Bearing Current Problem1 (Reliance) PDF
Bearing Current Problem1 (Reliance) PDF
Bearing Current Problem1 (Reliance) PDF
Inverter-Driven
Induction Motors
Shaft and Bearing
Current Solutions
Table of Contents
Executive Summary ............................................................................................................ 3
Sine Wave Bearing and Shaft Currents ............................................................................... 5
Inverter-Driven Motor Bearing Currents ............................................................................ 6
Grounding in Inverter-Driven Motor Systems .................................................................. 10
Field Measurement of Inverter-Induced Bearing Currents ............................................... 12
Table 1: Bearing Current Measurement Equipment Requirements .................................. 13
Bearing Damage from Currents ........................................................................................ 17
Bearing Current Remediation for Inverter-Driven Motors ............................................... 19
Table 2: Bearing Current Remediation (Motor-Based) ..................................................... 20
Improve High Frequency Grounding Connection
from the Motor to the Drive and from the Motor to the Load ........................................ 21
Insulated Coupling Between the Motor and the Driven Equipment ................................. 21
Shaft Grounding Brush without Insulated Bearing........................................................... 22
One Insulated Bearing on the Motor ................................................................................ 22
Shaft Grounding Brush with One or Two Insulated Motor Bearings ............................... 23
Electrostatically Shielded Induction Motor (ESIM) ......................................................... 24
Standard Practice for Bearing Protection .......................................................................... 25
Field Experiences .............................................................................................................. 26
Systems/Process Line Installation
Motors Coupled to Gearboxes or other Machinery
Case 1 ......................................................................................................................... 26
Case 1A....................................................................................................................... 27
Case 2 ......................................................................................................................... 28
Case 3 ......................................................................................................................... 29
HVAC Application Motors Coupled to Fans via Non-conductive Belts
Case 4 ......................................................................................................................... 29
Continuing Areas of Research .......................................................................................... 30
Drive Design Issues .......................................................................................................... 32
Conclusion ........................................................................................................................ 33
Table 3: Bearing Current Remediation For Motor And Coupled Equipment ................... 35
References ........................................................................................................................ 37
Appendix
Grounding/Bonding .................................................................................................... 38
Vibration Analysis of Bearing Damage due to Passage of
Current in a PWM Environment ................................................................................. 39
Capacitor Current Flow .............................................................................................. 41
Rolling Bearing Electrical Behaviors ......................................................................... 44
The vast majority of bearing failures in electric motors are due to mechanical and thermal
causes. Potential causes of these types of failures include misalignment of the motor and load,
vibration, incorrect lubrication, excessive radial or axial loading, lubricant contamination or
inadequate maintenance. In a small fraction of electric motor applications, bearings prematurely
fail due to electrical causes.
Currents flowing through induction motor bearings have the potential of creating premature
failure of these bearings. Shaft and bearing currents in sine wave driven motors are well
understood. These currents are either localized in the bearing or are driven through the bearing
due to asymmetries in the motor material properties or construction. The low frequency nature
of bearing and shaft currents in sine wave driven motors results in current paths through what
are generally considered to be conductive materials (motor shafts, frames, bearing races and
bearing balls). Interrupting the conducting current path with insulating materials can eliminate
these low frequency shaft and bearing currents.
Electric motors powered by fast switching pulse width modulated (PWM) voltage source
inverters experience high frequency voltage pulses with respect to motor ground. At these high
frequencies (up to several MHz transitions) capacitively coupled currents can flow through
paths that would normally be considered to be insulators. Currents now can flow through the
magnet wire insulation, stator slot liners, motor air gap, bearing grease and stator slot top sticks.
These high frequency current paths offer new opportunities for shaft and bearing current flow
that can result in premature bearing failure. With inverter driven motors, a clear understanding
of high frequency current paths from the motor terminals back to the inverter and to ground is
key to determining potential bearing current problems and remedies. Current paths both internal
to the motor and between the inverter, the motor and the driven equipment must all be considered
when looking for methods to reduce unwanted bearing current flow. Consequently system related
issues, such as grounding and cable shielding, become very important to inverter powered electric
motor bearing current remediation.
It should be pointed out that inverter-induced bearing currents have not been found to cause
significant problems in the majority of applications. This report focuses on that small percentage
of installations and applications in which damage as a result of bearing currents is possible.
Topics to be presented include:
1) root causes of inverter induced motor bearing currents,
2) potential current paths of capacitively coupled currents,
3) bearing damage due to these currents,
4) preferred current paths for capacitively coupled currents,
5) examples of application problems and solutions,
6) field measurements of important voltages and currents,
7) remediation methods and
8) on-going research into motor bearing current modeling.
as shown in Figure 2.
paths
are
normally considered
to be insulators, for
example: stator slot
liners, stator to rotor
air gap and the
bearing grease film
between race and
ball. A detailed model
Figure 3: Typical common mode voltage and motor shaft voltage wave forms.
(Horizontal scale - 40 microsec per div)
will
be
bearing.
dt transition occurs
in the common
to
load
of
creating damage in
the load bearing or,
for some types of
couplings,
the
Figure 5: High frequency flux encircling the rotor causing shaft and bearing current.
coupling itself.
This current can be measured by putting a high
Grounding In Inverter-Driven
Motor Systems
between
the
driven equipment to
other points in the
grounding system. If
there is a significant
10
11
12
Quantity
Measured
Equipment
Equipment
Specification
Example
Equipment
100 Megasamples/sec
5 to 10 MHz bandwidth
Tektronix TDS540,
TDS1354, TDS3054
Yokogawa DL1540,
DL4100, DL7100
Agilent 54622A,
54624A
LeCroy LT322,
LT224
Fluke PM 3394B,
PM 3390B
Shaft voltage/current,
common mode voltage/
current
50 MHz
Tektronix P5210
differential probe
Yokogawa 70924
differential probe
100 Megasamples/sec
50 MHz bandwidth
Tektronix P6139A
Yokogawa 700998
Agilent 10074C
100 Megasamples/sec
50 MHz bandwidth
Tektronix A6303/
CT-4/AM503B,
A6302/AM503B
current probes,
PEM high frequency
Rogowski Coil
Tektronix A6303/
CT-4/AM503B,
A6302/AM503B
current probes,
PEM high frequency
Rogowski Coil
5 to 10 MHz bandwidth
(2 or more channels)
13
shape.
Figure 9: Typical common mode voltage and motor shaft voltage wave forms.
14
be
sudden
15
voltage.
16
equipment.
17
is
somehow
related
to
the
18
19
Action
Comments
Insulate coupling
Faraday shield
Insulate coupling
Faraday shield
20
21
22
Figure 18: Drive end shaft grounding brush and opposite drive end insulated.
It is possible, of course, to
insulate both bearings on the
motor, protecting both from
current flow.
Figure 19: One insulated bearing, shaft brush and ground strap.
In this
To eliminate
23
Electrostatically
Shielded
Induction Motor
(ESIM)
An
electrostatically
22).
Ba l d o r - R e l i a n c e
developed this tech-nology
and has several patents in
this area, both in concept
and application. The basic
goal is to create a grounded
conductive path between
the stator and the rotor that
will bleed off capacitive
coupled current. While
Figure 22: Electrostatically shielded induction motore (ESIM).
ESIM
technology
it
doesnt
24
place
bearing
for
prev enting
current
damage
For large AC
Enhanced
ground
25
Figure 26.
Field Experiences
(Case 1)
26
recorded.
as is desired.
As a point of
comparison,
another similar
application
which had a
more rigorous
attention
to
cabling/
grounding/
Trace 1: Ground current. (5 amps/div)
27
28
29
Current
Baldor - Reliance
technology
T he
30
O nce
the
four
parameters in the
motor are measured,
the proposed model
31
mode voltage.
by a drive manufacturer.
32
With four-phase
Conclusion
33
applications.
Baldor-Reliance
provides
the
patented
Table 3
34
REMEDY
Well
Bonding
terminated
strap
cable
between
ground
motor and
connec- load frame
tions:
drive to
motor
One
insulated
motor
bearing
on
opposite
drive end
Two
insulated
motor
bearings
Shaft
grounding
brush
across
one motor
bearing
Faraday
shield
(ESIM)
Insulated
coupling
between
motor and
driven
load
Source of Current
Internal, circulating,
due to magnetic
dissymmetry leading
to net flux linking
shaft (fundamental
frequency or sine
wave). Generally
occurs on motors
above NEMA 400
frame.
X X
Avoid
without
opposite
bearing
insulated
Figure 2
Common mode
(ground) current
(induced by common
mode dv/dt) taking a
return path via the
motor shaft
extension and
coupled equipment.
X X
Figure 4 gold
current
Discharge through
bearing of
capacitively-coupled
common mode
voltage (scaled by
capacitor divider).
Figure 4 red and
green current
Only if in
combination
with motor
shaft ground
brush
Only if in
combination
with motor
shaft ground
brush or
insulated
coupling
Avoid
without
insulated
coupling or
bonding
strap
between
motor and
load to
protect
driven
equipment
X X
X
May cause
increased
motor
bearing
current
without
motor shaft
ground
brush,
faraday
shield or
insulated
motor
bearings
X indicates that the remedy will, by itself, reduce or eliminate motor bearing current damage and current
flow to coupled equipment.
Inverter-Driven Induction Motors Shaft and Bearing Current Solutions
35
quasi-continuous current.
B aldor-Reliance
continues
to
study
36
References
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 Applied Power Electronics Conference (APEC),
Dallas, TX, March 6-9, 1995, pp. 24-33. IEEE Transactions on Industry Applications, Vol. 32, No. 2,
March/April 1996, pp. 250-259.
Doyle Busse, Jay Erdman, Russel J. Kerkman, Dave Schlegel, and Gary Skibinski, Bearing Currents
and Their Relationship to PWM Drives, IEEE Industrial Electronics Conference (IECON), Orlando, FL,
Nov. 6-10, 1995, pp. 698-705. IEEE Transactions on Power Electronics, Vol. 12, No. 2, March 1997, pp.
243-252.
Doyle Busse, Jay Erdman, Russel J. Kerkman, Dave Schlegel and Gary Skibinski, The Effects of PWM
Voltage Source Inverters on the Mechanical Performance of Rolling Bearings, IEEE Applied Power
Electronics Conference (APEC), San Jose, CA, March 3-7, 1996, pp. 561-569. IEEE Transactions on
Industry Applications, Vol. 33, No. 2, March/April 1997, pp. 567-576.
Doyle Busse, Jay Erdman, Russel J. Kerkman, Dave Schlegel and Gary Skibinski, System Electrical
Parameters and Their Effects on Bearing Currents, IEEE Applied Power Electronics Conference (APEC),
San Jose, CA, March 3-7, 1996, pp. 570-578. IEEE Transactions on Industry Applications, Vol. 33, No.
2, March/April 1997, pp. 577-584.
Doyle Busse, Jay Erdman, Russel JKerkman, Dave Schlegel and Gary Skibinski, An Evaluation of the
Electrostatic Shielded Induction Motor: A Solution for Rotor Shaft Voltage Buildup and Bearing Current,
IEEE Industry Application Society Conference, San Diego, CA, October 6-10, 1996, pp. 610-617. IEEE
Transactions on Industry Applications, Vol. 33, No. 6, November/December 1997, pp. 1563-1570.
R. Kerkman, D. Schlegel and G. Skibinski, An Examination of Rotor Shaft Voltage and the Resulting
Bearing Current: A Consequence of PWM Inverter Operation, IEEE Industry Applications Magazine,
Vol. 3, No. 6, November/December 1997, pp. 21-32.
D. Schlegel, R. Kerkman and G. Skibinski, Turning The Tables On Shaft Voltage And Bearing Currents, EC&M, June 1998, pp. 60-71.
Sidney Bell, Timothy J. Cookson, Steven A. Cope, Richard A. Epperly, Annika Fischer, David W. Schlegel
and Gary L. Skibinski, Experience With Variable Frequency Drives and Motor Bearing Reliability, IEEE
IAS Petroleum and Chemical Industry Conference (PCIC), Indianapolis, IN, Sept. 28-30, 1998.
37
Appendix 1: Grounding/Bonding
be used.
38
39
bearing damage.
Cautions
In fact,
from
subsequent running
after false brinelling
can exhibit a pattern
such as shown in
Figure A2-4.
40
v(t) = Vmsin(2ft)
(3)
i = C dv/dt
(1)
Zc = 1/(2fC)
(5)
(2)
bearings.
41
winding
Vb
Csf
Csr
the rotor
Crf
Cb
(6)
42
43
compressibility.
44
Essentially, the bearing acts like a resistivecapacitive (R-C) circuit when a voltage is applied
across it. In a rotating bearing, the location of the
bearing balls and resistive state of the bearing
lubricant causes the bearing resistance and
capacitance to change with time. The random
contact of the bearings balls to the races as well as
the highly unpredictable resistance behavior of the
bearing grease make reliable prediction of bearing
current difficult, even if the shaft to frame voltage is
known.
45
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www.baldor.com
Baldor Electric Company
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