white paper
Finding Electrical Failure Modes
of Rotating Machinery with
Electrical Testing Mechanisms
Begin >
E
lectrical maintenance professionals are
expected to keep plant operations moving smoothly with an ever-decreasing
budget and greater demands on their time and
equipment. This dilemma creates a large and
daunting task. In order to develop a comprehensive reliability program a wide variety of requirements need to be met. A unified approach
to meeting these needs is to identify the most
common failure modes in electrical equipment
and then identify the tests that most effectively
find those failures. In accomplishing this task,
the amount of reactive maintenance due to unscheduled downtime is lessened.
There is myriad equipment available to the
maintenance professional for electrical predictive and preventative maintenance. Vibration
technology is a usual starting point for most
programs; however, there is also lubrication
analysis, infrared, ultrasonic, along with testers
that supply specific results on the motor circuit and insulation systems of electric motors.
These testers include capabilities like torque
and current signature analysis on the dynamic side of testing and inductance, impedance,
capacitance, phase angle, winding resistance,
Megohm, DC High potential (HiPot) and surge
on the static side of testing.
For the purpose of this article, the focus of
failure modes and appropriate testing procedures will be for static testing of electric motors.
These tests evaluate the integrity of the motor
to run in the environment that it is installed.
The instruments that perform these tests pro-
vide both low voltage and high voltage testing
capabilities that look at different types of failures in one instrument. Most of these tests are
governed by IEEE standards.
Failure Modes in Electric Motors
IEEE and EPRI have done independent studies
focusing on failure modes in electric rotating
machinery. Their conclusions state that over
40% of all motor failures are bearing related,
over 25% are stator related, 8-9% are rotor related and the remaining 14-22% are a combination of other smaller faults.
Insulation failure modes are broken up into
four motor aging categories: Thermal aging,
electrical aging, mechanical aging and environmental aging. An effective reliability program
will have the tools necessary to identify and
prevent these causes prior to catastrophic motor failure and unplanned downtime. For this
paper, we will focus on electrical aging faults
and how they affect the insulation of the motor.
Electrical Aging
Electrical aging occurs when voltage across
the insulation causes deterioration. This aging
occurs several ways. Overheating is a major
contributor to deterioration along with manufacturing defects, winding ground faults, broken rotor bars or short circuit end rings, electrical discharges, surface tracking and moisture
absorption, system surge voltages, transient
overvoltages in rotor windings, high resistance
connections among others. Voltage stress, i.e.,
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Be a r in g
41%
O ther
14%
IEEE Study
O ther
22%
EPRI Study
R ot or
9%
St a t or
36%
Be a r in g
44%
R ot or
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St a t or
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voltage overload, reduces insulation service
life. Causes include: bus transfers, switching
surges, reflected wave phenomena due to variable frequency drives or even volts per turn
that was not properly calculated in the motor
design.
Overheating
Under-excitation and over-excitation of core elements can cause excessive heat in the core
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laminate insulation. Higher temperatures reduce the dielectric strength of the inter laminar
insulation over time and causes other stresses
due to expansion and relative motion. Also circulating currents in the laminations can result
in voltage being developed between adjacent
core laminations. Even minor defects in the
inter laminar insulation may provide paths for
circulating currents causing further deterioration. This heat then affects the coils, turn-toturn and inter lamination insulation. A general
core failure. [1] In addition to core faults, this
increased temperature will also deteriorate the
groundwall and turn-to-turn insulation.
Manufacturing Defects of the Stator Core
Lamination shorts can be introduced during
manufacturing or refurbishment. Poor adhesion between the insulation and steel causes
flaking, which creates weakness. Poor edge
deburring of the laminations will cause sharp
edges that cut through the insulation and create
Having a unified approach to testing is the first step
in reaching goals to lessen reactive maintenance
and unscheduled downtime.
rule of thumb: Every 10° rise in temperature
cuts the insulation life in half.
The large volume of core behind the slot is
more prone to overheating due to increased
flux compared to the tooth area. This area has
less ventilation causing higher temperatures
particularly as the iron begins to saturate. The
increase in temperature creates a vicious cycle.
Once the temperature is elevated, inter laminar
breakdown of insulation increases, which gives
rise to faults and eddy currents, which cause
even higher temperatures to be produced.
These higher temperatures cause mechanical
stresses resulting in distortion and vibration.
When combined, these effects eventually lead
to fusing of laminations, melting of iron and
metal-to-metal contact. In addition, the shorting of the diameter of the stator bore caused by
smearing of the insulation in transportation or
installation or the overfilling of stator slots can
damage the core. All of these occurrences can
shorten the life of the motor and deteriorate
the insulation. [1] Shorting of the laminations
will also lessen the efficiency of the motor.
Winding Ground Faults in Core Slots
The energy and heat produced by stator winding faults in the slot region are often high
enough to melt and fuse the core laminations at the slot surface. If this core damage
is not repaired when the failed coil or bar is
replaced, the new coil might fail to ground
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because of the heat generated by the shorted
laminations. [1]
Stator Winding Insulation: Electrical Discharges
A winding fails when the dielectric strength
of the insulation can no longer support the
operating voltage or transient overvoltages
seen during startup or shut down. Electrical
discharges generally occur in windings with
voltage ratings of 5 kV rms or above and are
commonly known as Partial Discharge. This gas
breakdown phenomenon occurs in gas-filled
pockets, which have solid insulation boundaries. A characteristic of partial discharge is that
it requires a minimum voltage be met in order
for a discharge to take place. The rate of deterioration of insulating materials that occurs
due to partial discharge is a function of their
discharge resistance properties. The fewer the
voids produced due to the manufacturing process, the more resistant the insulation is to partial discharge. Generally, organic materials are
more susceptible to partial discharge then nonorganic materials.
Symptoms of aging due to partial discharge
are evident of treeing through and burning of
the groundwall insulation. This burning of the
turn insulation occurs at its interface with the
groundwall and is associated with turn-to-turn
and strand-to-strand short circuits. [1]
Surface Tracking and Moisture Absorption
The formations of permanent conductive
paths on the end-winding regions of the insu-
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lation are caused by ac electrical stress. The
surface of a stator coil in the end-winding region is highly resistive; however, small conductive areas can occur in normal operation
due to contaminates such as oil, coal dust,
chemicals or moisture. Leakage current will
travel along these conductive patches causing
degradation of the insulation surface. Failures
resulting from this type of aging are ground or
phase-to-phase.
When motors absorb moisture from sources
such as coolant systems, condensation or other moisture related problems, the resistance of
the groundwall insulation is reduced and can
create conductive paths especially in weakened or delaminated regions of the winding.
These conductive paths can cause ground or
phase-to-phase faults.
In synchronous machines wound rotor windings are susceptible to problems with contamination of conducting materials due to
proximity to these contaminates and that their
insulating materials are often just separated
instead of surrounded leaving items exposed
between conductors. Contamination from oil
dripping from a bearing and dust can cause
surface tracking, which can lead to turn-to-turn
or ground short circuits. [1]
System Surge Voltages/Transient Overvoltages
Turn-to-turn failures can be contributed to aging of turn insulation from exposure to power
surges during start up and shut down of motors. If a fast rise time voltage surge strikes the
stator winding from the switching of a motor,
a voltage of several kiloVolts above operating
voltage can appear across the turn insulation
for a short time.
When the turn-to-turn insulation is in a weakened state this voltage can puncture the turn
insulation-causing break down in the insulation. This will lead to a turn-to-turn short and
High Resistance Connections
If a joint between two conductors is poorly soldered it creates a high resistance to the current
flowing under load. This generates excessive
heat and causes extensive thermal damage.
This type of aging will develop turn-to-turn,
phase-to-phase or ground faults.
Quality control testing such as resistance
When the insulation between turns is weak, the result
is a low energy arc and a change in inductance.
high circulating currents ultimately resulting in
motor failure. These random switching events
can quickly age the insulation and degrade the
system.
High transient overvoltages may be introduced into rotor windings due to line-to-line
stator winding short circuits, faulty synchronization, or asynchronous operation. Such transient voltage, together with weakened insulation can cause failures, mainly turn-to-turn.
In a wound rotor induction motor there is
a transformer effect between the stator and
rotor windings. Consequently, power system
surge voltage imposed on the stator winding
will induce overvoltage in the rotor winding.
Providing there is adequate turn and ground
insulation on the rotor winding, such voltage
should not cause electrical aging, however,
these overvoltages will accelerate the failure
of insulation that is already weak. [1]
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testing or thermal imaging will detect these
joints and can help mitigate this type of fault
in the manufacturing or rewinding process. [1]
Fault Identification and Predictive Maintenance
Insulation systems are subjected to a wide variety of events that can cause possible problems on a daily basis. With the progression
of time, these issues turn into reality and can
cause unexpected downtime of electric rotating machinery. The condition of electrical insulation materials is often best assessed through
an electrical test. Such tests are broadly divided into two major categories: (1) high voltage
and (2) low voltage tests. The former are definitive tests performed at some elevated ac
or dc voltage to give assurance that rotating
equipment can withstand the voltages that are
typically seen during startup and shutdown.
Test voltages are given in several standards
and formulated by class and type of machine.
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The later, low voltage, yields information such
as an indication of moisture, dirt, or some other contaminant along with cracks or voids in
the insulation. Low voltage tests also evaluate
the motor circuit and identify failures such as
shorts, opens, miss connections and unbalances in the phases. Predictive maintenance programs have been developed in order to investigate motor faults, find insulation weaknesses
before it creates downtime and allows for root
cause analysis. In order to obtain a comprehensive predictive maintenance program both
high and low voltage testing is a necessity. The
performed tests look at and identify problems
in a portion of the insulating system. These
tests work in conjunction with each other to
produce an overall picture of the health of the
machines insulation. In the following section
Electrical Static Tests & Failure
Modes Found
Low Voltage Tests
Impedance is defined as a circuit’s total opposition to alternating current flow. It is a combination of the circuit’s resistance, inductance and
capacitance. When adding these terms we add
the values of Resistance, Inductive Reactance,
and Capacitive Reactance. The reactance is simply the resistive equivalents of the inductance
or capacitance.
In an AC circuit, we add an angle to the measurement to denote its effect on voltage to cur-
rent displacement. That is to say, that the values
of inductance and capacitance will cause the
resulting current to be phase shifted from the
applied voltage wave.
Inductance causes a negative shift in current
such that the current will tend to lag the applied
A circuit with many turns of very fine wire that
has a short turn to turn may show little effect
on the resistive balance measurement but that
same circuit will show a large difference in impedance due to the inductive imbalance created
by missing turns.
When a motor is wound, the inductive balance
of the circuit can be helpful in validating the
connections in the circuit.
voltage. The voltage rises first and the current
rises some time later based on the level of inductance in the circuit.
It is the inductor or coil and its opposition to
changes in current that causes this effect. The
changing magnetic field in the coil induces a
voltage and resulting negative current flow back
into itself. Lenz’s Law describes this.
Capacitance has the opposite effect and
causes the current to lead the voltage. It is the
capacitors opposition to changes in voltage that
cause this to occur.
Every motor circuit is defined by these parameters. The total impedance can be evaluated
in much the same way we evaluate resistance
in the motor. Where a resistive imbalance may
show us high resistance connections or shorted
turns due to variation between phases, Impedance can also be evaluated with respect to its
balance in the circuit.
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If we change the frequency of the applied AC
voltage, it can have an even greater effect. This
is because the Inductive Reactance (XsubL) is
equal to 2 x Pi x L x F. The higher the applied frequency the larger the effective resistance of the
circuit will be due to the inductor present. Capacitance has the inverse effect since it is equal
to 1/(2 x Pi x C x F). As the frequency is increased
in this circuit, the effective resistance to current
flow is reduced.
When using the parameters to analyze a motors condition it is important to understand how
they each fit into the circuit. A three-phase motor is made up of groups of windings that form
three phases. These three phases are connected
in a star or delta configuration. In theory, these
three circuits should be balanced perfectly. The
cross sectional area and length of the copper
should be even throughout all three phases. The
number of turns per coil and coils per group,
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and groups per phase should all be the same.
The amount of insulation around each part of
the winding and where the winding is laid in
the slot adjacent to the iron should all be evenly distributed. Each of these design and construction conditions has a bearing on each of
the three parameters (R, L, C).
When the Impedance is measured, a low AC
voltage is applied at a specific frequency. The
resulting current flow is then measured. This
measurement of current can then be used to
Capacitance is directly proportional to the
surface area on each side of the dielectric material, in this case the insulation, being tested.
Therefore, the level of cleanliness and moisture can be evaluated and trended over time.
When a motor is wound, the inductive balance of the circuit can be helpful in validating the connections in the circuit. If a coil or
group of coils were misconnected, this would
have an effect on the inductance in the circuit. Any imbalance present in the impedance
Lamination shorts can be introduced during
manufacturing or refurbishment.
calculate the circuit’s total opposition to current
flow at the applied voltage and frequency. This
is the impedance of the circuit under test. The
displacement of current and voltage (phase
angle) will allow us to determine how much
of the circuit is inductive or capacitive and values for each can be determined. Changing the
frequency of the voltage applied will cause the
values of capacitive and inductive reactance to
vary as described above.
This same type of measurement can also be
performed through the ground wall insulation
directly. When a motor is contaminated, the capacitance of the circuit will increase. This is due
to the increase of the effective surface area of
the winding.
measurement would indicate an unbalance
turn count or misconnection of the windings.
Since the impedance test is normally performed at a low voltage (<100V) this is not an
effective test for testing weakened insulation
between turns. The surge test is performed
for this reason.
The impedance and capacitance tests find
hard turn shorts, improper turns count, misconnections of windings and moisture and particulate contamination.
The Coil Resistance test consists of injecting
a known constant DC current through the winding, measuring the voltage drop across the
winding, and calculating the coil resistance using Ohm’s law. If a coil is shorted somewhere
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in the interior of the winding, the resistance will
be lower than normal. This lower coil resistance
can be compared to previous measurements of
the same coil, measurements of identical coils,
or to the motor nameplate value in order to
identify a bad coil. Low values indicate shorts,
less turns, less cross sectional area. Measured
values that are higher than normal can indicate
loose, corroded connections or opens.
The measured resistance is affected by the
variation of copper conductivity with temperature. Before comparing two different measurements, correct the measured resistance value
to a common temperature, usually 25° C per
IEEE 118.
Performing Resistance tests on the same motor over time provides early warning signs of
motor connection problems. Motors operated
in conditions that allow corrosion, contamination, or other physical damage may show initial warning signs of motor failure.
Since the windings found in many motors
have very low resistances, the injected current
may have to be many amps to measure accurately the voltage drop across the coil. Difficulties in measuring the voltage drop across the
coil itself are the affects of the contact resistance of internal relays and the contact resistance of the clip leads used to connect to the
motor’s winding. Contact resistances can be
comparable or even greater than the resistance
of some coils.
A practical lower limit of the coil resistance
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test exists to evaluate the copper winding conductors. The test instrument must be able to
resolve the change in copper resistance caused
by a short in the winding before conclusions
are made regarding the coil resistance.
The instrument should compare the percentage difference in resistance between leads with
the calculation of Max Delta R. The user defines
the acceptable Delta R tolerances for each motor,
thereby giving the instrument its pass/fail limits.
When the Resistance test results are displayed, measured resistance values, resistances corrected for temperature and the Delta Resistance percentage are listed. A problem with
the motor under test may be indicated when
Delta Resistance is high. The motor fails the
test when the instrument detects Delta Resistance values not within the prescribed limits.
The Meg-Ohm test consists of applying a DC
voltage to the windings of a machine after isolating the winding from ground. According to
IEEE 43 the test, voltage is usually near the operating voltage of the machine.
The intended purpose of the Meg-Ohm test
is to make an accurate measurement of the
insulation resistance of the ground wall insulation. The insulation resistance, abbreviated
IR, is a function of many variables: the physical properties of the insulating material, temperature, humidity, contaminants etc. The IR
value is calculated using Ohm’s law – the applied voltage is divided by the measured leakage current. This leakage current is the current
which passes from the winding through the
Equipment Rated Voltage (V)
Insulation Tester Voltage V DC
1000 and lower
500
1000-2500
500-1000
2501-5000
1000-2500
5001-12000
2500-5000
>12000
5000-10000
ground wall insulation to the motor’s steel core
plus any surface leakage currents. The surface
leakage currents flow through moisture or contaminants on the surface of the insulation. To
determine accurately the insulation resistance,
the surface leakage must be reduced to an inconsequential level.
The Meg-Ohm test is best used for finding
ground faults and the level of moisture or particulate contamination.
The Polarization Index Test (PI test) is best
used for determining if the winding is wet or
is contaminated. The PI test is performed in order to measure quantitatively the ability of an
insulator to polarize. When an insulator polarizes, the electric dipoles distributed throughout the insulator align themselves with an applied electric field. As the molecules polarize,
a polarization current, or absorption current, is
developed that adds to the insulation leakage
current. This additional polarization current decreases over time and drops to zero when the
insulation is completely polarized.
PI results become confusing when attempting to attribute variations in the PI value to the
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polarizability of the insulator or other affects
such as humidity or moisture, surface leakage
or instrument error. The result is even more
confusing when attempting to reconcile a PI of
1 when expecting a different PI result.
The PI test is typically performed at 500,
1000, 2500 or 5000 Volts. This depends on the
operating voltage of the motors being tested.
The duration of the test is 10 minutes. The PI
value is calculated by dividing the insulation
resistance at 10 minutes by the resistance at 1
minute as shown in the formula:
PI =
IR(10 min)
IR(1 min)
In general, insulators that are in good condition will show a high polarization index while
insulators that are damaged will not.
Unfortunately, most insulating materials recently developed (last 20 years) do not easily
polarize. For example, the newer epoxy resins
do not readily polarize. As recommended in
IEEE 43-2000, if the one-minute insulation resistance is greater than 5000 MOhms, the PI
measurement may not be meaningful.
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The Dielectric Absorption (DA) test is essentially a short-duration PI test and is usually
intended for smaller motors. Larger motors
whose insulation does not easily polarize are
also good candidates for the DA test. Other
than the shorter test time, all other principles
are the same as the PI test, explained in the
previous section.
While the PI test is recommended only for
motors 200 horsepower or greater, the DA test
is often useful for motors in approximately the
50 to 200 horsepower range. In the situation
where PI ratio may not be meaningful, the Dielectric Absorption (DA) is widely used. The DA
is the IR value at 3 minutes divided by the IR
value at 30 seconds as the formula states:
DA=
IR(3 min)
IR(30 sec)
The motivation for doing a DA test is to reduce the test time from 10 minutes to 3 minutes. To date there are no standardized accepted values for the DA test; however, useful
information can be obtained by trending the
DA values and graph over time.
The PI and DA tests find insulation embrittlement (deterioration) along with moisture and
particulate contamination.
High Voltage Tests
DC High-Potential (HiPot ) test
The DC High-Potential (HiPot) test consists of
applying a DC voltage to the windings of the
machine, same as a Meg-Ohm/PI test, but at a
higher voltage. The intended purpose of the DC
HiPot test is to prove that the ground wall insulation system can withstand a high-applied
voltage without exhibiting an extraordinarily
high leakage current. Therefore, the DC HiPot
test is often called a proof test. The observed
insulation resistance or leakage current is recorded and compared to acceptable limits. If
the insulation fails the DC HiPot test, the insulation to ground is determined to be unreliable.
Knowledge of the real behavior of insulators/resistors, not just ideal resistors, will help
the operator to test the winding insulation to
a point before insulation breakdown. For an
ideal resistor, good or poor, as the voltage is
increased, the leakage current will increase
proportionately. In real world applications, insulation resistance rarely behaves in this manner. Instead, the current in a typical resistor
will increase proportionately with voltage until the voltage is within as little as five percent
of breakdown voltage. Just before insulation
breakdown, the current will rise faster than the
voltage. At still higher voltages, the insulation
will completely breakdown and the current will
rise extremely fast. The key to DC HiPot testing
is to look for leakage current that is rising faster
than the increase in voltage that is applied to
the winding.
The HiPot test is considered a mainstay of
motor testing. A HiPot test can be performed
in one of two ways, AC or DC. The AC HiPot
test brings the entire motor winding up to the
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IEEE test voltages
same potential. Since all the windings are at
the same potential, there is no turn-to-turn, or
phase-to-phase insulation stress. There is uniform voltage stress applied between the winding insulation and the ground wall, throughout
the entire winding.
The HiPot test verifies groundwall insulation
integrity.
The Step Voltage test is similar to a HiPot in
that it looks at the integrity of the groundwall
insulation; however, at a less rigorous way. It is
performed to a voltage of what the motor typically sees during starting and stopping. The test
voltages are governed by IEEE and are posted
below for reference.
The DC voltage is applied to all three phases of the winding, raised slowly to a preprogrammed voltage step level, and held for a predetermined time period. It is then raised to the
next voltage step and held for the appropriate
time period. This is continued until the target
test voltage is reached. Typical steps for a 4160-
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Volt motor are 1000-volt increments, holding
at one-minute intervals. For motors less than
4160, the step voltages should be 500 Volts.
Data is logged at the end of each step. This is
to ensure the capacitive charged, polarization
current is removed, and only real leakage current remains, thus providing a true indication
of the groundwall insulation condition. If at any
point the leakage current (IµA) doubles, there is
an indication of insulation weaknesses and the
test should be stopped. If the leakage current
(IµA) raises consistently less than double, the
motor insulation is in good standing.
The Step Voltage test is necessary to insure
the ground wall insulation and cable can withstand the normal day-to-day voltage spikes the
motor typically sees during operation.
Whereas the Meg-Ohm/PI/ HiPot tests are
used to detect ground wall insulation weak-
ness, the Surge test is used to find turn-to-turn
insulation weakness. Motor winding insulation failures often start as turn-to-turn failures,
which eventually damage the ground wall insulation and lead to catastrophic failure. Surge
testing can detect the early stages of a problem
before it becomes severe.
The surge test consists of applying a fast rise
time, high current impulse to a winding. This
fast-rise time impulse will induce a voltage difference between adjacent loops of wire within
the winding.
If the insulation between the two loops of
wire is damaged or somehow weakened, and
if the voltage difference between the wires is
high enough, there will be an arc between the
wires. This arc shows up as a change in the
surge waveform.
The surge test is performed with an impulse generator and a display to observe the surge
waveform in progress. The
surge waveform is the voltage present across the test
leads of the instrument during the test. The indication
of a weak insulation is a
shift to the left, and/or a decrease in amplitude of the
waveform when the arc between loops of wire occurs.
The wave pattern observed during a Surge test
is directly related to the
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coil’s inductance. There are other factors that
influence the wave pattern, but inductance is
primary. The coil becomes one of two elements
in what is known as a tank circuit or an LC-type
circuit made up of the coils inductance (L) and
the surge tester’s internal capacitance (C).
The inductance (L) of a coil is determined by
its geometry, number of turns of wire and the
type of iron core. The frequency of the wave
pattern is approximated by the formula:
Frequency =
1
2π LC
This formula implies that when the inductance decreases, the frequency will increase.
A surge test can detect a fault between turns
by observing a jump in the resonant frequency
of this LC tank circuit. If the voltage potential is
greater than the weakened dielectric strength
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of the turn insulation, the current will find the
quickest path to ground, bypassing the weakened turns. In effect, the number of turns in the
coil is reduced. Fewer working turns reduce the
inductance of the coil and increase the frequency of the ringing pattern from the surge.
The voltage or amplitude of the surge wave
pattern is also reduced due to the decrease in
inductance of a coil with a fault between turns.
It is determined by the following formula:
Voltage = L
waveforms be to identify a bad coil? The Error
Area Ratio (EAR) was developed to answer that
question. The EAR values give a quantitative
number to how different two waveforms are.
EAR is defined as:
Npts
EAR1− 2 =
∑ Abs( F
i =1
Npts
When the insulation between turns is weak,
the result is a low energy arc and a change in
inductance. When this happens, the wave pattern becomes unstable – it may shift rapidly to
the left and right, and back to the original position. In modern surge testers, the instrument
will automatically register the fault, stop the
test and inform the user of the fault.
Added Capabilities of the Surge Test –
The Error Area Ratio
When testing three phase motors, the waveforms of the three phases can be compared
to each other. They should all be virtually the
same: same shape, same zero crossings, and
same amplitude. In practice; however, the three
waveforms will not be exactly the same. There
will be slight differences in the physical windings themselves as one phase is wound over
another. However, how different should two
(1)
− Fi ( 2 ) )
∑ Abs( F
j =1
di
dt
i
(1)
j
)
Where:
F (1) = Data points representing waveform 1.
F (2) = Data points representing waveform 2.
EAR 1-2 = error area ratio of waveform 2 with
respect to waveform 1.
If two waveforms are exactly the same, the
EAR value will be zero. Two waveforms that are
phases of a motor. A second application is to
use the EAR formula as a way to compare the
surge waveforms from a single lead or phase
to itself. This application of the EAR is called the
Pulse-to-Pulse EAR (abbreviated PP-EAR).
To explain the PP-EAR, recall that weak insulation turn-to-turn is identified by a shift to the
left of the surge waveform as the test voltage
is slowly increased. On a good coil, the waveforms from consecutive pulses would appear
almost the same – the only difference being the
increases in amplitude as the test voltage increases. On a weak coil, the consecutive pulses
would look nearly the same until an arc occurs.
At this voltage, the whole waveform shifts to
the left and possibly drops in amplitude.
Consider what a pp-EAR calculation of two
consecutive pulses could look like as voltage
increases. Since the amplitudes of the two
A characteristic of partial discharge is that
it requires a minimum voltage be met in order
for a discharge to take place.
almost exactly the same will have EAR values
of 3-4%. Waveforms with obvious separation
will have EAR values greater than 10%. This application of comparing one phase of a winding
to another is called a Line-to-Line EAR (LL-EAR).
The LL-EAR application above is used to compare two waveforms from two leads or two
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waveforms are different, there would be some
EAR value calculated, possibly around 2-5%.
Now consider doing the EAR calculation on the
pulse just before weak insulation is detected
and again on the pulse just after the detection.
The EAR value would jump to a significantly
higher value (10 to 100%).
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Conclusion
With the ever-increasing demands of maintenance professional’s time, budget and equipment to keep electrical rotating machinery
operating effectively understanding the failure
modes and tests available is highly important.
Having a unified approach to testing is the first
step in reaching goals to lessen reactive maintenance and unscheduled downtime.
Failure modes in electrical equipment are
broken into four major categories. Thermal,
electrical, mechanical and environmental issues cause electric motors to fail prematurely.
For this paper, we focused on the electrical aging processes. These include overheating due
to under or over-excitation, manufacturing
defects, winding ground faults in core slots,
broken rotor bars and short circuit rings, stator
winding electrical discharges, surface tracking
and moisture absorption, system surge voltages, transient overvoltages and high resistance
connections to name a few.
In order to investigate insulation and motor
circuit issues, a well-rounded battery of tests,
both high and low voltage should be performed on a regular basis. These tests include
inductance, impedance, capacitance, phase angle, coil resistance, meg-ohm, PI, DA, DC HiPot
or step voltage and surge testing. These tests
used together examine the health of the turnto-turn and ground wall insulation and circuit
issues within motors.
Having the tools available to perform a compre-
Failure Modes found by each test.
Static Test
What Is Being Evaluated
High Resistance connections
Resistive Balance Testing
Internal Shorts
Inductive Balance Testing
Impedance
Phase Angle
Frequency Response
Hard Turn to Turn Faults
Improper Turn Count
Misconnection of Winding
Ground Fault Determination
Meg-Ohm Testing
Particulate/Moisture Contamination Level
Insulation Embrittlement/Deterioration
Polarization Index Test/
Dielectric Absorption Test
Moisture Contamination
Particulate Contamination
Capacitance Testing
Moisture/Particulate Contamination
Step Voltage Test
Ground Wall Insulation Integrity
Turn-to-turn Insulation Integrity
Surge Testing
Motor circuit issues: shorts, opens, reversed coils
hensive predictive maintenance program is highly important in this age of increasing demands
and decreasing budgets. The maintenance professional has a myriad of tools available, but the
true task is picking the best tools to receive the
greatest benefit for the plant or operation.
References
[1] EPRI Handbook to Access Rotating Machines Insulation Condition, Volume 16, November 1988.
[2] IEEE Std 43-2000: Recommended Practice
for Testing Insulation Resistance of Rotating
Machinery
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Special Report: Electrical Testing | Page 11
[3] IEEE Std 95-2002: Recommended Practice
for Insulation Testing of AC Electric Machinery (2300 V and Above) with High Direct
Voltage
[4] IEEE Std 522-1992: IEEE Guide for Testing
Turn-to-Turn Insulation on Form-Wound
Stator Coils for Alternating-Current Rotating
Electric Machines
[5] IEEE Std 1415-2006: IEEE Guide for Induction Machinery Maintenance Testing and
Failure Analysis
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