background
While the 75 year old sources of bearing
currents are well understood and solutions
exist, it is important to keep them in mind to
avoid resurrecting them in trying to solve
the challenges brought on by common mode
voltages and currents.
Bearing Damage / Failure
Mechanisms
Individual arc damage spots
Fluting in outer race, from
prolonged operation after
damage from current flow
Bearing Damage / Failure
Mechanisms
Fluting on inner race, from prolonged
operation after damage from current flow
Fluting in outer race
Bearing Damage / Failure
Mechanisms
• Interrupted current causes melting and
“re-hardening” of the race material,
creating untempered martensite, which is
brittle and prone to fatigue
Bearing Damage / Failure
Mechanisms
Quenched and tempered ball
bearing inner ring
White Layer,
untempered
martensite
Bearing Damage / Failure
Mechanisms
• Interrupted current causes melting and “re-hardening”
of the race material, creating untempered martensite,
which is brittle and prone to fatigue
• The normal bearing loads are then capable of
breaking off small pieces of this brittle material
• Subsequent running on this brittle surface and
in the presence of the damage “trash” material
creates the “fluting”
Bearing Damage / Failure
Mechanisms
• If the damaged material does not progress to a
fluted pattern from subsequent running, two
other patterns may be seen
– A “frosted” surface may appear, or
– A number of “pits” may be visible under
high magnification
• The verification of current flow as the root
cause requires more than visual inspection
Sine Wave Bearing Currents
• “If it were possible to design a perfectly
balanced and symmetrical machine, both
theory and practice indicate that no
bearing current could exist” - C. T.
Pearce, Bearing Currents - Their Origin
and Prevention, The Electric Journal,
August 1927.
Sine Wave Bearing Currents
• Alternating flux “linking” the shaft …
• Net flux encircling the shaft is
typically due to asymmetric magnetic
properties of stator or rotor core
• Bearing current created by transformer
action in “single turn” secondary
(shaft, bearings, frame)
Sine Wave Bearing Currents
Flux path
Shaft
Boyd and Kaufman, 1959
Sine Wave Bearing Currents
• Currents flow thru shaft, bearings,
endshields, and frame
• Axial voltage on shaft can be measured if
a bearing is insulated (IEEE Std 112 1996)
• Small shaft voltage (500 mV) can lead to
bearing currents above 20 amps
• Bearing damage is more likely to occur in
larger machines
Common Mode Voltage / Current
• Modern PWM drives create switching
patterns where instantaneous average
voltage to ground is not zero.
• Voltage has a rapid change of magnitude
with respect to time (dV/dt)
• High dV/dt results in capacitively coupled
currents from motor windings to ground
through several paths
I = C x dV/dt
Common Mode Voltage / Current
PHASE
VOLTS
CMV
Common Mode Voltage / Current
PHASE
VOLTS
CMV
Common Mode Voltage / Current
PHASE
VOLTS
CMV
Common Mode Voltage / Current
High Frequency Current Paths
I = C x dV/dt
Common Mode Voltage / Current
High Frequency Current Paths
Common Mode Voltage / Current
High Frequency End-End Circulation
Common Mode Voltage / Current
Rotor Discharge Current
Common Mode Voltage / Current
High Frequency Current Paths –
Capacitive Charging of Rotor / Bearing
Stator Winding
Rotor
+
VCM
+
CSR
CRF
CSF
-
Frame
Bearing Voltage : Vb =
Cb
VCM
Bearing
Csr
Csr + Cb + Crf
Common Mode Voltage / Current
Rotor Discharge Current
Common Mode Voltage / Current
Transient Frame Voltage Discharge
Common Mode Voltage / Current
Peak Amps Through Bearing
Bearing Current Relative Magnitude
40
30
30
20
10
3
0.5
0
Stator Winding to
Frame/Shaft
Discharge
dv/dt Charging
Bearing Current Solutions
• Eliminate or reduce common mode
voltage / current (Drive design issue)
• Create best high frequency ground
paths between drive, motor, and load
• Electrostatic shielded induction motor
• Insulated bearings
• Shaft grounding brush
Bearing Current Solutions
Insulated Opposite Drive-End Bearing
for Circulating Type Currents
Bearing Current Solutions - Precautions
Shaft Brush Without Opposite End
Insulated Bearing (Larger Motor)
(One Bearing at Increased Risk)
b)
Rotor
Bearing at Risk
Bearing Current Solutions - Precautions
Insulated Opposite Drive-End Bearing
and Drive-End Shaft Brush
(Bearings in Coupled Equipment Still at Peril)
Bearing Current Solutions
Insulated Opposite Drive-End
Bearing, Drive-End Shaft Brush,
and Coupled Equipment Bond Strap
Bearing Current Solutions
Two Insulated Bearings, Drive-End
Shaft Brush, and Coupled Equipment
Bond Strap
Bearing Current Solutions –
Faraday (Electrostatic) Shield
• Add grounded conductive layer
between stator and rotor
• Eliminates stator to rotor coupling
• Will not eliminate stator winding to
frame coupling
– Still need good high frequency ground current
path from motor to drive ground
Bearing Current Solutions
Faraday Shield to Prevent Rotor Charging / Discharging
(Bearings Still at Peril from Transient Frame Voltage
Discharge when Shaft is Conductively Coupled to
Grounded Equipment)
Bearing Current Solutions
Faraday Shield
and Coupled Equipment Bond Strap
Bearing Current Solutions
• Internal, end-end from magnetic
asymmetry
 Insulate opposite drive-end
bearing
 Insulate both bearings
Bearing Current Solutions
• Shaft Extension Current (stray ground current)
 Insulate coupling
 Insulate bearings
 Bond strap from motor to load
 Better low impedance ground in
cable from inverter to motor
Bearing Current Solutions
• Discharge of voltage on rotor
 Faraday (electrostatic) shield
 Shaft brush
Measurements (Voltage)
250
V/div
12.5
V/div
Common
Mode
Voltage
Shaft
Voltage
Measurements (Current)
Internal End-End Circulation
2 A/div
Measurements (Current)
Shaft
Extension
Current
(30 Amp Pulse)
Measurements
• Other than internally-sourced
circulating currents, all data is at high
frequency
• Data tends to be non-repetitive
• Oscilloscope triggering technique
strongly influences perceived results
Conclusions
• Current flow in rotating bearings is
not new
• Common mode voltages and
currents from modern inverters can
cause current flow through bearings
(plus couplings, gears, etc)
Conclusions
• Corrective actions are dependent
upon the particular type of current
flow
• Transient (high frequency) nature of
the voltages and currents imposes
different requirements than traditional
60 Hz waveforms
Conclusions
• Since the sources of the currents as well
as the paths are typically outside the
machine whose bearings are taking the hit,
a thorough understanding of the system is
key
• Grounding is important, but more in the
sense of point to point (low impedance)
“bonding” rather than “earthing”
Plus ...
• Bonus material
Common-Mode Filters?
Virtual Brush? (doped or not)
SKF Insocoat Coated /
Insulated Bearings
SKF Insocoat Coated /
Insulated Bearings
• Developed for DC traction (rail) motors
• Originally 2 mils thick, with option for 4 mils
• Now standardized on 4 mil thickness, with option
for 12 mils offered
• Now available for ID coating, rather than OD
• Good DC insulation, but at pwm high
frequencies, current still flows. Whether a 4 or 12
mil coating reduces or eliminates pwm type
damage is not conclusively proven.
Hybrid / Ceramic Bearings
“Conductive” Grease
While the notion of a conductive grease as a solution may
sound appealing, the electrical behavior of bearing lubricants
is not as simple as "insulating" versus "conducting." Both
the behavior of the grease in bulk as well as the behavior of
the thin oil film separating races from rolling elements is
strongly dependent on external influences, including the
presence of a voltage. As a result, the current and voltage
characteristics seen in a rotating bearing are not simply
described by a resistive value. In fact, it is not simply
described by a combination of fixed resistors, capacitors, and
other circuit elements. It has a "memory" effect, based on
past applied voltages and current flow, as well as behaviors
that may best be described as "stochastic."
“Conductive” Grease
• Different greases can have varying electrical
characteristics, based on their chemical composition,
but still would have the "inconsistent" behavior as
described above.
• Any proposed grease would obviously need to not
degrade the "normal" properties expected in a
bearing.
• The conclusion of the points above is that a change
to a grease with different electrical properties is not
a solution to the basic problem of bearing currents
(for neither VFD nor line-fed motors).
Shaft Voltage
Shaft Voltage
Inverter Driven Induction Motor Bearing Current Solutions
References:
www.reliance.com/pdf/white_papers/ Inverter_Driven_Indct_Mtrs_Shft_Brngs.pdf
[1]
C. T. Pearce, “Bearing Currents – Their Origin and Prevention,” The Electric Journal, Vol.XXIV,
No.8, pp.372-376, August 1927.
[2]
IEEE, Std-112 -1996, Section 9.4, Shaft Currents and Bearing Insulation, New York, NY: IEEE.
[3]
J. M. Erdman, Russel J. Kerkman, David W. Schlegel and Gary L. Skibinski, “Effect of PWM
Inverters on AC Motor Bearing Currents and Shaft Voltages,” IEEE Transactions on Industry Applications, Vol.32,
No.2, pp.250-259, March/April 1996.
[4]
R.F. Schiferl, M.J. Melfi, J. S. Wang, “Inverter Driven Induction Motor Bearing Current Solutions,”
IEEE Petroleum & Chemical Industry Conference Proceedings, Sept., 2002.
[5]
Quirt, Richard C., "Voltages to ground in Load-Commutated Inverters," IEEE Transactions on
Industry Applications, Vol 24, No. 3, pp 526-530, May/June, 1988.
[6]
S. Chen, D. Fitzgerald and T. A. Lipo, “Source of induction motor bearing currents caused by PWM
inverters,” IEEE Energy Conversion, Vol.11, No.1, pp.25-32, March 1996.
[7]
A. von Jouanne and H. Zhang, “A Dual Bridge Inverter Approach to Eliminating Common Mode
Voltages and Bearing and Leakage Currents,” IEEE Trans. Power Electronics, Vol. 14, No. 1, pp. 43-48, Jan. 1999.
[8]
J. M. Bentley and P. J. Link, “Evaluation of motor power cables for PWM AC drives,” IEEE Trans.
Industry Applications, vol.33, No.2, pp. 342-358, March/April 1997.
[9]
Doyle Busse, Jay Erdman, Russel J. Kerkman, Dave Schlegel and Gary Skibinski, “An Evaluation of
the Electrostatic Shielded Induction Motor: A Solution for Rotor Shaft Voltage Buildup and Bearing Current,”
IEEE Transactions on Industry Applications, Vol.33, No.6, pp.1563-1570, Nov/Dec. 1997.
Inverter Driven Induction Motor Bearing
Current Solutions
Patents:
- Faraday Shield (various methods)
- Virtual Brush (labyrinth with or without
dielectric media)
Inverter Driven Induction Motor
Bearing Current Solutions
QUESTIONS ?