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Operating Efficiency Important Factor in Motors' Repair or Replace Decision
The most common factor managers need to consider when evaluating the repair-or-replace option is the motors' operating efficiencies. As the result of several rounds of energy performance standards developed for a wide range of motors, the operating efficiency of the types of motors found in most facilities has risen significantly. The latest round, The Energy Independence and Security Act of 2007, updated the mandated efficiency standards for general-purpose, three-phase AC industrial motors manufactured for sale in the United States from 1 hp to 500 hp.
The net effect of the improved efficiency standards is an increase in motor efficiency of 1-5 percent under full-load operating conditions. While a 1 percent increase in operating efficiency might not seem worthwhile, the economic impact depends on the operating hours per year for the motor and the local utility rate structure.
The calculation is straightforward — reduced energy use multiplied by the utility rate structure and the number of hours that the motor is in use each year. Time-of-day rates and electrical-demand charges will impact the calculations and make them more complex, but managers still can estimate the savings.
It is important to do a fair economic comparison between the energy requirements of a new, premium-efficiency motor and those of a rebuilt one. Rewinding a motor can change the properties of the stator, resulting in increased losses. Depending on the quality of the rewinding operation, these losses are typically 1-4 percent.
For most rewinds performed by a quality shop, managers can estimate the additional losses associated with the rewind operation to be 1 percent. This will provide a more realistic comparison of the efficiencies produced by the two options.
Rules Of Thumb
For small motors, in general, it is more cost-effective to replace the motor than repair it. For larger motors, managers need to make the decision on a case-by-case basis, factoring in the condition of the motor, how closely it matches the needs of the application, and the economics of the two options. And here, managers must compare the life-cycle economics, not just the first costs.
Managers also need to consider downtime. Most motors covered by current standards are stock items and are available on rather short notice. But special, non-standard motors typically are not stock items, so it can take weeks or even months to obtain a replacement, making repair the better option.
Managers also need to consider utility rebates for upgrading motor efficiency. These rebates might be large enough to tip the balance in favor of replacement in otherwise marginal applications.
James Piper, P.E., is a national consultant based in Bowie, Md. He has more than 25 years of experience with facilities maintenance, engineering and management issues.