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Finding ElEctrical FailurE ModEs oF rotating MachinEry with ElEctrical tEsting MEchanisMs

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Electrical maintenance professionals are expected to keep plant operations mov-ing 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 compre-hensive reliability program a wide variety of re-quirements 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 un-scheduled downtime is lessened.

There is myriad equipment available to the maintenance professional for electrical predic-tive 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 cir-cuit and insulation systems of electric motors. These testers include capabilities like torque and current signature analysis on the dynam-ic 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 proce-dures 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 fail-ures in one instrument. Most of these tests are governed by IEEE standards.

Failure Modes in Electric MotorsIEEE 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 re-lated and the remaining 14-22% are a combina-tion of other smaller faults.

Insulation failure modes are broken up into four motor aging categories: Thermal aging, electrical aging, mechanical aging and environ-mental aging. An effective reliability program will have the tools necessary to identify and prevent these causes prior to catastrophic mo-tor failure and unplanned downtime. For this paper, we will focus on electrical aging faults and how they affect the insulation of the motor.

Electrical AgingElectrical aging occurs when voltage across the insulation causes deterioration. This aging occurs several ways. Overheating is a major contributor to deterioration along with manu-facturing defects, winding ground faults, bro-ken rotor bars or short circuit end rings, elec-trical discharges, surface tracking and moisture absorption, system surge voltages, transient overvoltages in rotor windings, high resistance connections among others. Voltage stress, i.e.,

voltage overload, reduces insulation service life. Causes include: bus transfers, switching surges, reflected wave phenomena due to vari-able frequency drives or even volts per turn that was not properly calculated in the motor design.

Overheating Under-excitation and over-excitation of core el-ements can cause excessive heat in the core

EPRI Study

Sta tor36%

Rotor9%

O ther14%

Bear ing41%

IEEE Study

Rotor8%

Bear ing44%

O ther22% Sta tor

26%


laminate insulation. Higher temperatures re-duce the dielectric strength of the inter laminar insulation over time and causes other stresses due to expansion and relative motion. Also cir-culating 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 deteriora-tion. This heat then affects the coils, turn-to-turn and inter lamination insulation. A general

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

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 CoreLamination shorts can be introduced during manufacturing or refurbishment. Poor adhe-sion 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

metal-to-metal contact. In addition, the short-ing 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 wind-ing faults in the slot region are often high enough to melt and fuse the core lamina-tions 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

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 boundar-ies. 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 de-terioration 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 pro-cess, the more resistant the insulation is to par-tial discharge. Generally, organic materials are more susceptible to partial discharge then non-organic 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-

Having a unified approach to testing is the first step in reaching goals to lessen reactive maintenance and unscheduled downtime.


lation are caused by ac electrical stress. The surface of a stator coil in the end-winding re-gion is highly resistive; however, small con-ductive 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 oth-er moisture related problems, the resistance of the groundwall insulation is reduced and can create conductive paths especially in weak-ened or delaminated regions of the winding. These conductive paths can cause ground or phase-to-phase faults.

In synchronous machines wound rotor wind-ings are susceptible to problems with con-tamination 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 OvervoltagesTurn-to-turn failures can be contributed to ag-ing of turn insulation from exposure to power surges during start up and shut down of mo-tors. 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 weak-ened state this voltage can puncture the turn insulation-causing break down in the insula-tion. This will lead to a turn-to-turn short and

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 intro-duced into rotor windings due to line-to-line stator winding short circuits, faulty synchroni-zation, or asynchronous operation. Such tran-sient voltage, together with weakened insula-tion 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]

High Resistance ConnectionsIf a joint between two conductors is poorly sol-dered 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

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 MaintenanceInsulation systems are subjected to a wide va-riety of events that can cause possible prob-lems on a daily basis. With the progression of time, these issues turn into reality and can cause unexpected downtime of electric rotat-ing machinery. The condition of electrical insu-lation materials is often best assessed through an electrical test. Such tests are broadly divid-ed into two major categories: (1) high voltage and (2) low voltage tests. The former are de-finitive 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.

When the insulation between turns is weak, the result is a low energy arc and a change in inductance.


The later, low voltage, yields information such as an indication of moisture, dirt, or some oth-er 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 unbalanc-es in the phases. Predictive maintenance pro-grams have been developed in order to inves-tigate motor faults, find insulation weaknesses before it creates downtime and allows for root cause analysis. In order to obtain a compre-hensive 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 TestsImpedance is defined as a circuit’s total opposi-tion to alternating current flow. It is a combina-tion 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 sim-ply the resistive equivalents of the inductance or capacitance.

In an AC circuit, we add an angle to the mea-surement 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

voltage. The voltage rises first and the current rises some time later based on the level of in-ductance 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 pa-rameters. 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, Imped-ance can also be evaluated with respect to its balance in the circuit.

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 im-pedance due to the inductive imbalance created by missing turns.

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 fre-quency the larger the effective resistance of the circuit will be due to the inductor present. Ca-pacitance 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 mo-tors condition it is important to understand how they each fit into the circuit. A three-phase mo-tor 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,

When a motor is wound, the inductive balance of the circuit can be helpful in validating the connections in the circuit.


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 even-ly distributed. Each of these design and con-struction 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

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 val-ues 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 ca-pacitance of the circuit will increase. This is due to the increase of the effective surface area of the winding.

Capacitance is directly proportional to the surface area on each side of the dielectric ma-terial, in this case the insulation, being tested. Therefore, the level of cleanliness and mois-ture can be evaluated and trended over time.

When a motor is wound, the inductive bal-ance of the circuit can be helpful in validat-ing the connections in the circuit. If a coil or group of coils were misconnected, this would have an effect on the inductance in the cir-cuit. Any imbalance present in the impedance

measurement would indicate an unbalance turn count or misconnection of the windings. Since the impedance test is normally per-formed 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, mis-connections of windings and moisture and par-ticulate contamination.

The Coil Resistance test consists of injecting a known constant DC current through the wind-ing, measuring the voltage drop across the winding, and calculating the coil resistance us-ing Ohm’s law. If a coil is shorted somewhere

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 tempera-ture. Before comparing two different measure-ments, correct the measured resistance value to a common temperature, usually 25° C per IEEE 118.

Performing Resistance tests on the same mo-tor over time provides early warning signs of motor connection problems. Motors operated in conditions that allow corrosion, contamina-tion, or other physical damage may show ini-tial 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 accu-rately the voltage drop across the coil. Difficul-ties in measuring the voltage drop across the coil itself are the affects of the contact resis-tance of internal relays and the contact resis-tance 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

Lamination shorts can be introduced during manufacturing or refurbishment.


test exists to evaluate the copper winding con-ductors. 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 percent-age 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 dis-played, measured resistance values, resistanc-es corrected for temperature and the Delta Re-sistance 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 Resis-tance values not within the prescribed limits.

The Meg-Ohm test consists of applying a DC voltage to the windings of a machine after iso-lating the winding from ground. According to IEEE 43 the test, voltage is usually near the op-erating 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 insu-lation. The insulation resistance, abbreviated IR, is a function of many variables: the physi-cal properties of the insulating material, tem-perature, humidity, contaminants etc. The IR value is calculated using Ohm’s law – the ap-plied voltage is divided by the measured leak-age current. This leakage current is the current which passes from the winding through the

ground wall insulation to the motor’s steel core plus any surface leakage currents. The surface leakage currents flow through moisture or con-taminants on the surface of the insulation. To determine accurately the insulation resistance, the surface leakage must be reduced to an in-consequential level.

The Meg-Ohm test is best used for finding ground faults and the level of moisture or par-ticulate 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 or-der to measure quantitatively the ability of an insulator to polarize. When an insulator polar-izes, the electric dipoles distributed through-out the insulator align themselves with an ap-plied 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 de-creases over time and drops to zero when the insulation is completely polarized.

PI results become confusing when attempt-ing to attribute variations in the PI value to the

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:

min)1(min)10(

IRIRPI =

In general, insulators that are in good condi-tion will show a high polarization index while insulators that are damaged will not.

Unfortunately, most insulating materials re-cently 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 re-sistance is greater than 5000 MOhms, the PI measurement may not be meaningful.

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


The Dielectric Absorption (DA) test is essen-tially 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 Di-electric 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:

sec)30(min)3(

IRIRDA=

The motivation for doing a DA test is to re-duce the test time from 10 minutes to 3 min-utes. To date there are no standardized ac-cepted 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 embrittle-ment (deterioration) along with moisture and particulate contamination.

High Voltage Tests

DC High-Potential (HiPot ) testThe 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 in-sulation 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 re-corded and compared to acceptable limits. If the insulation fails the DC HiPot test, the insu-lation to ground is determined to be unreliable.

Knowledge of the real behavior of insula-tors/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, in-sulation resistance rarely behaves in this man-ner. Instead, the current in a typical resistor will increase proportionately with voltage un-til 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

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 uni-form voltage stress applied between the wind-ing 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 typi-cally 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 phas-es of the winding, raised slowly to a prepro-grammed voltage step level, and held for a pre-determined 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-

IEEE test voltages


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 cur-rent 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 with-stand 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 insula-tion failures often start as turn-to-turn failures, which eventually damage the ground wall in-sulation 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 dif-ference 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 im-pulse generator and a dis-play to observe the surge waveform in progress. The surge waveform is the volt-age present across the test leads of the instrument dur-ing the test. The indication of a weak insulation is a shift to the left, and/or a de-crease in amplitude of the waveform when the arc be-tween loops of wire occurs.

The wave pattern ob-served during a Surge test is directly related to the

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:

LCFrequency

π21=

This formula implies that when the induc-tance 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


of the turn insulation, the current will find the quickest path to ground, bypassing the weak-ened turns. In effect, the number of turns in the coil is reduced. Fewer working turns reduce the inductance of the coil and increase the frequen-cy 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:

dtdiLVoltage =

When the insulation between turns is weak, the result is a low energy arc and a change in inductance. When this happens, the wave pat-tern becomes unstable – it may shift rapidly to the left and right, and back to the original posi-tion. 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 RatioWhen testing three phase motors, the wave-forms 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 wind-ings themselves as one phase is wound over another. However, how different should two

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

jj

Npts

iii

FAbs

FFAbsEAR

1

)1(

1

)2()1(

21

)(

)(

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

almost exactly the same will have EAR values of 3-4%. Waveforms with obvious separation will have EAR values greater than 10%. This ap-plication 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 com-pare two waveforms from two leads or two

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 insu-lation 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 wave-forms from consecutive pulses would appear almost the same – the only difference being the increases in amplitude as the test voltage in-creases. 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

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%).

A characteristic of partial discharge is that it requires a minimum voltage be met in order for a discharge to take place.

< Previous Page Special Report: Electrical Testing | Page 11 Next Page >

ConclusionWith the ever-increasing demands of mainte-nance professional’s time, budget and equip-ment 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 main-tenance and unscheduled downtime.

Failure modes in electrical equipment are broken into four major categories. Thermal, electrical, mechanical and environmental is-sues cause electric motors to fail prematurely. For this paper, we focused on the electrical ag-ing 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 voltag-es, 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 per-formed on a regular basis. These tests include inductance, impedance, capacitance, phase an-gle, coil resistance, meg-ohm, PI, DA, DC HiPot or step voltage and surge testing. These tests used together examine the health of the turn-to-turn and ground wall insulation and circuit issues within motors.

Having the tools available to perform a compre-

hensive predictive maintenance program is high-ly important in this age of increasing demands and decreasing budgets. The maintenance pro-fessional 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 Ma-

chines Insulation Condition, Volume 16, No-vember 1988.

[2] IEEE Std 43-2000: Recommended Practice for Testing Insulation Resistance of Rotating Machinery

[3] IEEE Std 95-2002: Recommended Practice for Insulation Testing of AC Electric Machin-ery (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 Induc-tion Machinery Maintenance Testing and Failure Analysis

Static Test What Is Being Evaluated

Resistive Balance TestingHigh Resistance connections

Internal ShortsInductive Balance TestingImpedancePhase AngleFrequency Response

Hard Turn to Turn Faults

Improper Turn Count

Misconnection of Winding

Meg-Ohm TestingGround Fault Determination

Particulate/Moisture Contamination Level

Polarization Index Test/Dielectric Absorption Test

Insulation Embrittlement/Deterioration

Moisture Contamination

Particulate Contamination

Capacitance Testing Moisture/Particulate Contamination

Step Voltage Test Ground Wall Insulation Integrity

Surge TestingTurn-to-turn Insulation Integrity

Motor circuit issues: shorts, opens, reversed coils

Failure Modes found by each test.

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