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

During this revision effort, IEEE Standard 1068–2010 was modified to make clear common practices and was restructured to reflect the flow of a ty-pical machine through the repair process

B Y T R A V I S G R I F F I T H , A U S T I N H B O N N E T T , B I L L L O C K L E Y ,

C H U C K Y U N G , & C Y N T H I A N Y B E R G

Improvements in repairing and rewinding of ac electric motors

T HIS ARTICLE DETAILS THE UPDATESand modifications to the 1996 revision of

IEEE 1068, Recommended Practice for the Repair and Rewinding of Motors in the Petroleum and Chemical Industry It contains only selected topics present

within the standard and should not be treated as a substitute

for the entire standard A major change in the document

is its evolution to full standard status The IEEE Standards Association also granted the working group’s petition to broaden the scope and title to include process industries

in general Such recognition acknowledges its value to those employing machines in demanding services and severe envi-ronments, such as the cement trade and pulp and paper processing IEEE Standard 1068–2010 was restructured

Digital Object Identifier 10.1109/MIAS.2010.939428

Date of publication: 12 November 2010

© ARTVILLE

26

to better track the methodologies and

processes employed in present-day

repair facilities Substantive

improve-ments include incorporation of

cur-rently available technology, document

specific testing, evaluation criteria, and

clarification of end user and service

center responsibilities

It is commonly known that electric

motor drivers are the most significant

user of electric energy within a process

facility Such machines often take prime

consideration in the plant’s critical path

of operation Even in spared or

noncriti-cal service, the cost or long delivery

cycle of a new unit makes

refurbish-ment, repair, and rewinding an essential

part of plant reliability, uptime, and

profitability When ac machines (Figure 1)

require repair, an important

relation-ship exists between the motor user and a repair facility In

large plants, orders for machine repair may be repeated

several times during a normal year of operation The original

1068 recommended practice [1] provided basic guidance for

plants with few motors, personnel who were new to the

industry, and those less familiar with motor repair specifics

First published in 1990, it achieved acceptance in the

petro-leum and chemical industries and was then revised in 1996

IEEE guidelines advise that a recommended practice is,

by and large, distinguished by the verb should This style

of writing is critical to imply wording with more force than

the use of “may” in a guide It is also differentiated from a

standard employing “shall,” which indicates a single-accepted

method Consideration of 1068’s general use prompted the

IEEE Industrial Application Society (IAS) Petroleum and Chemical Indus-try Committee (PCIC) to establish a working group for the next revision cycle However, its potential within the IEEE IAS indicated the need for

1068 to become a standard This evo-lution required large-scale changes In most cases, more emphatic wording necessitated full rewriting rather than a simple change of might/may wording being replaced with will/shall

During this revision effort, IEEE Standard 1068–2010 was modified to make clear common practices and was restructured to reflect the flow of a ty-pical machine through the repair process Qualitative and quantitative test pro-cedures were included and importance placed on key aspects of each step References were updated and expanded to reflect the most recent versions of relevant documents from the American Petroleum Institute (API), American Society for Testing Materials (ASTM), Electrical Apparatus and Service Association (EASA), IEEE, International Elec-trotechnical Committee (IEC), International Standards Organization (ISO), and National Electrical Manufactur-ing Association (NEMA)

Of note was the working group’s focus on ac machines and the decision to remove dc types that are quite dissimi-lar to ac units and are less prevalent in the petroleum, chemical, and process industries

In short, IEEE Standard 1068–2010 [2] provides detailed procedures for ac machine evaluation and data interpretation

Air Deflector

Air Baffle

Shroud

Rabbet Fit Spigot Fit

End Turns Coil Extensions

Coils End Ring Stator Shroud

Belly Band

Eye Bolt Lifting Eye Grease Fitting Zerk Fitting Axial Thrust Washer External Cooling Fan

Bearing Cap Bearing Retainer Back Cap Fan Cover Fan Shroud Grease Drain Sator Laminations Satcked Stator Core Iron Core Plate Punchings Rotor Skew

Frame Stator Frame Foot

End Bracket End Bell Shaft

Keyway

Clearance Fit

Flame Path

Shaft Opening

1

Horizontal electric motor nomenclature (Illustration courtesy of EASA.)

IEEE STANDARD

1068 WAS RESTRUCTURED TO BETTER TRACK THE METHODOLOGIES AND PROCESSES EMPLOYED IN PRESENT DAY REPAIR FACILITIES.

27

through a higher degree of engineering language and the

establishment of a technical reasoning base

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

docu-ment to address all possible designs, construction methods,

or materials having occurred over the previous century Thus,

it does not supersede the manufacturer’s information,

direc-tives, 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

catego-ries of electric motors, each of which has unique repair

requirements, the document specifically excludes dc,

her-metic, nuclear, submersible, and hazardous (classified) area

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 peri-odic 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 subse-quent 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 cata-strophic 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 replac-ing of antifriction bearreplac-ings, or inspectreplac-ing and veri-fication 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 replace-ment) Level 5 includes misapplied motors, inad-equate enclosures, and pre U-frame motors A motor that should be replaced, if not for the owners’ inabil-ity to operate without it

The standard recognizes that in cases where replacement new unit or replacement component delivery time is unac-ceptable, or where substitutions are not possible, it may be necessary to repair machines usually considered catastroph-ically 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

2

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

28

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

back-ground 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

sec-tion describing procedures for preinspecsec-tion test runs,

when warranted

Importance of Machine History

For the repair facility, obtaining complete nameplate

infor-mation can be critical

Consider the example of a two-pole motor

manufac-tured 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

surpris-ingly, 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

bal-ance 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,

there-fore, 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

invalu-able 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

opportu-nity 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

evaluat-ing distinctive problem areas of rotatevaluat-ing 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

electri-cal and mechanielectri-cal components, and the presence,

arrange-ment, and condition of accessories, such as filters, surge

capacitors, lightning arrestors, and space heaters

User Guidance

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 wind-ing 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.) 29

evaluate vibration, bearing temperature, and thermal

stabil-ity There are instances where operation of a machine in

dangerous electrical or mechanical condition carries

suffi-cient risk that could preclude running Good

communica-tion 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

exam-ination 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

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 sta-tor, rosta-tor, 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 con-trol 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, band-ing, undercuttband-ing, and lamination stiffening

Rewind

The rewind section is divided into random- and form-wound machines See Figure 6 for a representative illustra-tion used in IEEE 1068 relating to coil types Most random

TABLE 2 INSULATION RESISTANCE TEST VOLTAGE.

Winding Rated

Voltage (V)*

Insulation Resistance Test Voltage (dc)

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.)

TABLE 1 MECHANICAL INSPECTION TOLERANCES.

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)

30

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

temper-ature 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

insu-lation, and refined rewind procedures

are employed to resolve waveform

dam-age issues IEEE 1068–2010 standard

expands this area of discussion by

pro-viding further particulars

Electrical Testing

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

541-2003 [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

pro-vides specific information on how to attain the

require-ments 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

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

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

TABLE 4 TURN INSULATION

RECOMMENDED VALUES.

Volts/Turn Turn Insulation

Up to 30 Film coating of wire

Up to 60 Fiberglass over film

>60 Mica turn tape

TABLE 5 RECOMMENDED GROUNDWALL INSULATION THICKNESS FOR COMMON VOLTAGE RATINGS.

Groundwall (kV)

Total (mm)

Per Side (mm)

7

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

1

1

2

2 2

5

5 5

6 4

4

3

6

6

6

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

31

Insulative Quality

The widely accepted polarization index

(PI) test has long been recognized as

use-ful for evaluating insulation condition

Prior to improved insulation materials

and VPI methods, interpretation of the

PI test was straightforward: the IR to

ground is measured at time 0 and again

at 1 min intervals for 10 min; the 10-min

resistance value is divided by the 1-min

resistance value and the resulting ratio

used to assess insulation condition

A ratio between two and five was

generally deemed acceptable with a ratio

below two indicating poor insulation, and a ratio above five

often interpreted as indicating a dry winding in need of

var-nish treatment

Improvements in insulation systems have resulted in

initial IR values measured in gigaohms (1 billion X or

13109X) It is unrealistic to expect such a high IR value

to double over the course of the PI test IEEE 1068 adopts

the caveat that states “If the initial resistance is 5,000 MX

(5 GX) or higher, the PI ratio may not be meaningful.”

Standard 1068 further stipulates that a PI ratio of 1.5 or

lower requires the repairer to notify the user

For random windings, a dielectric absorption ratio

(DAR) of the 1-min value divided by the 30-s value is used

instead This is due to the differences in the insulation

sys-tem design: insulation thickness, surface capacitance, and

other factors

Rotor Test

Rotor inspection should include a single-phase rotational

test, or growler test, to aid in the detection of open rotor

bars It is noted that all tests are indicative, some containing

hard information, and others providing subjective data and

requiring personal interpretation Here, current signature

analysis results obtained before the machine is removed from

service can be of high value in evaluating rotor condition

The standard includes guidance for recognizing many

symptoms of rotor cage faults (Figure 8), such as burned or

discolored laminations, evidence of arcing, electrical noise

under loaded conditions, and more obvious signs such as

visibly broken bars or lamination rubbing On a practical

note, the document directs attention to the fatigue-cycle

nature of rotor bar failure If one or more broken bars are revealed, it is highly probable that the remaining bars are at or near the end of their fatigue-cycle life For this reason, par-tial repairs are discouraged

Synchronous Machines

As synchronous machine stators are the same as those in induction units, Sec-tion 6.3.3 continues to address wind-ings located on the rotor Procedural instructions are provided for the inspec-tion, testing, removal, and connection

of rotating poles Slip rings and the more common meth-ods of excitation are also addressed

Mechanical Repair Topics Cleaning Methods

Machines are routinely cleaned of oil, grease, dirt, as well

as environmental and biological contaminants as part of a routine repair, while larger machines are sometimes cleaned

in place as part of a preventive maintenance program The standard covers steam cleaning, pressure washing, and dry-ice blasting of motor components, with particular cautions for windings

Mechanical Repairs

For machines equipped with antifriction (i.e., ball or roller) bearings (Figure 9), removal of bearings should be accom-plished by the use of a hydraulic or screw-type puller to prevent possible shaft damage The disassembly and removal

of babbitt bearings is also dealt with, for the benefit of those unfamiliar with them There is emphasis on identifying the location and orientation of the bearings, as well as inspection and bearing fits

With the prevalence of ASDs (particularly PWM drives)

in process industries, material has been included to diag-nose, evaluate, and understand various corrective measures

It is necessary to be familiar with capacitively generated circulating currents, know how to interpret symptoms found during the inspection process (Figure 10), and initi-ate effective repair processes

Balancing

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 technol-ogy, so tutorial information was added Included are the difference between mechanical runout and electrical run-out, 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 unfil-tered maximum relative shaft displacement

8

Failure of the upper cage of this dual-cage rotor indicates

a starting issue (Photo courtesy of EASA.)

JUST AS NOT ALL FAILURES ARE EQUALLY SEVERE, NOT ALL REPAIRS ARE EQUALLY EXTENSIVE.

32

Additional Topics

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

require-ments, 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 pro-vided 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

10

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

TABLE 6 UNFILTERED SHAFT DISPLACEMENT LIMITS.

Max r/min

Relative Displacement (Peak-to-Peak) of Shaft 1,8013,600 50 lm (0.0020 00 ) 1,2011,800 70 lm (0.002800)

Up to 1,200 76 lm (0.0030 00 )

Outer Ring

Cylindrical Roller

Cylindrical Roller Bearing Spherical Roller Bearing-Self-Aligning

Cage

(Machined Cage

with Rivet)

Cage (Machined Cage with Rivet)

Deep Groove Ball Bearing Tapered Roller Bearing

Outer Ring Roler (Tapered) Cage (Pressed Cage) Roller Small End Face

Small Rib Inner Ring (Cone) Inner Ring Front Face

Inner Ring Back Face

Inner Ring Raceway Surface

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

Inner Ring Raceway Surface

Rolling Surface Roller Large End Face Guide Rib Face

Outer Ring Back Face

Outer Ring Front Face Large Rib Ball

Inner Ring

Cage

(Pressed

Cage)

Rivet

Side Surface

Inner Ring Bore Surface Inner Ring Raceway Surface (Raceway Groove)

Outer Ring Outer Diameter Surface

Rivet

Rib

Center Rib

Guide Rib Face

Roller (Spherical) Guide Rib Face Roller Surface

Roller Large End Face

9

Types of antifriction bearings (Reprinted with permission from the EASA, Mechanical Repair Fundamentals of Electric

Motors, 2003.)

33

Acceptance Testing

Final Test and Documentation

An adage states “If it is important

enough to measure, it is important

enough to record.” Proper final testing

includes documentation—lots of

docu-mentation Shaft runout, vibration

lev-els, 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,

mis-aligned, incorrect end float, or otherwise misapplied Such

obvious items as bearing temperature should also be

moni-tored 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

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

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 0.25 in) Often overlooked is the fact that it also stipulates a maxi-mum 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 illus-trated in Figure 12, it is a self-indict-ment of the alignself-indict-ment practices used We note here that the IEC-based machines have a total end float of 6 mm (3 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 impor-tant in the last step of total quality management Reinstalla-tion of a repaired machine into an unsatisfactory mechanical

or electrical environment can quickly repeat the failure Attention to issues such as power quality, precision align-ment, 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,

11

Acceptance testing after repair establishes baseline

information for vibration levels and no load current (Photo

courtesy of EASA.)

12

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

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

34

ideally in a temperature- and

humidity-controlled environment Space

heat-ers (or some other means) should be

used to maintain the winding

temper-ature above the dew point When the

motor is placed into service, IR should

be measured, alignment to driven

equipment must be precise, and

vibra-tion and bearing temperatures ought

to be monitored for an appropriate

time to assure there are no problems

Poor installation practices could

neces-sitate the next repair

Informative Annexes

This IEEE standard would not be

com-plete without supportive

documenta-tion or extra informadocumenta-tion To this end,

Annex A provides a list of useful IEEE

PCIC technical papers and the

obliga-tory catalog of other IEEE standards

and recommended practices

Informa-tive Annex B provides an evaluation

form that equipment owners can use in

the process of screening repair facilities

Basic capabilities included in the questionnaire are electrical

and mechanical repair, lifting, technical and backup resources,

test facilities, housekeeping, and quality assurance

Conclusions

The repair process is important to both the repair facility

and the user Accepted high-quality procedures and

mate-rials must be used so as to maximize the machine’s

useful-ness 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

opera-tions, recognize that downtime is measured in the tens or

hundreds of thousands of dollars

It has long been recognized that higher quality

workman-ship and materials increase the life of both new and repaired

machines By making sure that repairs meet stringent

require-ments 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

par-ticular) recognize the relationship between quality control and

machine life, the extension of 1068 to include process

indus-tries is a logical way to expand the benefits of this standard

A key element of this standard deals with the

impor-tance 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

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

Petro-leum, Chemical and Process Industries, IEEE Std 1068-2009.

[3] A Bonnett and C Yung, “A repair-replace decision model for petro-chemical industry electric motors,” in Proc 2002 Petroleum and Chemi-cal Industry Conf., pp 55–66.

[4] “Root cause failure analysis,” Electrical Apparatus Service Association, 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 exist-ing 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 Con-stant (Rigid) State—Part 1: Specification and Verification of Balance Toler-ances, 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 (t.griffith@ieee.org) 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

Bottom Half of Bearing Housing/Oil Chamber/Bracket

Oil Ring Oil Ring

Bearing Shell

Babbitt Labyrinth Seal Top Half of Bearing

Bottom Half of Bearing

Assembled Flange-Mounted Sleeve Bearing

Top Half of Bearing Housing

Bearing Saddle Bearing

Shell

Babbitt

13

Sleeve bearing component nomenclature (Reprinted with permission from the EASA, Mechanical Repair Fundamentals of Electric Motors, 2003.)

35