AC Induction Motors
What are the options?
How to make the best choice
What's on the horizon?
Some of the largest opportunities to save energy and reduce operating costs in buildings and industrial facilities come from optimizing electric motor systems. About half of all electricity consumed in the U.S. flows through motors, 90% of which are alternating-current (AC) induction motors. The U.S. Department of Energy (DOE) estimates that, on average, the manufacturing sector could reduce industrial electric motor energy use 11% to 18% by using proven efficiency technologies and practices. In a single year, a motor often consumes energy worth about 10 times its initial cost. That's why even small improvements in efficiency can pay back quickly. The key is to choose the right-sized, energy-efficient motor and integrate it into an optimized drivepower system.
What are the options?
The AC induction motor is the dominant motor technology in use today, representing more than 90% 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 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, are also available. Although they are expensive, they offer greater control of the motor's performance characteristics and are therefore most often used for special torque or acceleration applications, and for adjustable-speed applications.
The major choice facing motor specifiers is whether or not to select a motor that complies with an efficiency specification developed by the National Electrical Manufacturer's Association (NEMA)—known as NEMA Premium. To meet NEMA Premium specifications, a motor must exceed the minimum efficiency mandated by law (through the 1992 Energy Policy Act in the U.S. and the Canadian Standards Association's 1995 Standard C-747) by between 0.4 and 3.0 percentage points, depending on the size and type of motor. More information about NEMA Premium specifications is available here. But note that in almost all motor classes, there are many models available that exceed even the NEMA Premium specs. Premium-efficiency motors often carry a price premium as well, but depending on motor size and type, some premium-efficiency models are available at little or no price premium (Figure 1).
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Figure 1: Normalized price premium versus efficiency gain, 10 and 100-horsepower TEFC motors |
In retrofit situations, users 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%—or more, in some cases.
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 2). 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" types of motors and motors up to 200 hp. To obtain maximum energy savings, specify NEMA Premium rather than just "high-efficiency." Premium-efficiency motors often cost 10 to 20% more than standard-efficiency models. Although the economics will 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 chart at the bottom of the page.
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Figure 2: Range of available motor efficiencies for 3,600 rpm TEFC motors |
For more detailed analyses, the DOE offers MotorMaster+, a free program that can help you perform a thorough economic analysis by drawing on its database of high-efficiency motors. The software can be downloaded, free of charge, or used online by going to the MotorMaster+ web site. MotorMaster+ can create a list of motors that meet a user's specific requirements, and it can be used to calculate the savings and simple payback period for premium-efficiency motors as compared to standard-efficiency units.
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 to ensure all components are correctly sized from the start. Properly optimized motor systems often use less than half the energy of systems designed according to 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. (The DOE offers a free Pump System Assessment Tool that can help industrial users assess the efficiency of their pumping system operations. For more information on sizing fans, see HVAC: Fans. Check to see 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% and 100% of their full-rated load, dropping off sharply in efficiency below 50% loading (see Figure 3). About one-third of motors in the field are so oversized that they operate below 50% of rated load most of the time. Motors only operate at their peak efficiency if they are sized correctly for the load that they drive. In addition to operating inefficiently, oversized motors 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.
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Figure 2: Efficiency versus load for a 10-hp induction motor |
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 as 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% can increase energy use by more than 33%. 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 help you correctly allow for speed differences in calculating energy savings.
Evaluate the cost-effectiveness of ASDs. Adjustable-speed drives (ASDs) are electronic or mechanical devices that allow a motor designed for single-speed operation to drive a load at variable speeds. By controlling motor speed so that it closely corresponds to varying load requirements, ASDs can reduce energy consumption—and, in some cases, energy savings can exceed 50%. Variable-frequency drives—electronic ASDs that vary the voltage and frequency of the power provided to the motor—can also improve power factor and provide performance benefits, such as soft-starting and overspeed capability. ASDs require a small amount of power to operate, 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 this difference. ASDs can be cost-effective in cases with average loadings as high as 90%, but an analysis should be performed for each individual case based on the time spent at part-load conditions, and 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-ampere reactive (VAR) devices.
| How to Calculate Efficient Motor Savings |
For applications in which a motor will operate at constant speed according to a well-defined schedule, calculating the savings for motor efficiency upgrades is a fairly straightforward exercise.
Here's how it is done:
Where:
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What's on the horizon?
A new class of super premium-efficiency induction motors has begun to roll off production lines in Germany, Denmark and India, and they may soon be in production in North America. What distinguishes these motors from their predecessors and allows them to achieve higher efficiencies is the fact that their rotor bars are made of cast copper rather than aluminum. The higher conductivity that is characteristic of copper has reduced motor losses by 10% to 20% in initial tests, boosting efficiencies by 1.0 to 5.0 percentage points. Although copper is a better material for rotor bars, almost all induction motors have aluminum rotor bars because the higher temperature necessary to melt copper would quickly destroy the components of the rotor die-casting process. Now that materials capable of withstanding these elevated temperatures have been identified, more manufacturers will likely begin to offer super-efficiency cast copper rotor motors.
Copyright © 2008 E Source Companies LLC
Last Modified: May 9, 2009
