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Industrial Practices in Repair, Maintenance, and Performance
Evaluation of three-Phase Induction Motors: A Practical Case Study
Sachin J P¹, Midhun Balaa C², and P.A. Gowri Sankar³*
¹,² Student, Department of Electrical and Electronics Engineering, Knowledge Institute of Technology
(KIOT), Salem – 637504, Tamil Nadu, India
³ Associate Professor, Department of Electrical and Electronics Engineering, Knowledge Institute of
Technology (KIOT), Salem – 637504, Tamil Nadu, India
DOI:
https://doi.org/10.51583/IJLTEMAS.2026.150600193
Received: 27 June 2026; Accepted: 02 July 2026; Published: 21 July 2026
ABSTRACT
Repair and rewinding of three-phase induction motors is a routine but technically demanding activity in
industrial maintenance workshops, and the quality of that work has a direct bearing on motor efficiency,
reliability, and service life. While international standards (e.g., IEEE 43, IEC 60034 series, NEMA MG-1, ISO
21940-11) define acceptance criteria for insulation resistance, surge/impulse withstand, and rotor balancing,
published literature rarely documents how these criteria are applied together on a single motor as it moves
through a workshop repair cycle. This paper addresses that gap through a practical case study of a burnt-out
three-phase induction motor processed at an industrial motor-servicing facility.
The objective of the study is to document, in a structured and reproducible format, the diagnostic and corrective
sequence applied to the motor — direct current resistance (DCR) testing, insulation resistance (Megger) testing,
surge testing, rotor static and dynamic balancing, mechanical reconditioning, and stator rewinding — and to
compare the pre-repair (faulty) and post-repair (healthy) results against recognised acceptance limits. The motor
was diagnosed with a grounded, turn-to-turn shorted stator winding (Megger reading of 0 MΩ and a shorted
phase pair) and unbalance readings far above the workshop's 9 g acceptance threshold. Following stripping,
rewinding with Class F insulation and vacuum-pressure impregnation (VPI), and two-plane dynamic balancing,
the motor achieved an insulation resistance of 998–1000 MΩ, a maximum inter-phase DCR deviation of 0.2 Ω,
superimposed surge waveforms across all three phases, and a final rotor unbalance below 9 g on both correction
planes.
The novelty of this work lies in presenting a single, fully-instrumented before/after case record that links
electrical diagnostics, mechanical balancing, and winding-design considerations (coil pitch, connection type)
within one workshop cycle, and in framing the acceptance criteria used against established international
standards rather than workshop convention alone. The findings are intended to serve as a practical reference for
motor-repair technicians, junior engineers, and students, and as a template for reporting repair-quality data in a
form suitable for later statistical or predictive-maintenance analysis.
Keywords: Three-phase induction motor; motor repair and rewinding; insulation resistance testing; direct current
resistance (DCR) test; surge test; rotor balancing; predictive maintenance; industrial case study.
INTRODUCTION
Background
Three-phase induction motors are the dominant electromechanical drive in industry, and motor-driven systems
account for a large share of global industrial electricity consumption (Waide & Brunner, 2011). Because
replacement is often costlier and slower than repair, industrial workshops routinely dismantle, diagnose, rewind,
and rebalance failed motors rather than discard them. The quality of that repair process — not merely the choice
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of materials — determines whether the motor returns to service at, below, or above its original efficiency and
reliability (APEC-CAST, 2011).
Problem Statement
Despite the maturity of individual test methods (DCR, insulation resistance, surge testing, rotor balancing), the
technical and academic literature more often treats each test in isolation, or discusses fault diagnosis without
following a single unit through a complete repair cycle with quantified before/after results. Workshop training
manuals describe procedures step-by-step but rarely benchmark the results against internationally recognised
standards, and research papers on condition monitoring rarely include the physical repair and mechanical
correction stages. This leaves a practical gap between diagnostic theory and workshop-floor decision-making
that this study aims to narrow.
Research Objectives
● To document the complete repair cycle of a representative three-phase induction motor, from dismantling
through electrical and mechanical diagnosis to final performance verification.
● To record and compare pre-repair and post-repair DCR, insulation resistance, surge-test, and rotor-
balancing results against defined acceptance criteria.
● To relate workshop acceptance criteria to relevant international standards and published condition-
monitoring literature.
● To identify the practical winding-design and mechanical-correction decisions (coil pitch, connection
type, balancing grade) that most influenced the outcome of the repair.
Novelty and Contribution
Unlike prior single-topic studies, this paper reports a single motor's diagnostic-to-verification trail with matched
pre/post data across four test domains (DCR, Megger, surge, balancing) in one internally consistent record, and
explicitly maps the workshop's pass/fail thresholds to the standards from which they are conventionally drawn.
This structure is offered as a reusable reporting template for future repair-quality case studies and for datasets
that could support machine-learning-based predictive maintenance (Abdulkareem et al., 2025).
LITERATURE REVIEW
Insulation aging in motor windings is a cumulative, largely irreversible process driven by combined thermal and
electrical stress, and understanding its mechanisms underpins any predictive model of winding life (Montanari
& Simoni, 1993). Building on this, partial discharge (PD) activity has been established as an early symptom of
insulation aging in stator windings, and continuous on-line PD monitoring is now preferred in industry precisely
because it gives the longest lead time before failure (Stone, 2005). More recent work extends this toward always-
on condition assessment: Zhou et al. (2019) review offline-to-online insulation health-monitoring methods based
on impedance and discharge-pattern analysis, while Florkowski et al. (2012) describe digital PD acquisition and
pattern-recognition techniques that distinguish internal, surface, and corona discharge across a machine's service
life. Read together, these studies justify treating the Megger and surge tests used in this case study not as isolated
pass/fail checks but as single data points on a longer degradation curve — a connection the present study makes
explicit by reporting both the pre-failure and post-repair values rather than only the final result.
At the system level, motor-driven equipment is estimated to consume roughly 45% of global electricity and
about 70% of industrial electricity use, which makes repair quality an energy-policy issue and not only a
workshop one (Waide & Brunner, 2011). APEC-CAST (2011) shows this concretely: poor rewinding practice,
in particular over-temperature stripping of old windings, damages the stator core and produces a measurable
efficiency penalty after "repair" — a finding that motivates this study's emphasis on documenting stripping and
rewinding parameters rather than treating them as incidental workshop detail.
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On the fault-diagnosis side, a broad taxonomy of induction-motor faults spans stator insulation failure,
mechanical bearing damage, and supply-side abnormalities, and reliable protection depends on recognising how
faults such as phase unbalance or overload accelerate thermal degradation (Electric Motor Faults, n.d.). Motor
Current Signature Analysis (MCSA) has become a standard non-invasive complement to the offline tests used
in this study, allowing broken rotor bars, bearing faults, and winding short circuits to be detected from current
harmonics during normal operation (Nandi et al., 2005). Most recently, machine-learning classifiers such as
Random Forest and Artificial Neural Networks have been applied to real-time equipment data to predict faults
before failure, shifting maintenance from reactive to proactive (Abdulkareem et al., 2025).
Taken together, the literature is strong on individual diagnostic principles but weak on reporting a single unit's
complete repair trail with quantified acceptance criteria — precisely the gap this case study addresses. The
present work also differs from the cited sources in scope: it is a workshop-level practical account rather than a
laboratory or fleet-level study, and its contribution is a transparent, standards-referenced record rather than a
new diagnostic algorithm.
MATERIALS AND METHODS
Case Motor Details
Parameter
Value
Parameter
Value
Manufacturer
General Electrical
Rated Power
5 kW
Rated Voltage
415 V
Rated Current
30 A (reported, no-load
9–10 A)
Number of Poles
04
Synchronous Speed
1500 RPM
Frame / Connection
Star
Insulation Class
F (post-repair, as
rewound)
Reported Fault
Grounded stator
winding; turn-to-turn
short (Phase U–V)
Service History
NA
Figure 1. Electrical and mechanical specifications of the case motor
Facility and Test Instruments
Testing and repair were carried out across the four functional sections of the servicing facility described in
Section 4: service and assembly, winding, balancing, and machine sections. The following instruments were
used:
● DCR (winding resistance) meter — phase-to-phase resistance measurement at the terminal box.
● Insulation resistance tester (Megger), DC output, for phase-to-ground testing.
● Surge tester, for turn-to-turn insulation and interphase comparison via decaying-oscillation waveform
capture.
● Precibalance dynamic balancing machine, for static (single-plane) and dynamic (two-plane) rotor
balancing.
● Digital laser tachometer, clamp-on ammeter/power analyser (LM/LME meter), micrometers, dial
indicators, and vernier calipers for mechanical verification.
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The workshop pass/fail thresholds used in this study correspond to commonly referenced industrial and
international practice for the following test types. Authors should confirm and cite the specific clause and edition
actually followed.
Test
Acceptance Criterion (as
applied)
Related Standard
DCR (phase balance)
Maximum deviation ≤ 0.2 Ω, or
within 2–5% of phase average
IEEE 118 / NEMA MG-1
(general practice)
Insulation Resistance (Megger)
500–1000 MΩ: Excellent; 100–
500 MΩ: Good; <1 MΩ:
Faulty; 0 MΩ: Grounded
IEEE 43-2013
Surge / Impulse Test
Superimposed waveforms
across all phases; no visible
shift or flat line
IEC 60034-15 / IEEE 522
(surge testing of coils)
Rotor Balancing
Residual unbalance below 9 g
(single-digit) on each correction
plane
ISO 21940-11 (balance quality
grade, e.g., G2.5/G1)
Figure 2. Standards and acceptance criteria used in this study
Test and Repair Procedure Overview
The motor was processed through the following sequence: (i) receipt and visual inspection; (ii) dismantling and
terminal-configuration identification; (iii) pre-repair electrical diagnostics (DCR, Megger, surge); (iv) rotor and
mechanical component inspection; (v) mechanical reconditioning (shaft, keyway, bearing housing) where
required; (vi) stripping and rewinding of the stator with Class F insulation and VPI varnish treatment; (vii) post-
repair electrical verification (DCR, Megger, surge); (viii) rotor static and dynamic balancing; and (ix) final no-
load/load performance verification. Each stage is described with its corresponding results in Sections 4 and 5.
Industrial Servicing Facility and Case Study Procedure
Overview of the Servicing Facility
The facility is organised into four functional sections that together carry out the servicing process described in
Section 3.4: the service and assembly section, the winding section, the balancing section, and the machine
section.
Figure 3. Hierarchical classification of induction motor faults, showing the relationship between major
fault categories and their sub-types.
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A. Service and Assembly Section — responsible for dismantling, inspection, cleaning, bearing replacement,
reassembly, and testing. The outer thermal box is removed first, terminal connections are inspected to identify
the star/delta configuration, and the stator and rotor are separated for individual testing.
B. Balancing Section — carries out rotor and fan static and dynamic balancing on a Precibalance machine
(capacity range: a few hundred grams to several thousand kilograms) to reduce vibration and noise and extend
service life.
C. Machine Section — houses lathe, drilling, milling, and keyway-cutting machines together with precision
measuring instruments (micrometers, dial indicators, vernier and depth gauges) used for shaft and housing
reconditioning.
D. Winding Section — performs stator winding inspection (DCR, Megger, surge) and determines the winding
layout from the relationship between speed, poles, slots, and coil pitch.
Figure 4. Key stages of the stator winding verification process.
Dismantling and Terminal Identification
The motor is received at the service and assembly section, the thermal box is removed to expose the terminals,
and the existing connection (star or delta) is identified from the arrangement of link plates before any wire is
disconnected.
● Delta connection: link plates placed vertically, each connecting a start terminal of one phase to the end
terminal of another (U1–U2, V1–V2, W1–W2), forming a closed-loop circuit suited to high-torque
applications.
● Star connection: link plates placed horizontally, joining the three end terminals (U2, V2, W2) at a
common neutral point while the start terminals (U1, V1, W1) receive the supply; this configuration
reduces starting current.
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Figure 5. Internal configuration of the motor terminal box, showing delta (left) and star (right)
connection links.
Electrical Diagnostic Testing
Three tests were used to characterise the stator winding condition: DCR, insulation resistance (Megger), and
surge testing. Results are reported below for the pre-repair (faulty) and post-repair (healthy) condition of the
same motor; the acceptance criteria applied are as listed in Table in Section 3.3.
Direct Current Resistance (DCR) Test
Measurements were taken phase-to-phase at the terminal box (U1–U2, V1–V2, W1–W2) with all link plates
removed to isolate each phase.
Test Connection
Resistance Read (Ω)
Status / Remark
R winding
1.62
Healthy
Y winding
1.61
Healthy
B winding
0.95
Faulty (Short)
Overall Result
0.67 Max Diff
Fault Detected
Figure 6. Initial diagnostic DCR test results (pre-repair).
Test Connection
Resistance Read (Ω)
Status / Remark
R winding
1.56
Healthy
Y winding
1.56
Healthy
B winding
1.55
Healthy
Overall Result
0.01 Max Diff
Ok / Ready
Figure 7. Final verification DCR test results (post-repair).
Insulation Resistance (Megger) Test
Condition
Phase U1–U2
Phase V1–V2
Phase W1–W2
Pre-repair (faulty)
Imbalanced
0.0 Ω (shorted)
Imbalanced
Post-repair (healthy)
Balanced
Balanced
Maximum inter-phase
difference 0.2 Ω
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The Line (L) probe was connected to a phase terminal and the Earth (E) probe to a clean, unpainted point on the
motor frame.
Measured Value (MΩ)
Condition
Status
500 - 1000
Excellent
Healthy
100 - 500
Acceptable
Good
1 - 100
Needs Cleaning
Faulty
Overall Result
Insulation Failure
Faulty
Figure 8. Standard insulation-resistance acceptance criteria used for evaluation (per Table in Section 3.3).
Condition
Phase R–Ground
Phase Y–Ground
Phase B–Ground
Pre-repair (faulty)
0 MΩ
0 MΩ
0 MΩ
Post-repair (healthy)
998–1000 MΩ
998–1000 MΩ
998–1000 MΩ
Test Connection
Resistance (MΩ)
Status / Remark
GND – R Phase
0 MΩ
Insulation Failure
GND – Y Phase
0 MΩ
Insulation Failure
GND – B Phase
0 MΩ
Insulation Failure
Overall Result
0 MΩ
Total Insulation Failure
Figure 9. Initial diagnostic insulation-resistance results (pre-repair).
Figure 10. Final diagnostic insulation-resistance results, post-rewinding and VPI varnish treatment.
A pre-repair reading of 0 MΩ across all three phases indicated a dead short (grounded winding), consistent with
completely degraded slot-liner/varnish insulation. Following complete stripping, rewinding with Class F
insulation, and a vacuum-pressure impregnation (VPI) cycle, readings recovered to 998–1000 MΩ, classified as
Excellent under the criteria in Section 3.3.
Surge Test
Condition
Observation
Diagnosis
Pre-repair
(faulty)
Visible separation between Phase U and
Phase V waveforms
Turn-to-turn short (carbonised inter-conductor
insulation)
Post-repair
(healthy)
All three waveforms (U, V, W)
superimposed
Balanced inductance/resistance; insulation
withstands high-voltage transients
A high-voltage pulse was applied to each phase (HV lead on U1/V1/W1, ground lead on the motor core) to
generate a decaying oscillation waveform for comparison across phases.
Test Connection
Resistance (MΩ)
Status / Remark
GND – R Phase
998 MΩ
Excellent (Pass)
GND – Y Phase
1000 MΩ
Excellent (Pass)
GND – B Phase
1000 MΩ
Excellent (Pass)
Overall Result
998 - 1000 MΩ
Successful Repair
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Figure 11. Surge test observations and diagnosis.
Figure 12. Final surge-test waveform comparison. (a) Healthy condition — superimposed waveforms;
(b) faulty condition — visible waveform separation indicating a turn-to-turn short.
Rotor and Mechanical Component Inspection
Following electrical validation of the stator, the rotor and associated mechanical components were inspected:
rotor laminations for smearing or contact marks indicating prior bearing failure; rotor-bar-to-end-ring joints for
cracks; shaft journals by micrometer against manufacturer tolerance; shaft run-out on V-blocks by dial indicator
(values above 0.03–0.05 mm total indicated reading were classed as faulty and referred for machining); and
bearing-housing bore diameter, with oversized or worn bores referred to the machine section for sleeving.
Rotor Balancing
Balancing was carried out on a Precibalance machine, calibrated against a master rotor before use. Both static
(fan-only, single-plane) and dynamic (full rotor, two-plane) balancing were performed, with correction weights
calculated by the ABC method (A: distance from first pulley to rotor end; B: full rotor length; C: distance from
rotor end to second pulley) and applied until residual unbalance fell below the workshop's 9 g threshold (the
"Single-Digit Rule"), consistent with an ISO 21940-11 balance-quality grade such as G2.5 or G1.
Figure 13. Graphical interface of the digital balancing analyser, showing unbalance magnitude, angular
position, and tolerance limits for the left and right correction planes.
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Figure 14. Dynamic balancing of a large industrial rotor assembly on a horizontal balancing machine.
Static (Single-Plane) Balancing Results
Stage
Left Plane (g)
Right Plane (g)
Status vs. 9.000 g
Threshold
Initial (pre-correction)
21.684
18.392
Faulty — exceeds
threshold
After first correction run
12.417
10.863
Faulty — exceeds
threshold
Final (post-correction)
6.274
5.891
Pass
Figure 15. Comparison of static balancing results at different correction stages.
Dynamic (Two-Plane) Balancing Results
Dynamic balancing was performed at an operating speed of 600 RPM to capture couple unbalance not detectable
in the static test.
Stage
Left Plane (g)
Right Plane (g)
Status vs. 9.000 g
Threshold
Initial (pre-correction)
24.736
20.548
Faulty — exceeds
threshold
After first correction run
13.284
11.462
Faulty — exceeds
threshold
Final (post-correction)
7.152
6.843
Pass
Figure 16. Dynamic balancing results, left and right correction planes, across correction stages.
Machining and Mechanical Repair
Shaft and housing reconditioning followed standard workshop practice: sleeve fitting for undersized bearing
journals (shaft turned down, sleeve shrink-fitted or pressed, then machined to OEM diameter); soft-cut correction
for minor eccentricity; and keyway repair by weld-filling the worn keyway and milling a new slot at a 90° or
Balancing Stage
Left Plane (g)
Right Plane (g)
Status
Initial (Pre - Correction)
21.684 g
18.392 g
Needs Balancing
After (First - Correction )
12.417 g
10.863 g
Fine Tuning
Final (Post - Correction)
6.274 g
5.120 g
Balanced (Pass)
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180° offset. Welding practice used arc welding for heavy structural repairs and gas/flame welding for thinner
sections, followed by controlled cooling, grinding, and inspection for porosity before final machining.
Figure 17. Lathe machining of the motor shaft and milling of a new keyway.
Stator Winding Design Considerations
Coil pitch was calculated as the ratio of total stator slots to total poles (e.g., 36 slots / 4 poles = 9 slots pitch, coil
spanning slot 1 to slot 10). Synchronous speed at 50 Hz follows directly from pole count (2-pole: 3000 RPM; 4-
pole: 1500 RPM; 6-pole: 1000 RPM). A long (full) pitch maximises flux and torque for a given pole
configuration; a short (fractional) pitch trades some speed/force for reduced copper use, suppressed harmonics,
and improved cooling. The rewound motor used an Opposite Type connection (phase coils in alternating R–Y–
B–R–Y–B sequence) rather than a standard Chain Type connection, on the basis that the interleaved layout gives
a more even magnetic-field distribution and lower vibration for this class of motor.
Figure 18. Winding layout comparison — double-mesh (double-layer) winding (left) and single-mesh
(single-layer) winding (right).
Figure 19. Digital automatic coil-winding machine used for stator rewinding.
Final Performance Verification
After rewinding and balancing, the motor was run on a 15 kW motor driver test bench under no-load and load
conditions. Speed was measured with a digital laser tachometer; current, voltage, and efficiency were measured
with an LM/LME meter. Reported no-load current was 9–10 A against a rated current of 30 A, remaining within
the rated limit under load.
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RESULTS AND DISCUSSION
Summary of Pre- and Post-Repair Condition
Test
Pre-Repair (Faulty)
Post-Repair (Healthy)
Acceptance Criterion
Met?
Insulation Resistance
0 MΩ (all phases)
998–1000 MΩ
Yes — Excellent (IEEE
43)
DCR (max. deviation)
0.0 Ω on one phase
(short)
0.2 Ω
Yes — within ≤0.2 Ω
limit
Surge Test
Waveform separation,
U–V
Waveforms
superimposed
Yes
Static Balance (L/R)
21.684 g / 18.392 g
6.274 g / 5.891 g
Yes — below 9 g
Dynamic Balance (L/R)
24.736 g / 20.548 g
7.152 g / 6.843 g
Yes — below 9 g
Figure 20. Summary of pre-repair and post-repair motor performance evaluation.
The consolidated results in Table above show a consistent pattern across all four diagnostic domains: every test
moved from a clear fail condition to a value comfortably inside its acceptance band after rewinding, VPI
treatment, and two-plane balancing. Restoration of insulation resistance from 0 MΩ to near 1000 MΩ,
combined with waveform superimposition on the surge test, indicates that both the phase-to-ground and turn-
to-turn insulation systems were fully restored — consistent with the aging-and-recovery mechanisms described
by Montanari and Simoni (1993) and the PD-based degradation model of Stone (2005).
Effect of Winding Design Choices
Adopting the Opposite Type connection over the Chain Type is reported to have improved thermal stability
(better heat dissipation, fewer localised hotspots) and speed accuracy at the rated synchronous value without
electromagnetic humming. This is consistent with the general principle that interleaved phase-coil sequencing
improves magnetic-field distribution (Section 4.7), though the present study does not include a controlled
comparison against an equivalent Chain Type winding on the same frame; this is noted as a limitation in Section
5.4.
Effect of Balancing Precision on Operating Noise
Bringing both static and dynamic unbalance below the single-digit (9 g) threshold was associated with reduced
mechanical noise and lower bearing temperature during the trial run. The workshop observation that accurate
entry of rotor weight into the Precibalance machine was the single most failure-prone step in the balancing
procedure is a practical finding worth carrying into future studies as a defined process-control check.
Limitations
● Motor nameplate data, exact instrument models/calibration records, and ambient test conditions were not
available for inclusion in this revision and are flagged for the authors to complete.
● Results are drawn from a single motor and repair cycle; no statistical replication or comparison against
a control (e.g., an equivalent motor rewound with the Chain Type connection) was performed.
● Vibration spectra and MCSA data, which would allow direct comparison with the condition-monitoring
literature cited in Section 2, were not recorded during this case study.
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CONCLUSION
This case study documents the complete repair cycle of a three-phase induction motor diagnosed with a
grounded, turn-to-turn shorted stator winding, from initial DCR/Megger/surge diagnostics through mechanical
reconditioning, rewinding, and two-plane balancing, to final performance verification. Applying the workshop's
acceptance criteria — themselves consistent with IEEE 43 (insulation resistance), IEC 60034/IEEE 522 (surge
testing), and ISO 21940-11 (balance quality) — the motor moved from a clearly faulty condition on every test
to a healthy condition on every test after repair. The Opposite Type winding connection and the single-digit
balancing rule are highlighted as the two practical decisions with the most direct influence on the final result.
Future work should extend this single-motor record into a multi-unit dataset, incorporating MCSA and vibration-
spectrum data, to support the kind of predictive-maintenance modelling described by Abdulkareem et al. (2025).
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