THE INFLUENCE OF AXIAL MAGNETIC CENTERING FORCES
ON SLEEVE BEARING INDUCTION MOTORS
Paper originally presented PCIC 2006 Conference
Paper published PCIC 2006 Conference Record
Javier Portos
Member, IEEE
TECO-Westinghouse
5100 N. IH-35
Round Rock, TX 77681
USA
portosj@tecowestinghouse.com
Sandra Turner
Member, IEEE
TECO-Westinghouse
5100 N. IH-35
Round Rock, TX 77681
USA
turners@tecowestinghouse.com
Abstract – All motor rotors have an axial position called a
magnetic center, which is the location where the motor’s axial
magnetic forces are balanced. The magnitude of these axial
magnetic forces that hold the rotor in this position can vary
greatly depending on the machine size, speed,
electromagnetic configuration and mechanical geometry.
Very little research has focused on this phenomenon and
other issues that can cause weak magnetic centers, floating
magnetic centers and multiple magnetic centers. This paper
presents comparisons of calculated and tested data to
describe the axial magnetic forces and their effects. It also
suggests methods to strengthen weak magnetic forces. This
condition exists in induction motors, but is most serious in
sleeve bearing configurations where mechanical rotor endplay
can permit violent and damaging rotor motion when weak
magnetic centers or multiple centers are present.
configuration. Therefore, extensive tests are required to
analyze the effect of each category. This paper focuses on
typical large induction motor construction in a horizontal
configuration with radial cooling vents in the stator and rotor
cores, fabricated rotor bars, sleeve bearings, and internal
blowers mounted on the rotor. This paper also discusses
variations in rotor configurations and their effects on the
magnetic center using calculations and tests.
II.
Magnetic center - The axial position the rotor will take
when running at no load condition (Uncoupled and
energized with full voltage and frequency at the motor
terminals) with axial forces balanced.
Mechanical endplay - Total distance that a rotor assembly
can move axially between the limits set by the sleeve
bearing thrust faces and shaft collars.
Mechanical center - The location of the rotor shaft
assembly exactly half way between the physical limits of
its possible endplay movement [3].
Magnetic centering force - The magnitude of the axial
magnetic re-centering force exerted on the rotor when
any external influence moves it from its magnetic center
while the machine is running.
INTRODUCTION
A rotor’s magnetic center position is determined by the
influence of the stator and rotor geometry and the magnetic
flux established in the core and air gap. Following are some
parameters for the mechanical and electrical influences of the
magnetic center phenomenon.
A. Mechanical:
1) Stator core and rotor core length alignment
2) Stator cooling vents and rotor cooling vents aligned or in
an offset configuration
3) Stator or rotor skewed slots
4) Aerodynamic forces
5) Allowable mechanical endplay
B. Electrical:
1) Magnetic flux distribution inside the machine
2) Load current in the rotor
Each of the above has its own effect on the magnetic
center. Predicting each influence is a difficult task due to the
differences in each motor’s design, geometry, and
1-4244-1192-0/07/$25.00 ©2007 IEEE
GENERAL
The following definitions are used to understand the
magnetic centering analysis:
Index Terms – Axial Magnetic Force, Weak Magnetic
Centers, Double Magnetic Centers, Stator and Rotor
Configurations, Solving Magnetic Center Issues.
I.
Bill Veerkamp
Member, IEEE
The Dow Chemical Company
400 W. Sam Houston Parkway S.
Houston, TX 77042
USA
bveerkamp@dow.com
On the drawing board, the machine is designed with the
mechanical center coinciding with the magnetic center.
However, to account for machining and assembly variances,
engineers have to establish tolerances on the manufacturing
drawings. This can result in the mechanical center and the
magnetic center being mis-aligned, but within acceptable
limits. For machines that are running on their magnetic center
position, the sum of the axial magnetic centering force is zero.
However, during the uncoupled starting and accelerating
condition, these forces will be unbalanced and can have a
magnitude that will move the rotor axially from its mechanical
and magnetic center position to the extent that the shaft
collars make momentary contact with the sleeve bearing
thrust face. This is considered a normal occurrence and
bearings are designed to momentarily accommodate this
265
bumping force. To measure these forces, across the line
testing (ATL) is included in this study with time and transitory
effects recorded. In all cases, the behavior of the rotor and
the axial force present equalize during starting and drops to
zero once the magnetic center is found. In other words, when
the machine is energized and reaches full speed, the final
centering force is zero. Experimental testing for the running
condition is used to correlate results with mathematical
modeling.
III.
AXIAL MAGNETIC FORCE VERSUS
ROTOR AXIAL POSITION (MAGNETIC
CENTER)
As mentioned previously, the magnetic centering force is
influenced by machine geometry, assembly tolerance, as well
as, mechanical and electrical characteristics. When the
machine is uncoupled and energized, the magnetic centering
force may be classified as follows:
acceptable tolerances.
However, if the machine is
manufactured outside of these tolerances, it is possible that
the machine may experience a weak magnetic center or may
have more than one magnetic center position.
Fig. 1
illustrates discrepancies in manufacturing that may position
the rotor on one or two magnetic centers. When the motor is
energized the rotor will try to search for a magnetic running
neutral [1]. A characteristic of a motor with more than one
magnetic center is its failure to seek center. For example,
when the machine is energized uncoupled it will oscillate as
normal, but will continue to oscillate searching between the
primary and secondary magnetic center. Another possibility is
to axially push the shaft from its float in position to the float out
position. The rotor may find another magnetic center within
the endplay limits. The force applied to move the rotor from
its original position is low to medium.
1) Light or Weak: The rotor can easily be moved off its
magnetic center, changing the rotor position, by applying an
external force to the end of the shaft. The rotor will then
oscillate axially for a period of time searching for its magnetic
center.
2) Medium: A moderate force is required to move the rotor
off its magnetic center. When forced off center, the rotor will
oscillate a few times before it locks once again onto the
magnetic center.
3) Strong: A significant applied force is required to move
the rotor off its magnetic center location. The rotor will
immediately return to its magnetic center without observable
axial oscillations.
Test experience shows that the main contribution to the
magnetic re-centering force is due to the variations in the air
path permeance in the air gap and radial cooling vent areas
when the rotor is displaced from its magnetic center.
Although mechanical airflow imbalance will be considered, it is
usually a very small influence on the magnetic center
phenomenon.
For high speed motors, such as 2 pole machines where a
few stator and rotor radial cooling vents are required or no
rotor vents are required, the magnetic center has a tendency
to be weak. Conversely, the magnetic center tends to be
strong in machines with a large number of poles due to the
required higher magnetization current [1]. However, for 4 pole
and slower speed machines, the magnetic center can still fall
into any one of the above 3 categories (weak, medium, or
strong) depending on how the radial vents are configured.
Motors with radial cooling vents aligned are generally
considered to be designs with strong magnetic centers. The
magnetic centering forces associated with radial aligned vents
are significantly stronger than for those machines that have
misaligned vents because there are more edges of the rotor
packs aligned with the edges of the stator packs.
Unfortunately, the technique of having the stator and rotor
cooling vents aligned can lead to other undesirable effects,
such as an increase in noise level.
When non-aligned stator and rotor cooling vents are used,
it is expected to have a magnetic center from medium to
strong if the stator and rotor are manufactured within
Fig. 1 – Illustration of Double Magnetic Center
Another factor that may influence axial rotor position is the
airflow balancing forces. The axial force created by the
velocity of the air from cooling fans mounted on the rotor can
have an effect on the axial magnetic forces. This effect will
mostly be noticeable on TEFC machines with an external fan
mounted on the shaft and interior axial single end ventilation.
Open machines with two fans of identical construction and
double end ventilation have differences that are so minimal
that the circuits will appear to be balanced and the forces
developed by the fans can be considered equal and opposite
to each other. However, if the external fan is mounted on one
end of the rotor shaft, such as a TEAAC enclosure, and the
pressure of the external fan produces forces that exceed the
re-centering magnetic force established inside the motor, the
resulting forces may lead to oscillation on the shaft and an
unstable magnetic center. This situation is especially true
when the magnetic center is light or weak.
266
There are methods that have been developed to help
strengthen the magnetic center forces on a machine with
minimum effect on overall motor performance.
These
methods will be discussed in Section VI of this paper.
IV.
as shown in Figs. 3 and 4, the characteristic of the rate
change, equation (3), is plotted in Fig. 5.
CALCULATION OF FORCES
The calculations used in this paper are based on the
method developed by Bradford and Rhudy in the AIEE report
from 1953 [2].
There are several factors that influence the axial magnetic
force resulting from the air gap permeance of the machine as
the rotor is displaced axially.
1) Core Alignment: Changes in alignment of the ends of
the stator core with respect to the rotor core ends.
Equation (1) represents the force in pounds (lbs.) for a
relatively small axial displacement of a few air gap lengths.
Fends = 0.0117α ×
where
Eo
α
Imo
f
Lo
L
∂L/ ∂x
60
f
×
E o Im o
Lo
⎛ Lo ⎞
×⎜
⎟
⎝ L ⎠
2
×
∂L
∂x
(1)
Fig. 2 – Idealized End Configuration [2]
Phase voltage, volts
Number of phases
No load phase current, amps
Frequency, hertz
Gross core length, inches
Effective core length per unit air gap, inches
Rate of change of effective machine length
with axial displacement
0.0117α ×
60
F
x
E o Imo
Lo
Fig. 3 – Rate of change of effective machine lengths
when stator overhangs rotor at each end [2]
(2)
The expression (2) represents the energy stored in the air
gap per unit length, and is a unit of force. The second term of
2
equation (1), (Lo/L) , is dimensionless and is the ratio of the
rotor displacement from its magnetic center. This value is
nearly equal to unity for relatively small displacements of a
few air gap lengths. The term [∂L/ ∂x] is also dimensionless
and its contribution is established by the following relationship.
−
where
h
g
d
∂L
∂x
⎡
2
⎣
π
= ⎢1 −
× Ctn
−1⎛ h ⎞⎤
⎜ ⎟⎥
⎝ g ⎠⎦
(3)
Fig. 4 – Rate of change of effective machine length
when the stator overhangs the rotor [2]
displacement, inches
air gap length, inches
width of the duct (vent), inches
The value [∂L/ ∂x] represents the rate of change of effective
length with respect to axial displacement of the core ends.
The idealized stator and rotor end configuration is referenced
in Fig. 2. When the rotor and stator are not identical lengths
2) Vent Alignment: The force resulting from changes in
alignment of the edges of the radial cooling vents.
Equation (4) will apply for the force resulting from changes
of the edges of the radial vents. Its contribution of [∂L/ ∂x]
will be for each duct of the machine and its associated vent
pack correction per (Lo/L)2. The component for opposite
267
1
6
0.9
d/ g=10
5
0.8
d/ g=8
4
0.7
3
0.6
2
0.5
1
0.4
0
d/ g=6
d/ g=4
d/g=2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.3
AXIAL DISPLACEM ENT PER UNIT AIR GAP (h/g)
0.2
d/g=2
d/g=4
d/g=6
d/g=8
d/g=10 d/g=12
Fig. 7 – Rate Of Change Of Effective Machine Length With
Axial Displacement Of Rotor [2]
d/g=14
0.1
Equation (5) and (6) are correct only when stator and rotor
core ends are aligned [2].
0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
Axial Displacement per unit air gap (h/g)
Frx = −
Fig. 5 – Rate of change of effective machine length with
axial displacement of rotor [2]
stator and rotor ducts to the term, [∂L/ ∂x], is represented by
the idealized configuration in Fig. 6. The reduction of effective
machine length per axial displacement of the rotor is plotted in
Fig. 7.
Fducts = 0.0117α ×
where
n
60
f
×
E o Im o
Lo
⎛ Lo ⎞
×⎜
⎟
⎝ L ⎠
2
×
∂L
∂x
×n
(4)
Fsx =
12T
D
12T
D
where
T
D
θrk
θsk
(
× θ rk + θ sk
(
× θ rk + θ sk
)
)
(5)
(6)
Torque, ft-lbs
Rotor diameter, inches
Angle of rotor skew, radians
Angle of stator skew, radians
This magnitude of force can be evaluated based on the stator
and rotor currents and physical dimensions of the machine.
However, due to the limited application of large machines with
skewed cores and the low axial force effect at no load
condition, this particular force component was not analyzed.
number of radial vents
4) Air Gap Flux vs. Ring Current: Axial force is influenced
by the interaction of the air gap flux with the current on the
rotor cage end rings. For the steady state condition, this
effect is negligible and its contribution only arises when the
rotor is shifted from its magnetic center by an external force or
during the starting condition where the rotor will move axially
until it achieves magnetic center.
All of the above forces contribute to the final resultant
magnetic centering force.
The magnitude is directly
influenced by the energy stored in the air gap, the force
generated by the core ends, the radial ducts and the position
of the rotor with respect to the stator.
V.
Fig. 6 – Idealized duct configuration [2]
3) Skewed Core: A third component arises when either the
stator or rotor core is skewed.
The force in pounds resulting from skewing a rotor or stator
will be proportional to the skew and torque transmitted.
TEST PROCEDURES AND RESULTS
Tests were performed to several machines to measure the
axial force and its axial displacement. The machines were
selected based on size, type of enclosure, and speed, as well
as stator and rotor cooling vents configurations shown in
TABLE I.
268
TABLE I
Case
1
2
3
4
5
HP
4550
2000
3500
4000
8000
Poles
4
2
2
4
6
TEST CASES
Voltage
Freq.
13800
60
4160
60
4160
60
4160
60
13800
60
Enclosure
TEAAC
WP2
WP2
WP2
TEWAC
A load cell was mounted to the shaft end to measure the
axial force for different conditions. Fig. 8 illustrates a typical
set up for a load cell device.
Per Fig. 9, the axial forces for each case show different
results when the rotor is locked away from its magnetic
center. As the rotor is moved in and out, the re-centering
forces are opposed to the direction of displacement indicating
that the rotor wants to return to its magnetic center. Cases 1
through 4 are designs with the stator and rotor core ends
aligned and in line radial cooling vents. Cases 1 and 4 are 4
pole machines that demonstrate stronger magnetic recentering forces in both rotor positions as compared to the 2
pole machines shown in Cases 2 and 3. Case 5 has the
lowest tested re-centering force as a result of being designed
with non-aligned radial cooling vents.
AXIAL FORCE (lbs.)
600
400
Case 1
Case 2
Case 3
Case 4
Case 5
200
0
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
-200
Fig. 8 – Load Cell Configuration
-400
Components marked in Fig. 8 are as follows:
-600
AXIAL DISPLACEMENT OF ROTOR FROM ITS MAGNETIC CENTER
A: The Flange is bolted to the drive end of the motor
shaft and rotates with the shaft.
B: The Thrust Bearing is a non-rotating part that
transfers force to the load cell.
C: The Load Cell is locked in place with the motor at
magnetic center and offset to zero.
Fig. 9 – Axial Force Test at Full Voltage
Fig. 10 represents testing of the axial forces for Case 4
when the rotor is locked away from its magnetic center at
different voltages. All other cases tested demonstrate the
same axial force profile when the voltage is reduced from
100% to 60% of rated voltage. The axial force magnitude
Testing protocol is divided into 2 different conditions, Running
and Starting.
600
A. Running Condition
100%V
The motors were operated at rated voltage and frequency
under no load conditions with the rotor positioned and held on
its magnetic center. Under these conditions, the motors were
brought up to speed, with the axial forces measured and
recorded using high speed data acquisition equipment. While
running, the voltage was dropped from 100% to approximately
60% of the rated voltage to see the rotor behavior under the
weaker flux conditions. Symmetrical radial cooling vents as
well as stator and rotor offsets were measured during
manufacturing for record purposes.
Machines designed with stator and rotor cooling vents
aligned were also selected for testing. The axial magnetic
forces were measured while the rotor was moved axially,
inboard and outboard, along the complete travel path of the
end play. Fig. 9 is a summary of the test results for the axial
force in pounds versus the rotor axial displacement at full
voltage condition.
80%V
90%V
400
70%V
60%V
AXIAL FORCE (lbs.)
200
0
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
-200
-400
-600
AXIAL DISPLACEMENT OF ROTOR FROM ITS MAGNETIC CENTER
Fig. 10 – Axial Force Test at Different Voltages for Case 4
269
magnetized at the same rate as the winding during the
starting condition. Therefore, the vent arrangement has
minimal contribution to the axial forces.
12 0
Case 1
10 0
AXIAL FORCE (lbs.)
decreases approximately to the square of the voltage as the
voltage drops.
Table II is a comparison of the calculated forces using
equations from Section IV and the test results. The tested
values are similar to calculated for machines with core ends
and radial cooling vents aligned. However, Case 5, which is a
non-aligned vent arrangement, did not follow the same pattern
due to multiple axial forces working against each other. To
predict accurate results, it is necessary to evaluate the
contribution for each individual rotor duct with respect to the
stator ducts. For this arrangement (non-aligned radial cooling
vents), the axial force may vary in sign between individual
packs and it is possible to have more than one equilibrium
position in the rotor with respect to the stator [2].
TABLE II
AXIAL FORCE (lbs.) - CALCULATED VS. TESTED
Displacement
Calculated
Tested
(inches)
0.1
0.2
0.1
0.2
Case 1
189
386
244
485
Case 2
60
230
80
160
Case 3
142
337
125
242
Case 4
320
599
340
469
Case 5
84
151
45
95
Case 2
80
Case 3
60
Case 4
40
Case 5
20
0
0
10
20
30
40
-20
-40
-60
-80
TIME (Seconds)
Fig. 11 – Axial Force vs. Time for ATL Starting
270
7000
140
Amps (A)
6000
120
Voltage (V)
5000
100
Speed (RPM)
Load (Lbs.)
4000
80
3000
60
2000
40
1000
20
0
0
0
2
4
6
8
10
12
-1000
14
-20
TIME (SECONDS)
Fig. 12 – ATL Starting for Case 3
AXIAL FORCE (lbs.)
When a motor is energized, the magnetic field can not be
established or changed instantaneously in the magnetic iron.
The instantaneous field created by applying voltage to the
winding is forced to interact with the air or leakage paths of
the machine due to the concentration in the end turn regions
of the stator winding. A strong magnetic field is present on
each end of the winding [5]. The magnetic material in the
presence of these magnetic fields tries to position itself to
shorten the flux path and the rotor reacts by trying to move
into the end turn regions to shorten the air lines of force.
Thus, the rotor will axially move and thrust between the
bearing and shaft collar until the magnetic field is established
in the stator iron before it will return to its magnetic center.
Testing axial force during starting has not been reported in
the literature. To accomplish this test, the motor was started
across the line with a load cell device mounted on the shaft
end to measure and record the axial force. Refer to Fig. 8 for
load cell configuration.
All five of the cases were started across the line (ATL) with
the rotor locked by the load cell on the magnetic center while
the axial force, voltage, amps and speed were recorded. In
each case, additional starts at reduced voltages were
performed to analyze the axial force behavior. Figure 11 is a
summary of all the cases started across the line.
In all cases the rotor has a behavior of exhibiting positive or
negative axial forces until the machine reaches full speed.
Fig. 11 shows that the magnitude of the axial forces measured
are transitory and decay rapidly to zero indicating that the
magnetic center has been located. Data was recorded using
a data acquisition system with a high speed sample rate to
demonstrate this transitory effect. The results show that there
is no correlation between the magnitude of the force, the vent
arrangement of the machine or the number of poles. This lack
of correlation is due to the core lamination packs not being
Fig. 12 illustrates how the axial force rises rapidly when the
voltage is applied during start up due to the magnetic field
concentration on the end turns versus the lack of
magnetization in the iron laminations. Once the machine
reaches full speed the laminations become fully magnetized
and it returns to magnetic center where the axial forces are
zero. The intention of performing the ATL starting test was to
study the rotor behavior, not to establish the relationship of
the flux in the machine in comparison with the axial force.
This is due to the lab limitations of the voltage source which
did not recover quick enough to correlate the magnitude of
force with the voltage applied during acceleration.
RPM,VOLTS,AMPS
B. Starting Condition
All cases were tested to show the difference in axial force
when started at different voltages.
Fig. 13 plots the
magnitude of the axial force against the voltage applied during
the starting condition. The 4 pole machines show minimal
change in the axial force with relation to the starting voltage.
However, the 2 and 6 pole machines exhibit a magnitude
close to linear correlation to the applied voltage. Due to the
size of the machine for case 5 and the test facility limitations,
the motor was tested at 52% of rated voltage.
125
Case 1
100
Case 2
Case 3
75
Case 4
AXIAL FORCE (lbs.)
50
Case 5
25
0
0
20
40
60
80
100
120
-25
-50
load test. The contribution of this re-centering force will most
likely happen on machines with weak magnetic centers.
VI.
METHODS TO STRENGTHEN AXIAL
MAGNETIC FORCES.
As stated in Section III, the axial forces associated with
magnetic centering are influenced by the alignment of the
stator core and rotor core ends and the edges of the stator
and rotor radial ducts. When double magnetic centers are
found, as represented in Fig. 1, or when the magnetic center
is considered weak, the rotor can shift off its magnetic center
producing an axial force sufficient enough to damage the
couplings. To minimize this effect by strengthening the
magnetic center, a “dummy” or grooved vent can be
incorporated on the rotor. The dummy vent is approximately
0.075 inches deep and is positioned on the rotor to equal or
be slightly larger than the width of the stator vent. The
machining of the dummy vent location is dependent on the
radial stator and rotor vent configuration, but most often is
placed exactly opposite the stator vent location. See Fig. 14
as an example.
Dummy vents can be used to correct manufacturing
variations where the rotor vents are mis-aligned with the stator
vents creating nonsymmetrical balancing forces.
-75
-100
-125
STARTING VOLTAGE (%)
Fig. 13 – Starting Voltages vs. Axial Force
C. Additional Tests
An additional experimental test was performed to study the
rotor behavior while controlling the external airflow of the
machine.
1) Open Enclosures: Top mounted open enclosures with
double end ventilation had one air inlet partially blocked to
simulate an unbalanced aerodynamic effect. The results
conclude that the rotor did not experience any change in the
magnetic center in either direction of the end travel for cases
2 through 4.
2) TEAAC Enclosures: For case 1, the air inlet of the
external blower was blocked to increase the back pressure of
the blower and the axial force was recorded at steady state
condition resulting in 31 pounds of axial force. This suggests
that the external blower, depending on its size and speed, is
capable of producing a mechanical axial force that could
effect the machine re-centering forces. There was no attempt
to calculate the external force due to the use of an external
fan.
3) Load Effect: The five test machines were too large to
test under actual mechanical load conditions to see the effect
of the rotors under load. Previous testing experience reveals,
however, that the magnetic center on machines under load
condition can vary compared to the no load case, due to the
additional axial force produced by the current on the end
rings. This axial force component is not present during a no
Fig. 14 – Illustration of “Dummy” Rotor Vent
Note that these radial machined grooves do not provide a
radial ventilation passage. The purpose of the dummy vent is
to simulate alignment between the rotor vent and the stator
vent to increase the axial centering force between the radial
vent edges. The addition of the machining groove can be
easily obtained on fabricated rotor bar designs where the bars
are driven into each rotor slot [1].
The number of dummy vents to provide is dependent on the
number of stator and rotor vents and the magnitude of the
desired magnetic force increase. In general, providing a
dummy vent in the center of the rotor core and at least one on
each extreme end of the rotor will improve the magnetic
271
center force. Providing more rotor grooves will only increase
the strength of the magnetic center. However, special
attention should be paid to motor performance since the
average radial air gap is being modified with every dummy
vent added. Past experience shows that the changes in
performance on large induction machines are minimal. The
main parameter that will be effected is the power factor due to
the increase in the average air gap. The expected
performance with the addition of the grooves (or offset depth)
can be calculated by averaging the equivalent air gap through
the effective core length of the machine.
VII.
AXIAL ALIGNMENT OF FLEXIBLE
COUPLINGS WITH SLEEVE BEARING
MOTORS
The sleeve bearing machines designed and applied in
accordance with NEMA MG1-20.29 for large motors, greater
than 500hp, establish a rotor end float limit of 0.5” and notes
to limit the maximum coupling end float to 0.19” [4]. This limit
is to ensure that clearance is maintained under all operating
conditions between the journal shoulders on the motor shaft
and the end faces of the bearings. A machine with sleeve
bearings running on its magnetic center will not show axial
forces. If the rotor position is displaced from its magnetic
center, the re-centering force will develop and try to return the
rotor to its magnetic center position.
For most large motor applications, limited-end float flexible
couplings are used for connecting motors with sleeve
bearings to driven loads containing thrust bearings. When the
motor is uncoupled and then energized, the centering force
will move the rotor to its magnetic center position as described
in Section V. B ‘Starting Condition’. However, when the
machine is coupled the motor will transmit torque to the load.
Under this condition the centering force is usually insufficient
to overcome the friction in the coupling and therefore, will
continue to operate in the position set by the coupling. In
other words, the rotor requires a small force to move off
center, but requires a very large force to slip the coupling and
return to magnetic center when transmitting motor starting
torque to the load.
VIII.
CONCLUSIONS
Although the information and results discussed in this paper
are performed on induction motors, the principles can be
applied to any AC electrical machine, including synchronous
and wound rotor motors.
Several conclusions can be derived based on the testing
performed.
A. The equations presented in this paper provide a
reasonable, order of magnitude prediction of the resultant
axial forces when the machines are influenced by the
magnetic circuit and the iron effects. These calculations can
help to determine the strength of the magnetic re-centering
force and its consequences. These equations are intended to
be used exclusively for machines with aligned radial vents or
when core ends (stator or rotor) overhang each other.
B. Both electrical and mechanical components influence the
magnetic centering force. Machines constructed with internal
vents and core ends aligned have a stronger magnetic center
compared to those with non-aligned vent arrangements.
C.
All tested machines experienced axial movement of the
rotor within the endplay limitations during across the line
starting. The magnitude of the axial force is transitory and
decays once the machine reaches full speed. The shifting of
the rotor may exceed the end play limits, but the sleeve
bearing and the shaft collar are designed to accommodate
this momentary effect [5].
D.
Methods to strengthen the magnetic center where the
re-centering forces are relatively weak or minimize a double
magnetic center have been successfully applied with the use
of “dummy” or grooved vents. The machining of the dummy
vents is relatively easy to accomplish and can contribute
tremendously to strengthen weak magnetic centers. The recentering force effect created by the grooves will vary
depending on the speed of the motor and the number of
grooves. High speed machines may require two or more
dummy vents while low speed machines may only need one
or two to strengthen the magnetic center.
E.
Because of the inherent nature of electric machines,
small unbalanced forces that are dynamic or magnetic can
physically disrupt the force equilibrium. This will result in
unstable rotor positions and shaft oscillations within a few
thousandths of an inch and the magnitude of the axial force
can be as low as one pound of force. At first glance, these
oscillations could be misconstrued as axial force by the
human eye; however, the magnitude is negligible and
considered normal for most machines.
IX.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the assistance of
Bryan Evans, Terry Evans, Gabriel Sosa, Pat Hyzak for their
help and use of the test facility. The authors would also like to
show appreciation to Dennis Kosar, Russell King, and MP
Reddy, for their help to support the technical aspect of this
documentation.
X.
REFERENCES
[1] EASA technical paper 1996, Some Aspects of Magnetic
Centering effects on sleeve bearing induction Motors by Bob
Brozek.
[2] AIEE Rotating machinery committee, Published in 1953
Paper 53-124, Axial Magnetic forces on Induction Machine
Motors, By C.E. Bradford and R.G. Rhudy
[3] Managing Motors, The plant engineering’s guide to
induction industrial electric motors, Richard L. Nailen, P.E.
[4] NEMA Standards Publication MG 1-2003, Motors and
Generators
[5] Axial Alignment of Flexible Coupled Sleeve Bearing
Motors, September 1985, Internal Westinghouse Memo by E.
F. Merrill, PE
XI.
VITAE
Javier Portos graduated from U.A.N.L, Mexico with a BSEE
degree in 1990. He joined Unimega-Hitachi Monterrey Plant
in 1990 as an Electrical Design Engineer. He moved in 1996
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to GE Canada as an Electrical Design Engineer with the large
motor technology group. In 1998 he pursued a career with
Louis Allis company in Milwaukee as a Electrical Design
Engineer of large and special induction motors. In December
1998, he joined Teco-Westinghouse Motor Company as an
Advanced Design Engineer and currently holds a position as a
Senior Design Engineer for large Induction and Synchronous
machines.
Sandra Turner graduated from Texas A&M University in
December 1994 with a BSEE degree. She joined TecoWestinghouse Motor Company in January 1995 as an
Electrical Engineer and has held various positions in sales,
applications and engineering. She is currently a Senior
Design Engineer for large Induction Machines.
Bill Veerkamp received the B.S. degree with honors and the
M.S. degree from the University of Missouri-Rolla in 1988 and
1989, respectively, both in electrical engineering. In 1989 he
joined The Dow Chemical Company, where he has worked in
a variety of positions.
He currently provides electrical
technical support in their Engineering Solutions office in
Houston. He is a member of the IEEE Industry Applications
(IAS) and Power Engineering Societies as well as the
Standards Association. He is Vice-Chair of the Awards
Nominating Subcommittee of the IAS Petroleum and
Chemical Industry Committee (PCIC).
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