Revisions to IEEE standard 1068 improvements in repairing and rewinding of ac electric motors

BY TRAVIS GRIFFITH, AUSTIN H. BONNETT, BILL LOCKLEY,
CHUCK YUNG, & CYNTHIA NYBERG

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© ARTVILLE

26

Improvements in repairing and
rewinding of ac electric motors
HIS ARTICLE DETAILS THE UPDATES

for the entire standard. A major change in the document

and modifications to the 1996 revision of

is its evolution to full standard status. The IEEE Standards

IEEE 1068, Recommended Practice for the Repair

Association also granted the working group’s petition to

and Rewinding of Motors in the Petroleum and

broaden the scope and title to include process industries

Chemical Industry. It contains only selected topics present

in general. Such recognition acknowledges its value to those

within the standard and should not be treated as a substitute

employing machines in demanding services and severe envi-

T

Digital Object Identifier 10.1109/MIAS.2010.939428
Date of publication: 12 November 2010

ronments, such as the cement trade and pulp and paper
processing. IEEE Standard 1068–2010 was restructured
1077-2618/11/$26.00©2011 IEEE


End Turns
Coil Extensions

Coils

End Ring

Stator Shroud
Belly Band

Eye Bolt
Lifting Eye
Grease Fitting
Zerk Fitting

Rabbet Fit

Spigot Fit

Axial Thrust Washer
External Cooling Fan

Air Deflector
Air Baffle
Shroud

Bearing Cap
Bearing Retainer
Back Cap

Clearance Fit
Flame Path
Shaft Opening

Fan Cover
Fan Shroud

Keyway
Grease Drain
Shaft
End Bracket
End Bell
Rotor Skew

Foot

Sator Laminations

Satcked Stator
Core Iron
Core Plate
Punchings

Frame
Stator Frame

1
Horizontal electric motor nomenclature. (Illustration courtesy of EASA.)

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IEEE Industrial Application Society
to better track the methodologies and
(IAS) Petroleum and Chemical Indusprocesses employed in present-day
IEEE STANDARD
try Committee (PCIC) to establish a
repair facilities. Substantive improveworking group for the next revision
ments include incorporation of cur1068 WAS
cycle. However, its potential within
rently available technology, document
the IEEE IAS indicated the need for
specific testing, evaluation criteria, and
RESTRUCTURED TO
1068 to become a standard. This evoclarification of end user and service
lution required large-scale changes. In
center responsibilities.
BETTER TRACK THE
most cases, more emphatic wording

It is commonly known that electric
necessitated full rewriting rather than a
motor drivers are the most significant
METHODOLOGIES
simple change of might/may wording
user of electric energy within a process
AND PROCESSES
being replaced with will/shall.
facility. Such machines often take prime
During this revision effort, IEEE
consideration in the plant’s critical path
EMPLOYED IN
Standard 1068–2010 was modified to
of operation. Even in spared or noncritimake clear common practices and was
cal service, the cost or long delivery
PRESENT DAY
restructured to reflect the flow of a tycycle of a new unit makes refurbishpical machine through the repair process.
ment, repair, and rewinding an essential
REPAIR FACILITIES.
Qualitative and quantitative test propart of plant reliability, uptime, and
cedures were included and importance
profitability. When ac machines (Figure 1)
placed on key aspects of each step.
require repair, an important relationship exists between the motor user and a repair facility. In References were updated and expanded to reflect the
large plants, orders for machine repair may be repeated most recent versions of relevant documents from the
several times during a normal year of operation. The original American Petroleum Institute (API), American Society
1068 recommended practice [1] provided basic guidance for for Testing Materials (ASTM), Electrical Apparatus and
plants with few motors, personnel who were new to the Service Association (EASA), IEEE, International Elecindustry, and those less familiar with motor repair specifics. trotechnical Committee (IEC), International Standards
First published in 1990, it achieved acceptance in the petro- Organization (ISO), and National Electrical Manufacturleum and chemical industries and was then revised in 1996. ing Association (NEMA).
Of note was the working group’s focus on ac machines

IEEE guidelines advise that a recommended practice is,
by and large, distinguished by the verb should. This style and the decision to remove dc types that are quite dissimiof writing is critical to imply wording with more force than lar to ac units and are less prevalent in the petroleum,
the use of “may” in a guide. It is also differentiated from a chemical, and process industries.
In short, IEEE Standard 1068–2010 [2] provides detailed
standard employing “shall,” which indicates a single-accepted
method. Consideration of 1068’s general use prompted the procedures for ac machine evaluation and data interpretation

27


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through a higher degree of engineering language and the
establishment of a technical reasoning base.

28

Description of IEEE 1068
To demonstrate the flow of a machine through the various
individual or combined modification processes, a brief
document outline is illustrated:
n scope
n qualification of service centers
n define user and repair facilities responsibility
n identify information to obtain before the machine
is removed from service
n incoming inspection (prior to dismantling the
machine)
n accessory device inspection
n disassembly and inspection key points

n electrical tests (stator and rotor)
n mechanical inspection
n rewind guidelines
n balancing of rotating element
n assembly and final test
n post repair work.
In the scope, the first significant change was to focus on
ac induction and/or synchronous machines (e.g., motors)
and to add dc machines to the list of excluded apparatus.
Should consensus determine the need, a future dc repair
standard might be developed. As noted above, a midstream
alteration broadened the usefulness of IEEE Standard 1068–
2010 to associated IAS constituents and other unrelated
process industries. The document is now titled Standard for
the Repair and Rewinding of AC Electric Motors in the Petroleum,
Chemical and Process Industries.
As with the original recommended practice, IEEE
Standard 1068–2010 is a supplement to manufacturers’
designs, tests, and instructions. It is not possible for the document to address all possible designs, construction methods,
or materials having occurred over the previous century. Thus,
it does not supersede the manufacturer’s information, directives, or cautions. To quote from the revised scope: “The
standard covers recondition, repair, and rewind of horizontal
and vertical induction motors and synchronous machines.”
Recognizing that there are certain specialized niche categories of electric motors, each of which has unique repair
requirements, the document specifically excludes dc, hermetic, nuclear, submersible, and hazardous (classified) area

2
Example of a Level 5 failure. (Photo courtesy of EASA.)

machines from coverage. While large portions of Standard

1068 are still applicable to such repairs, those specialized
machines require unique treatment.
It is self-evident that a working motor has no use for this
document. Aside from the few programs that require periodic cleaning of large motors, an operating machine is not
likely to be sent to a repair facility until something breaks.
When a damaging event occurs, the usual preliminary focus
is to return the unit to running condition. Repair extends
machine life at reduced cost and in less time than obtaining
a new unit. On some occasions additional goals arise subsequent to teardown and component evaluation. A simple case
is upgrading components to accommodate a manufacturer’s
current design, but more likely are changes to mitigate the
cause of the failure and, particularly, redesign of the winding
to improve any one of several operating parameters.
Just as not all failures are equally severe, not all repairs are
equally extensive. The standard adopts a practical description
of graduated levels of repair, ranging from Level 1 (routine
maintenance) through Level 5 (machines that suffered catastrophic failure and would normally not be repaired). These
levels of repair as in [3] are defined as follows:
n Level 1: Basic Reconditioning: It includes replacing of antifriction bearings, or inspecting and verification of hydrodynamic bearings, cleaning all parts,
and replacing lubricant. Also, the repair includes
addition of seals and other accessories as agreed with
the customer.
n Level 2: This includes Level 1 with the addition of
varnish treatment of stator windings, repair of worn
bearing fits, and straightening of bent shafts.
n Level 3: This includes Level 2 as well as rewinding
the stator (replacing windings and insulation).
n Level 4: This includes rewinding of the stator plus
major lamination repair or rotor rebar. It may also
include replacement of the stator laminations or

restacking of laminations. Shaft replacement would
normally fall into this category. In short, Level 4
involves major repairs that are costly enough to
justify examining the option of replacement.
n Level 5: Motors that would normally be replaced
except for special circumstances faced by the customer
(i.e., no spare or unacceptable lead time for a replacement). Level 5 includes misapplied motors, inadequate enclosures, and pre U-frame motors. A motor
that should be replaced, if not for the owners’ inability to operate without it.
The standard recognizes that in cases where replacement
new unit or replacement component delivery time is unacceptable, or where substitutions are not possible, it may be
necessary to repair machines usually considered catastrophically failed (Figure 2).
Summary of the Standard
The importance of communication between the end user
and the repair facility is recognized and emphasized. If the
repair is more complex, then more importance is placed on
good communication. Certainly, this is important not only
to avoid misunderstandings but also to have a complete
performance and repair history of critical machines. This
is, especially, necessary in identifying cases where previous
changes have impacted performance or present modifications


can increase reliability. Negative results are to be avoided and
positive ones considered as best practice.
User and repairer responsibilities are set forth in
detail. Where practical, the standard contains background information and guidance for the user. There is a
specific checklist useful for prequalifying a service center,
material about in-plant machine diagnostics, and a section describing procedures for preinspection test runs,
when warranted.
Importance of Machine History

Aside from expanding the initial list of recommendations,
the document includes practical courses of action to benefit
the user, for example, reporting coupling damage so it can
be replaced in a timely fashion and the mating half be
inspected and replaced if necessary.
Noting the importance of root cause failure analysis, the
standard now includes guidelines for evaluating less common
failures, such as an open rotor or certain types of stator winding failures. For example, when a rotor bar fractures because
of fatigue-cycle life, the remaining bars are also likely near
the end of their fatigue-cycle life. The entire rotor should be
rebarred, rather than a repair performed on the open bar [4].
Airgap

The physical airgap between stator and rotor is electrically
and mechanically important. Experience has shown that the
airgap should be uniform within 10% of the average value.
Determining the status of the airgap during the incoming
inspection is critical to determining the complete work
scope. This is necessary for several reasons, not the least of
which are focusing on the correct components contributing
to the problem, projecting a practical completion date, and
establishing a realistic cost of the repairs.
When practical, one predisassembly inspection step is
the performance of an uncoupled test run (Figure 4) to

3
Squirrel cage rotor after dismantling for inspection. (Photo
courtesy of EASA.)

4
Incoming test run can reveal some problems. (Photo
courtesy of Chuck Yung.)

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For the repair facility, obtaining complete nameplate information can be critical.
Consider the example of a two-pole motor manufactured for 50-Hz operation, where the rotor resonant
frequency is 20% above the operating r/min. The machine
is eventually moved to North America, where it operates
on 60-Hz power. Chronic vibration problems, not surprisingly, plague the machine. Absent the original nameplate
and/or knowledge of the machine’s history, the user would
lose production attempting to correct the vibration.
Lacking knowledge of the machine’s history, a repair
facility—and possibly a succession of repairers—would balance the rotor. Yet, it is unlikely that the resonant frequency
problem would be immediately revealed. The user is in the
best position to know the machine’s history and is, therefore, responsible for retaining documentation and, where
practical, sharing repair and maintenance history with the
repair facility.
This is one example where on-site diagnostics are invaluable to a complete root cause failure analysis. A complete
vibration spectrum, voltage and current records, and accurate
description of the operating and environmental conditions
are valuable aids in determining the repair requirements.
Presented with as much machine history as possible,
and a good description of the reason the machine was
removed from service, the repair facility has the opportunity to better evaluate the machine with attention
toward those issues that might contribute to the user’s
experience with the machine. Unless otherwise agreed in
advance with the user, the repair facility shall provide a
detailed inspection report with estimated repair costs

prior to proceeding with repairs. Toward that end, IEEE
Standard 1068 includes sample inspection and repair
report forms.
Where possible, consensus approaches toward evaluating distinctive problem areas of rotating equipment are
provided. These include practical tests for squirrel cage
induction rotors, insulation and winding tests, rotor thermal
sensitivity tests, and evaluation of laminated stator cores for
eddy-current losses.
Incoming inspection (Figure 3) is necessary to verify
machine condition and detect items needing repair. When
there is no spare for the machine, a sense of urgency can
cause routine items to be overlooked. The new standard
suggests best practice procedures for those initial steps,
with emphasis on those which experience has shown to
cause later delays. These procedures include details such as
lead markings, the location and position of critical electrical and mechanical components, and the presence, arrangement, and condition of accessories, such as filters, surge
capacitors, lightning arrestors, and space heaters.

User Guidance

29


evaluate vibration, bearing temperature, and thermal stability. There are instances where operation of a machine in
dangerous electrical or mechanical condition carries sufficient risk that could preclude running. Good communication between user and repair facility can avoid this risk to
the machine, test equipment, and personnel.
Where the user advises the reason the machine was
removed from service, a predisassembly test run can aid in
evaluation of the machine and justify a more lengthy examination into specific phenomena or a component. When
vibration is the concern, it is often possible to duplicate

operating thermal conditions. The new standard provides
detailed instructions for this step as well as acceptance
criteria to aid in evaluation of the results.
Inspection

During the disassembly process, the mutual experience of
users and repair shops illustrates that there are common
key areas where problems can develop. Identifying these
enables a directed approach to problem resolution. The
standard describes these in the sequence in which they are
encountered during the disassembly and inspection process.
For the stator, these include presence and condition of air
baffles, evidence of a core loose in the frame, damage to (or
loose) stator wedges, condition of winding ties, blocking,
evidence of arcing, or partial discharge.

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TABLE 1. MECHANICAL INSPECTION TOLERANCES.

30

Foot flatness

0.0127 mm

Shaft bearing journal
diameter

0.005 mm

Sleeve bearing inside
diameter (ID)

0.005 mm

Sleeve bearing outside
diameter

0.01 mm

Bearing housing ID

0.01 mm

Bearing cartridge

0.01 mm

Bracket to stator fit

0.03 mm

Shaft extension runout
(total indicated
runout)

Manufacturer’s values
or Table 3 (by r/min)

For those users without a comprehensive document to
assure quality repairs, IEEE Standard 1068 includes specifics
as to which components should be measured and to what
degree of accuracy. A partial list is included in Table 1.
Organization
The material in IEEE 1068 is separated into electrical and
mechanical sections, with subparagraph identification for stator, rotor, shaft, and bearing information.
Electrical Repair Topics
Insulation Evaluation

Important through the evaluation, repair, and final test phases
of a repair, recommended voltages for measuring insulation
resistance (IR) are noted in Table 2.
Core Evaluation and Repair

Significant areas of the rewind process are described and control guidelines provided. Removal of the failed winding is
an area where improper procedures can be detrimental not
only to the duration and cost of the repair but also extended
to permanent or nonrepairable damage. The three methods
for winding removal (burnout oven, water blasting, and
mechanical removal) are described, with procedural tips to
control and evaluate the results for each method.
When inspection and testing reveals that a stator core has
lamination damage (Figure 5), the corrective measures are
dictated by the extent and nature of the damage. IEEE
Standard 1068 provides descriptive paragraphs to detail
these methods. The methods described are
n pneumatic vibration of the core to separate fused
laminations
n use of a die grinder to remove small areas of fused

laminations
n a complete or partial restack of the core, cleaning,
and reinsulating the individual laminations
n installation (or adjustment of) pressure plates, banding, undercutting, and lamination stiffening.
Rewind

The rewind section is divided into random- and formwound machines. See Figure 6 for a representative illustration used in IEEE 1068 relating to coil types. Most random

TABLE 2. INSULATION RESISTANCE TEST VOLTAGE.
Winding Rated
Voltage (V)*
<1,000

Insulation Resistance
Test Voltage (dc)
500

1,000–2,500

500–1,000

2,501–5,000

1,000–2,500

5,001–12,000

2,500–5,000

>12,000

5,000–10,000

*: Rated line-to-line voltage for three-phase ac machines.

5
Ground failure that may result in core damage. (Photo
courtesy of EASA.)


windings are rated 600 V or lower,
with an increasing portion of these
machines being operated from an
adjustable speed drive (ASD). The
most commonly applied ASD is the
pulse width modulated (PWM) type,
which may subject the winding to fast
rise times and voltage overshoots.
The insulation shall be capable of
continually operating at rated temperature with repetitive spikes having a
0.1-ls rise time and a magnitude of
1,600-V peak for motors operating on
a 480-V system and 1,900-V peak for
motors operating on a 600-V system.
Enhanced insulation additives (spike
resistance), mechanically robust insulation, and refined rewind procedures
are employed to resolve waveform damage issues. IEEE 1068–2010 standard
expands this area of discussion by providing further particulars.

1

3
5
4
2

25 6
4

6
5
4
3
2
1

3
6

(a)

(b)

6

Coil types: (a) random wound and (b) form wound. (Reprinted with permission
from the EASA, Mechanical Repair Fundamentals of Electric Motors, 2003.)

Electrical Testing

TABLE 3. SINGLE-COIL SURGE TEST VOLTAGES.
Rated
Voltage

At 0.1 ls

At 0.5 ls

At 1.2 ls

460 V

650 V

760 V

945 V

2.3 kV

3.3 kV

3.8 kV

4.7 kV

4 kV

5.7 kV

6.6 kV

8.2 kV

6.6 kV

9.4 kV

10.9 kV

13.5 kV

13.2 kV

18.8 kV

21.8 kV

27 kV

Lacing and bracing methods are described, with some
general description of coil spacing, brazing, vacuum
pressure impregnation (VPI), and resin-filled insulation
methods. Figure 7 illustrates an in-process rewind of a
form coil stator. The tape on each coil comprises the
groundwall insulation.
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Electrical testing methods for ac and dc high potential,
and surge testing, draw on the IEEE Standards 432-1992

[5], 43-2000 [6], and 112-2004 [7], as well as API 5412003 [8], and ANSI/EASA AR100-2006 [9].
For form coil windings, surge test voltages, and rise times
(based on phase–phase voltage) are specified in Table 3.
As with the random winding section, the standard provides specific information on how to attain the requirements where the “as found” materials and thicknesses are
shown to be insufficient. Table 4 indicates the types of turn
insulation required to provide proper protection for the
noted steady-state volts per turn levels.
Recommended groundwall insulation thicknesses [10],
based on standard voltage ratings, are provided in Table 5.

TABLE 5. RECOMMENDED GROUNDWALL
INSULATION THICKNESS FOR COMMON
VOLTAGE RATINGS.
Groundwall
(kV)

Total
(mm)

Per Side
(mm)

2.3

3

1.5

4

3.56

1.78

6.6

4.57

2.29

TABLE 4. TURN INSULATION
RECOMMENDED VALUES.
Volts/Turn

Turn Insulation

Up to 30

Film coating of wire

Up to 60

Fiberglass over film

>60

7

Mica turn tape
Form coil insertion in process. (Photo courtesy of EASA.)

31


nature of rotor bar failure. If one or
more broken bars are revealed, it is
The widely accepted polarization index
JUST AS NOT ALL
highly probable that the remaining
(PI) test has long been recognized as usebars are at or near the end of their
ful for evaluating insulation condition.
FAILURES ARE
fatigue-cycle life. For this reason, parPrior to improved insulation materials
tial repairs are discouraged.
and VPI methods, interpretation of the
EQUALLY SEVERE,
PI test was straightforward: the IR to
ground is measured at time 0 and again
Synchronous Machines
NOT ALL REPAIRS
at 1 min intervals for 10 min; the 10-min
As synchronous machine stators are the
ARE EQUALLY
resistance value is divided by the 1-min
same as those in induction units, Secresistance value and the resulting ratio
tion 6.3.3 continues to address windEXTENSIVE.
used to assess insulation condition.
ings located on the rotor. Procedural
A ratio between two and five was
instructions are provided for the inspecgenerally deemed acceptable with a ratio

tion, testing, removal, and connection
below two indicating poor insulation, and a ratio above five of rotating poles. Slip rings and the more common methoften interpreted as indicating a dry winding in need of var- ods of excitation are also addressed.
nish treatment.
Improvements in insulation systems have resulted in Mechanical Repair Topics
initial IR values measured in gigaohms (1 billion X or
1 3 109 X). It is unrealistic to expect such a high IR value Cleaning Methods
to double over the course of the PI test. IEEE 1068 adopts Machines are routinely cleaned of oil, grease, dirt, as well
the caveat that states “If the initial resistance is 5,000 MX as environmental and biological contaminants as part of a
(5 GX) or higher, the PI ratio may not be meaningful.” routine repair, while larger machines are sometimes cleaned
Standard 1068 further stipulates that a PI ratio of 1.5 or in place as part of a preventive maintenance program. The
lower requires the repairer to notify the user.
standard covers steam cleaning, pressure washing, and dryFor random windings, a dielectric absorption ratio ice blasting of motor components, with particular cautions
(DAR) of the 1-min value divided by the 30-s value is used for windings.
instead. This is due to the differences in the insulation system design: insulation thickness, surface capacitance, and Mechanical Repairs
other factors.
For machines equipped with antifriction (i.e., ball or roller)
bearings (Figure 9), removal of bearings should be accomplished by the use of a hydraulic or screw-type puller to
Rotor Test
Rotor inspection should include a single-phase rotational prevent possible shaft damage. The disassembly and removal
test, or growler test, to aid in the detection of open rotor of babbitt bearings is also dealt with, for the benefit of those
bars. It is noted that all tests are indicative, some containing unfamiliar with them. There is emphasis on identifying the
hard information, and others providing subjective data and location and orientation of the bearings, as well as inspection
requiring personal interpretation. Here, current signature and bearing fits.
With the prevalence of ASDs (particularly PWM drives)
analysis results obtained before the machine is removed from
in process industries, material has been included to diagservice can be of high value in evaluating rotor condition.
The standard includes guidance for recognizing many nose, evaluate, and understand various corrective measures.
symptoms of rotor cage faults (Figure 8), such as burned or It is necessary to be familiar with capacitively generated
discolored laminations, evidence of arcing, electrical noise circulating currents, know how to interpret symptoms
under loaded conditions, and more obvious signs such as found during the inspection process (Figure 10), and initivisibly broken bars or lamination rubbing. On a practical ate effective repair processes.

note, the document directs attention to the fatigue-cycle

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Insulative Quality

32

Balancing

8
Failure of the upper cage of this dual-cage rotor indicates
a starting issue. (Photo courtesy of EASA.)

Rotor balance procedures are described, with reference to
NEMA MG1 Part 7 [11], ISO 1940 [12], and API 541
[8]. There are specific procedural details, such as where and
how weight can be safely added or removed.
The use of proximity probes to monitor vibration when
a machine is in service requires special consideration. Not
all users or repair facilities are familiar with this technology, so tutorial information was added. Included are the
difference between mechanical runout and electrical runout, the need to burnish the area of the shaft beneath the
probe(s) tip, and avoidance of invasive repair methods, such
as welding or metalizing.
In addition to the standard machine vibration limits
established in NEMA MG1 [11], a table designated for
special machines is included (Table 6). These are for unfiltered maximum relative shaft displacement.


Deep Groove Ball Bearing

Tapered Roller Bearing
Outer Ring

Outer Ring

Roler (Tapered)
Inner Ring
Cage (Pressed Cage)
Bore Surface
Roller Small End Face
Inner Ring
Raceway Surface
Small Rib
(Raceway Groove)

Ball
Inner Ring
Cage
(Pressed
Cage)

Inner Ring (Cone)

Rivet

Outer Ring
Outer Diameter
Surface

Side Surface

Inner Ring Front Face
Outer Ring Back Face

Cylindrical Roller Bearing

Rolling Surface
Roller Large
End Face

Center Rib

Inner Ring
Raceway Surface

Inner Ring

Rib

Rivet

Inner Ring
Raceway Surface

Spherical Roller Bearing-Self-Aligning

Outer Ring

Cage

(Machined Cage
with Rivet)

Guide Rib Face

Outer Ring

Cylindrical Roller
Inner Ring

Outer Ring
Front Face
Large Rib
Inner Ring Back Face

Guide Rib Face

Guide Rib Face

Roller (Spherical)

Roller Surface
Cage
(Machined Cage
with Rivet)

Roller Large
End Face

Inner Ring

Raceway Surface
Roller Filling Slot
Small Rib
Rolling Surface
Roller Large
End Face

9
Types of antifriction bearings. (Reprinted with permission from the EASA, Mechanical Repair Fundamentals of Electric
Motors, 2003.)

Electrical Connections

The standard includes gasket and minimum spacing
requirements as well as torque values for electrical fasteners
in both standard and metric bolt sizes. Because there are
many connection variations dictated by machine size and
type, plus the many possible user instrumentation requirements, this section was limited to general guidance.
Accessories

The handling of auxiliary components, devices such as
space heaters, pressure sensors, and vibration probes, is

addressed. Also included are temperature sensors, such as
resistance temperature detectors (RTDs), thermocouples,
and bimetallic thermal elements. Practical guidance is provided for both incoming inspection and final assembly,
including device location, verification of proper operation,
and correct lead marking.
The associated issue of lead characteristics is important
for other stator, rotor, and other line leads. Observance

of original markings and comparison with NEMA and
industry standard labels shall be observed. Final assembly
must also consider wires or cables that are connected by
terminal lugs, which were installed with a compression or
crimping tool.
This includes verifying that all strands are held within
the lug barrel, insuring the barrel is properly crimped with
the correct tool and the strands are securely held so as to
avoid a high-resistance connection, which could overheat
and fail.
TABLE 6. UNFILTERED SHAFT DISPLACEMENT LIMITS.

Max r/min

10
Fluting resulting from shaft currents. (Photo courtesy of EASA.)

Relative Displacement
(Peak-to-Peak) of
Shaft

1,8013,600

50 lm (0.002000 )

1,2011,800

70 lm (0.002800 )

Up to 1,200

76 lm (0.003000 )

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Additional Topics

33


Acceptance Testing
Final Test and Documentation

THE PHYSICAL
AIRGAP BETWEEN
STATOR AND
ROTOR IS
ELECTRICALLY
AND
MECHANICALLY
IMPORTANT.

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An adage states “If it is important
enough to measure, it is important
enough to record.” Proper final testing
includes documentation—lots of documentation. Shaft runout, vibration levels, IR, voltage, and current on each
phase during the test are extremely
important. These readings must dovetail

with expected values and approach or
equal original manufacturer performance
data. Also, establishing this repaired/
refurbished baseline data is crucial to
the comparison of historical data.
Significant differences between
the in-shop (Figure 11) and on-site values for any of these
items should trigger an investigation to determine the
cause. For users with duplicates of the same machine, a
comparison of like units is helpful. There are many times
when prompt inspection of apparent deviations reveals a
problem, which, untended, would have resulted in another
machine failure.
The authors attest to many cases where a machine was
connected to the wrong voltage, reversed rotation, misaligned, incorrect end float, or otherwise misapplied. Such
obvious items as bearing temperature should also be monitored and recorded. Bearing temperature should be allowed
to stabilize, which is defined to be no more than a 1 °C
increase over a 30-min time frame.

34

Winding Resistance

Winding resistance between phases should not vary by
more than 3% [9]. High-resistance connections, broken
strands, and incorrect winding connections are some of the
more common causes of excessive variation in resistance.
However, the root cause must be considered.
Especially for smaller machines, the cause could be no
more than the use of a concentric winding. Machine-wound

11
Acceptance testing after repair establishes baseline
information for vibration levels and no load current. (Photo
courtesy of EASA.)

concentric windings rarely have the
same mean length of turn (MLT), so the
resistance may differ as much as 5%.
Bearing End Float for
Sleeve Bearing Machines

NEMA MG1 [11] prescribes that a
machine fitted with babbitt bearings
have a minimum total end float of 1/2
in (or Æ0.25 in). Often overlooked is
the fact that it also stipulates a maximum coupling end float of 0.190 in.
Thus, when a machine operates on its
magnetic center on the test bed and
the user complains that a machine is
running against the thrust face as illustrated in Figure 12, it is a self-indictment of the alignment practices used. We note here that
the IEC-based machines have a total end float of 6 mm (Æ3
mm), which would cause problems if not observed prior to
installation. Figure 13 shows a representative illustration
used in IEEE 1068 relating to sleeve bearings.
Quality Assurance Measures

Comparison of in-shop performance criteria to those same
items measured after the machine’s installation is important in the last step of total quality management. Reinstallation of a repaired machine into an unsatisfactory mechanical
or electrical environment can quickly repeat the failure.

Attention to issues such as power quality, precision alignment, belt tensioning, and piping stresses is critical to future
machine operability and life. Quality assurance at every step,
including final installation and operation, is necessary to
obtain full value from a first-class repair.
Toward that end, the repair report should be suitably
detailed to inform the reader as to the probable cause of
failure, the method(s) of repair, the repaired condition, and
final test results. The user also has the responsibility to
appropriately protect the machine. This means that a
motor placed into storage should be kept in a clean area,

12
Sleeve bearing thrust face damage is the result of improper
coupling practices. (Photo courtesy of EASA.)


ideally in a temperature- and humiditycontrolled environment. Space heaters (or some other means) should be
used to maintain the winding temperature above the dew point. When the
motor is placed into service, IR should
be measured, alignment to driven
equipment must be precise, and vibration and bearing temperatures ought
to be monitored for an appropriate
time to assure there are no problems.
Poor installation practices could necessitate the next repair.

Bottom Half of Bearing Housing/Oil Chamber/Bracket

Oil Ring
Assembled
Flange-Mounted

Sleeve Bearing

Oil Ring
Bearing Shell

Bottom Half of
Babbitt
Informative Annexes
Babbitt
Bearing
Labyrinth
This IEEE standard would not be comBearing Saddle
Bearing
Seal
plete without supportive documentaShell
Top Half of Bearing
tion or extra information. To this end,
Top Half of Bearing
Annex A provides a list of useful IEEE
Housing
PCIC technical papers and the obliga13
tory catalog of other IEEE standards
Sleeve
bearing
component
nomenclature.
(Reprinted
with
permission
from

the
and recommended practices. Informative Annex B provides an evaluation EASA, Mechanical Repair Fundamentals of Electric Motors, 2003.)
form that equipment owners can use in
the process of screening repair facilities.
[3] A. Bonnett and C. Yung, “A repair-replace decision model for petroBasic capabilities included in the questionnaire are electrical
chemical industry electric motors,” in Proc. 2002 Petroleum and Chemiand mechanical repair, lifting, technical and backup resources,
cal Industry Conf., pp. 55–66.
test facilities, housekeeping, and quality assurance.
[4] “Root cause failure analysis,” Electrical Apparatus Service Association,

References
[1] Recommended Practice for the Repair and Rewinding of Electric Motors for
the Petroleum and Chemical Industry, IEEE Std. 1068-1990.
[2] Standard for the Repair and Rewinding of AC Electric Motors in the Petroleum, Chemical and Process Industries, IEEE Std. 1068-2009.

Inc., St Louis, MO, 2002.
[5] IEEE Guide for Insulation Maintenance for Rotating Electric Machinery (5
hp to less than 10 000 hp), IEEE 432-1992.
[6] IEEE Recommended Practice for Testing Insulation Resistance of Rotating
Machinery, IEEE Std. 43-2000.
[7] Standard Test Procedure for Polyphase Induction Motors and Generators, IEEE
Std. 112-2004.
[8] Form-Wound Squirrel-Cage Induction Motors—500 Horsepower and Larger,
4th ed., ANSI/API Standard 541-2003, June 2004.
[9] Recommended Practice for the Repair of Rotating Electrical Apparatus,
ANSI/EASA AR100-2006.
[10] C. Yung, “Opportunities to improve reliability and efficiency of existing medium-voltage electric motors,” in Proc. 2005 Petroleum and
Chemical Industry Conf., pp. 199–208.
[11] Motors and Generators, NEMA MG1, 2006.
[12] Mechanical Vibration—Balance Quality Requirements for Rotors in a Constant (Rigid) State—Part 1: Specification and Verification of Balance Tolerances, ISO 1940-1, 2003.

[13] T. Griffith, C. Yung, and C. Nyberg, “Recent revisions of IEEE 1068
standard for the repair and rewinding of AC electric motors in the
petroleum, chemical and process industries,” in Proc. 2007 Pulp and
Paper Industry Technical Conf., pp. 191–196.

Travis Griffith () is with GE Oil and Gas
in Houston, Texas. Austin H. Bonnett (retired) was with
Emerson Electric in Gallitin, Missouri. Bill Lockley is with
Lockley Engineering in Calgary, Alberta, Canada. Chuck
Yung is with EASA in St Louis, Missouri. Griffith, Yung,
and Nyberg are Senior Members of the IEEE. Bonnett is a Life
Fellow of the IEEE. Lockley is a Fellow of the IEEE. This
article first appeared as “Revisions to IEEE 1068: Standard
for the Repair of AC Electric Motors in Process Industries” at
the 2009 Petroleum and Chemical Industry Conference.

IEEE INDUSTRY APPLICATIONS MAGAZINE  JAN j FEB 2011  WWW.IEEE.ORG/IAS

Conclusions
The repair process is important to both the repair facility
and the user. Accepted high-quality procedures and materials must be used so as to maximize the machine’s usefulness and reduce mean time between failures. For process
industries, the repair cost is typically a small portion of the
total cost of a machine failure. Process industries, such as
pulp and paper, petroleum companies, and chemical operations, recognize that downtime is measured in the tens or
hundreds of thousands of dollars.
It has long been recognized that higher quality workmanship and materials increase the life of both new and repaired
machines. By making sure that repairs meet stringent requirements and pass tests designed to provide quality assurance, the
user and repairer can increase machine life. IEEE Standard
1068 is designed to aid both repairer and user toward that
goal. Given that most manufacturers (process industries in particular) recognize the relationship between quality control and

machine life, the extension of 1068 to include process industries is a logical way to expand the benefits of this standard.
A key element of this standard deals with the importance of doing a root cause failure analysis to assure that
repeat failure do not occur. Also, this analysis may suggest
modifications to prevent future failures.

35