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SMART IDEAS FOR YOUR BUSINESS ® VARIABLE SPEED DRIVES FACT SHEET Variable speed drives (VSDs) allow motor-driven devices such as fans, pumps and automation equipment to vary the rate of speed at which they operate based on the actual needs of the equipment, rather than operating at a constant full speed. Many motors operate at a constant speed all the time, regardless of load demand, but constant speed means constant energy consumption. When constant speed isn’t necessary, VSDs allow a motor’s operation to be continually tuned to match the varying demand. Adding VSDs to the motors that run fans and pumps can reduce energy use significantly, and incentives from ComEd’s Smart Ideas for Your Business® program can help get you going. HOW VSDS SAVE ENERGY The most common type of VSD is a variable frequency drive (VFD). The most efficient VFD systems include three-phase induction motors combined with a VFD controller, a solid state electronic power conversion device that changes the motor input frequency to control the speed of the motor. Electric motors that drive equipment like pumps and fans normally operate at a constant speed. Fluctuations in demand are usually adjusted by some form of mechanical throttling, such as a valve or louvers, to control water or air flow speed and volume. A VSD handles these fluctuations by varying the current and motor speed–thereby making effective use of the motor without wasting energy.1 To quantify the savings that can be achieved with the use of VSDs, consider that when a shaft speed is reduced by 10 percent, power consumption is reduced by 27 percent. If speed is reduced by 20 percent, power is U.S. Department of Energy Energy Efficiency and Renewable Energy, Improving Motor and 1 Drive System Performance: A Sourcebook for Industry (Golden, Colorado: National Renewable Energy Laboratory, 2008), 30. 2 3 Ibid., 89-90. Phil Gregory, “Take Control of Your Processes with Variable Speed Drives.” (Paper presented at ComEd’s Energy Efficiency Expo, Rosemont, IL, 19 May 2010). 4 Ibid. ComEd.com/BizIncentives reduced by 49 percent. Understanding those ratios requires a little math: the fluid or air flow varies directly with the pump or fan rotational speed. However, the fan’s input power requirement varies as the cube or third power of the speed ratio. Thus if 100 percent flow requires full power, 75 percent flow theoretically requires (0.75)3 = 42 percent of full power.2 Energy use is also optimized as the motor starts up. VSDs are often equipped with “soft starting” features that decrease motor starting current to about 1.5 to 2 times the operating current, dramatically reducing the impact of fan starts on an electrical system. VSDs also reduce the “voltage sag” that can occur when a large motor starts quickly. Voltage sags can dim lights and cause other equipment to shut down or restart.3 Soft starts typically allow a motor to develop 150 percent of its rated torque while the VSD is drawing less than 50 percent of its rated current. The controlled start-up also helps extend motor life and reduce maintenance costs.4 APPLICATIONS FOR VSDS The most common application of VSDs is on HVAC pumps and fans. In a typical building, maximum capacity is required only about 10 percent of the time; most of the time, buildings require 40 to 70 percent of maximum energy. The VSD delivers its savings by controlling motor speed to provide only the precise airflow and pressure required to maintain desired heating or cooling levels. Another common application of VSDs is on air compressors. A VSDis ideal for “trimming”—matching pressure to demand. A compressor equipped with a VSD varies compressed air flow in response to detected changes in air system pressure to maintain an exact and constant pressure level. As demand falls and more air is delivered into the air system, the system pressure will rise and the VSD will reduce the compressor’s speed and output. An increase in compressed air demand causes system pressure to fall and the VSD to increase output. Demand control ventilation is a growing application for VSDs. DCV systems regulate exhaust fan speed based on need, reducing the fan’s energy consumption by 30 to 50 percent. Because these systems keep heated and airconditioned air from being lost, HVAC system energy consumption can also drop by 20 to 30 percent as well.5 DCV systems for office space, parking garages and commercial kitchens may be eligible for standard incentives based on exhaust fan HP or square feet of conditioned space. Other applications for VSDs include: » Cooling tower fans » Industrial or process equipment » Pumps » Supply fans » Compressed air systems » Exhaust fans 5 U.S. Environmental Protection Agency, ENERGY STAR ® Guide for Restaurants: Putting Energy into Profit (www.energystar.gov, 2009), 6. According to the U.S. Department of Energy, motordriven equipment accounts for 64 percent of the electricity consumed in the U.S. industrial sector. Industrial plants can often reduce their current electricity use and costs by about 5 to 15 percent or more by improving the efficiency of their motor-driven systems. CONTACT US For more information about Smart Ideas for Your Business, including applications and incentive amounts, visit www.ComEd.com/BizIncentives. You can also reach us by phone at 888-806-2273 or email us at [email protected]. Smart Ideas® is funded by ComEd customers in compliance with Illinois Public act 95-0481. © Commonwealth Edison Company, 2012