EASA Senior Technical Support Specialist
EASA’s Recommended Practice for the Repair of Rotating Electrical Apparatus, designated as ANSI/EASA AR100, was first approved as an American National standard in 1998. Since then, it has been revised and approved four more times: 2001, 2006, 2010 and now in 2015.
A condensed outline of the review and approval process for AR100 is as follows: The EASA Technical Services Committee (TSC) reviews the Recommended Practice and proposes changes; then a canvass group approves and often comments on the TSC proposals. The canvass group has representation from service centers, end users, testing laboratories, government and those having a general interest. Per ANSI requirements, there must be balanced representation among the canvass group representatives. After the canvass group and the TSC find consensus agreement, the revised document is submitted for approval to the EASA Board of Directors. Following Board approval, ANSI is requested to approve the revision as an American National Standard. The entire process must be completed within five years following the previous revision.
The 2015 edition of AR100 contains nearly 100 revisions from the previous version. A PDF version of AR100 with the changes highlighted is available. The focus of this article will be on the more significant changes, noted in clause order, and some of the reasons for making these changes. Also noted will be explanations of the effects on the EASA Accreditation Program.
1.12 Authorization to Ship (new clause): Added a recommendation to indicate an authorization to ship for a completed repair, e.g., “OK to ship.” It is a good practice to have someone “sign off” on a repair to indicate that all repairs are complete and the finished machine is ready to be returned to the customer. The EASA Accreditation Program Checklist already included this step as a requirement.
2.6 Balancing: Changed balance quality level for machines rated above 2500 rpm to balance quality grade level G1.0; machines rated 2500 rpm and below remain at the grade level of G2.5. This change reflects an indus.try trend of both service centers and end users to provide more precise dynamic balance levels for rotating elements that operate at higher speeds. This change will affect service centers that are EASA accredited or are considering accreditation. Effective August 2016, the balance level of G1.0 for motors rated above 2500 rpm will be a requirement for the EASA Accreditation Program.
2.7 Slip Rings: Added a specific tolerance for slip ring surface finish to be between 40 and 60 micro-inches (1.02 and 1.52 microns). The previous guidance was somewhat subjective, calling for the finish to be smooth and polished.
2.8.1 Machining (of commutators): Added a specific tolerance for commutator surface finishto be between 40 and 60 micro-inches (1.02 and 1.52 microns). As with the slip ring finish, the previous guidance for com.mutators was somewhat subjective, calling for the finish to be smooth and burnished.
2.12 Air Gap of Machines: Expanded the scope of air gap tolerances. The previous edition only addressed DC machines. The revised clause provides a 10% tolerance for variation from the average for all machines.
3.1.1 Core Laminations: Clarifies that stators and armatures should be core tested before and after winding removal. Further, the revision states that cores should be tested for hot spots and losses. The primary change is the expansion in scope to include armatures; the reason is that armature cores are part of an AC circuit and may have frequencies up to and above typical AC motor frequencies. Thus the core condition can significantly affect heating and losses.
3.3 Stripping of Windings: Added temperature limits of 700°F (370°C) for cores with organic coreplate and 750°F (400°C) for inorganic coreplate. These temperature limits have been in the Technical Manual for many years. Since they have been, or should have been, standard practice, they were adopted into AR100. Also, a good practice is to assume the coreplate is organic un.less it is known to be inorganic. Thus the 700°F (370°C) should be used in almost all cases. Further, as a conservative approach, the EASA Accreditation Program prescribes using the 700°F (370°C) limit.
3.7 Field Coils: For clarity, the text of the main clause was deleted. The text was moved to the appropriate location in clauses 3.7.1 and 3.7.2.
4.2.5 Winding Surge Test (previously 4.2.6): Clarified that the winding surge test, formerly termed “surge comparison test,” applies to complete new or used windings. Previously the reference of applicability was to “winding circuits.” The recommended test level was not changed, but the applicability was clarified.
4.2.6 Interlaminar Insulation Test (previously 4.2.7): The content of this clause regarding interlaminar insulation testing was expanded to include the types of tests and parameters for evaluating cores, including maintain.ing the same test flux level within 5% for the after-winding removal test versus the before-winding removal test. The reasons for these changes were to make the clause more specific and less subjective.
4.2.7 Bearing Insulation Test (previously 4.2.8): Changed evaluation criteria for the bearing insulation test (see Figure 2), noting that there is no industry consensus for bearing insulation in motors supplied by variable frequency drives (VFDs). Although the 1 megohm minimum from the previous edition still applies to motors operating from sinusoidal (e.g., utility line) or DC power supplies, the change reflects the uncertainty regard.ing a bearing insulation test for motors supplied by variable frequency drives (VFDs). If and when there is industry consensus on a bearing insulation test for VFD supplied motors, the Recommended Practice will consider adopting such a test.
Clauses 4.2.8 through 4.2.11: All of the clauses in main clause 4.2 describe tests that are used in AR100. Clauses 4.2.8 through 4.2.11 were added to provide descriptions of the phase balance (4.2.8), polarity (4.2.9) and the dummy rotor [previously ball rotation] (4.2.10) tests (see Figure 3) that had not been previously described; and to describe the new impedance (4.2.11) test.
4.3.1 Stator and Wound-Rotor Windings: Winding resistance and surge tests should now be performed on every repair; previously they were optional. Insulation resistance testing also should be performed on every repair. This “tightening” of the recommendation brings AR100 in line with generally accept.ed industry practice. That is, almost all service centers perform insulation resistance tests and many perform winding resistance tests. Although not all service centers have surge testers, the surge test is the only winding test that checks for turn-to-turn, coil-to-coil and phase-to-phase defects or anomalies. This change will affect service centers that are EASA accredited or are considering accreditation. Effective August 2016, a calibrated surge tester will be required equipment for the EASA Accreditation Program.
Clauses 4.3.3 and 4.3.4: Insulation resistance testing should now be per.formed on every repair; previously it was an option for armatures (4.3.3) and shunt, interpole, series, compensating and synchronous rotor windings (4.3.4). The reasoning for the change to these test clauses is the same as for the insulation resistance test change in clause 4.3.1.
4.4 High-Potential Tests: As a further step toward internationalizing AR100, the term “earth(ed)” or “earthing” has been added whenever “ground(ed)” or “grounding” is referenced. This change applies throughout the document and is mentioned here because this clause contains many references to these terms.
126.96.36.199 Accessories of Machines with Windings Not Reconditioned: Added to this subclause was an acceptance criterion for insulation resistance testing of accessories. The previous text was somewhat subjective, indicating only that an insulation resistance test be performed. It is noteworthy to mention that other standards do not address a specific insulation resistance level for accessories, which this clause now provides.
4.5 No-Load Tests: Added that motors should be secured on a base plate or on a resilient pad for the no-load test. Also, added recommendations for testing AC and DC motors that operate at above base speed. The reason for the first change in this clause was to clearly bring AR100 in line with existing NEMA and IEC requirements. The second change applies to motors that are operated above base speed, such as some AC motors on VFDs with output frequencies above line frequency and DC motors with field weakening. The primary reason for checking the operation of AC motors on VFDs at their maximum rated frequency (and speed) is to confirm that vibration levels are acceptable. For DC motors with field weakening, the primary reasons are to check for acceptable (sparkless) commutation and to check that vibration is acceptable.
Table 4-5 Unfiltered Vibration Limits: Unfiltered vibration limits were revised to conform to current NEMA and IEC standards values. Further, the changes simplify and clarify the information in the table. The table continues to provide levels for resilient mounting and notes that a 0.8 multiplier is to be used for rigid mounting conditions.
As time goes on, the Technical Services Committee will continue to revise and improve AR100. Within a few years the revision process will become an active agenda item for the committee. One of the foremost goals with AR100 is to include as many good practices in it as possible. Further, when it is desired or necessary to add new good practices to the EASA Accreditation Program, AR100 is the conduit since it is the primary source document for the EASA Accreditation Program. Because AR100 is revised periodically, it is a “living document.” Changes to AR100 not only aid with the EASA Accreditation Program, its good practices and other guidance help enable service centers that use it to provide quality repairs that maintain or sometimes even improve rotating electrical apparatus reliability and efficiency.
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