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How Energy Efficiency Affects the Motor Repair-Replace Decision

Energy prices may have decreased lately, but this is no time to relax conservation efforts, especially those involving electric motors. Energy costs often climb rapidly when the economy rebounds, thus it’s just as important now as ever before to make well-informed decisions about whether to repair, rewind or replace energy-efficient motors when they fail. The choices, however, are not always so simple and straightforward as some would have you believe.

What Makes A Motor “Energy-Efficient?”

There’s nothing magical about energy-efficient or premium efficiency motors. In the process of converting input power (electricity) into useful (mechanical) work, some energy is always lost – to heat, friction and windage. By reducing losses in these areas, motor manufacturers have made their products more efficient.

Compared with standard efficiency motors, for example, some energy-efficient models use longer stator and rotor cores to reduce core losses. They also have more copper in the windings, which decreases copper losses. To minimize the power diverted to windage, totally enclosed, fan-cooled (TEFC) designs use the smallest fan that can handle the job.

Efficiency of Rewound and Repaired Motors

When it comes to repair, there’s a mistaken perception among some that energy-efficient and premium efficiency motors cannot be rewound without reducing their efficiency. In fact, a rewind study commissioned by the Electrical Apparatus Service Association (EASA, U.S.A.) and the Association of Electrical & Mechanical Trades (AEMT, U.K.) in 2003 scientifically proved that the energy efficiency of high-efficiency NEMA and IEC motors is maintained by following the good practices identified in the study. The efficiency levels of the motors tested ranged from the original EPAct level (U.S. federal law enacted in 1992) to the NEMA Premium (required by EISA US federal law enacted 2007) and IEC IE3 levels.

The study, which was performed at the University of Nottingham, tested the efficiency of 22 motors ranging in size from 50 hp to 200 hp (37 kW 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 power ratings can be maintained during repair, dispelling the notion that, of themselves, winding burnout and removal damage the core.

Both studies also found that following best practices during all phases of the repair process is critical to maintaining motor efficiency. Bearing friction loss, for example, must be controlled by replacing with the original bearing type, repairing or maintaining the proper bearing journal and housing fits, and using the correct quantity of lubricant.

Service centers that combine the wide range of recommendations in ANSI/EASA AR100-2010, Recommended Practice for the Repair of Rotating Electrical Apparatus, with the more specific recommendations of the Rewind Study’s “Good Practice Guide,” will be providing repairs that have a proven record of maintaining motor efficiency.

The Repair–Replace Decision Process

As mentioned earlier, deciding whether to repair or replace a failed motor often involves more than might be obvious at first glance. Even the accompanying flowchart doesn’t cover every possibility, because each application has unique circumstances. Here are the critical steps:

Application review: The first step is to determine if the failed motor is suitable for the application. A motor with an open enclosure, for instance, may not be practical for a sawmill application with lots of airborne dust and debris. A better choice might be a totally-enclosed, fan-cooled (TEFC) replacement. Processes and duty cycles often change over time, so it always pays to reexamine the application when deciding whether to repair or replace a failed motor.

Motor examination: If the failed motor is a “good fit” for the application, assess the condition of the stator core. Has it sustained significant damage? Prior to failure did the motor exceed its rated temperature rise (i.e., high core losses)? Absent special features that might affect price or availability, it may cost less to buy a new motor than to repair a badly damaged stator core.

Next, consider these decision points simultaneously:

  • Has catastrophic failure occurred?
  • 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?
Failed Motor Flowchart

Failed Motor Flowchart

 

Catastrophic failure: If a catastrophic failure has occurred, weigh the cost of repairing the motor against that of replacing it. Such failures typically do significant damage to the stator core and windings, as well as to the rotor, shaft, bearings and end brackets. In such cases, replacement may be the most economical option, especially if you question the suitability of the motor for the application.

Rotor damage varies widely, from surface smearing due to contact with the stator, to melted bars and end rings on die-cast designs, to lifted bars or broken end rings on fabricated designs. Surface smearing can often be repaired economically. Other kinds of rotor repair may not be feasible unless the motor is very large or has special features.

The shaft, frame or other mechanical parts may also be damaged so badly that they must be replaced. Here again, the cost of buying or making a new shaft, or of purchasing a new frame, may make repair a less attractive choice than replacing the motor, unless the motor is very large or has special features.

Prior catastrophic failure: Sometimes evidence of a prior catastrophic failure is discovered only after disassembly. Examples include a bent shaft that has bent again; a damaged rotor core or damaged rotor bars or end rings; and damaged stator core laminations. Whether you choose to repair or replace the motor, be sure to identify the contributing causes of failure to prevent a recurrence.

Energy-efficient motors: The points discussed so far have shaped motor repair-replace decisions for more than 50 years. The advent of energy efficient motors introduced another consideration: whether to replace the failed motor with a more energy-efficient model.

Broadly speaking, energy-efficient motors are those covered by earlier U.S. federal regulations (EPAct), IEC motors labeled EFF1, as well as newer, premium-efficient models (e.g., NEMA Premium, covered by EISA US federal regulations). Repair considerations for these motors are the same as for standard-efficiency models. Following the good practices of the EASA/AEMT rewind study, qualified service centers can repair any of these motors without affecting the efficiency rating.

Before repairing a standard-efficiency motor, consider the return on investment for a more energy-efficient replacement, based on the expected life of the motor or process. If your analysis shows that replacement is preferable to repair, the next consideration is whether you have the money in your budget. If not, you may still opt for repair as long as it costs less than a new motor.

Assuming you have the funds for a new motor, the next decision point is availability. Motors such as those that fall under EISA rules are normally stock items. Delivery times for larger motors, or for those with special features, often range from a few weeks to several months. If the delivery time is longer than you require, a qualified service center usually can repair the original motor in far less time. It also may be able to add the special features you need to a stock motor, for example, by converting it to a C-face mounting.


Download “The Effect of Repair/Rewinding on Motor Efficiency: EASA/AEMT Rewind Study & Good Practice Guide to Maintain Motor Efficiency” by the Electrical Apparatus Service Association, Inc. and Association of Electrical and Mechanical Trades.
 
Download “ANSI/EASA AR100-2010: Recommended Practice for the Repair of Rotating Electrical Apparatus” by the Electrical Apparatus Service Association, Inc.

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