Evaluation of Form Wound Insulation with
Motor Circuit Analysis
Howard W Penrose, Ph.D., CMRP
President, MotorDoc LLC
Lombard, IL, USA
hpenrose@motordoc.com
Abstract—Motor Circuit Analysis techniques involve the use
of low voltage, frequency-based and high voltage turn and
insulation to ground tests. These methods primarily rely upon
circuit inductance, capacitance and impedances as well as
insulation to ground capacitive measurements. In this paper the
common methods will be outlined as they relate to form wound
assembled machines from 4160 Volts to 13.8 kV systems
including examples of applications at both voltages.
Keywords—Motor Circuit Analysis; MCA; Electric Machine;
Inductance; Capacitance; Impedance; Resistance; Surge
Comparison; PD Surge; Insulation Resistance; Polarization Index;
Insulation Resistance Profile
I. INTRODUCTION
Low and high voltage Motor Circuit Analysis (MCA)
techniques have been commercially available since the 1950s
with newer technologies in the low voltage MCA space since
the 1980s. Since this time MCA has become more prevalent as
a field predictive maintenance (PdM) and testing technology
performed on assembled electric machines versus it’s primary
use by the machine repair industry and some larger industries,
such as utilities, in the past. The description used within the
testing industry has commonly been ‘low and high voltage’
testing technologies, giving the incorrect impression that the
technologies focus on low and high voltage equipment.
Most of the manufacturers of MCA technologies
recognized the increased use of these testing methods by
maintenance organizations and their applications to fully
assembled machines. This required some modifications to
existing instruments as the rotating components of the
machines had a tendency to impact the test results due to
mutual inductance with the stationary component over the
airgap. The volume of the airgap, the distance across the
airgap between components in particular, the sensitivity of the
device being used, and the type of winding style, all have an
influence on the balance of readings between phases.
The balance and influence of the mutual inductance allows
users of MCA technologies to evaluate both the condition of
the stator (stationary windings) and rotor (rotating component)
when testing can be performed in such a way that the rotor can
be moved. However, proper field interpretation requires
experience in applications where the rotor cannot be moved
due to resulting imbalances in different types of machines.
In this paper we will be discussing the application of low
and high voltage MCA on form wound machines through
13.8kV including field experience. These machines include
industrial machines from induction, wound rotor to
synchronous and are applicable to such industries as wind.
Each technology will be compared.
II. TECHNOLOGIES APPLIED
For purposes of this study, three devices were used in the
field as part of an evaluation of 42 electric machines ranging
from 4160 Vac to 13.8 kV. The devices used in this study
included in which all testing methods are outlined in IEEE Std
1415-2006:

Amprobe 5kV Insulation Resistance Tester
(AMB5): Insulation resistance, polarization index,
dielectric absorption, dielectric discharge, and
capacitance to ground.

ALL-TEST PRO 5: Resistance, inductance,
impedance, phase angle, current/frequency
response, insulation resistance, capacitance to
ground, dissipation factor.
All testing but
insulation resistance is performed with ~10V
output at frequencies through 1200 Hz.

Electrom iTIG II D12: Resistance, inductance,
impedance, phase angle, capacitance, Q-factor (all
low voltage), insulation resistance, polarization
index, dielectric absorption, high potential test,
surge comparison testing (performed 50-60Hz),
and PD Surge. High voltage tests up to 12kV.
Each of the above systems is designed to be applied in the
field, as well as in the OEM and service industries. It is noted
that low voltage equipment is applied across all machine
voltages and is generally small and light weight. High voltage
test equipment must increase in size in order to provide the
output voltage necessary to test machines.
©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current
Sponsors
of this including
paper are ALL-TEST
Pro LLC and Electrom
or future
media,
reprinting/republishing
this material for advertising or promotional purposes, creating new collective
Instruments.
works, for resale or redistribution to servers or lists, or reuse of any copyrighted components of the work in other works.
III. CASE STUDIES
The following case studies are relatively typical of what is
found in the field as part of maintenance testing or PdM.
Results are often more severe when troubleshooting, which
will be covered in the second case study.
Fig. 2. Motor Evaluated in Case Study 1
A. Case Study 1: 4160 Volt Induction Machine
Evaluation of an open drip proof 300 horsepower, 885
RPM, 4160 Vac, form wound electric motor as part of a
predictive maintenance program. Leads are disconnected at the
motor and testing performed using the Electrom iTIG (iTIG)
and ALL-TEST Pro 5 (ATP5) with the winding temperatures
at room temperature, approximately 20oC.
TABLE I.
LOW VOLTAGE MCA TEST RESULTS (ATP5)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
I/F (percent)
Insulation Resistance
(1000V)
Capacitance
Dissipation Factor
Test Frequency
TABLE II.
T1-T2
1.15
43.7
69.5
81.2
-46.8
T1-T3
T2-T3
1.15
1.15
43.8
43.7
69.7
69.5
81.2
81.1
-46.9
-46.8
>999 MegOhms
35.7 nF
1.13%
100 Hz
HIGH AND LOW VOLTAGE MCA TESTS (ITIG)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
Q Factor
Partial Discharge (pC)
Insulation Resistance
(1000V corrected to 40oC)
High Potential Test
Capacitance
Capacitance D Factor
T1-T2
T1-T3
T2-T3
1.1530
1.1528
1.1526
369.3
369.0
367.9
58.47
58.43
58.26
84.2
84.3
9.93
9.85
9.93
9.89
114,145
97,839
114,145
8,333 MegOhms, 0.03 uAmps
10,300 V, 1.54 uAmps
0.128 nF
0.437
Fig. 1. Surge Comparison at 8,902 Volts
A visual inspection of the windings indicated that there was
some level of contamination while test results in both the low
and high voltage MCA testing indicated that no serious
concern had yet presented itself. The unbalanced partial
discharge tests identified in Table II may be an early indicator.
However, no other tests performed indicated that there were
insulation degradation issues beyond visible contamination.
Other than the insulation resistance tests identified in
Tables I and II, all other tests are comparative between phases
or can be compared to similar machines. Small unbalances
between phases are normally due to the rotor being present
with specific data patterns identifying faults regardless of the
unbalance, such as found in Case Study 2.
B. Case Study 2: 13.8 kV Synchronous Machine
Evaluation of a 4500 horsepower, 13.8kV, 1800 RPM
synchronous machine due to degradation over time was
performed using the Amprobe, ATP5 and iTIG systems.
Visual inspections had identified degradation of both the stator
and rotating fields due to contamination and corona effects. It
was determined that the high voltage tests would be performed
at the same value as tests performed a year prior.
TABLE III.
LOW VOLTAGE MCA TEST RESULTS ON STATOR (ATP5)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
I/F (percent)
Insulation Resistance
(1000V)
Capacitance
Dissipation Factor
Test Frequency
T1-T2
0.718
67.8
108
78.2
-46.4
T1-T3
T2-T3
0.771
0.770
62.7
54.2
99.7
86.3
77.7
78.6
-46.6
-47.2
>999 MegOhms
76.9 nF
1.12%
100 Hz
©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current
or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective
works, for resale or redistribution to servers or lists, or reuse of any copyrighted components of the work in other works.
TABLE IV.
HIGH AND LOW VOLTAGE MCA TESTS STATOR (ITIG)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
Q Factor
Partial Discharge (pC)
Insulation Resistance
(1000V corrected to 40oC)
High Potential Test
Capacitance
Capacitance D Factor
T1-T2
T1-T3
T2-T3
0.7195
0.7186
0.7204
519.2
437.7
437.7
81.23
68.77
76.42
79.4
80.8
79.9
5.35
6.19
5.62
21,742
10,871
8153
25000 MegOhms, 0.01 uAmps
0.62 uAmps at 11,000 V
0.051
-0.701
Fig. 3. Surge Comparison at 11,000 Volts
Capacitance
170 nF
Dissipation Factor
1.38%
Test Frequency
100 Hz
This change in results between Tables V and III is
indicative of issues with the rotor. As a result, additional rotor
tests were performed and compared. The unbalance in PD tests
between phases also indicated additional degradation of the
stator windings, requiring continued monitoring.
TABLE VI.
LOW VOLTAGE MCA TEST RESULTS ON ROTOR
COMPARISON BETWEEN PRESENT AND 4 MONTHS PRIOR (ATP5)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
I/F (percent)
Insulation Resistance
(500V)
Capacitance
Dissipation Factor
Test Frequency
Present
0.171
42.0
33.4
70.9
-41.2
727
MOhms
9.60 nF
1.12%
100 Hz
Prior
0.153
45.8
72.9
63.4
-39.1
>999
MOhms
9.80 nF
1.23%
100 Hz
Difference
0.018
3.8
39.5
7.5
2.1
Reduced
0.2
0.11
N/A
As found in Table VI, all of the circuit values changed
including the insulation to ground test results. A polarization
index curve was performed in order to evaluate the condition
of the rotor to ground insulation. It was determined, due to
contamination, and the significant changes in low voltage
MCA results, that high voltage MCA would be dangerous as it
could fail the fragile insulation system and eliminate the
machine owner’s ability to bring the machine online.
The unbalance shown in both of the MCA methods,
including the surge test waveforms, was determined to be due
to the rotor, even though the rotor circuit was disconnected for
testing purposes. Comparison to other machines indicated that
when set up this way, the stator windings indicated that they
were balanced. The rotor had been tested four months
previously with MCA only, allowing a comparison.
TABLE V.
The insulation resistance profile identified in Fig. 4
indicates contamination and some possible aging of the rotor
winding insulation to ground, as there are capacitive discharges
across the full time of the test being performed. Fig. 5
identifies that the discharges have become far more severe
during the four-month period. This indicates that the rotor
insulation system has rapidly degraded during the four-month
period.
Fig. 4. Insulation Resistance Profile 4 Months Earlier
LOW VOLTAGE MCA TEST RESULTS ON STATOR 4 MONTHS
PRIOR (ATP5)
Resistance (Ohms)
Impedance (Ohms)
Inductance (mH)
Phase Angle (degrees)
I/F (percent)
Insulation Resistance
(1000V)
T1-T2
0.726
55.4
88.2
78.7
-47.1
T1-T3
T2-T3
0.727
0.724
54.3
54.7
86.4
87.1
76.8
76.6
-46.2
-46.0
>999 MegOhms
©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current
or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective
works, for resale or redistribution to servers or lists, or reuse of any copyrighted components of the work in other works.
Fig. 5. Present Insulation Resistance Profile
potential tests, surge comparison tests, and partial discharge
tests, were dramatically dissimilar from the low voltage MCA
test regimen. As a result, trending between the technologies is
not possible. Trending between the low voltage tests of both
machines, from the standpoint of phase to phase comparisons,
is possible. However, it was noted that the inductance and
impedance readings were dissimilar between machines even
though confirmation of the calibration of both machines was
sought following the identification of these differences.
In order to bring the high voltage MCA device up to the
voltage required for 13.8 kV, if the test was performed
properly, would have required the addition of a second power
supply. No such requirements were needed for low voltage
MCA. Both technologies successfully identified the same
conditions on all of the machines tested, including those in the
case studies.
REFERENCES
1.
When the results of the stator and rotor tests are reviewed,
while there are conditions in the stator, it appears that a
developing short and insulation to ground problems in the rotor
are the primary issues. The motor was also seeing a vibration
issue that appeared to be from misalignment and related
causes, but the conditions of the rotor may be contributing to
the issues.
IV. DISCUSSION
In both cases low voltage and high voltage MCA provided
similar results. In Case 2, both technologies identified that
there was an issue with the machine, including the ability to
compare previous to present test results. As the machine had
been operating, additional testing was performed on the rotor
to see if the issue existed there versus in the stator. The
conditions found were significantly different in low voltage
MCA such that it was determined that the application of high
voltage MCA would most likely put the machine in a condition
where it would not operate.
2.
3.
4.
5.
IEEE, IEEE Std 43-2013: IEEE Recommended Practice for Testing
Insulation Resistance of Electric Machinery, Electric Machinery
Committee of the IEEE Power Engineering Society, 2013
IEEE, IEEE Std 522-2004: IEEE Guide for Testing Turn Insulation of
Form-Wound Stator Coils for Alternating-Current Electric Machines,
Electrical Machinery Committee of the IEEE Power Engineering
Society, 2004
IEEE, IEEE Std 1415-2006: IEEE Guide for Induction Machinery
Maintenance Testing and Failure Analysis, Electrical Machinery
Committee of the IEEE Power Engineering Society, 2006
IEEE, IEEE Std 1434-2000: IEEE Guide to the Measurement of Partial
Discharges in Rotating Machinery, Electrical Machinery Committee of
the IEEE Power Engineering Society, 2000
H. W. Penrose, “Study of Insulation Resistance Profiling Use on
Random and Form Wound Machines under 6kV,” Conference Record of
the 2012 International Symposium on Electrical Insulation, San Juan,
PR, 2012
The study was performed on 42 similar machines at one
location with similar results. Both the high voltage and low
voltage MCA systems produced similar results with the
differences involving the amount of time to perform testing.
For the full suite of tests identified with high voltage MCA, the
testing took approximately 45 minutes and required a plug in
power supply. Low voltage MCA took less than 5 minutes
with an internal battery. When coupled with insulation
resistance profiling, testing time took approximately 20
minutes total.
In both cases, any surge arrestors or power factor correction
capacitors had to be removed from the circuit or they would
compromise the test results. A majority of the tests were
performed at the machines at the request of the machine owner.
However, a number of tests were performed from the MCC in
which capacitance testing was dropped on both instruments.
With the exception of the low voltage tests provided by
both instruments, the high voltage tests including the high
©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current
or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective
works, for resale or redistribution to servers or lists, or reuse of any copyrighted components of the work in other works.
©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current
or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective
works, for resale or redistribution to servers or lists, or reuse of any copyrighted components of the work in other works.