Repair Guidelines for Motors

8/12/2019 Repair Guidelines for Motors

1/5

Repair Guidelines for Motors & Generators

Best practices of ANSI/EASA AR100-2010: Recommended Practice for the Repair ofRotating Electrical Apparatus

Jun 22, 2011Tom Bishop, P.E., EASA| Electrical Construction and Maintenance

EMAIL

INSHARE

COMMENTS0

Best practices of ANSI/EASA AR100-2010: Recommended Practice for the Repair of RotatingElectrical Apparatus

A new edition of the only American national standard for repair of motors and generators ANSI/EASA AR100-2010: Recommended Practice for the Repair of Rotating Electrical

Apparatus (ANSI/EASA AR100-2010) was published recently by the Electrical ApparatusService Association (EASA). The best practices it provides for mechanical repair, rewinding, andtesting help apparatus rebuilders maintain or enhance the energy efficiency and reliability of bothAC and DC motors and generators. This article focuses strictly on the electrical aspects of ACmachine repair that ANSI/EASA AR100-2010 prescribes, as well as their importance for end-users.

Photo 1. A random wound stator damaged by contact with the rotor.

Compared to the 2006 edition, ANSI/EASA AR100-2010 contains more than three dozenchanges. Many of these are best practices for maintaining motor efficiency that were identifiedduring a comprehensive rewind study published in 2003 by EASA and the Association ofElectrical and Mechanical Trades (AEMT), a United Kingdom service center association.

For end-users, one value of ANSI/EASA AR100-2010 is that in just 22 pages it conciselydescribes good repair practices. It also provides six pages of supplemental information. End-users who require service centers to comply with the recommended practices in AR100 can alsobe assured that repairs will be made in accordance with a recognized American nationalstandard. The result should be a good practice repair i.e., a quality repair without shortcuts.

General guidelines

General guidelines provided by AR100 include making sure the machine has a nameplate andrecording the nameplate data. By reviewing this data, the service center can ensure the machineis suited for its application and that repairs will maintain its original ratings. AR100 alsorecommends that the service center determine the root cause of failure and actions that can helpprevent a recurrence. This requires careful inspection and testing of the machine before repairsare made.
http://ecmweb.com/forward?path=node%2F11065http://ecmweb.com/forward?path=node%2F11065http://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/motors/repair-guidelines-motors-generators#commentshttp://ecmweb.com/forward?path=node%2F11065


2/5

Photo 2. Open rotor bars detected by visual inspection.

While some of the good practices in AR100 may seem inconsequential, their combined effectestablishes the document as the good practice repair standard for motors and generators. It alsohas efficiency as a strong underlying theme, even though it was not specifically written tomaintain or improve motor or generator efficiency.

RewindingAR100 concisely describes the requirements for a good practice rewind in just two pages,beginning with inspection of the windings (Photo 1) and rotor squirrel cages. Its easy to forgetthat the rotor is an electrical component the rotating secondary of a transformer, with thestator being the primary. This is important, because defective rotor bars or end rings (Photo 2)could reduce output torque or cause vibration.

Winding data. Exact duplication of the original winding is crucial to maintaining motorperformance and energy efficiency. Thus, as a preventive measure, AR100 recommendsrecording and checking the accuracy of the as-found winding data before destroying the oldwinding. In this regard, it also recommends that in the new winding the average length of the coilextensions should not increase and that the cross-sectional area of the conductors should bethe same (or increased, if possible). Following these good practices will maintain or reducewinding resistance and losses, thereby maintaining or increasing winding life and energyefficiency.

Stator core testing. Stator cores consist of a stack of thin steel discs called laminations, each ofwhich is insulated on all surfaces and has a circular opening for the stator bore. Notches aroundthe circumference of the opening form slots to hold the winding. If shorts develop between thelaminations, circulating currents will increase stator heating and losses.

Photo 3. Core testing a stator prior to rewind.


3/5

AR100 provides good practices for core inspection and testing, with a focus on detecting coredegradation, such as shorted laminations. For example, it prescribes loop or core testing (Photo3) before and after winding removal, investigation of any increase in core losses, and repair orreplacement of damaged laminations. This helps identify a faulty core before (not after) repair or worse, after the customer places the repaired machine in service.

Winding removal. How to remove (strip) the old windings from the stator core without damagingthe laminations receives special attention in AR100. One specific practice it provides is to firstthermally degrade the winding insulation in a temperature-controlled oven, while monitoring thetemperature of the part (typically the stator), helping to prevent damage to the stator core whenthe windings are removed.

Insulation system. AR100 also recommends ensuring that the insulation system of the newwinding is equal to or better than the original and that all components are compatible. Thebetter than option is normally achievable, because service centers typically use Class Hsystems (180C) for random windings and Class F systems (155C) for form coil windings. Mostoriginal manufacturers use either Class F (155C) or class B (130C) random windings and classB (130C) form coil windings.

Rewind procedure and slot fill. Regarding the rewind process, AR100 says the new windingshould have the same electrical characteristics as the original. This is best accomplished bycopy rewinding for example, using the same size conductors (wire cross-sectional area), thesame number of turns per coil, and the same coil dimensions as the original.

Another way AR100-2010 can help end-users improve efficiency in some cases is to increasethe wire cross-sectional area, which increases conductivity and reduces losses. They can alsoreduce the average length of coil turns, which, in turn, reduces resistance and losses.

Photo 4. This rotor end ring was not properly brazed to the bars, resulting in a complete open circuitfailure.

Guidance on how to repair rotor squirrel cage and amortisseur windings reinforces the need tomaintain the machines original performance characteristics. This includes making certain thatrotor bars fit tightly in the core slots, that bar-to-end ring connections are welded or brazed(Photo 4), and that the rotor cage retains its original electrical characteristics and can withstandnormal thermal and mechanical forces.

Winding impregnation. AR100 also stresses good practices for winding impregnation. Key pointsinclude preheating the stator winding, using varnish/resin with an adequate thermal rating, andensuring that the treatment is both compatible with the insulation system and suitable for theapplication environment. Although every part of the rewind process is important, the cured

varnish/resin literally is the tie that binds the winding components together. It also ensures goodheat transfer from the winding to the stator core and to cooling air.


4/5

Testing and Inspection

Following the good practice procedures in AR100 builds quality into the repair. As an example,the document devotes an entire section to inspecting and testing repaired machines oftenprescribing multiple tests to verify their suitability to perform in accordance with nameplateratings. Recommended procedures include careful inspection, followed by winding resistance,

surge comparison, and high-potential testing. As explained below, these procedures may detecta fault or anomaly that could cause premature winding failure.

Inspection. AR100 recommends that the windings and insulation system be carefully inspectedfor damage or degradation before performing insulation resistance, surge comparison, or high-potential tests. The main purpose of this procedure is to detect and correct existing damage thatmight escalate under test and possibly destroy a new or overhauled winding.

Insulation resistance test. Following inspection, testing begins with the insulation resistance test.This test can detect an incorrect winding if the resistance value of the original manufacturer isknown; and it may detect a high-resistance connection (e.g., if one pair of leads has higherresistance than the others).

Often called a megger test (a trade name of Megger Group, Ltd.), it measures winding

insulation resistance in megohms after a constant test voltage has been applied for 1 minute.This is long enough for insulation dielectric stress conditions to begin to stabilize, which results inrepeatable test values.

Photo 5. A form coil stator being surge tested.

AR100 recommends testing the insulation resistance of the winding prior to high-potential testing;this could save a winding with weak ground insulation from a test that could cause it to fail. Thedocument includes acceptable test ranges for various machine ratings, as well as recommended

minimum insulation resistance values, corrected to 40C(click here to see Table).If a windingdoes not meet these minimum values, a high-potential test should not be performed.

Surge comparison tests. Whereas insulation resistance tests apply only to the ground insulationsystem, surge comparison tests (Photo 5) can detect shorts within the winding (e.g., turn-to-turn,coil-to-coil or phase-to-phase). AR100 provides a suggested test level for surge comparisontesting two times the circuit rating plus 1,000V. This essentially breaks new ground, becausethis criterion is not dealt with specifically in other standards.

High-potential tests. High-potential testing stresses the insulation system of the windingconductors to ground, so AR100 cautions that it should not be done unless acceptable inspectionand insulation resistance test results have been obtained.

The standard provides test levels for new, reconditioned, or not reconditioned windings, as well

as comprehensive tables illustrating AC and equivalent DC test voltages. (The AC test voltagelevel is multiplied by 1.7 to obtain the equivalent DC voltage.) Among its advantages, the DC
http://ecmweb.com/images/106ecmMFfig1.jpghttp://ecmweb.com/images/106ecmMFfig1.jpghttp://ecmweb.com/images/106ecmMFfig1.jpghttp://ecmweb.com/images/106ecmMFfig1.jpghttp://ecmweb.com/images/106ecmMFfig1.jpghttp://ecmweb.com/images/106ecmMFfig1.jpg


5/5

high-potential test requires an instrument with a much smaller capacity than the AC version.Therefore, it does less damage if a failure occurs.

For a new winding, the test level is the maximum value (100%) given in the tables. After machineassembly, the test level is 80% of the maximum. Both the 100% and 80% test levels are limitedto one-time tests of a winding. To prevent insulation damage, that means a winding may be

subjected to each test level only once in its lifetime.If subsequent high-potential tests are desired (or for reconditioned windings), AR100 suggeststesting at a 65% of maximum (new winding) level. This is another example of a recommendedpractice that other standards do not address. For windings that have not been reconditioned, thedocument says testing should be limited to insulation resistance tests a good practice thatcould prevent a winding failure under test.

No-load testing. Following repair and assembly, a motor is normally no-load tested. AR100provides details on tests that should be performed at this critical point. For example, the exactoperating speed should be checked, typically with a digital tachometer.

Instrument calibration. The section on testing concludes with another good practice instrumentcalibration. AR100 stresses the importance of having instruments calibrated at least once a year

to a national standard as well as clearly labeling them with the vendors name and calibrationdate. This helps users avoid issues such as a winding failure due to a high-potential tester thatoutputs a higher voltage than indicated.

Although this article describes only the electrical aspects of AC machine repair, the ANSI/EASAAR100-2010 standard also provides good practices for DC machine repair, as well formechanical repair of rotating electrical apparatus. By specifying that apparatus rebuilders followthe procedures in ANSI/EASA AR100-2010, end-users can be assured of receiving qualityrepairs that are made in accordance with a recognized American national standard.

Bishop is a licensed professional engineer and senior technical support specialist at the ElectricalApparatus Service Association (EASA) in St. Louis. He can be reached [email protected].
mailto:[email protected]:[email protected]:[email protected]:[email protected]

Documents

Repair Guidelines for Motors