IEEE APEC Conference Dallas. TX March, 1995
Effect of PWM Inverters on AC Motor Bearing Currents and Shaft Voltages
Jay Erdman, Russel J. Kerkman, Dave Schlegel, and Gary Skibinski
Allen Bradley Drives Division
6400 W. Enterprise Drive P.O Box 760
Mequon, WI 53092
(414) - 242 - 7151 (414) - 242 - 8300 Fax
Abstract - This paper investigates AC induction motor shaft voltage
problems, current flow thru motor bearings and electric discharge
current problems within bearings when operated under both pure
sinewave and Pulse Width Modulated (PWM) inverter sources.
Recent experience suggests that PWM voltage sources with steep
wavefronts especially increase the magnitude of the above electrical
problems, leading to motor bearing material erosion and early
mechanical failure. Previous literature suggests that shaft voltage bearing current problems under 60 Hz sinewave operation are
predominantly electromagnetically induced. It is proposed that
under PWM operation these same problems are now
predominantly an electrostatic phenomenon. A system model to
describe this phenomenon is characterized and developed.
Construction and test of a new Electrostatic Shielded Induction
Motor (ESIM) verifies this model and is also a possible solution to
the bearing current problem under PWM operation.
I. Introduction
Bearing currents and shaft voltages under 60 Hz sinewave
operation has been a recognized problem since 1924 [1-3].
The bearing impedance characteristic largely determines the
resulting bearing current that will flow for a given shaft
voltage magnitude and waveform present. A number of
surveys have indicated that 30 % of all motor failures operated
with 60 Hz sinewave voltage are due to bearing current
damage [4]. All rotating machines potentially have a bearing
current problem whether it is DC or AC, and either large or
small horsepower in size. These rotating machines have three
basic sources of shaft voltage - electromagnetic induction,
electrostatic coupled from internal sources or electrostatic
coupled from external sources.
Electromagnetic induction from the stator winding to the
rotor shaft was recognized by Alger [1] and is more prevalent
in long axial machines. The shaft voltage is due to small
dissymmetries of the magnetic field in the air gap that are
inherent in a practical machine design. Most induction
motors are designed to have a maximum shaft voltage to frame
ground of < 1 Vrms with recommended practice limits stated
in [5]. The induced shaft voltages cause bearing current flow
in a circulating path from the shaft, thru side A grounded
bearing, thru the stator frame, thru side B grounded bearing
and back to the shaft. The induced shaft voltage, although low
in magnitude, results in a high circulating current thru both
motor bearings since the impedance of the circulating path is
low. Modern day induction motors less than 250 horsepower
have grounded bearings but have minimized steady state shaft
voltage to extremely small values. However, during transient
start and stop conditions across the AC line, magnetic
dissymmetries appear as increased shaft voltage, resulting in
bearing current flow and reduced life [4]. This transient
bearing current flow for line started motors was
experimentally verified. The traditional electromagnetic
solution to induced shaft voltage on larger frames is to insulate
the non drive end bearing. This does not mitigate shaft
voltage but rather the resulting bearing current.
Electrostatic induced shaft voltage may be present in any
situation where rotor charge accumulation can occur.
Examples are belt driven couplings, ionized air passing over
rotor fan blades or high velocity air passing over rotor fan
blades as in steam turbine [6]. The electrostatic solution is to
keep the shaft and frame at the same potential by installing a
shaft grounding brush to reduce electrostatic build up and
reduce shaft voltage to 70 - 400 mV. This value is not enough
to cause damaging bearing current to flow.
Electrostatic coupled shaft voltage from external rotor
sources, such as a static exciter in a turbine generator, is
possible and historically solved with the application of a shaft
grounding brush [6]. Electrostatic coupled shaft voltage from
external stator sources, such as a PWM inverter, is
investigated in this paper.
A. Present Theory of Bearing Current with AC Line
The shaft voltage magnitude measured is commonly used
as an indicator of the possible bearing current that results. It is
the magnitude and passage of electrical current thru the
bearing that results in ultimate mechanical damage [7].
Bearing damage caused by electrical current is characterized
by the appearance of either pits or transverse flutes burnt into
the bearing race. Electrical pitting continues until the bearing
loses its coefficient of friction, further increasing the losses
and breaking up bearing surface. Typical fluting results in a
washboard like formation that appears on the race as shown in
IEEE APEC Conference Dallas. TX March, 1995
a) Low Speed
b) High Speed
c) Perfect Bearing
Fig. 3 Asperity Contact Possibilities [8]
Fig. 1 Fluting of AC Drive Motor Bearings
Fig. 1. It has been proposed that the current density of the ball
bearing contact area with the race is a better identifying factor
for permissible peak amps allowed without pitting or fluting.
However, this contact area is difficult to analyze since it varies
with bearing speed and load, vibration, method of installation,
viscosity and temperature of the lubricant. It is known that the
contact area increase is proportional to the bearing load raised
to approximately the 1/2 power [8].
Thus, it is important to characterize the impedance of the
bearing under different loading conditions to determine the
problem severity. Surface contact is made in three ways: metal
to metal, quasi-metallic surface contacts and metal point
contact thru electrically insulating surfaces between the ball
surface roughness and race roughness.
The actual bearing contact zone area in a slow moving or
non-rotating bearing is large and consists mostly of
Fig. 2 Bearing Resistance vs. Speed
quasi-metallic surfaces. The lubricant film is only 50
Angstroms (1 Ao = 10-10 m) while quasi-metallic surfaces have
metallic oxides of 100-120 Ao. Quantum mechanical tunneling
effects enable the current to pass thru the contact zone with
series resistances < 0.5 Ω. This is evidenced by the low
bearing resistance measurement made at low speeds in Fig. 2.
Reference [7] suggests that large current may pass thru
non-rotating bearings without damage.
The actual bearing contact zone area in a rotating bearing
is smaller and depends on bearing surface roughness. The
contact area comprises primarily of asperity point-like contact
of ball metal to race metal as shown in Fig. 3a for low speed
operation. High speed operation in Fig 3b has fewer asperity
contact points. Asperity contact duration is typically 100 µs at
low speed and 33 µs at high speed. The increased bearing
resistance with rotation shown in Fig. 2 suggests that the
lubricant is introducing a partially insulating film between
ball and race at speeds greater than 10% of rated. Typical
surface roughness of the race and ball from Fig. 4 is seen to be
in the 1 - 10 micron (1 micron = 1 µm) range while the typical
lubricating film of 0.1 - 2 micron depends on speed, lubricant
characteristics and to a lesser extent on load [7]. Fig. 5 shows
the relationship between oil film and surface roughness in a
Fig. 4 Waviness and Vibration Spectra From Inner
Ring With Accentuated Waviness [8]
IEEE APEC Conference Dallas. TX March, 1995
Fig. 5 Percent Film vs. Gamma for a Bearing [8]
bearing [8]. Percent film is the time percentage during which
the "contacting " surfaces are fully separated by an oil or
lubricant film while Gamma is the relationship of lubricant
film thickness to rms value of contacting surface roughness.
Most bearing applications operate in the Gamma = 1 to 2
region. This implies that high quality bearings look like a high
resistive impedance 80 % of the time with the oil film acting
as a capacitor ready to charge to breakdown potential. A
lower quality bearing will have low resistance metal to metal
contact a majority of the time and in the presence of high
resistivity lubricant acts as a race to ball junction capacitor
that may charge only randomly during non contact peak to
valley points.
The magnitude of the shaft voltage will determine the
bearing current present in lower quality bearings having
asperity contacts the majority of the time or high quality
bearings that use low resistivity lubricants. A high shaft
voltage causes increased current and pits or craters to form
since bearing current flows thru a number of points. Heating
can occur at point contact to such a degree that the material
melts creating craters, thus liberating wearing metal particles
into the lubricant. A low shaft voltage has lower current
amplitudes but has been found to still cause corrosive type of
pitting due to grease decomposition.
In high quality bearings with high resistance grease, the
junction bearing capacitor may discharge into a low
impedance circuit when the electric field exceeds the
breakdown strength in the lubricant asperity points . The
bearing breakdown voltage threshold is 0.4 volts since mineral
oil field strength is 106 v/m, a typical oil film is 0.2 microns
and there are two films in series. On occasion the bearing
capacitor voltage, charged by the shaft voltage present,
becomes high enough ( > 0.4 volts) to break down the grease
and a short (nanoseconds) high current impulse flows from the
charged oil film capacitor within the bearing as shown in Fig.
6. This discharge current pulse, if it occurs, is a prime source
of bearing erosion and is commonly referred to as fluting or
Electric Discharge Machining (EDM ). The washboard craters
Fig. 6 EDM Capacitive Charging Characteristics
of Fig. 1 are formed from the microscopic pits that soften
under repetitive heating of the race to its melting temperature.
Several authors suggest that shaft voltage < 0.3 volts is
safe, while 0.5 - 1.0 volts may develop harmful bearing
currents, and shaft voltages > 2 volts may destroy the bearing.
The rotating bearing breakover threshold voltage (when
bearing current starts to flow) was measured under DC source
voltage to be 700 mv peak.
B. Proposed Theory of Bearing Current with PWM Inverters
The preceding analysis was based on steady state, low
frequency and low dv/dt shaft voltage sources. However, PWM
inverter modulation causes high frequency step-like voltage
source waveforms and high dv/dt's to be impressed across the
stator neutral to frame ground. It is shown that a portion of
this waveform is also present as rotor shaft voltage to ground
due to capacitor divider action. The preceding sinewave
analysis applies to PWM operation but with the change that
the experimental static breakdown threshold voltage on the
rotor shaft increases to 8-15 volts ( Fig. 6) vs. 700 mv for the
same bearing monitored under 60 Hz sinewave operation (Fig.
10). This increase is explained using dielectric breakdown
theory for pulsed sources [9]. Fig. 7 shows that the impulse
breakdown strength of hexane (1.1 106 v/m) increases
dramatically over the static value for short step-like pulse
durations. The bearing voltage breakdown threshold also
increases as a function of shaft voltage rate of change [10].
This increased breakdown level under PWM operation is
undesirable since during bearing discharge the resulting EDM
bearing currents are much higher than with sinewave
operation. Fig. 8 shows that rough surfaces typically seen in
bearings will have a statistical time lag of 3 us prior to
breakdown, which agrees with measured value of Fig 6.
It is theorized that the high quality bearings of Fig. 5
(Gamma = 2 ) give long mechanical life when used under
sinewave operation but may lead to premature bearing current
IEEE APEC Conference Dallas. TX March, 1995
II. Effect of PWM Drives on Bearing Current
1.9
A. Test Structure and Instrumentation
Pulse Strength ( MV / cm )
Pulse Shape
1.8
1.7
1.6
1.5
1.4
0
0.5
1.0
1.5
2.0
Pulse Duration ( uS )
Fig. 7 Increased Dielectric Strength with Impulse Sources [9]
failure under inverter operation due to the bearing junction
capacitor being impulse charged 80 % of the time to higher
impulse shaft voltages. This will result in higher destructive
EDM discharge currents. The low quality bearings of Fig. 5
(Gamma =1) give low mechanical life bearings when used
under sinewave operation but may actually be better for
inverter operation since the destructive capacitive EDM
currents only occur 5 % of the time due to asperity contact
resistance shorting the bearing.
Test results of a 15 HP motor ( with grounded motor
bearings) under 60 Hz steady state sinewave operation showed
no evidence of EDM current occurring, except on across the
line starting. Test results on the same motor under Bipolar
Junction Transistor (BJT) and Insulated Gate Bipolar
Transistors (IGBT) PWM inverter sources however did show
evidence of EDM and fluting on a continuous basis.
The measurement of the contributors to bearing roughness
induced by PWM voltage source inverters requires detecting
signals within a noisy environment. The identification of the
contributors requires an experimental structure with test
instruments that provide isolation, but adequate sensitivity.
Fig. 9 shows the test fixture and instrumentation employed for
the investigation presented in this paper. The motor was a 15
Hp, 460 volt, 8 pole, induction motor. The drive and non
drive bearings were insulated. A grounding strap simulated
normal grounded bearings. A carbon brush sensed the rotor
shaft voltage. The stator neutral was available for measuring
the stator neutral to ground voltage. High voltage probes with
an isolation amplifier performed voltage measurements and a
current probe detected the current through the grounding
strap. A digital sampling oscilloscope with mass storage
provided a tracking of the desired signals. A spectrum
analyzer detected the frequency and phase content of the
voltages and current.
B. Sine Wave Operation of the Induction Motor
Bearing and shaft currents are not specific to motors
operating from PWM voltage source inverters. Alger
investigated shaft and bearing currents in the 1920's. Exciting
the induction motor with sine waves provided a reference
condition. Measurements of the stator neutral to ground and
rotor to ground voltages and rotor current were made while
operating the induction machine at no-load and 60 Hz. The
L1
AC Line
460 Volt
L2
L3
AC Drive
GND
U
V
W
GND
U
V
W
Earth Ground
16
Step Function
Pulse
14
Oscilloscope
and
Spectrum Analyzer
Time Lag ( us )
12
10
8
Stator
Neutral
Voltage
200 X Differential Probe
Shaft
Voltage
50 X Differential Probe
6
Shaft
Current
4
Neutral
Carbon
Brush
2
AC Motor
0
Rough
Cathode
Smooth
Cathode
Rough
Cathode
Fig. 8 Surface Roughness Effect on Statistical Time Lag
to Breakdown [9]
Grounding
Strap
Current Probe
Fig. 9 Test Fixture and Instrumentation
IEEE APEC Conference Dallas. TX March, 1995
Fig. 10 AC Line Operation
results of those tests are shown in Fig. 10. EDM currents were
not detected. The 60 volt stator neutral voltage induced a 1
volt rotor voltage, a 60 to 1 reduction. This rotor shaft voltage
level is at the upper end of the standards.
C. Evidence of Electric Discharge Machining (EDM)
Limiting the number of variables is essential in preventing
unjustifiable conclusions from experimental results, especially
when investigating the effects of high frequency IGBT
inverters. To accomplish this: The power cable was fixed to a
length of ten feet with four conductors and the braided shield
grounded at the drive end. A 4 KHz carrier frequency was
selected. Common mode chokes were not inserted in the input
or output of the drive.
Tests were performed on the drive system of Fig. 9. The
stator neutral to ground voltage, rotor shaft to ground voltage,
and bearing strap current were monitored. Fig. 11 shows
experimental results when operating the AC drive at rated
volts per hertz and 48 Hz. The stator neutral to ground
voltage displays the typical per carrier cycle waveform
associated with PWM voltage source inverters. The rotor
Fig. 11 AC Drive Operation
voltage, however, shows a quite different profile. For a
majority of the time, the rotor is grounded, but occasionally
the rotor tracks the stator neutral to ground voltage. Then
quite suddenly, the rotor voltage collapses, producing a current
pulse. Fig. 6 is an expanded plot of an EDM discharge. As
the stator to neutral voltage increases, the rotor voltage
responds with a capacitive charging characteristic. In fact, the
rotor voltage rises to a value fifteen times larger than the
measured value when operating on sine waves. At the instant
of discharge, an impulse of current occurs with the rotor
voltage simultaneously collapsing.
A number of bearings were removed from motors operating
on AC drives and the AC mains. The bearings were examined
for evidence of EDM fluting. Fig. 1 shows examples of
bearings from motors operated on AC drives after being
sectionalized. The fluting is quite pronounced. The outer
bearing race on the left shows a random EDM discharge. The
outer race on the right shows a continuous etching of the race
surface.
The normal dv/dt switching current is in the hundreds of
milli-amp range and occurs with the rise in rotor potential. A
review of the technical literature does not indicate a consensus
on the effects of this relatively small current. However, the
large current following the rapid collapse of the larger rotor
voltage is believed to cause EDM. The value of the EDM
shown is limited by the inserted grounding strap and its surge
impedance. A standard drive system's bearing current would
be limited by the bearing short circuit impedance. This
current, its cause, modeling, and control, are the focus of the
remainder of this paper.
III. An Equivalent Circuit for Bearing
Displacement and EDM Currents
A. The Model
Fig. 12 shows the physical construction of the test motor.
Both the drive and non drive ends of the rotor were outfitted
with an insulated bearing support sleeve, which isolated the
rotor bearings from the motor frame. This provided a
measurement of the rotor open circuit voltage, and when
shorted by the grounding strap, simulates an actual bearing
mounting. In addition, the grounding strap provides a
mechanism for measuring the bearing to ground current. Fig.
12 shows a carbon brush for measuring the rotor voltage and
investigating solutions to the EDM bearing current problem.
The motor had 48 stator slots and 64 rotor bars. Fig. 13
depicts the capacitive coupling relevant to the development of
the model. The stator to frame capacitance (Csf) is a
distributed element representing the capacitive coupling to
frame along the length of the stator conductors. For most
investigations, magnetic coupling of the stator and rotor is
sufficient. But with the high dv/dt present with modern power
IEEE APEC Conference Dallas. TX March, 1995
MOTOR FRAME
Rb
Stator Laminations
Insulating Sleeve
R inner race
Outer
Race
Carbon Brush
C ball,i
n Balls
in Parallel
Inner
Race
ROTOR SHAFT
C gap,i
Z
ball,i
Z
,i
C ball,i
ROTOR
Ground
Strap
C sleeve
R outer race
Inner
Race
Per Ball Model
Outer
Race
Grounding
Strap
R
Cb
Reduced Model
Stator Laminations
Insulating Sleeve
Fig. 14 Motor Bearing Models
Current
Probe
MOTOR FRAME
Fig. 12 Physical Construction of the Test Motor
devices, capacitive coupling considerations cannot be ignored.
Therefore, the stator to rotor capacitance (Csr) and the rotor to
frame capacitance (Crf) are included.
The bearings, lubricating film, and insulating sleeve present
a combination of capacitances, resistances, and a nonlinear
impedance, Fig. 14. First there exists an inner and outer race
resistance. Then, depending on the physical construction, the
bearing consists of n balls in parallel; each ball having an
effective resistance (Rball,i). In addition, each ball is immersed
in the lubricating film; thus, each ball develops two
capacitances (Cball,i) linking the ball to the inner and outer
Frame
Csf
Crf
Stator
Winding
Csr
Csr
Stator
Winding
Crf
Csr
Csf
races. The ball portion of the bearing model, therefore,
consists of n parallel combinations of (Cball,i) and (Rball,i).
Between balls, the inner and outer races are separated by the
lubricant, which forms a dielectric barrier. Therefore, a
capacitance (Cgap,i) is formed between each pair of balls,
resulting in n parallel capacitors. The nonlinear impedance
(Zl,i) accounts for the mechanical and electrical abnormalities
and randomness of the bearing.
Combining the individual components results in a reduced
order bearing model, which is compatible with the motor drive
models employed in simulations and analyses. The reduced
order model consists of a resistance (Rb) in series with the
parallel combination of an effective capacitance (Cb) and a
nonlinear impedance (Zl). Finally, the insulating sleeve adds a
series capacitance (Csleeve) that is shorted when the grounding
strap is employed.
Combining the bearing model with a simple inverter/motor
model yields the model of Fig. 15. Here, the inverter is
modeled as three line to neutral voltages with a neutral to
ground zero sequence source. This model allows the inverter's
voltages to be examined as positive, negative, and zero
sequence sets. The motor is represented as two sets of three
phase windings; one each for the stator and rotor windings.
The capacitive coupling from stator to frame is lumped at the
neutral of the stator winding and the capacitive coupling
Drive
Stator
Rotor
Csr
Crf
Csr
Csf
Zero
Sequence
Source
Csr
Rotor
Line to Neutral
Sources
Csr
Csf
Rb
Crf
Cb
Stator Winding
Crf
Fig. 13 Motor Capacitive Coupling
Fig. 15 Inverter / Motor Model
Z
IEEE APEC Conference Dallas. TX March, 1995
between the stator and rotor connects the stator and rotor zero
sequence networks. Finally, the rotor to frame capacitance
and bearing provide the paths to ground from the rotor shaft,
here represented by the neutral of the rotor.
B. An Explanation of the Cause of Bearing Displacement and
EDM Currents
Examining the bearing model in the context of the
experimental results shown in Fig. 11, the significance of the
nonlinear impedance Zl is apparent. Because the bearing
capacitor normally exhibits a dv/dt or displacement current
when the stator voltage changes, the nominal dv/dt current is
limited by the impedance given by the model of Fig. 15 with Zl
equal to a low non zero value. This corresponds to the bearing
in a position of low impedance between outer and inner race.
However, occasionally the bearing rides the lubricating film,
which allows the rotor to track the source voltage with a
random duration. This condition corresponds to a substantial
increase in Zl . When Zl collapses, reflecting the preferred
bearing position or the breakdown of the film, the capacitor Cb
is discharged and an EDM current occurs, with the current
through the bearing limited by the zero sequence or common
mode impedance. Thus, the bearing's impedance is statistical
in nature and depends on the position of the balls, the
condition of the bearing and its lubricant.
C. Model Parameter Values
Inputs to the model of Fig. 15 include relevant bearing and
motor parameters, and the zero sequence forcing function.
Calculations and tests provided parameter values and the
source voltage. To calculate the stator to rotor capacitance,
two parallel conductors were analyzed with a separation equal
to the distance between the centers of the conductors. This
value was modified to reflect the number of stator slots and
slot opening area. To establish the rotor to frame capacitance,
the rotor and stator were considered to be parallel cylinders
with an air gap. Fig. 16 shows the Crf as a function of
Fig. 17 Stator - Rotor Capacitance - Measured
horsepower for 4 and 6 pole motors. The bearing film
capacitance was calculated assuming a spherical construction
for the ball with respect to the race surface. A typical value
for the ball bearing capacitance is 190 pf [11]. The calculated
values for the test motor and bearing are contained in Table 1.
Tests were performed to establish the accuracy of the above
calculations. With the stator unexcited and the rotor coupled
to a drive motor, measurements of the effective capacitance
from rotor to frame were made with a RLC meter at various
speeds. The tests consistently produced a capacitance of 1400
pf. This value represents the equivalent of Csr // ( Csf + (Crf //
Cb)). Although the Cb depends on the speed of rotation, the
invariance of the measurement suggests Crf dominates. The
Csf is obtained by measuring the capacitance from the stator
terminals to frame with the rotor removed. To establish the
Csr, measurements were made of the effective capacitance from
stator terminals to frame with rotor shaft and frame connected.
The Csr is obtained by subtracting Csf . Fig. 17 shows Csr for
the test motor as a function of frequency. Finally, the bearing
impedance Zl was measured as a function of rotational speed,
the results of which are shown in Fig. 2. This in combination
with the measured value of Cb allowed for the determination of
Crf . The measured values are included in Table 1.
Verification of the parameter values consisted of tests with
the insulating sleeve grounding strap open circuited and the
drive operating at various frequencies at no-load. The stator
neutral to ground voltage and rotor voltage to ground were
measured; the stator voltage from the neutral of the stator
windings and the rotor voltage from the rotor brush
Table 1 Motor Model Capacitances
Fig. 16 Rotor - Frame Capacitance - Calculated
Calculated
Measured
Csr
100 pF
100 pF
Csf
-----
11 nF
Crf
1 nF
1.1 nF
Cb
200 pF
200 pF
IEEE APEC Conference Dallas. TX March, 1995
Experimental
Simulation
Fig. 18 AC Drive Operation - Open Bearings
attachment. Typical results of the tests are displayed in Fig.
18. With the grounding strap open, the rotor voltage is
strikingly different from the rotor voltage of Fig. 11, where the
grounding strap was in place. The tracking of the stator to
neutral voltage by the rotor voltage confirms the existence of
zero sequence paths as indicated by the model of Fig. 15.
The stator to rotor voltage ratio confirmed the relative
weighting of the capacitors Csr and Crf in Table 1.
D. Simulation Results
For simulation and analysis purposes, the model of Fig. 15
was reduced to a zero sequence approximation, which is the
shaded portion of Fig. 15. A simulation was developed with
the parameters of Table 1 for the bearing model. The
simulation provided an analytical tool for examining the
effects of PWM waveforms, verifying the system model and
parameters by correlating simulation results with experimental
data, and for evaluating various solutions to EDM. Fig. 19
shows an expanded portion of Fig. 11 and a simulation
employing the zero sequence model. The forcing function for
the simulation was the stator neutral to ground voltage from
the experimental results. The outputs include the rotor voltage
and probe current as shown.
Comparing the simulation results to the experimental
results shows good agreement. The dv/dt and EDM currents
are representative of experimental results. The rapid rise in
rotor voltage at the point of EDM discharge is in very good
agreement with the data. To obtain this accuracy, an estimate
of the nature of Zl is necessary. For the results presented
above, Zl was modeled as a diac (Fig. 2); high impedance until
the voltage threshold is met; thereafter it is voltage limited.
The threshold voltage was experimentally determined. The
value of the impedance while voltage tracking, determined
from the rate at which the experimental rotor voltage of Fig.
19 decayed, was found to be in good agreement with the
results of Fig. 2.
Fig. 19 EDM Discharge Top) Experimental Bot) Simulation
One area where the simulation fails to predict the observed
response occurs in the transient response of the dv/dt and
EDM currents. Close examination of the experimental results
shows a 12.5 MHz oscillation in the measured current;
however, the oscillation does not appear in the simulation
results. One explanation for this discrepancy is the
measurement technique. Inserting a grounding strap modifies
the system impedance. The characteristic impedance of the
grounding strap alters the natural frequency and establishes an
oscillation in the dv/dt and EDM currents.
IV. The Electrostatic Shielded Induction Motor:
A Solution to EDM Bearing Currents
The previous section's experimental results suggest
electrical discharge as a principal contributor to bearing
roughness. A bearing model was developed and interfaced
with the model for the electrical source and interconnecting
network. The model reflects the observed electrical behavior,
which suggests the source of PWM induced bearing roughness
is the common mode or zero sequence voltage.
Using the model developed above, the task of proposing
solutions to EDM discharge becomes simply one of disrupting
the discharge either through the source voltage,
interconnecting impedance, or the bearing design. Thus three
design areas are available for investigation.
IEEE APEC Conference Dallas. TX March, 1995
Fig. 20 Stator Shield - Open Bearing
Because of the capacitive coupling from stator to rotor, the
most likely candidate is the coupling mechanism from stator to
rotor - the Csr in Fig. 15. If an electrostatic shield is inserted
between the stator and rotor, the coupling capacitance from
stator to rotor is defeated; thus reducing the dv/dt and
preventing voltage tracking by the rotor. Because the
induction machine generates torque through magnetic
induction, the presence of the shield will not affect motor
output ratings. A shield was constructed by inserting 1 inch
adhesive backed copper foil tape strips to cover the stator slot
area. The shield was grounded to the motor frame.
Fig. 20 shows the stator neutral to ground and shaft voltage
for an identical operating condition as shown in Fig. 18. With
the shield in place, a rotor voltage of 18 volts peak exists when
the outer race grounding strap is open circuited - a 56%
reduction when compared to the 40 volts peak of Fig. 18.
With the strap grounded (Fig. 21), the dv/dt currents were
reduced from 500 ma to 50 ma. No EDM currents were
detected.
Employing the copper foil strips as indicated above reduced
the rotor exposure to the stator windings in the precise
proportion by which the rotor voltage is reduced. By
Fig. 21 Stator Shield - Sleeve Shorted
Fig. 22 Full Shield - Open Bearings
extending the Faraday shield to enclose the stator end
windings and duplicating the tests above, a near complete
shielding of the rotor voltage was observed. As results of Fig.
22 show, the rotor voltage with grounding strap open is
reduced 98% when compared to the unshielded case.
Connecting the grounding strap (Fig. 23), virtually zero dv/dt
current was measured and no EDM current detected.
The experimental results presented above confirm bearing
currents, both dv/dt and EDM, are induced primarily by
electrostatic coupling. The stator to rotor capacitance couples
the zero sequence or common mode source from stator to
rotor. The bearing provides a return path for the common
mode source, thus allowing dv/dt and EDM discharge
currents.
V. Conclusions
The paper presented a review of electrically induced bearing
roughness for AC machines under low frequency sine wave
operation. A theory was proposed for lubricant dielectric
breakdown under PWM excitation. Electrostatic coupled
discharge or displacement (dv/dt) and electric discharge
Fig. 23 Full Shield - Sleeve Shorted
IEEE APEC Conference Dallas. TX March, 1995
machining (EDM) currents were identified and experimentally
measured. Electrical models were developed and
experimentally verified for the source voltage, coupling
network, and bearing. An electrostatic shielded induction
motor was described and experimentally demonstrated as a
solution to the bearing current problem.
The technical literature and experience show unloaded
motors at high speed provide the worst case scenario for
bearing currents. In addition, applications with coupled loads
tend not to exhibit the problem because of parallel paths for
electrostatic discharge.
ACKNOWLEDGMENT
The authors wish to thank Mr. Steve Stretz for his research
assistance in the bearing current phenomenon from a motor
design point of view.
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[3]
Lawson,
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