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 Apparatus Service Association (EASA). The best practices it provides for mechanical repair, rewinding, and testing help apparatus rebuilders maintain or enhance the energy efficiency and reliability of both AC and DC motors and generators. This article focuses strictly on the electrical aspects of AC machine repair that ANSI/EASA AR100-2010 prescribes, as well as their importance for end-users.
Compared to the 2006 edition, ANSI/EASA AR100-2010 contains more than three dozen changes. Many of these are best practices for maintaining motor efficiency that were identified during a comprehensive “rewind study” published in 2003 by EASA and the Association of Electrical 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 concisely describes “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 also be assured that repairs will be made in accordance with a recognized American national standard. 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 and recording the nameplate data. By reviewing this data, the service center can ensure the machine is suited for its application — and that repairs will maintain its original ratings. AR100 also recommends that the service center determine the root cause of failure and actions that can help prevent a recurrence. This requires careful inspection and testing of the machine before repairs are made.
While some of the good practices in AR100 may seem inconsequential, their combined effect establishes the document as the good practice repair standard for motors and generators. It also has efficiency as a strong underlying theme, even though it was not specifically written to maintain or improve motor or generator efficiency.
Rewinding
AR100 concisely describes the requirements for a good practice rewind in just two pages, beginning with inspection of the windings and rotor squirrel cages. It’s easy to forget that the rotor is an electrical component — the rotating secondary of a transformer, with the stator being the primary. This is important, because defective rotor bars or end rings could reduce output torque or cause vibration.
Winding data. Exact duplication of the original winding is crucial to maintaining motor performance and energy efficiency. Thus, as a preventive measure, AR100 recommends recording and checking the accuracy of the “as-found” winding data before destroying the old winding. In this regard, it also recommends that in the new winding the average length of the coil extensions should not increase — and that the cross-sectional area of the conductors should be the same (or increased, if possible). Following 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 discs called laminations, each of which is insulated on all surfaces and has a circular opening for the stator bore. Notches around the circumference of the opening form slots to hold the winding. If shorts develop between the laminations, circulating currents will increase stator heating and losses.
AR100 provides good practices for core inspection and testing, with a focus on detecting core degradation, such as shorted laminations. For example, it prescribes 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 (not after) repair — or worse, after the customer places the repaired machine in service. Read More
