Can you repair energy-efficient motors? Maybe.
By Thomas H. Bishop P.E.
April 19, 2016 - Most plant engineers and maintenance staff can attest to the reliability of standard-efficiency motors that have been repaired or rewound using industry best practices. They also know repair can cost far less than replacement, especially when the motor has special features. Despite this, some of them hesitate to have failed energy-efficient motors (NEMA Premium models, in particular) repaired because they’ve heard it degrades efficiency.
By Thomas H. Bishop P.E.
So, what’s the right answer? Is the decision to repair, rewind or replace a failed energy-efficient motor as simple and straightforward as you may have heard?
What makes a motor more energy-efficient?
Some energy is always lost (to heat, friction and windage) when motors convert electricity into useful (mechanical) work. To improve motor efficiency, manufacturers have found ways to reduce these losses—not by changing raw materials or production methods, but through design modifications: first with higher-efficiency EPAct motors and, more recently, with NEMA Premium models.
Compared with standard-efficiency motors, for example, some higher-efficiency models have longer stator and rotor cores to reduce core losses, and more copper wire area in the windings to decrease copper losses. Using the smallest fan that can keep the winding temperature within design limits also minimizes windage losses in totally enclosed, fan-cooled (TEFC) models.
Repaired motor efficiency
The mistaken view that motors—including energy-efficient and NEMA Premium models—cannot be repaired or rewound without a loss of efficiency was scientifically disproven by a study in 2003 that identified good practices that maintain the energy efficiency of repaired motors. Commissioned by the Electrical Apparatus Service Association (EASA, United States) and the Association of Electrical & Mechanical Trades (AEMT, United Kingdom), the study tested the efficiencies of motors ranging from the original EPAct level to NEMA Premium and IEC IE3 levels.
Conducted at the University of Nottingham under the direction of engineering executives from motor manufacturers in the U.S. and U.K., the study measured the efficiencies of 22 motors ranging in size from 50hp to 200hp (37kW to 150kW) before and after multiple winding burnout processes and rewinds. A 1998 study by AEMT also proved that the efficiency of motors with lower hp/kW ratings can be maintained during repair, dispelling the notion that, of themselves, winding burnout and removal damage the core.
Among the good repair practices identified by the two studies were making certain the overall length of the turns in the winding does not increase (more resistance increases loss), and increasing the wire area (lower resistance means lower loss) when slot fit allows it. These steps maintain, or may even reduce, the copper losses (I2R) in the winding.
Service centres that follow the guidelines in ANSI/EASA AR100-2010 “Recommended Practice for the Repair of Rotating Electrical Apparatus” and the more-specific recommendations of the EASA/AEMT Rewind Study’s “Good Practice Guide” will provide repairs that have a proven record of maintaining motor efficiency. Both documents are available as free downloads at www.easa.com/energy to assist service centres, end users and energy advocates in obtaining this critical information.
Review the application
When a motor fails, the first step is to determine its suitability for the application. A motor with an open enclosure, for example, may not be practical for a paper mill application with a great deal of airborne moisture and debris. Rather than repair, a better choice in this instance would be a TEFC (totally-enclosed, fan-cooled) replacement. Processes and duty cycles can change over time, so it’s always best to reassess the application when deciding whether to repair or replace a failed motor. An even better approach would be to assess all critical applications prior to failure as part of a motor management plan.
When the failed motor suits the application, assess the condition of its stator core. Is there significant damage? Prior to failure, did the motor exceed its rated temperature rise (e.g. due to high core losses)? Unless the motor has special features that might affect replacement price or availability, it may be more economical to buy a new motor than to repair a seriously degraded stator core.
Next, consider these decision points simultaneously. Has catastrophic failure occurred during this failure? Is there evidence of a prior catastrophic failure? Is the rotor damaged? Are other mechanical parts severely damaged?
Is it an EPAct, NEMA Premium or IEC IE3 motor?
Catastrophic failure (present)
When the motor has had a catastrophic failure, compare the cost of repair and replacement. Such failures typically do extensive damage to the stator core, windings and other parts of the motor, including the rotor, shaft, bearings and end brackets. In such cases, replacement may be the most economical option, especially when the motor’s suitability for the application is questionable.
Catastrophic failure (prior)
Evidence of a prior catastrophic failure may be apparent only after disassembling the motor. Examples include damaged stator core laminations; a damaged rotor core or damaged rotor bars or end rings; and a bent shaft that has bent again.
Rotor damage varies widely, from surface smearing due to contact with the stator to melted bars and end rings on die-cast designs, to broken bars or broken bar-to-end ring joints on fabricated designs. Surface smearing of the outside diameter can often be repaired economically. Other types of rotor repair may not be cost-effective, however, unless the motor is very large or has special features.
Mechanical parts condition
The shaft, frame or other mechanical parts may also be damaged beyond repair. Here, again, the cost of buying or making a new shaft, or of purchasing a new frame, may make replacing the motor the logical choice, unless the motor is very large or has special features.
The factors discussed so far have shaped motor repair/replace decisions for more than a half-century. A consideration added more recently is whether to replace the failed motor with a more energy-efficient model.
Broadly speaking, higher-efficiency motors are those covered by earlier U.S. federal regulations (EPAct, 1992), IEC motors labelled IE3, as well as NEMA Premium motors covered by newer Canadian (NRCan, 2011) and U.S. federal regulations (EISA, 2007). Repair considerations for these motors are the same as for standard-efficiency models.
Before repairing a standard efficiency motor, consider the return on investment for a more energy-efficient replacement (e.g. NEMA Premium) based on the expected life of the motor or process, hours of operation and energy costs. When the analysis favours replacement, determine whether the cost fits within your budget. If not, the best option may be a good-practice repair (so long as it costs less than a new motor).