(1,775 / 1,745)³ = 1.05

Or, a 5% increase in load.

Fortunately, it may be possible to find a new energy-efficient motor with a full-load speed closer to that of the original motor. If not, an impeller trimming can provide the required adjustment.

Rules and exceptions

The U.S. Energy Policy Act of 1992 (commonly called "EPAct") eliminated the least efficient motors from the market and raised the bar for many motors manufactured since October 1997 (Table 2). That does not, however, negate the need for motor shopping for optimum efficiency. There is an acceptable range of efficiencies for any given motor size. Your application may warrant a selection in the premium end of the range. In many cases, though, any new motor will have a higher efficiency than the old motor.

EPAct does not guarantee a good motor efficiency selection since many motors are exempt from EPAct requirements. Exempt motors include motors larger than 200 hp; non-foot-mounted motors; rebuilt, repaired and rewound motors; and NEMA Design C and D motors [3].

Check your spec

Chances are your organization already has a motor specification. Is it up-to-date?

If you need a motor specification, consider referencing IEEE Standard 841 for severe duty, TEFC, squirrel-cage induction motors up to 500 hp [4]. You'll get heavy-duty motors with energy efficiency higher than EPAct requirements. Consider adding the NEMA Premium efficiency standard [5] for even higher efficiency (Table 2) and adding IP 55 bearing isolators under 40 hp. (They are already included for 40 hp and above.)

How high a quality will that get you? Hint: Most motor manufacturers provide a 60-month warranty for IEEE 841 motors instead of the usual 12-month warranty for standard EPAct motors.

Don't forget to "clip coupons"

Electric deregulation eliminated many rebate and incentive programs for installing energy-efficient motors. However, some programs still exist. To determine whether rebates or incentives are available, call your account representative at the electric company.

Be sure to get program details before specifying new motors because the rebates or incentives may require approval in advance of purchase. The program requirements may partially guide your choices. For example, the rebates or incentives often only apply in the premium end of the efficiency range, and you may find that the rebates or incentives cover much of the premium price associated with the premium efficiency.

Learn all you can

There are many excellent sources of information about motor selection and upgrade for energy efficiency. Your preferred motor supplier is one source.

Another source is the U.S. Department of Energy (DOE). DOE's free non-commercial source for guidance is the "BestPractices" program of the Office of Industrial Technologies (OIT). You can find motor tip sheets, case studies, technical publications, software to download, and training and event schedules on their Web site at http://www.oit.doe.gov/bestpractices/motors.

OIT downloadable publications include titles such as:

"Buying an Energy-Efficient Electric Motor"

"Optimizing Your Motor Drive System"

"Replacing an Oversized and Underloaded Electric Motor"

"Determining Electric Motor Load and Efficiency"

"The Impacts of the Energy Policy Act of 1992 on Industrial End Users of Electric Motor-Driven Systems"

"Improving Fan System Performance: A Sourcebook for Industry"

"Reduce Pumping Costs through Optimum Pipe Sizing"

"Eliminate Voltage Unbalance"

"Reducing Power Factor Cost"

There are also more extensive publications, such as "Energy Management for Motor Driven Systems."

OIT software includes:

MotorMaster+ 3.0, a free motor selection and management tool that includes a database of more than 20,000 AC motors.

Pump System Assessment Tool (PSAT), an analysis tool that uses pump performance data and motor data to predict potential energy cost savings.

A rewarding replacement

Properly pursuing electric motor replacement for energy efficiency requires an awareness of numerous potential opportunities and pitfalls. Opportunities such as ASD installation and process optimization may have a significantly better return on investment than motor replacement. Lack of advance attention to motor replacement details, such as small speed differences in centrifugal equipment applications, can be costly. Fortunately, detailed motor replacement information and analysis tools are readily available, and many of them are free.

If you consider all these details before replacing an electric motor, you will rev up energy savings every time.

Darrell T. Mears, P.E., is principal for Optimum Utility Systems in Langhorne, Pa., and consults on industrial energy auditing and energy management.