IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO.

5, SEPTEMBER/OCTOBER 2003 1467

Motor Primer—Part III

Gary Donner, Senior Member, IEEE, Bryan K. Oakes, and Steve T. Evon, Member, IEEE

Abstract—In recent years, much has been written about motors Torque Versus Speed Requirements of the Load: The initial
on variable-speed drives, high-speed rigid shaft motors, impact of motor size must reflect the fact that in many cases the load
API Standard 541, motor diagnostics, etc. Most of these papers torque increases as the equipment ages or operation conditions
and articles assume that the reader has significant knowledge of
motor theory and operation. However, this assumption is overly change. Failure to accurately establish the load torque require-
optimistic, considering that only a few colleges teach motor theory ments is the most frequent cause of incorrectly sized motors.
today, and that application experience at motor user locations has Area Classification Where the Motor Will Be Located: If the
been reduced in recent years. motor is started in an atmosphere that may be combustible, the
Index Terms—AC induction motor, bearing currents, bearing internal temperature of the motor should be limited to no more
life, motor sizing, motor torque, rotor bars, shaft currents. than 80% of the lowest autoignition temperature of the gas that
will be present during starting. Special precautions may be nec-
I. INTRODUCTION essary to ensure safety of the installation. Some examples are as
follows:
HIS PAPER is the third in a series of papers where the
T authors provide answers to questions that are routinely
asked by working engineers in industry. The authors will present
• limiting rotor bar and rotor hot-spot temperature total rise
to 200 C;
• providing special seals to allow purging of the motor be-
motor theory and application information with an extensive ref-
fore starting;
erence list that will help working engineers increase their gen-
• ensuring that all covers and attachments are bonded so no
eral understanding and knowledge of motors. This series of pa-
sparking occurs;
pers also serves as a valuable reference for those who apply and
• using a totally enclosed machine to minimize winding
specify motors.
contamination, thus reducing air-gap and end-turn
sparking.
II. HOW DO YOU SIZE A MOTOR?
Number of Motor Starts: The number of starts the motor can
Considerations for Selecting a Motor: Motor selection is a be expected to make during its life has a definite limit. For larger
process containing numerous tradeoffs. The objective of motor motors the number of starts is limited to 5000. If the motor will
selection is to arrive at the best possible installation, taking into be expected to make more starts, the design will usually require
account the following criteria: life-cycle cost, horsepower and the use of stronger shaft material, a larger diameter shaft, or
frame size for the specified life expectancy, load torque, load both.
inertia, and duty cycle of the specified application. Voltage: It is essential to have an accurate model of the power
The following discussion assumes that the motor to be se- system to limit voltage drop in order to prevent some motors
lected will be a single-speed induction motor, operating from from shutting down when another motor is started. The voltage
normal power and not connected to a variable-frequency drive at the motor terminals during both starting and running must be
(VFD). If a motor–drive combination is required, it is recom- known in order to insure that the new motor will start without
mended that the motor and drive be supplied by the same man- affecting other equipment on the power system. If starting other
ufacturer to insure that a compatible system is obtained. motors will depress the voltage of the new motor below its crit-
For other types of motors, application assistance from the ical recovery voltage, the new motor will stall. Also, low voltage
motor manufacturer is suggested. Before the size of a motor can can result in the motor being overloaded.
be determined, several factors must first be evaluated. They are Uniform application of single-phase loads can help assure
as follows. proper voltage balance in the electrical distribution system
that supplies polyphase motors. Unbalanced voltage affects
Paper PID 03–05, presented at the 2002 IEEE Petroleum and Chemical the motor’s current, speed, torque, temperature rise, and effi-
Industry Technical Conference, New Orleans, LA, September 23–25, and ap- ciency. A relatively small unbalanced voltage will significantly
proved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS
by the Petroleum and Chemical Industry Committee of the IEEE Industry
increase motor losses and decrease motor efficiency. The gains
Applications Society. Manuscript submitted for review September 15, 2002 achieved by purchasing a premium-priced premium-efficiency
and released for publication June 25, 2003. motor that reduces losses by 20% will be negated by a voltage
G. Donner is with Shell Oil Products US, Los Angeles Refinery, Wilmington,
CA 90748-0817 USA (e-mail: gldonner@shellopus.com).
unbalance of only 3.5%, because that small voltage unbalance
B. K. Oakes is with Reliance Electric, Kings Mountain, NC 28086 USA will decrease the efficiency of the motor by 20%.
(e-mail: bkoakes@powersystems.rockwell.com). Load Profile: An accurate evaluation of the load profile is
S. T. Evon is with Reliance Electric, Greenville, SC 29615 USA (e-mail:
stevon@powersystems.rockwell.com). essential. Reciprocating or cyclic loads will have an impact on
Digital Object Identifier 10.1109/TIA.2003.816557 rotor, shaft, bearings, winding and housing design.
0093-9994/03$17.00 © 2003 IEEE
1468 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 5, SEPTEMBER/OCTOBER 2003

Fig. 1. Motor speed–torque curves at 100% voltage and 80% voltage versus
driven load speed–torque requirements. Fig. 2. Motor speed and torque for NEMA design motors.

One of the most common sources of motor losses is a motor conveyors, crushers, stirring machines, agitators, reciprocating
that is not properly matched to its load. In general, for standard pumps, and compressors.
NEMA frame motors, motor efficiency reaches its maximum at NEMA Design D Motors: This design is intended for high
a point below its full-load rating. Efficiency peaking below full peak loads, with or without a flywheel, such as punch presses
load is a result of the interaction of the fixed and variable motor shears, elevators, extractors, winches, oil well pumpers, and
losses. Power factor is also load variable and increases as the wire drawing machines. Design D motors deliver high starting
motor is loaded. If the motor is operated above full load in order torque and are designed with high slip (more than 5%) so that
to take advantage of its service factor, the power factor begins to motor speed can drop when fluctuating loads are encountered.
decrease because the motor’s resistance to reactive ratio begins Although Design D motor efficiency can be less than other
to decrease and the power factor declines. Motor loading and NEMA designs, it is not possible to replace a Design D motor
motor power factor have to be weighed against each other in with a more efficient Design B motor, because the Design B
order to obtain optimum motor efficiency. motor would not meet the performance demands of the load.
In some applications where motors run for an extended period See Table I for a comparison of NEMA motor characteristics.
of time at no load, shutting down the motor and restarting it at Fig. 3 explains where each value of torque occurs on the motor
the next load period could save energy. speed–torque curve.
Motor Sizing: Here are two methods that can be used to se- Other Considerations for Motor Sizing When Using the
lect the actual motor rating once a complete analysis of the Second Method:
motor service has been performed. Motor efficiency is not the most important consideration
The most accurate method, regardless of motor horsepower, is when selecting a motor because the motor with the highest
to obtain an accurate speed–torque curve from the driven equip- operating efficiency does not always provide the lowest energy
ment manufacturer. This curve will have an end of curve horse- cost. If the motor is in cyclic service, a higher slip motor may
power rating. Create a graph that shows both the driven equip- actually save energy. Selecting the most efficient motor of a
ment speed–torque curve and the motor speed–torque curve at given size and type does not insure that energy savings are
80% voltage (see Fig. 1). being optimized. Every motor is connected to some form of
Locate the pinch point, which is the point where the differ- driven equipment: a crane, a machine tool, a pump, etc., and
ence between the speed–torques of the 80% voltage curve of motors are often connected to their loads through gears, belts,
the motor and the load curve of the driven equipment is the or slip couplings. By examining the total system efficiency, the
least. The motor torque must be at least 10% greater than the component which offers the greatest potential improvements
load torque at the pinch point. The final check is to insure that can be identified and purchased.
the motor full-load torque is always above the driven equipment Energy-efficient motors may be the most cost-effective an-
full-load torque, as it is in Fig. 1. swer for certain applications. Here are simple guidelines to keep
The second method is for NEMA size machines, usually in mind when making this determination.
500 hp and less. There are four standard speed–torque char- • Choose applications where motor running time exceeds
acteristics available. They are NEMA A, B, C, and D. Each idle time.
classification of motors has its own distinctive speed–torque • Review applications involving large-horsepower motors,
relationship (see Fig. 2). where energy usage is greatest and the potential for cost
NEMA Design A and B Motors: The NEMA Design A motor savings can be significant.
is a variation of the B design, having a higher locked-rotor • Select applications where loads are fairly constant and
current than the B design. These two designs are intended for where load operation is at or near the full-load point of
general applications such as fans, blowers, centrifugal pumps, the motor for the majority of the time.
compressors, motor–generator sets, etc. • Consider energy-efficient motors in areas where power
NEMA Design C Motors: This design is intended for appli- costs are high. In some areas power rates can run as much
cations where the motor will be starting under load, such as as $0.18 per kilowatthour. In these cases, the use of an en-
DONNER et al.: MOTOR PRIMER—PART III 1469

TABLE I
COMPARISON OF NEMA MOTOR CHARACTERISTICS

Fig. 3. Explanation of motor torques.

Fig. 4. Homopolar shaft flux.
ergy-efficient motor might be justified in spite of long idle
times or reduced-load operations.
• Utility rebate programs can also have a strong influence on
the decision to purchase a high-efficiency motor. In some
areas of the U.S. and Canada the net cost of an energy-
efficient motor after rebate is less than that of a standard-
efficiency motor.
The simple guidelines outlined above will lead to the selec-
tion of correctly sized motors.

III. WHAT CAUSES SHAFT AND BEARING CURRENTS?

Currents that flow through the shaft and bearings of a motor
are the results of a voltage potential between the motor shaft
and frame. All motors have some voltage potential between the Fig. 5. Homopolar bearing current.
shaft and frame. It is only when this voltage potential is large
enough to cause damaging currents through the motor bearings Homopolar Flux: The homopolar flux is direct or alternating
or driven equipment bearings that failures can occur. flux flowing in the shaft, through the motor bearing into the
Bearing currents are not new and have existed since motors frame, and through the same bearing back into the shaft. It can
were first built. In [1], Pearce remarked “If it were possible also be referred to as shaft magnetization flux or “through flux.”
to design a perfectly balanced and symmetrical machine, both Figs. 4 and 5 show the axial shaft flux and bearing current path.
practice and theory indicate that no bearing currents, could Homopolar flux is usually constrained to high-speed sleeve-
exist.” It may be possible to manufacture a perfectly balanced bearing machines, as it requires the bearing surface to span the
and symmetrical machine, but it is not cost effective. flux path. It is caused by an unbalanced ampere turn encircling
Today, motors are powered by two sources of power, the shaft. Possible causes of the unbalance ampere turn are as
sine-wave power and variable-frequency power. The power follows:
source of the motors is often used to distinguish between two • asymmetrical winding connection;
categories of shaft currents. Since many papers have been • broken rotor bar;
published in recent years about shaft currents due to VFDs, • sectionalized end ring;
most of this section will discuss shaft currents in sine-wave • shaft residual magnetization;
applications. • to a lesser extent, uneven air gaps (rotor eccentricity).
Shaft currents in sine-wave applications can be divided into The bearing current induced is usually minor. There is no
two categorizes, homopolar flux and alternating flux linking the established method to measure this type of bearing current
shaft. While both types of shaft currents can occur, the second flow since it is normally contained in the bearing. An insulated
type is much more common. bearing may reduce the bearing current, but does not eliminate
1470 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 5, SEPTEMBER/OCTOBER 2003

recommends insulated bearings if shaft axial voltage exceeds

300-mV peak under sine-wave operation.
Unlike homopolar shaft currents, circulating shaft currents
can be measured by shorting both ends of the shaft with a low-
impedance cable. (see IEEE Standard 112-1996). The circu-
lating shaft current can only be present when both motor bear-
ings are conducting or when the opposite drive-end bearing of
the motor is uninsulated.
Protecting bearings from circulating shaft currents can be
accomplished by insulating the opposite drive-end bearing or
insulating both bearings. Insulating only the drive-end bearing
may still result in circulating shaft currents. Since the shaft
Fig. 6. Circulating shaft current. voltage is generated in the rotor, the driven equipment bearing
may complete the conductive path through the shaft to the op-
posite drive-end bearing. Therefore, circulating shaft currents
continue to flow through the bearings, perhaps damaging the
bearings in the driven equipment.
Shaft Current from VFDs: Bearing currents may also be
generated when motors are powered by VFDs). The previously
discussed internally sourced bearing currents may still exist in
VFDs. The externally source currents are a result of the wave
shape from the VFD. Bearing currents from VFDs can occur,
although the majority of motors on VFDs do not have bearing
currents levels that are high enough to damage motor bearings.
Many papers have been written over the last several years
Fig. 7. Example of stator dissymmetry.
about VFD-sourced bearing currents. Proper high-frequency
grounding is critical to limit the currents caused by VFDs.
homopolar flux. Installing nonmagnetic bearings, bearing When bearing currents from VFDs are present, techniques
housings, and/or shaft can isolate the source. such as insulated bearings, shaft-grounding brushes, good
Circulating Shaft Currents: Circulating shaft/bearing high-frequency grounds, and/or electrostatic shielded windings
currents due to time-varying flux linking the shaft are more may provide protection for the bearings. Since so many papers
common than homopolar flux. The axial flux is generated are available on this subject and it would consume the entire
through transformer action. Since electrical steel is not totally paper to properly address the subject, it will not be addressed in
homogenous, flux paths in the motor are not entirely sym- detail. The best method of avoiding bearing currents on VFDs
metrical. An asymmetric rotor and/or stator core construction may be to purchase the motor and drive from one manufacturer.
may cause a net flux to encircle the shaft. The shaft, bearing
and frame may be seen as a one-turn secondary winding of a
IV. WHAT CAUSES ROTOR BARS TO BREAK?
transformer and an electromotive force (EMF) along the shaft
will be induced. The resultant current will flow through the There are many causes of rotor bar failures, but most can be
shaft, down the bearing, through the frame, and back through divided into three categories: thermal stress, magnetic stress, or
the second bearing. The flux path and current path can be seen mechanical fatigue. Both cast aluminum and copper bar rotors
in Fig. 6. can be subjected to these types of stresses. However, the con-
Many of the causes of shaft current can be controlled. Lami- struction and design of both will have an impact on the thermal
nations are designed as symmetrically as possible. One common and mechanical stress levels they can withstand before failure.
cause of high shaft currents is the number of straps or welds Rotor failures may appear to be sudden, but with a few ex-
holding a stator together. Even symmetrical combinations such ceptions they usually occur over a long period of time. Early
as four and six welds or straps may induce shaft current for a detection of rotor bar failures is difficult. The primary reason
certain number of poles. For example, six stator straps or welds for most rotor failures is operation of the motor beyond its de-
will induce shaft currents in four- and eight-pole motors. Dis- sign limits.
symmetries in the rotor construction may also cause shaft cur- Thermal Stress: Thermal stress failures occur when the
rents. An example of a weld location dissymmetry can be found motor is pushed beyond its designed capability. Thermal stress
in Fig. 7. failures can occur quickly or over time. The most common
When both bearings are uninsulated and conducting, shaft causes of thermal stress are excessive number of consecutive
voltages as small as 500 mV can cause shaft currents greater starts, long periods of rotor acceleration, rotor stalling, exces-
than 20 A. Properly designed motors will not have shaft cur- sive overloads, and rotor rubbing the stator.
rent levels that are high enough to cause bearing damage. It has Perhaps the most common cause of motor thermal stress
been the experience of the authors that current levels of 20 A or failure is from excessive consecutive starts. Induction motor
greater require at least one insulated bearing. NEMA MG1-1998 currents during starting are usually 5–7 times full-load current
DONNER et al.: MOTOR PRIMER—PART III 1471

aluminum cage is cast directly into the lamination core, allowing

the material to maintain intimate contact with the laminations.
The intimate contact improves the heat transfer from the alu-
minum to the cast rotor laminations, reducing the amount of
heat transferred to the end rings. The rotor fans are integrally
cast into the end rings and also provide excellent heat sinks for
the end rings. The lower end-ring temperature will limit radial
end-ring expansion. However, excessive consecutive starts will
also fatigue the rotor bar and end-ring joint similarly to a bar
rotor and cause rotor bar cracking and failures.
NEMA MG-1 12.54.1 and 20.43.1 states that motor should
be capable of :
a) Two starts in succession, coasting to rest between starts,
with the motor initially at ambient temperature.
Fig. 8. Copper bar rotor. b) One start with the motor initially at a temperature not ex-
ceeding its rated load operating temperature.
If more consecutive starts are required, the motor manufac-
turer should be consulted at the time of purchase to ensure that
premature rotor failures do not occur.
Consistent excessive overloads affect the rotor similarly to
excessive consecutive starts. The excessive overloads cause
thermal cycling in the rotor-bar and end-ring joints. If the
overloads were not considered in the design of the motor, the
thermal cycles may causes excessive radial stresses in the bar
and end-ring joints, leading to rotor bar cracks and rotor failure.
Consistent excessive overloads should be avoided or addressed
at the time of motor purchase.
Rotor stalling may also cause rotor bar failures. Rotor stalling
occurs when the load starting torque requirements exceed the
torque produced by the motor. Single phasing of the motor, low
Fig. 9. Cast aluminum rotor.
voltage, or improper motor sizing are some causes of stalling.
The motor will either not start or will run at a speed below break-
and produce high I R losses in the rotor. Since the heat is gen- down. In either case the motor current is normally 5–7 times
erated quicker than it can be dissipated to the laminations, the rated current. If the motor is not taken offline and is allowed to
rotor bars, and end rings, the temperature of the rotor rapidly run under these conditions for a short period of time, excessive
increases. It is not uncommon for rotor-bar and end-ring rotor and stator heating will occur. Rotor temperature increases
temperatures to rise more that 20 C/s during starting. The end of 20 C/s or greater often occur. Rotor failures can then occur
rings become a heat sink for the rotor bars causing the end ring as the rotor-bar temperature reaches the melting point of the alu-
to expand radially, subjecting the rotor cage to high thermal minum or braze material.
stresses and dynamic loads. Rotors rubbing the stator may also cause rotor failures. Rotor
While the temperature increase in the end rings is rapid, the rubs can occur during bearing failures, from improper air-gap
temperature increase in the laminations is gradual. The combi- alignment and from unbalance magnetic pull. The heat from the
nation of the coefficient of expansion for copper being greater friction of the rotor and stator can lead to rotor bar failures.
than steel and the large difference in temperatures between the Magnetic Stress: Magnetic stresses can be grouped into
rotor laminations and rotor end rings causes a radial bending two categories: unbalanced magnetic pull and electromagnetic
force to be exerted on the rotor bars. This results in a high stress stress. Magnetic stresses are usually due to a defect in the
point where the bars exit the rotor core. Fig. 8 shows an exag- motor. While application conditions may accelerate the failure,
gerated example of the force for a copper bar rotor. Fig. 9 shows the cause is usually manufacturing or design related.
the force on a cast rotor. All motors have some amount of unbalanced magnetic pull.
The bars usually extend from 1 to 3 in beyond the end of the In the manufacture of a motor some dimension tolerances are
rotor laminations on bar rotors. This bar extension allows for a allowed. Therefore, the motor air gap is usually not uniform.
certain amount of radial growth. However, when the designed The nonuniform air gap results in a greater magnetic pull toward
limit of radial growth is exceeded on a regular basis, as in the the small side of the air gap. If the motor air-gap eccentricity is
case of excessive consecutive starts, the bar material will fatigue too great or the shaft too flexible then rotor pull-over may occur.
and crack, which will over time lead to a rotor failure. Rotor pull-over results in the rotor striking or rubbing the stator.
Cast aluminum rotors (see Fig. 9) do not have bar extensions. The friction of the strike will eventually result in a rotor failure
Cast aluminum rotors do have other features that the bar rotors due either to excessive temperature or the forces from the strike.
lack. A cast aluminum rotor assembly is one integral unit: the Motor manufactures normally design motors with low air-gap
1472 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 5, SEPTEMBER/OCTOBER 2003

Fig. 10. Typical rotor bar shapes. Fig. 11. Centrifugal force on the lamination slot.

eccentricities and shaft stiffness with adequate safety margin to

prevent magnetic pullovers.
Electromagnet stresses occur when a vibration occurs as a
result of electromagnetic forces. One type is the movement of
the bar radially between the top and bottom of the slot. The vi-
bration occurs at twice rotor current frequency and is a result
of the slot linkage flux generating electrodynamic forces on the
bars. The deflection normally occurs during starting and results
in a fatigue failure of the bars. Failures will not occur in prop-
erly designed rotors with controlled clearances. Another type of
electromagnet stress occurs from eccentric air gaps. The eccen-
tric air gap causes electromagnetic vibrations or noises. These
vibrations may lead to fatigue failures or may simply cause in-
creased noise levels at rated load. The vibration or noise will (a)
increase with an increase in air-gap eccentricity.
Mechanical Stress: Rotor failures related to mechanical fail-
ures of the rotor bars, end rings, and lamination may also occur.
These failures are normally related to centrifugal forces. Cen-
trifugal force is proportional to the product of the mass times
the square of the speed.
The Centrifugal Force Equation is

where
mass;
radius;
speed in revolutions per minute.
Large motors are where centrifugal forces become large
enough to be of concern. Variable-speed motors running (b)
above safe operating speed also may have rotor failures due
Fig. 12. Lamination stresses.
to centrifugal forces on the rotor. Excessive centrifugal forces
place the rotor end ring in hoop stress and the bar extension in
shear with a bending moment. Figs. 8 and 9 show examples of The rotor laminations are subjected to high stresses induced
these stresses on bar and cast rotors. by centrifugal forces on the rotor cage (Fig. 11). The centrifugal
Motor designers can reduce the stress due to centrifugal forces act directly on the rotor bars forcing them against the
forces by modifying the rotor design. Modifying the bar bridge. This force places the bridge in bending and in shear at
extension lengths reduces the compression and tensile stresses the outside edges of the slot.
in the lower and upper edges of the bar. Modifying the slot High stresses can also cause yielding or the fracturing of
shape and height (see Fig. 10) can redistribute the stresses and the laminations between the slots, bridge bending, etc. [see
extend the rotor life. Modifying the short-circuit end-ring shape Fig. 12(a) and (b)].
or incorporating shrink rings can minimize the radial expansion Minimizing the stress levels prevents possible vibration prob-
of the end ring. Alloy changes are also possible to obtain higher lems by a loss of the interference fit, which can create uneven
endurance limits. thermal expansion, imbalance, and lower critical speeds. When
DONNER et al.: MOTOR PRIMER—PART III 1473

TABLE II
BEARING LIFE EXAMPLE

lamination stresses exceed stress limits, mechanical engineers where

will work with electrical engineers to minimize the stresses. L basic rating life, operating hours;
Minimizing stresses may involve a new slot design, changing basic dynamic load rating;
the number of bars, modifying the bridge thickness, etc. equivalent dynamic bearing load;
Rotor failures due to centrifugal force can usually be operating speed;
attributed to poor motor design, manufacturing issues, or life equation exponent:
operating the motor above its maximum safe operating speed. — ball bearings use 3;
Proper modeling tools allow the motor designer to predict force
— roller bearings use 10/3.
and stress levels and design motors that will not have failures
Many factors affect the overall bearing life, with the major
due to centrifugal force.
ones being the following:
• proper installation;
V. WHAT DOES THE BEARING L LIFE REALLY REPRESENT? • lubrication;
• operating temperature;
It represents bearing life and reliability. Bearing performance
• operating alignment;
revolves around evaluating the rating and average life of the
• shaft and housing fits;
bearing based on the loading of the bearing. As in all rotating
• material quality.
machinery exposed to stresses, rolling bearings have a definite
To address some of the factors affecting bearing life, the Ad-
life span and will eventually fail due to fatigue.
justed Rating Life Equation was developed to provide a more
The basic dynamic load rating for a bearing defines the
detailed evaluation of the bearing life by taking into considera-
bearing load that will provide a basic rating life of 1 000 000
tion factors for reliability, material and operating conditions.
revolutions of the inner ring. The rating life (L ) of an identical
The Adjusted Rating Life Equation is as follows:
group of bearings is the life where 90% of a given bearing
population will not fail due to material fatigue. The average
L
life (L ) of an identical group of bearings is the life that will
result in a 50% survival rate. where
Material fatigue is the result of high cyclical stresses occur- L adjusted rating life, operating hours;
ring in the load-carrying zones of the bearing. When the en-
reliability life adjustment factor:
durance life of the material is exceeded, a crack initiates beneath
— 90% reliability use 1.00;
the area of high stress and propagates to the surface, resulting in
— 95% reliability use 0.62;
a spalling away of the surface material. The spalling or flaking
is actually bearing material separating from the raceways and — 99% reliability use 0.21;
rolling elements. material and operating condition life adjustment
Bearing life can be calculated in various degrees with the factor:
Basic Rating Life Equation being the simplest method: — bearing steel improvements allow 1 for this factor;
operating conditions life adjustment factor:
L — factor based on the amount of lubrication;
1474 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 5, SEPTEMBER/OCTOBER 2003

— a viscosity ratio ( ) is the ratio of the actual viscosity [3] J. Boyd and H. N. Kaufman, “The causes and control of electrical cur-
to the viscosity required for adequate lubrication; de- rents in bearings,” Lubrication Eng., vol. 15, pp. 28–35, Jan. 1959.
[4] S. Evon and B. Oakes, “Variable frequency drive principles and practices
pending on the viscosity ratio , the factor can (Above NEMA) AC motors for variable frequency applications,” Trans.
range from approximately 0.7 to 2.6. TAPPI, vol. 39, 1999.
In recent years the New Life Theory was developed that [5] A. Bonnett and G. Soukup, “Rotor failures in squirrel cage induction mo-
tors,” IEEE Trans. Ind. Applicat., vol. IA-22, pp. 1165–1173, Nov./Dec.
expands upon the adjusted rating life equation to evaluate the 1986.
bearing life concerning lubrication type and contamination [6] M. Bradford, “Unbalanced magnetic pull in a 6-pole induction motor,”
values Proc. Inst. Elect. Eng., vol. 115, no. 11, Nov. 1968.
[7] SKF General Catalogue, 3rd ed., SKF USA, Inc., Flowery Branch, GA,
1999.
L L [8] R. McFarland, “Use vibration analysis to increase bearing life,” Maint.
Technol., Dec. 1997.
where
L adjusted rating life according to new life theory,
operating hours;
reliability life adjustment factor: Gary Donner (S’68–M’70–SM’97) received the
— 90% reliability use 1.00; B.S.E.E. degree from California State Polytechnic
University, San Luis Obispo, in 1970.
— 95% reliability use 0.62; He is currently the Supervising Engineer for
— 99% reliability use 0.21; Instrument and Electric Utilities with Shell Oil
life adjustment factor based on new life theory: Products US, Los Angeles Refinery, Wilmington,
CA. He provides consulting services for Equilon’s
— this factor takes into consideration the viscosity Southern-California-based operations. He has been
ratio ( ) along with the level of contamination ( ) a member of the State of California High Voltage
and the fatigue load limit ( ) that represents the Advisory Committee and is on several standard
drafting committees of the American Petroleum
load below which fatigue will not occur in the Institute. He is the past Chairman of the API 541 Induction Motor Committee.
bearing; values of are given as a function of He is the holder of three patents.
( ) for different values of the viscosity ratio ; Mr. Donner is the past Committee Chairman of the IEEE PCIC Manufac-
turing Subcommittee and the Chairman of the IEEE P 1458 Circuit Breaker
the level of contamination ( ) can range from 1.0 Committee. He has also authored several IEEE papers. He is a Registered En-
for a very clean environment to 0.00 for a heavily gineer in the State of California.
contaminated environment.
Application of All Three Methods—Example: Assuming a
purely radial load of 2000 lbs what is the Basic, Adjusted,
and New Life Theory L life of a 6222 bearing operating at
1500 r/min. Bryan K. Oakes received the B.S. degree in mechanical engineering from the
University of North Carolina, Charlotte.
From Table II, the Adjusted Rating Life A has sufficient vis- Since 1989, he has been with Reliance Electric Company. From 1989 to 1996,
cosity, whereas in B the bearing is operating with an insufficient he was a Mechanical Engineer at the Kings Mountain, NC, motor plant, where,
viscosity resulting in a lower life. The New Life Theory exam- since 1996, he has been Mechanical Engineer Manager.
ples show New Life A operating in a clean environment with no
contamination. The New Life B example is operating in a con-
taminated area resulting in a significant life reduction as com-
Steve T. Evon (M’98) received the Bachelor degree
pared to the New Life A example. The tabular results indicate in electrical engineer technology from Southern In-
for the Adjusted and New Life Theory that the operating condi- stitute of Technology, Marietta, GA, in 1985.
tions must be well defined as estimations of factors can lead to Since 1985, he has been with Reliance Electric
Company. From 1985 to 1997, he was an Electrical
errors and magnify life calculations. Engineer at the Kings Mountain, NC, motor facility,
where, from 1997 to 1999, he was Electrical
Engineering Manager for large ac and hermetic
REFERENCES motor products. He is currently Senior Development
[1] C. T. Pearce, “Bearing currents—Their origin and preventions,” Elect. Engineer for Rockwell Automation-Reliance
J., vol. XXIV, no. 8, pp. 372–376, Aug. 1927. Electric Company, Greenville, SC. He is currently
[2] P. L. Alger and W. Samson, “Shaft currents in electric mahines,” AIEE Engineering Manager of Industry Applications for Rockwell Automation,
Trans., vol. 43, pp. 235–245, 1924. Power Systems.