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Motor Circuit Analysis
Concept and Principle by Howard W Penrose, Ph.D. General
Manager, ALL-TEST Pro, Old Saybrook, CT
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Abstract
Low
voltage Motor Circuit Analysis (MCA) techniques involve
the collection and analysis of resistance, impedance,
inductance, phase angle, current/frequency response and
insulation to ground faults. Output voltage of the test
instruments are less than 9 Vac, sinusoidal output. The
resulting low level alternating magnetic fields excite
the dielectric dipoles and surrounding magnetic steel
dipoles, in both the stator and rotor. Winding defects,
including developing shorts, cause changes to the
dielectric and resulting dipolar ‘spin,’ which changes
the capacitance of the winding circuit at the point of
defect. The resulting change effects the phase angle
and current/frequency response in the corresponding
phase, causing a difference when comparing phase, or
coil grouping, to phase. As the defect progresses, the
changes to the insulation system continue, allowing
trending of the defect over time. The purpose of this
paper is to cover the concepts and principles in the
physical changes to the windings in winding short and
winding contamination faults.
Introduction
Motor Circuit Analysis (MCA)
techniques utilizing resistance, impedance, inductance,
phase angle, current/frequency response and insulation
resistance have been in practice since 1985. The
technique has been successfully applied for the
detection of winding defects (shorts, resistive
unbalances and insulation to ground), cable defects and
rotor defects. It has also been found to be able to
trend and estimate winding failures with a high degree
of accuracy.,
In this paper we will
discuss the physical properties that allow for the
detection of these motor electrical circuit faults using
MCA.
The Motor Circuit
Figure 1: Equivalent Circuit
One Phase
The three phases of an
electric motor are separated by 120o
electrical. The supply voltage phases are also,
optimally, separated by 120o electrical.
Within each phase, as the voltage increases, current
increases due to the impedance of the motor circuit (see
Figure 1). As the current increases, the two magnetic
poles increase (or sets of poles), then decrease as
current decreases. The stator backiron acts to
strengthen and direct the magnetic fields within the
airgap between the stator and rotor. As the fields pass
through the rotor bars (conductors) of the rotor, a
second current develops in the rotor which interacts
with the rotating fields in the air gap. The rotor
follows the rotating fields, although lags behind the
synchronous speed of the stator (slip) in order to
maintain a rotor current, and resulting rotor magnetic
fields.
As this is occurring,
changes also occur to the insulation system and backiron
steel. As the current increases in each phase:
ü
There is a
skin effect within the copper conductors that forces
more current towards the surface of the conductor.
ü
Insulation
dipoles line up between conductors as the phase voltage
and current increases then decreases, causing constantly
changing capacitance within the circuit between
conductors.
ü
Insulation
dipoles line up between conductors and ground as the
phase voltage and current increases then decreases,
causing constantly changing capacitance between the
winding circuit and ground.
ü
Magnetic
dipoles line up in the area of effect of each pole
within the stator core steel. The reluctance to change
in direction is termed as hysterisis.
Operating voltages force the
changes to occur fairly rapidly. Changes to the
circuit, or to the dielectric or magnetic properties of
the motor effect its operation and the force of the
operating voltage causes the defective areas of the
insulation or steel to heat. Continued breakdown of the
dielectric occurs based upon the severity of the fault.
Insulation and Magnetic
Field Effects
Figure 2: Insulation Model
of Motor Winding System
The electrical insulation
circuit is modeled as a series of RC circuits between
conductors and conductors and ground. As changes occur
to the insulation system, the values of R and C change.
The values of A, B, C, in Figure 2, are the sum of the
turn to turn and coil to coil RC values of each phase.
G is the sum of the insulation between the windings and
ground for the complete circuit.
Figure 3: Dipolar Effect of
Insulation
As current passes through
conductors near electrical insulation, the insulation
reacts by polarizing the atoms (dipoles) within the
insulation. As the dipoles line up, there is less
leakage (capacitance) between the conductors and
ground. This also occurs in the insulation system
between conductors when there is a difference of
potential. In a good insulation system, the
polarization of the insulation happens quickly. Once
the potential is removed, the dipoles oppose each other
and randomize.
Figure 4: Magnetic Dipoles
    
The same effect occurs in a
magnetic field. The magnetic dipoles of the backiron
and teeth of the stator core line up in the direction of
the magnetic field. This helps direct the magnetic flux
and adds to the strength of the fields within the airgap.
The reluctance of the steel to change polarity shows up
as hysterisis losses from the steel. Once the field is
removed, the magnetic dipoles of the steel quickly
randomize.
The above descriptions for
the polarization of electrical insulation and core steel
represent the steady-state application of an applied
voltage potential. In an operating three phase system,
the effects get far more exciting. As each sinusoidal
phase of voltage is impressed across the windings:
ü
As the voltage
starts from zero, the beginning of the coil energizes,
the insulating dipoles between the insulation to ground
and the conductors within the coil are forced to line
up.
ü
As the voltage
continues to rise, the potential at the beginning of the
coil is higher than the end of the coil, insulating
dipoles continue to line up and the magnetic dipoles
begin to line up in the direction of magnetic flux
generated by the coils.
ü
As the voltage
hits its peak at the beginning of the coil, a majority
of the magnetic and insulating dipoles associated with
the start of the coil have aligned and the ones at the
end of the coil continue to align. There is a lag in
the fields between the beginning and end of the coil,
which causes a potential between conductors to exist.
ü
As the voltage
begins to decrease, the insulating and magnetic dipoles
begin to randomize at the beginning of the coil and
release energy back into the system as the fields
collapse. The fields at the end of the coil hit their
peak then start to decrease.
ü
The voltage approaches
zero, then passes into the negative sequence of the sine
wave. The dipoles and fields continue to react, but
align in the opposite direction. The result is a
‘dipolar spin’ of both the electrical insulation and
magnetic steel dipoles.
The high potential of most
electric motors force the changes to the fields and
dipoles to happen quickly. As a result, work is
performed and heat is generated.
The Capacitance of each
portion of the circuit is given, at any time, as:
Where an insulator exists
between the conductors and conductors and ground. The
induced charge, q, increases the capacitance by the
ratio Q/(Q-q). The dimensionless ratio q/(Q-q) is a
property of the polarizability of the material and is
referred to as the electric susceptibility, Xe.
At the boundary of each insulation system (conductors,
slot, phase, etc.), the boundary conditions are such
that
Where єr represents the
relative permittivity of the boundary of the insulation
surface.
By dividing each phase into
tubes and slices,
the total Capacitance for m slices and n tubes through
the system would be
The inductance of the
circuit can be figured as the flux linkage per unit of
current:
and is represented by the
unit henry (H).
For a motor with n coils,
the inductance may be defined
where Kpq is
referred to as the coupling coefficient between two
coils (p and q). When p and q are equal, the inductance
is termed as self-inductance, when unequal it is termed
mutual inductance.
The total impedance per
phase as viewed from the stator input terminals is given
where X refers to the
leakage reactances (capacitive).
In simpler form, impedance
can also be viewed as
 =
Inductive Reactance
 =
Capacitive Reactance
 =
Simplified Circuit Impedance
When looking at a balanced
system, a wye circuit would appear as in Figure 5.
Figure 5: Good Winding
Balanced Wye
The circuit impedance would
appear
Armed with this information,
we can now review the effects of winding related faults
on the operation of the motor.
Winding Faults
When a defect occurs in a
winding due to a developing short, winding contamination
or severely damaged core steel, it effects the
electrical properties of the insulation system. In the
case of a winding defect, changes to either capacitance
or resistance will cause a reactive problem due to
changes in the makeup of the insulation system. For
instance, in a developing short, the changes to the
insulation system cause changes to capacitance due to
changes in how the dipoles are excited (dipole spin).
As a result, there are changes to how the insulation
reacts in that area, causing a leakage reactance
variance and heating due to forcing the insulation to
polarize with higher applied potential. Winding
contamination causes changes to the resistive and
capacitive reactances between insulating surfaces, as
well.
At design voltage, most
defects do not become apparent until a distinct change
occurs, which may be represented by a severe current
unbalance, nuisance tripping or a direct short circuit
(smoke). In the case of winding contamination, the end
result is the same as a winding short – either a short
between conductors or across the insulation system to
ground.
As a result, as faults occur
due to thermal deterioration (overheating),
contamination, moisture absorption or other reactive
faults, the circuit impedance will change, slightly, at
first, then more dramatic as the fault progresses.
Traditional Test Methods
Most of the traditional test
methods require a significant voltage application in
order to work. The purpose is to stress the insulation
system by forcing a reaction of the insulation dipoles
or to force a potential across a resistive or capacitive
fault. In this section, we will review a few of the
test methods in brief, including: Insulation to ground
testing; Polarization Index; Resistance Testing; and,
Surge comparison tests.
Insulation to Ground Testing
(Meg-Ohm meters)
As described in Figure 3, a
DC potential is placed across the motor winding
conductors and ground. The applied potential is set and
a value of current (leakage) crosses the insulation
boundary. This value is converted to resistance,
usually in meg-ohms. It is, in effect, a method of
measuring leakage across the insulating boundary, but
only between the surfaces of the conductors and ground.
As the insulation dipoles are only excited with DC, some
time is required for them to polarize. Standards
normally indicate a winding charging time of about 1
minute and, as insulation resistance is directly
effected by temperature and moisture, normalization for
temperature.
Polarization Index
The polarization index (PI)
test is a measurement of leakage at one minute then at
ten minutes. The results are shown as a ratio of the
ten minute to one minute reading. It is assumed that a
fault will polarize slowly (high ratio) or rapidly (low
ratio) due to contamination and changes to the circuit
capacitance.
Resistance Testing
Resistance tests use a low
voltage DC output and a bridge. The primary purpose is
to detect high resistant joints, loose connections,
broken connections (or conductors) and direct shorts.
Surge Comparison Testing
An older method of
evaluating windings for shorts. A series of
steep-fronted higher voltage pulses are sent from the
instrument to the stator. The higher voltages force the
dipoles in one direction leaving the ability to detect a
reactive fault as creating enough potential to cross the
barrier (paschens law) either being shut down after
partial discharge occurs or an arc is drawn. Both
methods of detecting cause a change to the properties of
the insulation at the point of defect either
accelerating the fault or completing the fault. In
order to force slight defects, a greater potential must
be applied, stressing the complete insulation system.
Due to the steep fronted pulses, the applied voltage is
normally impressed only on the first 2-3 turns in the
first coil of each phase.
“The situation is quite
different for detecting the breakdown of the turn
insulation in a winding (parallel or phase) having many
coils. The breakdown of the turn insulation in a single
coil in a winding of many coils produces a very small
relative change in the characteristics (L, C, R) of
total load impedance seen by the surge generator. Hence
the change in the VFW [voltage wave form] shape produced
by the breakdown of the turn insulation somewhere in a
winding of many coils is relatively very small. Hence
the surge tests may not reliably verify the presence of
one shorted turn in a single phase winding or three
phase winding in a machine. The surge tests on windings
in a machine may probably lead to wrong conclusions.
Perfectly intact windings may appear to have a turn
short. More importantly, a turn short induced by the
surge test by breaking down the weakened turn insulation
may not be detected. In such a case, the stator winding
would likely fail after the machine is put back into
service.
“In view of the above facts,
caution is advised in surge testing of the turn
insulation in complete windings. These tests carry very
significant risks, which should be carefully
considered. Such caution is more important for
diagnostic tests on machines in service as such tests
are carried out quite infrequently in contrast to
frequent tests on new, or refurbished, or repaired
machines in a manufacturer’s plant.”
As shown, traditional
testing has specific flaws in the ability to detect, and
the ability to detect defects in a non-destructive
manner.
Modern Low-Voltage
Testing: Motor Circuit Analysis
Modern MCA devices use a low
voltage sine-wave output designed to excite the
insulation system dipoles and surrounding magnetic steel
dipoles with low current. There are several key
benefits to this approach: Size and voltage rating of
the machine being tested do not matter; Specific
pass/fail criteria can be applied to phase comparison;
and, Degradation can be trended over time without any
adverse effects to the existing condition.
“Based upon the physical and
electrical properties of coil windings, insulation,
systems, transformer theory and electric motor theory, a
set of electronic measurements can provide the necessary
information to determine the condition of electrical
equipment. The measurements must include circuit DC
resistance, circuit inductance, circuit impedance, phase
angle, current/frequency response tests and insulation
resistance readings. Resistance readings are used for
open or poor connections, inductance and impedance are
used to evaluate winding condition in electric motors
and phase balance in all other applications, phase angle
and current/frequency response tests evaluate windings
for shorts and insulation resistance readings are used
to detect winding to ground shorts.”
Detection of Winding
Contamination
Due to the fact that one of
the last measurements to change due to a fault is
inductance (L), a test result of L can be used as a
comparative baseline. This is important as the relative
position of the rotor in an assembled machine will
effect the reading due to mutual inductance.
Where 1 represents the
stator winding factors and phases and 2 represents the
rotor bar factors and bars per phase. The result is a
ratio, the same as a transformer ratio. When a rotor is
stationary in an electric motor, the ratios are
different for each phase.
Winding contamination causes
small changes to the capacitance of the winding
circuit. In most cases, the capacitance increases
within the circuit. When referencing the simple
impedance formula, earlier in this paper, it identifies
that an increase in capacitance will have a negative
effect on impedance. Also, as the applied voltage is
very low, capacitive reactance has a more significant
impact on the impedance (Ohms Law) as the capacitive
value is more dominant. The result, using a relatively
low frequency and sinusoidal output, is a collapse of
impedance towards inductance in the phase which has
capacitive effects from the contamination or water
absorption. In cases of high humidity, the insulation
has to have fissures or defects in order to cause the
change.
Overheating Windings
Overheated windings have a
similar impact as winding contamination. The difference
is that the insulation is thermally degrading causing
increased resistance to dipolar action. In this case,
the capacitance may decrease, causing an increase in
impedance in one, or more, phases.
In both winding
contamination and overheated windings, the end result
would be a winding short. Winding contamination can be
corrected if detected in its early stages. However,
once changes occur that allow for the detection of a
winding fault, the winding will have to be replaced.
Winding Shorts
One of the keys to proper
MCA testing is that inductance is not used as a primary
method of detection for developing shorts. Instead, two
specific measurements are used in combination to
determine the type and severity of the defect. These
measurements are: The circuit phase angle; and, A
Current:Frequency response method.
When a defect occurs in the
winding, it changes the effective capacitance of the
complete circuit. The change to capacitance will
directly effect how the low level current lags behind
voltage with the usual result being an increase to
capacitance and a reduction of the phase angle in the
effected phase. Once the fault becomes more severe, it
will begin to effect the surrounding phases. This
normally occurs when the defect exists in one coil or
between coils in the same phase. A very small change to
capacitance within the circuit can be detected, allowing
the detection of single turn faults and pinhole shorts
when using very low frequencies.
A second method of fault
detection uses a current ratio, similar in method to the
frequency response method used for transformer testing.
However, the low voltage current is measured, then the
frequency is exactly doubled and a percentage reduction
in the low-level current is produced. When the
frequency is doubled, small changes to capacitance
between turns or between phases are amplified, causing a
change to the percentage reduction when compared between
phases.
The combination of phase
angle and current frequency response allow for the
detection of winding shorts and the type of short being
detected in any size machine. Also, due to the use of
low voltage and the result that only a small change to
circuit capacitance is required to detect the faults,
early winding defects can be detected quickly and
trended to failure.
Additional Tests
In combination with the
above tests, MCA utilizes resistance readings and
insulation to ground tests. This allows the technology
to detect approximately half of the potential faults in
the overall motor system and allows for the comparison
of any two sets of insulated coils. Faults and defects
can be detected in cables, coils, transformers, motors
and rotor defects.
Rotor Testing and Back Iron
Effects on MCA
The effect of being able to
evaluate the condition of the motor rotor is “based upon
Faraday’s law of electromagnetic induction, according to
which a time-varying flux linking a coil induces an emf
(voltage) in it.”
 for
the primary emf
 for
the induced secondary emf
 for
the turns ratio
Which is the ratio of the
primary and secondary impedances of the circuit.
The motor circuit analyzer
excites the core steel based upon the amount of current
available to the circuit and reacts across the airgap:
The direct relationship to
the ability to detect the rotor across the airgap
depends upon the distance across the airgap, the area of
the steel magnetized and the length of the stator core.
In longer cores, the effect will carry across the airgap
and excite the rotor core and induce the instrument
frequency into the rotor circuit. In very short cores,
the fringing effect of the magnetic field from the
stator has a similar effect. In large machines, the
amount of energy available from an MCA device allows for
the detection of rotor defects only above the area
immediately surrounding each coil side.
This produces multiple
effects:
-
The mutual inductance
changes as the rotor position changes as a direct
result of the change to the transformer ratio between
the primary (stator) and secondary (rotor).
(Reference Figure 1 and Mutual Inductance). A good
rotor will show as a repeating pattern, a bad rotor
will change the transformer ratio and a defect will
appear as a non-repeating pattern.
-
Fractures will be readily
detected as the induced energy is relatively low and
the oxides on the surface of the defect will change
the transformer ratio. Whereas, in higher voltage
rotor tests, the energy may be significant enough to
pass through the defect.
-
In rare instances, the
airgap may be too significant and very little to no
variation of the mutual inductance occurs. In this
case, larger defects, such as multiple fractures or a
broken bar, will show as a variation in the straight
line.
-
MCA technology has the
ability to detect wound rotor, synchronous rotor field
and other wound-rotor defects across the airgap.
Because of the impedance ratio between the primary and
secondary, rotor winding defects will show as a change
to phase angle and current/frequency response and will
vary based upon rotor position.
Armature
and Commutator Contamination Detection
One of the unique abilities
of MCA is the ability to detect carbon buildup in DC
motor armatures. Due to the dielectric (capacitive)
properties of carbon, capacitance values of the circuit
become unstable. This causes test results of impedance,
phase angle, current/frequency response and insulation
to ground to become unstable and non-repeatable. As a
result, armature circuit contamination is detected by
noting non-repeatable test results. This is important
in that, if detected early, this type of defect may be
corrected by blowing out the armature with low pressure
air.
Conclusion
Based upon the engineering
principles of motor and transformer design, utilizing
low voltage test technologies allows for the detection
of incipient defects in the electric motor circuit
including cable insulation, coils, transformers,
connections, motor and rotor windings, armatures, air
gap issues and squirrel cage rotor defects, covering
over 50% of all potential motor system faults
(electrical and mechanical) of any size or voltage
machine through the motor circuit and cabling or
directly at the machine. This is achieved by utilizing
a low potential sinusoidal output (pulsed outputs do not
work) from the instrument which excites the insulation
and magnetic dipoles of the circuit. The low potential
allows defects to become more readily apparent at early
stages as it does not force dipolar spin, causing
changes to the circuit impedance, phase angle and
current at varying frequencies, depending upon the type
of fault. These properties of the technology allow for
long term trending of developing defects due to
insulation breakdown and contamination without any
harmful effects on the circuit condition.
About the Author
Dr. Penrose joined ALL-TEST
Pro in 1999 following fifteen years in the electrical
equipment repair, field service and research and
development fields. Starting as an electric motor
repair journeyman in the US Navy, Dr. Penrose lead and
developed motor system maintenance and management
programs within industry for service companies, the US
Department of Energy, utilities, states, and many
others. Dr. Penrose taught engineering at the
University of Illinois at Chicago as an Adjunct
Professor of Electrical, Mechanical and Industrial
Engineering as well as serving as a Senior Research
Engineer at the UIC Energy Resources Center performing
energy, reliability, waste stream and production
industrial surveys. Dr Penrose has repaired,
troubleshot, designed, installed or researched a great
many technologies that have been, or will be, introduced
into industry. He has coordinated US DOE and Utility
projects including the industry-funded modifications to
the US Department of Energy’s MotorMaster Plus software
in 2000 and the development of the Pacific Gas and
Electric Motor System Performance Analysis Tool (PAT)
project. Dr. Penrose is the Vice-Chair of the
Connecticut Section IEEE (institute of electrical and
electronics engineers), a past-Chair of the Chicago
Section IEEE, Past Chair of the Chicago Section Chapters
of the Dielectric and Electrical Insulation Society and
Power Electronics Society of IEEE, is a member of the
Vibration Institute, Electrical Manufacturing and Coil
Winding Association, the International Maintenance
Institute, NETA and MENSA. He has numerous articles,
books and professional papers published in a number of
industrial topics and is a US Department of Energy
MotorMaster Certified Professional, as well as a trained
vibration analyst, infrared analyst and motor diagnostic
specialist.
Bibliography
Sarma, Mulukutla S., Electric Machines: Steady-State
Theory and Dynamic Performance, PWS Publishing
Company, 1994.
Bodine
Electric Company, Small Motor, Gearmotor and Control
Handbook, 1993.
Nasar,
Syed A., Theory and Problems of Electric Machines and
Electromechanics, Schaum’s Outline Series, 1981.
Edminster,
Joseph, et.al., Electric Circuits Third Edition,
Schaums Electronic Tutor, 1997.
Hammond, et.al., Engineering Electromagnetism,
Physical Processes and Computation, Oxford Science
Publications, 1994.
Penrose, et.al, “Static
Motor Circuit Analysis: An Introduction to Theory and
Application,” IEEE Electrical Insulation Magazine,
July/August 2000.
Penrose, Howard W, Ph.D.,
Motor Circuit Analysis: Theory, Application and
Energy Analysis, Success by Design Publishing, 2001
Penrose, Howard W,
“Estimating Motor Life Using Motor Circuit Analysis
Predictive Measurements,” IEEE Electrical Insulation
Conference Proceedings, 2003
Stone, et.al, Electrical
Insulation for Rotating Machines, IEEE Press Series
on Power Engineering, 2004.
Nailen, Richard L,
Managing Motors, Second Edition, Barks Publications,
1996.
Fink, et.al., Standard
Handbook for Electrical Engineers, Fourteeth
Edition, McGraw Hill, 2000.
Humphrey, Dave, “Which Road
Will You Take,” IEEE Electrical Insulation Conference
Proceedings, 2003.
Gupta, Bal, “Risk in Surge
Testing of Turn Insulation in Windings of Rotating
Machines,” IEEE Electrical Insulation Conference
Proceedings, 2003.
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