A perennial question in maintenance circles is whether it’s best to repair or replace an electric motor that fails. For most plant engineers and maintenance staff, experience attests to the reliability of standard efficiency motors that have been repaired or rewound using industry best practices. Repair also can cost far less than replacement, especially if the motor has special features.
But some decision makers opt to replace failed energy efficient motors (NEMA Premium models in particular) because they’ve “heard” these motors can’t be repaired without a loss of efficiency. So, what’s the right answer? It turns out that the decision to repair, rewind or replace a failed energy efficient motor is not always so simple and straightforward as you may have heard.
What makes a motor more energy efficient?
Motors convert input power (electricity) into useful (mechanical) work, and in the process some energy is always lost–to heat, friction and windage. To improve motor efficiency manufacturers must therefore find ways to reduce these losses. Interestingly, though, they did not change the raw materials or production methods with the advent of higher efficiency (EPAct) motors, or with the introduction of the NEMA Premium models. Instead, they achieved efficiency gains through design changes.
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, which decreases copper losses. Totally enclosed, fan-cooled (TEFC) designs use the smallest fan that can keep the windings within the design temperature limit to minimize the power diverted to windage.
Repaired motor efficiency
The mistaken view of some that energy efficient and NEMA Premium motors cannot be repaired or rewound without reducing their efficiency is almost synonymous with electric motor repair. A rewind study in 2003, though, scientifically proved that the good practices identified in the study maintain the energy efficiency of higher efficiency NEMA and IEC motors.
The study, which was commissioned by the U.S.-based Electrical Apparatus Service Association and the Association of Electrical & Mechanical Trades from the U.K. tested the efficiencies of motors ranging from the original EPAct level to NEMA Premium and IEC IE3 levels.
The EASA/AEMT study, which was performed at the University of Nottingham under the direction of engineering executives from motor manufacturers in the U.S. and U.K., measured the efficiencies of 22 motors ranging in size from 50 to 200 hp (37 to 150 kW) before and after multiple winding burnout processes and rewinds.
An earlier study by AEMT (1998) also proved that the efficiency of motors with lower horsepower/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 centers 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 to assist service centers, end users and energy advocates in obtaining this critical information.
Repair–replace decision-making process
A well-informed decision to repair or replace a failed motor often involves more than might be readily apparent. Even the rather complex flowchart in Figure 1 doesn’t encompass every possibility, because each application has unique characteristics. Read More