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Ways to Save on Motor Energy Costs
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Turn it off – The simplest and most obvious method of saving motor energy is simply to turn it off when its not needed. Motors often run unnoticed when they are not needed, increasing energy costs. Motors can be switched manually and this is a fine solution for many applications, but there are also timers and sensors available that will turn them off automatically. Examples of motors that could be turned off at night include those for service hot water circulation, air compressors and ventilation fans. Reduce the speed on a fan motor – Another simple method of reducing motor energy costs is to reduce the speed of an HVAC fan. Energy consumption of fans and pumps varies according to the speed raised to the third power, so small changes in speed can make big changes in energy consumption. The chart below shows the relationship of fan speed in revolutions per minute (RPM) to power consumption. Most HVAC equipment comes from the factory with fan motor speeds preset, although they can
Use variable speed drives (VSD) for variable loads – Some loads driven by motors don’t need to operate at the same speed all the time. For example, pumps and fans often don’t need to produce the same flow all the time. These types of loads offer big opportunities for savings by moderating their speed according to their load. For example, reducing a fan’s average speed by 20 percent with a VSD can reduce energy consumption by more than 40 percent. Actual savings will be slightly less than those shown on the power consumption graph since variable speed drives themselves aren’t 100 percent efficient. Some examples of potential VSD savings are provided in the table below. |
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Specify energy-efficient motors – When replacing an existing motor or when specifying new equipment, consider using a high-efficiency motor. High-efficiency motors use better quality materials and are manufactured to higher quality specifications than standard-efficiency motors. They are five to 10 percent more efficient on average than standard motors in the smaller sizes (25 horsepower or less). Federal efficiency standards now require minimum efficiencies for electric motors so the older motors in your facility are likely to be less efficient than the lowest efficiency motors of that size you can buy now.¹ Be sure to check with the manufacturer or your building engineer when installing energy-efficient motors on fan and pump applications because some energy-efficient motors have higher speeds than standard motors. As discussed above, increasing fan or pump speed can actually result in an increase in energy use. So, it’s important to specify that the new motor has a full load speed no greater than that of the motor it’s replacing. The following table provides a comparison of the efficiencies of standard- and high-efficiency motors, and examples of potential savings with high-efficiency motors. |
Source: Efficiencies: MotorMaster. Assumes 3500 hours per year of operation. |
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Properly size motors – Many motor systems are oversized, and a significantly oversized motor will run at low efficiency increasing energy costs. An oversized motor also costs more to buy. The efficiency of most motors peaks around 75 to 80 percent of full load and drops off sharply below 40 to 50 percent of full load, although these ranges vary by design and manufacturer.² High-efficiency motors tend to maintain their efficiency over a wider range of loads than standard motors. Motors loaded below 50 percent arealmost always attractive candidates for replacement. However, because the relationship between efficiency and load variesamong different types and sizes of motors, be sure to check with the manufacturer or building engineer before replacing an oversized motor. Reduce the load – Often it’s possible to reduce the load on a motor and save energy by reducing pressure losses in pipe and duct runs with low-pressure loss elbows and fittings. Duct and pipe systems with lower pressure losses (usually expressed as “static pressure”) can often use a slower speed fan or pump to deliver the same amount of flow. As shown above, this can result in big savings. Other ways to reduce the load on a motor system include aligning the motor drive, and replacing inefficient drivetrains such as belts, chains, and gears with direct drive systems.
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¹ The Energy Policy Act of 1992 went in to effect October 24, 1997 establishing minimum efficiency standards for motors of NEMA design A and B, from 900 to 3600 rpm, between 1 and 200 horsepower.
² Energy-Efficient Motor Systems, ACEEE, 1991.