CR4 - The Engineer's Place for News and Discussion®
Login | Register for Engineering Community (CR4)

Previous in Forum: Universal Voltage Ballast for T12 Flourescent Bulbs   Next in Forum: question

Comments Format:


Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.


Rating Vote:


Join Date: Dec 2008
Posts: 3

motor winding status

12/20/2008 12:00 AM

How we can measure the healty condition of AC & DC motor windings

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Join Date: Dec 2007
Location: Cypress Calif
Posts: 741
Good Answers: 24

Re: motor winding status

12/20/2008 2:15 AM

You will need a meter capable of measuring resistance, to do the job properly, however a simple continuity tester is useful if you do not have access to in to a meter capable of measuring resistance.

The procedure is pretty much the same for an AC or a DC motor, the only difference that an AC motor will have more than one winding, or coil.

You will need to measure continuity through the winding, also you will need to check and see if any of the windings are shorted to ground.

For an AC motor, or a DC motor with two power connections (negative and positive) you could check for ground by taking one lead of your meter, or of your continuity tester to the power connection to the motor windings, the other lead makes the connection with the motor case, there should be no continuity, and your meter should register virtually infinite resistance. If you have continuity between the windings and the motor case you're windings are grounded, and the motor is no good.

If the DC motor has a single power connection, a typical example would be a automotive starter motor, the continuity tester is of limited use, you will need a meter capable of measuring resistance, since resistance varies with motor design and specifications , without detailed specifications on the motor, you will be looking for some resistance,if the meter indicates low resistance (less than .2 ohm, only a guess without specifications) the winding is suspect, if no resistance is indicated there is a shorted winding.

With either a AC or DC motor you will need to check continuity through the windings. AC motors will have two or three power connections there should be continuity between all the power connections, if you're checking resistance it, will vary with the motor, if you do not have the specifications for the motor the resistance through the windings for a three phase motor can be approximated If the motor has a locked rotor amp rating, by dividing the the supply voltage by the locked rotor amp's.

For example: R=E/I using a 120 V motor as an example with a locked rotor amp rating of 10 amps, 120/10= 12 Ohm's .

Single phase AC motors generally will have a start wanting, if the start winding comes out of the circuit, the same technique can be used to determine the run windings resistance, however this technique will not work for a permanent split phase capacitor motor. For a permanent split phase capacitor motors or DC motors without going into detail ,the easiest way is just to make sure there is continuity through the windings.

"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man" George Shaw
Register to Reply
Safety - Hazmat - New Member Safety - ESD - New Member Engineering Fields - Transportation Engineering - New Member Popular Science - Evolution - New Member Technical Fields - Procurement - New Member Hobbies - Target Shooting - New Member Popular Science - Cosmology - New Member Engineering Fields - Architectural Engineering - New Member Technical Fields - Marketing/Advertising - New Member Engineering Fields - Food Process Engineering - New Member

Join Date: Dec 2005
Location: Mariposa Ca
Posts: 5869
Good Answers: 114
In reply to #1

Re: motor winding status

12/21/2008 12:17 AM

It all looks good, until you suggested using a simple ohm's law calculation for a AC winding. The combination of 60Hz & an inductor [winding] yields impedance not resistance. An ohm reading will be close, not accurate though.

megger or ohm meter only help if you have an open, a short or a known good winding to compare to. A clamp on is good too. you can check against the nameplate amps. Use a stethoscope to listen for bad bearings. Check the temp of good motors [touch em] so you have a baseline. If it smells burnt it's probably been hurt.

Register to Reply

Join Date: Dec 2007
Location: Cypress Calif
Posts: 741
Good Answers: 24
In reply to #3

Re: motor winding status

12/21/2008 12:48 AM

Point taken and granted, as far as it being impedance versus straight resistance, and certainly there would be a little difference, I was merely suggesting it is being close enough to put you in the ballpark.

I have checked winding resistance by using locked rotor rating as described, and found it to be very close. Most of the time motor windings are either open or or shorted to ground, I may have found one flaky winding in the last 20 years using the describe method, but most of the time is just an exercise in academia.

Good point about the heat and the smell, if you've got a hot smelly motor the meter readings are usually just something to put on the death certificate.

"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man" George Shaw
Register to Reply
Panama - Member - New Member Hobbies - CNC - New Member Engineering Fields - Marine Engineering - New Member Engineering Fields - Retired Engineers / Mentors - New Member

Join Date: Dec 2006
Location: Panama
Posts: 4289
Good Answers: 213
In reply to #3

Re: motor winding status

12/21/2008 1:05 AM

A megger is a good tool, if used properly. For testing a motor, one needs to test it at intervals (once a year for a new motor, more frequently as the motor ages), and look at trends. Done properly, one can determine when it is time to dip and bake the windings before one encounters the "burned and smelly" condition. Not all that important for smaller motors, but a real money saver for larger ones (and for generators...)

Register to Reply

Join Date: Dec 2007
Location: Cypress Calif
Posts: 741
Good Answers: 24
In reply to #5

Re: motor winding status

12/21/2008 1:17 AM

In the HVAC industry, the majority of the motors we use are either relatively small, or in the case of semi- hermetic compressors that are large horsepower it would be impractically expensive to remove them from the machine until failure.

It sounds like a megger would be very useful in plant maintenance with large motors, in preventative maintenance. So just a curiosity question, what is the minimum size motor that you consider it practical to perform the type of maintenance you've suggested?

"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man" George Shaw
Register to Reply
Safety - Hazmat - New Member Safety - ESD - New Member Engineering Fields - Transportation Engineering - New Member Popular Science - Evolution - New Member Technical Fields - Procurement - New Member Hobbies - Target Shooting - New Member Popular Science - Cosmology - New Member Engineering Fields - Architectural Engineering - New Member Technical Fields - Marketing/Advertising - New Member Engineering Fields - Food Process Engineering - New Member

Join Date: Dec 2005
Location: Mariposa Ca
Posts: 5869
Good Answers: 114
In reply to #5

Re: motor winding status

12/21/2008 2:31 AM

Trending is certainly the key.

The more familiar you are with your equipment the easier it is to recognize when something is amiss. take some time to observe proper operation.

meggers [insulation testing], current readings are great tools, but only if you have context.

Register to Reply
Anonymous Poster

Re: motor winding status

12/20/2008 11:02 PM


Register to Reply

Join Date: Nov 2008
Posts: 158
Good Answers: 1

Re: motor winding status

12/21/2008 7:25 AM

The best resource will be the manufacturer's specs. The procedure (and general values) are as follows. Test each winding to ground; value should be greater than 15 Mohms. Test each winding; values should be equal + - 5%. All performed with a good quality and calibrated Megohm Meter.

Register to Reply
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 16547
Good Answers: 183

Re: motor winding status

12/21/2008 8:04 AM

You need a Megger for the condition of the insulation and a micro ohmmeter for the coil resistances. The motor needs to be identified from new and the measurements made and logged. Repeat at least once a year. That way if insulation starts to drop for example, you know well in advance of a "REAL" problem.

Bearings are also a point that need to be checked, noise is an indication of problems. Maybe there is some sort of device around that you can place a probe on the bearing cap and make a recording for analysis. (I am speaking here as though I know what I am talking about, I really don`t......but something along those lines may be available today!)

To sin by silence, when we should protest, makes cowards out of men. Ella Wheeler Wilcox
Register to Reply

Join Date: Apr 2007
Posts: 26
Good Answers: 3

Re: motor winding status

12/21/2008 12:35 PM

I have found that- In the automotive industry- an Ohm meter that is capable of accurately reading the resistance of motor windings in a starter motor, alternator or generator is a very expensive tool. A tool capable of measuring these small resistances must be 10 times more accurate than the least significant figure, and Ohm meters capable of of measuring typically .5 to 6 ohms is not an off-the-shelf meter.

There are better ways to confirm motor windings health. Although a simple Ohm meter is a good tester of continuity and simple shorts, a "Growler" is an armature specific test tool. I have an ancient growler made by SUN automotive. On it is a magnetic test bed in the form of multi-laminated steel table built in a fashion to allow the cylindrically shaped armature to rest in its "V" shaped bed. When turned on, a 60 cycle current is generated in the magnetic test bed that induces magnetism in the armature. A swing-out two probe test arm contacts the commutator, and a range adjustment potentiometer is adjusted to a meter that indicates each winding's current output as the armature is rotated in the test bed. A % difference of one coils reading over another indicates one of many possible shorts or incapacities. I watched a reliable rotating electric repair facility test the windings in an alternator rotor without a growler by placing a known wattage light bulb ( similar in amperage draw to the draw of the motor)in series with a coil and watch the glow of the lamp. Knowing the brightness of a known good rotor gave indication of the rotors condition under it's typical load.

If you say that you can, or say that you cannot, you are right either way.
Register to Reply

Join Date: Jun 2006
Location: Ottawa Canada
Posts: 1981
Good Answers: 118

Re: motor winding status

12/21/2008 2:36 PM

A growler.

If it was easy anybody could do it.
Register to Reply

Join Date: Apr 2007
Posts: 26
Good Answers: 3
In reply to #11

Re: motor winding status

12/21/2008 4:54 PM


you did not say "Growler?" with a question mark. Perhaps you do know what this ancient diagnostic tool is? Here is a brief. The reason it is called a growler, is because 60 cycle induced magnetism causes the armature being tested to jump up and down (very slightly) in the inductive bed. During this activity, the armature "growls" or hums a 60 cycle tune as it jumps or vibrates at that frequency. Another test indicating the condition of the windings is the blade test. Attached to this 1949 machine by a bead chain is a piece of metal similar in size to a hacksaw blade. When rotating the armature in the growler's bed, the thin magnetic blade clings to the open face of the armature with the same 60 cycle buzz. As the armature is rotated, if a shorted winding, or any of a number of incorrect magnetic anomalies is present, the blade will jump erratically, like a dousing stick. On the other hand, if you meant to say "Growler!"- In a world where everything has already been done- where all thoughts have been thought- and all invention already been invented, or at least considered- I find that studying an old technology is "hands -on useful, and often leads to the pieces of the puzzle that are missing. In this case, "feeling " and "hearing" a magnetic field is a reliable diagnostic tool. I would not be surprised if some major motor designer uses its modern day design extrapolation "magnetic reasoning imagery" right now in modeling and containing of these illusive lines of magnetic flux- that may be out of our ability to otherwise perceive given our limited senses in this bazaar quantum space/time continuum. I can give many practical examples that earn me a living- Yet I am now off topic.

If you say that you can, or say that you cannot, you are right either way.
Register to Reply

Join Date: Jun 2006
Location: Ottawa Canada
Posts: 1981
Good Answers: 118
In reply to #12

Re: motor winding status

12/22/2008 8:04 PM

What are you on about?

The guy asked me how to check a motor. A Megger would normally test the condiditon of the insulation in the stators, nothing beats a stethescope (or its modern equivalent, a thump tester), and nothing has ever been better than a growler with an old hack saw blade to find shorted windings in an amature.

I didn't see a need to explain how to use these tools...I presume he is literate and can read the owner's manual.

If it was easy anybody could do it.
Register to Reply
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 10661
Good Answers: 678

Re: motor winding status

12/22/2008 2:24 PM

I have two related scenarios for you depending on your conditions.

If you are testing off line motors (a motor not attached to its mechanical load) build a test bed that includes a mechanical load (fly wheel and possible brake) and power the motor through a current shunt. If you must test motors in-situ add the current shunt to the wiring of the motor. Measure the voltage vs current over the start up cycle and running time. For an AC motor one must also consider the angle between current and voltage so you will require an oscilloscope. A tachometer to read the shaft rotation is the last item you might need. Now measuring the peak speed with a known inertial load and comparing real power drawn will determine if bearing failures are occurring. Rapid fluctuations in current amplitude or phase will indicate possible winding shorts. Familiarity in the startup currents versus running currents will reveal problems with the starting circuit (bad contacts, capacitor break down, centrifugal switch failure.) Time from start to running speed will reveal status of starting windings if they exist.

The only criteria not testable by such a test bench is the condition of the motor brushes if they exist.

As I implied, if you include a current shunt in the wiring of the fixed installation you can measure the health of an operating system. The drawback is the additional wiring adds cost and depending on the choice of shunt a small percentage of peak torque available from the motor.

The simplest motor test though is to measure the operating temperature of the motor under expected load. Bad motors get hot.


"Don't disturb my circles." translation of Archimedes last words
Register to Reply
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 23759
Good Answers: 619

Re: motor winding status

12/23/2008 11:56 AM

Simply use an insulation tester in accordance with the instructions.

"Megger" is a colloquial name for this equipment in the same way as "Hoover" is a colloquial name for a vacuum cleaner (other insulation testers and vacuum cleaners are available).

"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Engineering Fields - Power Engineering - Siswanto

Join Date: Mar 2008
Posts: 73
Good Answers: 15

Re: motor winding status

12/23/2008 12:02 PM

Dear Friend

Following is a guideline that used when I performed Outage for HV Rotating Electric machinery, this paper is excluding for DC machine. but some test items can also be used to inspect of DC machine, such as:

1. Visual inspection

2. Insulation Resistance and PI

3. Winding dc resistance

4. Surge comparison test

To our CR4 administrator, how I can attached the .pdf file in this forum.......


( HV Generator and Motor Machine)

A. Introduction

The paper high light will discuss the specific mechanisms involved how to inspection , testing, and analyzing in the stator and rotor winding insulation on the HV electric rotating machine, distribution and power transformer will be discussed in the other paper issued.

Inspection, Testing and analysis method would be referred to "paper references as describe at chapter 15, pages 11".

Wound stator windings operating in utility and industrial plants have failed when exposed to the fast rise-time voltage surges coming from unbalance load, drives, breaker, lightning impulse, etc. Machine failure is due to a combination of bad luck in winding insulation process and material quality show that these surges create partial discharges (also called corona) and these discharges may eventually destroy the turn-to-turn and/or phase-to-phase insulation, resulting in premature machine failure. The paper will discuss the specific mechanisms involved in the stator winding failure due to corona and present the measurements and analysis from surge monitoring installed on many different electric machine

The fast rise-time voltage surges can lead to an electrical breakdown of the turn insulation in motor stator windings . If the turn insulation is of an insufficient thickness, or has aged in service, the insulation will puncture when a short rise-time voltage surge occurs. Punctured turn insulation allows for a very high circulating current to flow into the affected copper turn, rapidly melting the copper conductors, which, in turn, results in a consequent burning/melting of the slot liner insulation, thus leading to a stator winding ground fault .

B. Inspection and Test

This test procedure is using for rotating electric machine with all voltage range , some of the test activities as describe bellows will be applied for HV motor / generator only such as : PDA (Partial discharge analysis ,EL CID, Wedge mapping, Frequency Mapping, etc)

Test performed:

1. Winding dc resistance (applied for all voltage machine)

2. Insulation resistance and Polarization Index (PI ) (applied for all voltage machine)

3. Surge comparison test (applied for all voltage machine)

4. Dissipation power factor tangent delta (applied for HV machine)

5. Dielectric over-potential test (Hi Pot ) (applied for all voltage machine)

6. Core Test by EL CID (Electric Core Imperfection Detector) (applied for big capacity machine)

7. PD test (applied for HV machine)

8. Wedge Mapping (applied for big capacity machine)

9. AC Impedance test for rotor generator

10. Frequency Mapping (applied for HV machine)

11. Pole Drop Testing (applied on the rotor generator RR and SP type)

12. Retaining Rings Inspection (applied on the Round Rotor / RR. Generator)

13. Visual Inspection (specified for HV machine)

Test explanation

1 Rdc Winding Resistance Test

During this test, the measurement of the ohmic value between terminals winding is carried out. Given the relatively low series dc resistance of winding.

The purpose of this test is to detect shorted turns, bad connections, wrong connection and open circuit. Acceptable test result consist of three resistance value each phase to be balanced in according standard or factory measurements.

Correction to the base temperature is normal to be applied.

When the resistance,

Rt2 : of a winding has been determined by test at winding temperature,

t1 : the resistance may be corrected to a specified temperature,

t2 : the temperature test of winding

k : is 234.5 for 100% IACS conductivity copper, or 225 for aluminum, based on a volume conductivity of 62%

2 Insulation Resistance and PI Test

The Insulation Resistance test is purpose for this test is the measurement of the ohmic value between the conductors and the iron core [normally grounded]. The measurement in the Mega Ohm region after the winding is subjected to a DC voltage for 60 second. The insulation resistance test is refer to IEEE transaction No. 43 – 2000 standard.

Polarization Index [PI ] test is purpose, to evaluate how clean and dry a winding is. The Polarization Index [PI] is the ratio between the resistance reading at 10 minutes and the reading 1 minute. Winding with the low PI are usually relate to moisture and / or dirt contamination. The recommended minimum PI values with refer to EPRI EL – 5036, or IEEE 43 -2000 are:

- Class A insulation : 1.5

- Class B insulation : 2,0

- Class F insulation : 2,0

* EPRI : Electric Power Research Institute

3. Surge comparison test

The test is designed to detect incipient breakdown of the inter-turn , turn to ground , phase to phase, coil to coils, open circuit insulation in stator winding.

Surge comparison equipment will showing sinus wave form during test are applied, lead cable of surge equipment (L1, L2, L3] and ground to be connected to phase (T1, T2, T3] and ground and test switch to be positioned on T1-T2, T1-T3, T2-T3.

4. Insulation Power Factor Test

This test will be applied on the stator winding only for voltage 4000 volts or more, this test measures the dielectric loss per unit of volume of the insulation. It provides an additional measure the insulation condition, the quantity of this test is present in percent. An increase of power factor in the reading of the machine can be attributed to an increase in internal voids, and or increased slot –coil contact resistance (deterioration of the semi conductive paint)

Test Circuit, can be performed when machine complete assembled or rotor pull-out

5. Dielectric (Over-Potential or Hi-Pot) Test

This is test ascertaining if the winding is capable of sustaining voltage levels of the magnitude of the rated voltage without a breakdown of the insulation.

The test consist of applying high voltage to the winding (the three phases together or one at a time, with the other two grounded) for one minute.

** This test is distructive, not recommended when spare coil is not available at site.

Maintenance test must check first the winding conditions and year of manufacture.

6. Core Test by using Digital EL CID

Digital EL CID Tests

Electromagnetic Core Imperfection Detection (EL CID) is used to check the integrity of inter-lamination insulation for stator cores of generators and motors. A fault current occurring within the laminated stator core results in a hot spot and can cause a catastrophic failure of a generator and motor in service. Concerning to the cost, this test normally for big capacity machine only. An EL CID test can :

• detect faults and defects in the stator core;

• check effectiveness of core repairs;

• test the stator core before and after a stator rewind to check any damages caused by rewind;

• check the quality of a new stator core;

• trend the stator core condition.

In comparison with a traditional loop (ring flux) test, an EL CID test offers the following


• Uses a portable excitation transformer instead of a large excitation transformer in a loop test to considerably reduce test cost;

• Saves 30% of test time and labor cost of a loop test.

• Conducts low power testing (EL CID uses only 4% of the rated magnetic flux) to avoid further damage to fault areas;

• Does not have safety concerns to test personnel;

• Records test data by a computer for trending of test results.

• Provides an easy and in-expensive test method to check the quality of a core repair or a re-wedging operation.

7 Partial Discharge Test

Why PD test is important.

Operating generators and motors are subjected to electrical, mechanical, thermal and environmental stresses that cause aging and degradation of the stator insulation system. Deterioration of the stator winding insulation is an important factor in the lifetime of high voltage generators and motors. Partial discharges (PD) are a sign of stator insulation deterioration.

Corona activity

Corona activity normally can be found in machine having voltage of 4 KV or higher.

PD testing can identify the following generator / motor insulation problems:

(a). Semi-conductive coating/slot discharge

Slot discharge is a surface discharge that occurs between the surface of the stator bar and the core iron.

(b). Internal groundwall discharge

Groundwall discharge occurs at delamination or areas where the bonding material is missing or incompletely cured.

(c). Discharge at conductor surfaces

Deterioration of the bond between the conductor stack and the groundwall insulation is usually associated with this type of discharge.

(d). Endwinding discharge.

Discharge at the endwinding portion can be identified from the PD phase graph.

PD activity can eventually cause breakdown of generator insulation. Partial discharge (PD) testing is a recognized tool to diagnose stator insulation deterioration and the other related problems. The tests can be performed under the on-line condition (when a machine is running) or the off-line condition (when a machine is stand still).

Off line test (stand still) the test circuit is same with Tan D test (see item 4)

PD phenomenon on end winding HV machine

PD activities on winding when inspected by boroscope camera

Corona probe measurements

Corona probe testing can pinpoint partial discharge location in each slot. It supplements on-line/off-line partial discharge tests by precisely locating partial discharge to a particular coil or a spot. The corona probe can also be used as a testing tool to measure levels of PD activity. we has extensive experience to assess insulation condition based on corona probe tests and visual inspection. Corona probe testing can be performed either with the rotor in or out of the machine

Corona probe tests are used to screen out good bars (coils) and bad bars (coils) before new stator windings are installed in generators and motors. Before new bars (coils) are installed on generators and motors, bar screening tests can identify which bar (coil) has the better quality and which one has the worse one. The better bars (coils) should be installed at the high voltage terminals of the stator winding while the poor bars (coils) near the neutral terminals. Therefore risk of machine failure is considerably reduced. We provide on-site partial discharge tests to screen out poor bars (coils) to optimize arrangement of winding installation in generators and motors.

PD test by using corona probe

PD test by using C-Coupler, can be performed when machine off line or "on-line"

Following items can causing Partial Discharge activities on the winding

Top fig. Void in the winding insulation Bottom fig. Winding loose in the slot

Top fig: Insulation Delamination, Bottom fig : Winding not bottomed in slot

Winding not bottomed in slot or filler slippage out is indicate winding loose in the slot, when winding loose, its can cause semiconductive damaged due to friction between winding to slot wall.

Winding of HV machine, black color is semi conductive layer, damaged on this layer or loose contact this layer to slot wall can causing corona activities.

PD activities on the winding, when inspected by UV camera, (this figure is taken from OFIL)

8. Wedge Mapping

Wedge mapping is using for checking of wedge tight, loss or hollow, mapping can be applied by manual tapping or by WTD..

Wedge mapping using manual tapping

Wedge will be inspected by manual tapping is to tap on one side of the wedge with a small hammer and sense the amount of movement with the other hand touching the other side of wedge. Given the response the wedge condition can be classified either as tight, loose or hollow. A hollow wedge indicate a clearance exists in the radial direction between the slot and the coils. The clearance could be due to poor packing during installation of the coil or coil shrinkage. A substantial number of hollow or loose wedge in a row indicate a coil is loose and will probably tend to move within to slot during operation.

9. End Winding Frequency Mapping

Premature winding failure may occur due to excessive end winding vibration. Large turbine generators and hydro generators are most susceptible. Routine measurement of vibration amplitude and frequency is important in detecting looseness, which can lead to fatigue and failure of copper conductors and insulation systems.

10. AC Impedance / only for round rotor of generator

This test is designed to determined the existences of shorted turn in the DC excitation field winding in round rotor of generator, the individual windings are for all practical purpose inaccessible, unless the retaining rings are removed,

The Generator Field AC Impedance Test is performed on Field to determine the winding impedance at various voltage. Under some circumstances it is possible to detect the presence of shorted turn or changes in the number of shorted turn in the winding by changes in impedance at various test voltage when compared to data taken previously

11. Pole Drop Testing (for Rotor Generator only)

A pole drop test is performed by inducing a voltage into a rotor winding, measuring the voltage drop across each individual pole, and then comparing the two against one another. If the readings are off more than 5-percent, turn-to-turn shorts are suspected in the pole generating the lower reading

13. Visual Inspection

Frame and frame extension

Important information on the general condition of the machine may be obtained from a general view of the bore area and frame.


HV Bushing sometimes have passages built inside them to allow the flow of air or hydrogen for cooling purpose. It is important that the vent are inspected to see if they are uncloged, cracking, dust deposit loose part, its could be arise from any sudden load change, vibration over long time period, overheating of the lead,

End Winding and Connection Rings

Inspection Coil Cleanliness

Contamination causes degradation of the insulation by providing a medium for currents to flow on the surface of the coil insulation. The result in tracking and reduction of the insulation properties.

Blocking condition, Ties between coils tight ?, Ties between coils too dry ?

The blocking consist of the material used to separate the side of the coils at the end-winding and between end-winding connection.

Blocking coils are used to determine the clearance between the coil side at the end region, to eliminate mechanical stresses arising coil movement, that will be resulting coil touching each other at the end-winding.

Inspect blocking coil, ties, support rings, surge rings, wedges, etc

Surge ring insulation condition, Ties to surge rings tight ?, Ties surge rings too dry ?

The end-winding of electric machine are subject to substantial movement during sudden change load, vibration on normal operation. To minimize of end-winding movement the side coils are tied to a circular rings are commonly called : "surge-rings or support rings ".

Surge ring support assembly

The surge rings restrain the movement of the oils by distributing the forces by one coil onto other coils, and by transmitting them to the frame of the machine. In order to accomplished that the surge rings are supported by support assembly.

Core compression nuts and bolts

In large machine he stator core is almost always compressed by " core-compression bolt ". These are torqued to values specified by the manufacturer during the assembly.

Normally compression to around 150 psi, if part of the initial compression within the core is lost. The looseness of core is indicate by movement of nuts and bolts on the core compression against the compression plate.

Insulation Condition

Thermoplastic, resin / epoxy polyester binder, insulation system when exposed temperature will become dry and brittle, the exposed temperature may arise from overload condition, poor cooling, damage core section, negative sequence current due to unbalance, etc.

Overload condition will show-up as an external discoloration of the insulation, brittle winding will show powder accumulation arising from the movement of the shrunken coil within the slot.

The insulation degradation will be rapid growth-up if partial discharge activity are developed.

Tape separation (girth-cracking ?)

A problem common to machines with thermoplastic insulation is that the normal thermal cycling is causes coil movement in the slot, tend to generate crack within the ground wall insulation.

A tape separation is a separation of tape covering the wall insulation of the coil due to axial expansion and contraction of the conductors and the opposing forces of the slot applied to the wall insulation.

Insulation Galling/necking beyond slot

Necking is a lack of insulation which has been cracked and separated causes by thermo cycling in thermoplastic insulation.

Necking is always a sign of a weak point in the groundwall insulation. Some of the coils are made in the slot portion with a slot wrapper, while at the end-winding region a tape is used.

The interface between the two region close to the end of core is called a " scarf joint. ".

Insulation bulging into Air Duct

Bulging of the insulation right outside the slot and in the cooling vents is an indication of a soft spot , tape separation, girth cracks or asphalt migration.

Corona activity

Corona is define as the ionization of gas when exposed to an intense electric field , the discharge have a wide range of frequencies [ 40 kHz to 100 MHz ].

Corona activity in the rotating machine :

Corona at end winding ( End-winding Discharge )

Corona at inside of the insulation ( Internal Discharge )

Corona at ground-wall insulation ( Slot Discharge )

Corona at overhang of winding ( Surface Discharge )

RTD and TC

Resistance Temperature Detector ( RTD ) and or Thermocouple ( TC ) are mainly found in the winding, cooling gas flow path, cooling water paths and bearings.

Winding temperature detector normally of RTD type are located between the coils in inaccessible area.

Fan Baffle support studs

Most of large machine (gas and or air cooled) are provide an baffle to directing of the cooling from and to the fan. Fan baffle are supported stud is mounted on the tip of the core-compression finger with spaced studs are placed around the bore

The fan baffle are subjected to continuous of vibration, stud and bolts should be inspected for stresses-fatigue cracks, if broken during machine in operation they will probably causes extensive damage to the windings or rotating elements particularly the fan blade.

End Winding Support Hardware

In large synchronous machine, the end winding connection normally supported by support hardware.

Circumferential Bus Insulation

Circumferential buses is circular shape phase connection are normally separated from the rest of the winding by relatively electric clearance and commonly insulated and they are supported by a structure mostly made of steel bolted or welded to the frame.

The circumferential busses are subjected to movement by vibration, tend to separate of their insulation and together with the contaminant they will be produced electrical tracking and will result in phase to phase failure. Occasionally low megger reading due to the fault this insulation.

Bar Bottomed in Slot

Is bar seated tightly on the bottom of the slots. Coils not bottomed in slots indicate a loose-coil condition with all the problems and consequences coils movement within the slot, thus the semi conductive and wall insulation will be damaged.

A mirror or AV camera permits inspection around the end-winding and observation of the end core area and the bottom of coils.

Core inspection

All parts in generator are exposed to continual vibration, temperature change and other mechanical stresses. They may become loose, fractured or broken.

Its important to search for these abnormalities during the inspection before they develop into major troubles.In particularly all components of the core assembly are subjected to mechanical stresses due to sudden load change, such as during loss load, short-circuit, closing out of synchronism. In general condition core can be inspected with visual, beside inspection with a measurement ( EL CID, Loop).

Lamination Bent or Broken in the Bore ?

Core lamination are often damaged during removal rotor, when this occurs its become partially short-circuit, if left on this condition they may become reach excessive temperature during operation, and will resulting damage to the insulation between of lamination'

Its most important to identify lamination that are broken or in the bent, any broken of pieces lamination will got loose and most probably will damage the insulation of coils

Core Looseness

Core looseness usually will occur at the inside diameter near the ends of the core. However, it is possible for looseness to be general and/or exist on the core outside diameter.

Looseness may be accompanied by dust generation, punching and spacer movement, or small pieces of punchings flaking or cracking off.

Wedge Function and Tightening Test

The wedge is one of the main elements controlling the tightness of the coils in the slots, maintain the positive pressure on the coils reduces their movement within the slots to minimizing loss of semiconducting coating and wall insulation.

Slot electromagnetic forces are high and deterioration is common and can be severe. The slot portion of the stator winding is probably the most critical and difficult area of a generator to properly inspect. Most of the important areas of interest are enclosed behind the wedges and the core iron.

Filler Slipping Out at End

Another indication of loose coils at least radial direction is movement out of the bottom and or top slot filler .

As with the movement of end wedge large number of fillers slipping out of core by several inches may indicate a loose winding condition. However filler movement can also be result of elongation and contraction of the coils due to thermal cycles, even in tight coils.

15. Main Paper References

1. IEEE 43 – 2000, Recommended Practice for Testing Insulation Resistance of Rotating Machinery

2. DC Resistance test, using KEMA, Appendix to NEN-EN-ISO / IEC 170245 accreditation certification number L 218

3. NEMA MG 1. Motor Generator

4. ANSI / EASA AR100-2006: Recommended Practice for Repair of Rotating Electrical Apparatus

5. VDE [Verband Deutscher Electrotechniker] 0530

6. EPRI Power Plant Electrical Reference

7. IEEE Power Engineering Series , Inspection of Large Synchronous Machine

8. Maughan Engineering Consultants, Schenectady, New York , Visual Inspection of Large Electric Generator

9. Electro Mechanical Engineering Associates. Pittsburgh, PA, Generator Core Investigation

10 Hugh Zhu Ph.D.P.E. Power Tech Lab.Inc. Canada. "Insulation condition assessment of Generators and Motors"



13. S. E. Zocholl, Schweitzer Engineering Laboratories, Inc. INDUCTION MOTORS: ANALYSIS

14. Ronald J. Zawoysky. William M. Genovese. GE Power Systems.Schenectady, NY . enerator Rotor Thermal Sensitivity — Theory and Experience

15. the Swiss Insulating Work Ltd. Switzerland. "Corona Protection"

16. Von Roll Switzerland Ltd. • CH-4226 BREITENBACH "Resin rich insulation systemsfor High-Voltage rotating machines"

17 Von Roll Switzerland Ltd. • CH-4226 BREITENBACH. "VPI systemfor high voltage motors"

18 EPRI – Steam Turbine-Generatortech. Workshop and Vendor Exposition. August 25-27, 2003. Nashville, TN. JD Albright and DR Albright, Generatortech, Inc. "Generator Field Winding Shorted Turns: Moisture Effects"

19. W. Howard Moudy. National Electric Coil. 800 King Avenue, Columbus, Ohio 43212, USA "DETERIORATION MECHANISMS IN RECENT AIR COOLED TURBINE GENERATORS"

20 ISOVOLTA AG. A-2355 Wiener Neudorf. "E ISOSEAL MF 0611_MF ME 2411"

21. G. Klemner. Kinectrics.Inc. EPRI International Conference On Electric Generator Predictive Maintenance and Refurbishment , January 20 – 23, 2003. Orlando Florida. " Rotor Shorted Turn Detection and Diagnostic"

22. W Ahn,1 BS (MechEng), MESc (MechEng), PhD (MechEng). G D Robinson,2 BE Hons (MechEng), MIPENZ. "Life assessment and future inspection programme for 18-5 MnCr generator retaining rings"

23. National Electric Coil Bulletin. William G More. P.E. Engineering Manager. "Predictable and preventable on generator failure"

24 TGE Energy Services. " Gegard 600 Flex End Insulation Systems"

16. Site Works Project Experiences for the Electric Rotating Machinery's in INDONESIA

1. Stator Core Repair and Test Siemens Generator 7.5 MVA, 500 RPM, 6.6 KV. PT Branta Mulya Citereup Bogor

2. Major Overhaul incl. Insulation Condition Analysis and ELCID test on the GE Generator 49.5 MVA / 11500 V / 3000 RPM, PT. Petro Kimia Gresik

3. Stator wedge replacement and up grade to NEMA G-11, and Insulation Power Factor Tan Delta on the Toshiba Generator 250 MVA / 11500 V / 3000 RPM, PT PLN PJB Gresik

4. Re Insulated Stator winding, PD Test, EL CID Test, Insulation Power Factor Tan Delta Test, Anti Corona improvement on the winding Alstom Generator 87.5 MVA / 11000 V / 3000 RPM. PT DSS Serang

5. Rewinding Rotor Generator 7.5 MVA / 11000 V / 3000 RPM. PT Pura Barutama KUDUS.

6. Insulation Condition Analysis incl Power Tangent Delta Test on the 7 units Electric Motors with capacity from 1.5 MW up to 10 MW, 10 KV, PT Tanjung Jati Power Services Jepara Central Java

7 Off Line Partial Discharge Test and Analysis Mitsubishi Generator 100 MW, 11 KV , 3000 RPM. PT PLN PJB Unit #3 Muara Karang. Jakarta

8 Major Outage including, Off line Partial Discharge and EL CID analysis of Alstom Generator 46.5 MVA , 11 KV , 3000 RPM, PT Kaltim Daya Mandiri. Bontang. East Kalimantan

9. Etc

Register to Reply Good Answer (Score 4)

Join Date: Jun 2006
Location: Ottawa Canada
Posts: 1981
Good Answers: 118
In reply to #16

Re: motor winding status

12/23/2008 1:10 PM

wow. That deserves a plus vote.

If it was easy anybody could do it.
Register to Reply Off Topic (Score 5)
Anonymous Poster

Re: motor winding status

12/23/2008 10:45 PM

with series connection

Register to Reply
Anonymous Poster

Re: motor winding status

12/26/2008 9:18 PM

Looks like lots of comments! I will add one that is sometimes quick and useful. SIMPLE WITH A BATTERY CHARGER AND COMPASS.

A friend with a small hydroelectric plant bought a couple hundred Horsepower electric motor as scrap, yet it looked good. He asked me if it was good. Took about two minutes, with zero disassembly. HereĀ“s how. We hooked his 12 volt battery charger up to the leads, and then ran a compass around the outer periphery of the windings. It showed alternating north and south (the needle swung around) until it hit the one bad winding. We thus answered his question, and found the location of the bad winding.

Jesse Blenn

Register to Reply
Engineering Fields - Mechanical Engineering - New Member

Join Date: May 2008
Posts: 1601
Good Answers: 51

Re: motor winding status

02/15/2015 9:49 AM

Dear Mr.hkchand,

There is an instrument/equipment called SURGE TESTER. This instrument is capable of dveloping 1000 Volts (single phase)/ 2000 Volts. This 1000 or 2000 Volts is injected in to the winding independently for 100 Milli Seconds to 500 Milli seconds for the motor winding and this developes a SURGE and this surge is measured in the form of wave which can be seen on the screen and also can be recorded, and the wave amplitude finally, becomes zero.

The Surge Wave Form will vary in amplitude, frequency, and may take 5 to 7 seconds and the amplitude becomes zero. The wave form obtaine is compared with a standard wave form - say for a healthy winding, partially damaged winding, and about to get burnt out. Based on the above analysis the condition of the winding is decided.

The same equipment can be used for any say motors, transformers of 11 KV or 22 KV. The power supply for this instrument is single phase 220 plus or minus 5 Volts.

One important precaution is to be taken for this instrument is that the phase and the neutral should not be interchanged. If interchanged it will not work.


Register to Reply
Register to Reply 20 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

Andy Germany (1); Anonymous Poster (3); cwarner7_11 (1); dhayanandhan (1); DRFREON (1); Garthh (2); ODD Parts (2); PWSlack (1); redfred (1); Siswanto (1); Yusef1 (3); YWROADRUNNER (3)

Previous in Forum: Universal Voltage Ballast for T12 Flourescent Bulbs   Next in Forum: question

Engineering Newsletter Signup
Get the IHS Engineering360
Stay up to date on:
Advancements in components for batteries, diodes and LED's, and innovations in consumer and commercial products.