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What Are the Options? The AC induction motor is the dominant motor technology in use today, representing more than 90 percent of installed motor capacity. Induction motors are available in single-phase and polyphase configurations, in sizes ranging from fractions of a horsepower to tens of thousands of a horsepower. They may run at fixed speeds--most commonly 900, 1,200, 1,800, or 3,600 rpm--or be equipped with an adjustable-speed drive. The most commonly used AC motors by far have a squirrel-cage configuration, so named because of the shape of the rotor bar structure. Wound-rotor models, in which coils of wire turn the rotor windings, are also available. These are expensive but offer greater control of the motor's performance characteristics, so they are most often used for special torque and acceleration applications and for adjustable-speed applications. The major choice facing motor specifiers is whether or not to select a premium-efficiency model. Premium-efficiency motors typically cut losses by 10 to 40 percent (increasing motor efficiency by 1 to 10 points) and carry a cost premium of 15 to 30 percent compared with a standard motor. In retrofit situations, users also have the choice of repairing failed motors or replacing them. It is becoming common practice among energy-conscious companies to replace all failed, moderate-duty induction motors up to about 125 horsepower (hp) with new premium-efficiency models rather than repairing and rewinding the failed motor. This is because motor rewinds often degrade motor efficiency by 1 to 3 percent. How to Make the Best Choice Determine the cost-effectiveness of a premium-efficiency motor. AC motors are available in a wide range of efficiencies (see Figure 1). The Energy Policy Act of 1992 (EPACT) mandates that nearly all three-phase, general-purpose motors manufactured for sale in the U.S. after 1997 meet new minimum-efficiency levels. But EPACT only applies to certain "general-purpose" motor types and motors up to 200 hp. To obtain maximum energy savings, specify "premium-efficiency" rather than just "high-efficiency." Premium-efficiency motors often cost 10 to 20 percent more than standard-efficiency models. Although the economics vary by application, a premium-efficiency motor under typical operation will often pay for its price premium in reduced energy bills within a year or two. A quick calculation to determine motor savings is outlined in the sidebar.
Think systematically. The full potential of an efficient motor can best be captured if it is integrated into an optimized drivepower system. This may be difficult to do in retrofit applications but is very important when designing new systems, for which all components can be right-sized from the start. Properly optimized motor systems often use less than half the energy of systems designed with standard rules of thumb. To create an efficient drivepower system, select efficient, properly sized models of the equipment that the motor will drive, such as pumps and fans. (DOE offers a free Pump System Assessment Tool to help industrial users assess the efficiency of pumping system operations. For more information on sizing fans, see Energy Advisor Technology Buyer's Guide: Fans. Check that pressure drops in coils, heat exchangers, or other auxiliary devices are optimized for good lifecycle economics. Use efficient, properly aligned belts, cogged belts, or direct-drive connections between the motor and the equipment to minimize power loss through friction. Select the right controls to regulate motor and equipment operation.
Buy the right size motor. Motors operate at their highest efficiency between about 60 percent and 100 percent of their full-rated load, dropping off sharply in efficiency below 50 percent loading (see Figure 2). About one-third of motors in the field are so oversized that they operate below 50 percent of rated load most of the time. Motors operate at their peak efficiency if they are sized correctly for the load that they will drive. Oversized motors not only operate inefficiently, but they also carry a higher first cost than right-sized units. They can also contribute to reduced power factor, which increases loads on the building's electrical systems. Watch your speed. When replacing an old motor with a new premium-efficiency model in fan and pump applications, make sure the new motor's full-load speed is the same or slower than the old motor (making certain, of course, that it meets the minimum speed necessary for the application). The energy required by many fan and pump applications varies with the cube of the rotational speed of the fan or pump, so increasing its speed by only 10 percent can increase energy use by more than 33 percent. Therefore, putting in a premium-efficiency motor that rotates faster than the old standard-efficiency one may negate predicted energy savings. It may be necessary to adjust fan sheaves or pump impeller diameters to achieve the correct motor speed. The MotorMaster software can correctly allow for speed differences in calculating energy savings.Evaluate the cost-effectiveness of ASDs. Adjustable-speed drives (ASDs)--also known as variable-speed drives, variable-frequency drives, or variable-frequency inverters--use electrical waveform modification to vary the voltage and frequency of the alternating current that drives the motors. By controlling motor speed so that it closely corresponds to varying load requirements, ASD installations can reduce energy consumption (in some cases energy savings can exceed 50 percent). ASDs can also improve power factor and provide other performance benefits such as soft-starting and overspeed capability. ASDs require a small amount of power to operate, and so motors with an ASD consume more power at full load than single-speed motors. However, it takes very little time operating at part load to make up the difference. ASDs can be cost-effective in cases with average loadings as high as 90 percent, but an analysis should be performed for each individual case based on the time spent at part-load conditions and the efficiency with and without the ASD. (For more information about ASDs, see Adjustable-Speed Drives.)
Account for the motor's impact on power factor. Power factor is an indicator of how much of a power system's capacity is available for productive work. Low power factor is undesirable because it increases the load on a building's electrical system, and utilities sometimes charge customers a penalty for low power factor facilities. Because power factor is lower when a motor is lightly loaded, be sure to choose the right-sized motor. You can also specify a motor with a high power factor, but such models sometimes have lower efficiency. The ultimate selection depends in part on whether a facility is subject to power factor penalty charges. A facility with a significant number of induction motors and a low power factor can solve the problem with premium-efficiency motors that are properly sized. If new motors are not an option, other power factor correction methods are available, including static capacitor banks, rotary condensers, and static and dynamic volt-amperes reactive (VARs). What's on the Horizon? Induction motors are a mature technology. Manufacturers continue to make slow improvements in efficiency and performance, but no major changes in the technology are on the horizon. Most of the major innovations in motor technology are in the development of new types of motors, such as switched-reluctance and written-pole designs, which will gradually supplant induction motors in some market niches. Who Are the Manufacturers? The U.S. motor industry is relatively fragmented--roughly 20 manufacturers make most of the integral-horsepower induction motors sold in the U.S., and none of them dominate the market. Many of the following selected manufacturers also sell variable-speed motors, which can help to match the motor to variable loads. (Neither this list, nor any mention of a specific vendor or product in this guide, constitutes an endorsement or recommendation of any vendor or product by E Source, nor does this guide constitute an endorsement or recommendation, explicit or otherwise, of your service providers' various technology-related programs.)
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