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A Systematic Approach to Ac Motor Repair

Tom Bishop, PE, Electrical Apparatus Service Association - Plant Engineering

Figure 1: A random-wound stator damaged by contact with the rotor. (All images courtesy of EASA)The only national standard for repair of motors and generators is ANSI/EASA AR100-2010: Recommended Practice for the Repair of Rotating Electrical Apparatus (AR100). It provides best practices for mechanical repair, electrical repair including rewinding, and testing that help apparatus rebuilders maintain or enhance the performance, reliability, and energy efficiency of ac and dc motors and generators.

The focus here is on the electrical aspects of ac machine repair that this standard prescribes, and that form the basis of EASA’s new service center accreditation program.

Many of the good practices in AR100 that help maintain motor reliability and efficiency were identified through a comprehensive rewind study that was published in 2003 by EASA and the Association of Electrical and Mechanical Trades (AEMT), a United Kingdom-based service center association.

One value of AR100 for end users is that it describes “good repair practices” in just 22 pages. Another is that by requiring service centers to comply with these practices, end users can be sure repairs will conform to the requirements of a recognized American national standard. Further, the good practice recommendations in AR100 cited in this article are mandatory requirements in the EASA accreditation program. End users who choose EASA-accredited service centers also have the assurance of a third-party audit that these requirements will be met.

Figure 2: Open rotor bars detected by visual inspection.Rewinding

AR100 concisely states the requirements for a good practice rewind in only two pages, beginning with inspection of the windings (Figure 1) and squirrel-cage rotor bars and end rings. Since the rotor is an electrical component—the rotating secondary of a transformer, with the stator being the primary—defective rotor bars or end rings (Figure 2) could reduce output torque or cause vibration.

Winding data. Exact duplication of the original winding characteristics is crucial to maintaining motor performance, reliability, and energy efficiency. AR100 therefore recommends recording and checking the accuracy of the “as-found” winding data before destroying the old winding. It also advocates keeping the cross-sectional area of the conductors the same (or larger, if possible) in the new winding, and not increasing the average length of the coil extensions. These good practices will maintain or reduce winding resistance and losses, thereby maintaining or increasing winding life and energy efficiency.

Stator core testing. Stator cores consist of a stack of thin steel laminations that are insulated on all surfaces and have a circular opening for the bore. Evenly spaced notches around the circumference of the bore form slots to hold the winding.

The good practices for core inspection and testing in AR100 focus on detecting core degradation (e.g., shorts between laminations cause circulating currents that increase stator heating and losses). Among them are loop or core testing before and after winding removal, investigation of any increase in core losses, and repair or replacement of damaged laminations. This helps identify a faulty core before repair—or worse, after the repaired machine is put in service.

Winding removal. AR100 gives special directions on how to remove or strip the old windings from the stator core without damaging the laminations. For instance, it recommends first thermally degrading the winding insulation in a temperature-controlled oven, while closely monitoring the temperature of the part (typically the stator). The accreditation program goes beyond this recommendation and provides a specific temperature limit of 700 F (370 C). This helps prevent damage to the stator core when the windings are removed.

Figure 3: Class H random coils being made on a semi-automated winding machine.Insulation system. AR100 recommends that the new winding’s insulation system be equal to or better than the original, and use only compatible components. Service centers typically achieve the “better than” option by using class H systems (180 C) for random windings (see Figure 3) and class F systems (155 C) for form coil windings. Most original manufacturers use either class F (155 C) or class B (130 C) random windings and class B (130 C) form coil windings.

Rewind procedure and slot fill. Regarding the rewind process, AR100 states that the new winding should have the same electrical characteristics as the original. This is best accomplished by copy rewinding. This requires using the same size conductors (wire cross-sectional area), the same number of turns per coil, and the same coil dimensions as the original.

One good practice in AR100 that can improve efficiency is to increase the wire cross-sectional area. This increases conductivity and reduces losses. Another is to reduce the average length of coil turns, which reduces winding resistance and losses.

Guidance on how to repair rotor squirrel cage and amortisseur windings reinforces the need to maintain the machine’s original performance characteristics. This requires three things:

  • Rotor bars fit tightly in the core slots.
  • Bar-to-end ring connections are welded or brazed.
  • The rotor cage retains its original electrical characteristics and can withstand normal thermal and mechanical forces.

Winding impregnation. When applied properly, the varnish/resin treatment binds winding components tightly together while ensuring good heat transfer from the winding to the stator core and cooling air. AR100 therefore stresses the importance of winding impregnation practices that include preheating the stator winding; selecting a varnish/resin with an adequate thermal rating; and using a treatment that’s both compatible with the insulation system and suitable for the application environment.

Testing and inspection

The good practice procedures in AR100 build quality into the repair. To verify the machine’s ability to perform in accordance with its nameplate rating, for example, the document recommends careful inspection, followed by winding resistance, surge comparison, and high-potential testing. As explained later, these procedures may detect faults or anomalies that could cause premature winding failure.

Inspection. AR100 recommends that the windings and insulation system be carefully inspected before insulation resistance, surge comparison, or high-potential tests are conducted. The main purpose (and benefit) of doing so is to detect and correct existing damage that might escalate under test and possibly destroy a new or refurbished winding.

Figure 4: Use of a digital megohmmeter to check winding insulation resistance.Insulation resistance test. Following inspection, testing begins with the insulation resistance test (see Figure 4). 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 one minute. This is long enough for insulation dielectric stress conditions to begin to stabilize, which results in repeatable test values.

AR100 recommends testing the insulation resistance of the winding prior to high-potential testing (which could damage or destroy a winding with weak ground insulation). The document includes acceptable test ranges for various machine ratings, as well as minimum insulation resistance values, corrected to 40 C. If a winding doesn’t meet these minimum values, a high-potential test should not be performed.

Surge comparison tests. Whereas insulation resistance tests apply only to the ground insulation system, surge comparison tests can detect shorts within the winding, such as turn-to-turn, coil-to-coil, or phase-to-phase. AR100 suggests a surge comparison test level of two times the circuit rating plus 1000 V. This breaks new ground, because this criterion isn’t dealt with specifically in other standards.

High-potential tests. High-potential testing stresses the insulation system of the winding conductors to ground, so AR100 cautions against using it without first obtaining acceptable inspection and insulation resistance test results.

The standard provides test levels for new, reconditioned or not reconditioned windings, as well as comprehensive tables illustrating ac and equivalent dc test voltages. Among its advantages, the dc high-potential test requires an instrument with a much smaller capacity than the ac version. It therefore does less damage if a failure occurs.

For a new winding, the test level is the maximum value (100%) given in the tables. After machine assembly, the test level is 80% of the maximum. Both test levels are limited to one-time tests of a winding. That is, to prevent insulation damage, 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 suggests testing at 65% of the maximum (new winding) level. This is another example of a recommended practice that other standards don’t address. For windings that haven’t been reconditioned, the document recommends limiting testing to insulation resistance tests—a good practice that could prevent a winding failure under test.

No-load testing. Following repair and assembly, a motor is normally no-load tested. AR100 provides details on tests that should be performed at this critical point. For example, the exact operating speed should be checked, typically with a digital tachometer.

Instrument calibration. The testing section concludes by stressing the importance of another good practice—having instruments calibrated to a national standard at least annually. This helps users avoid such issues as a winding failure due to a high-potential tester that outputs a higher voltage than indicated.

Although this article describes only the electrical aspects of ac machine repair, the ANSI/EASA AR100 standard also provides good practices for dc machine repair, as well for mechanical repair of rotating electrical apparatus. By specifying that apparatus rebuilders follow the procedures in AR100, end users can be sure of receiving quality repairs that are made in accordance with a recognized American national standard.

Thomas Bishop, PE, is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, Mo.; 314-993-2220; 314-993-1269 (fax); easainfo@easa.com. EASA is an international trade association of more than 1,900 firms in 62 countries that sell and service electrical, electronic, and mechanical apparatus. For more information, visit www.easa.com.

The Bottom Line:

  • The ANSI/EASA AR100 standard provides a baseline for proper repair of ac motors. The standard is concise, covering just 22 pages.
  • Following good motor repair practices will ensure repairs are made with acceptable American standards.
  • The standard covers proper rewind, testing, and inspection procedures, and can also be applied to dc motors and rotating electrical equipment.

Key Words

EASA accreditation

More information about the EASA-accredited service centers for ac machine repairs will have the added assurance of independent, third-party audits that all requirements will be met. To view or download this standard, go to www.easa.com/energy. Information about the EASA accreditation program is also available at www.easa.com/accreditation.

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