Consideration when deciding to repair NEMA Premium motors

Further investigation needed before making a decision on motor repair

08/19/2013


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. 

Even this complex flowchart in doesn’t encompass every possibility in the repair-replace decision-making process because each application has unique characteristics. Courtesy: EASA

Review the application. When a motor fails, the first step should be to determine its suitability for the application. A motor with an open enclosure, for instance, 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 totally-enclosed, fan-cooled (TEFC) 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.

If 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. If the motor as-received for repair has had a catastrophic failure, evaluate the cost of repair versus that of replacement. Catastrophic failures typically do considerable 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 if the motor’s suitability for the application is questionable. 

Catastrophic failure–prior. Evidence of a prior catastrophic failure may be apparent only after disassembly of 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 condition. 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, however, may not be cost-effective 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. Whether the choice is to repair or replace the motor, be certain to identify and address the underlying causes of failure to prevent a recurrence.

Higher efficiency motors. The factors discussed to this point have shaped motor repair-replace decisions for more than a half-century. The advent of higher efficiency motors introduced another consideration–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 labeled IE3, as well as NEMA Premium motors covered by newer U.S. federal regulations (EISA, 2007). Repair considerations for these motors are the same as for standard efficiency models.

Following the good practices of ANSI/EASA AR100 and the EASA/AEMT Rewind Study, qualified service centers can repair any of these motors and maintain the efficiency rating.

Before repairing a standard efficiency motor, consider the return on investment for a more energy-efficient replacement such as NEMA Premium, based on the expected life of the motor or process, hours of operation, and energy costs. If the analysis favors replacement, determine whether the cost fits within your budget. If not, the best option may be a good practice repair, as long as it costs less than a new motor.

Assuming funds are available for a new motor, the next decision point is availability. Motors such as those that fall under EISA rules are predominantly stock items. Delivery times for larger motors or those with special features often range from a few weeks to several months. If the delivery time exceeds your requirements, a qualified service center usually can provide a good practice repair of the original motor in far less time.

Alternatively, the service center may be able to add the special features needed to a stock higher efficiency motor, such as converting it to a C- or D-face mounting. 

Thomas H. Bishop, P.E., is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), EASA is an international trade association of more than 1,900 firms in 59 countries that sell and service electrical, electronic, and mechanical apparatus, and a CFE Media partner.



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