Motors Matter

These underappreciated - but growing - consumers of household energy deserve more attention.

July 01, 2000
July/August 2000
A version of this article appears in the July/August 2000 issue of Home Energy Magazine.
Click here to read more articles about HVAC

        Appliance motors—the engines that drive air conditioners, refrigerators, washing machines, and other home appliances—account for about a quarter of all residential energy use. A new Department of Energy report estimates that in 1995, these motors—most of them less than 1 horsepower (hp)—used approximately 445 billion kWh, or about 4.9 quadrillion Btu of primary energy. Altogether, a home may contain dozens of motors, but consumers rarely think of motors when they look for ways to save household energy. This is because appliance manufacturers, who do not face the energy cost consequences of using inefficient motors in the home, purchase the overwhelming majority of motors for home appliances.
        Appliance manufacturers do face appliance efficiency standards created by the federal government, however. Minimum appliance energy efficiency standards established under the National Appliance Energy Conservation Act (NAECA) in the early 1990s have led to improved motor efficiencies for many residential appliances—especially refrigerators and freezers. That is why replacing an older refrigerator or freezer can be such a great step for energy efficiency, (see “Monitoring Refrigerator Energy Usage” HE May/June ’00, p. 32). Currently, more than 84% of residential motor energy consumption occurs in appliances that are covered by NAECA. Although there are other ways to improve the overall efficiency of an appliance, using a better motor is often one of the most costeffective ways for manufacturers to meet the standards.
        More efficient motors are also often cost-effective for the consumer, although retrofitting existing appliances with more efficient motors is not practical in many cases. However, there are occasions when retrofitting an appliance with a new motor is necessary—when a furnace’s fan motor dies or when a new well or pool pump is needed, for instance.
        Furnace fans can use quite a lot of energy. Replacing their motors with more efficient ones can bring real savings. Most forced-air furnaces and air conditioners currently in use are powered by multiple-speed shaded-pole or permanent-split capacitor (PSC) induction motors, with efficiencies between 50% and 60%. One of the available speed levels is selected when the unit is installed, to match the blower output with the flow resistances in the space-conditioning equipment and the duct system. Table 1 shows the potential energy savings for the average user from replacing the existing motor with each of three options: a high-efficiency PSC motor, an electronically commutated permanent magnet rotor (ECM), (see “Types of Motors,” p. 33), and a variable-speed ECM. Variable-speed motors can save the most energy and can be cost-effective to the consumer if they are installed in high usage equipment. Recognizing the value of these savings, Canada’s new standards for the R- 2000 program (the Canadian equivalent of Energy Star) will require the use of more efficient ECMs in furnace fans that are used in wholehouse mechanical ventilation systems.

Room for Improvement

        While NAECA has resulted in improved appliance motor efficiency, these motors are still not nearly as efficient as they could be. In the DOE study, researchers examining motor energy use and efficiency found much room for improvement, particularly in the appliances that use the most motor energy in the residential sector: HVAC equipment, refrigerators, and freezers.
         Table 2 shows the average efficiency of several common residential motors, their potential practical efficiency, and the energy savings and payback periods that would result from using the more efficient motor.

Air Conditioners

        The efficiencies of central air conditioners and heat pumps have been steadily increasing since the 1970s. The NAECA standards have driven the latest increases in the efficiency levels of these appliances in the last decade, raising them from 10.5 to 11.5 energy efficiency ratio (EER). Any further increases in the efficiencies of these appliances may be hard to achieve through motor improvements alone. Compressor motor efficiencies are now within 2% to 3% of the practical limits, leaving little room for increases in simple efficiency. For example, an 80% efficient compressor driven by a 90% efficient motor would have an EER of 11.8, only slightly higher than current products. However, simply improving the efficiency of the motors themselves is not the only way to save on appliance motor energy use.
        Most motors operate at a single speed, cycling on and off to meet a load, or controlling the output with valves and dampers. VSDs can greatly improve how efficiently air conditioners’ motors operate in applications with highly variable loads (see Table 3). They save energy by matching motor speed to load. VSDs also extend motor and system life.
        Manufacturers have been reluctant to take this approach to increasing motor efficiency, because switching to VSDs tends to be more involved than incrementally increasing the motor efficiency. It often requires a redesign for the whole appliance. But for certain appliances, such as air conditioners, a VSD can meet the appliancefs load much more efficiently.
        The capacity of an air conditionerfs motor is usually based on the anticipated maximum cooling load of the house. But most of a homefs operating hours occur under much milder conditions, requiring only a fraction of the cooling capacity of the air conditioner. In a typical air conditioner with a single-speed compressor, the unit cycles on and off to maintain the conditioned space at a desired set temperature. Continuous operation modulated by a VSD would use the heat exchangers more efficiently, limit losses associated with on-off cycling, and reduce the indoor air-flow rate.
        Using a continuously variable-speed compressor with an ECM indoor blower could improve an air conditionerfs seasonal energy efficiency ratio (SEER) by 50% (for example from SEER 10 to SEER 15). Using a two-speed motor would yield only a 30%.40% SEER increase but would cost somewhat less.
        Motors also drive the fans that blow air over the condenser and evaporator coils. Using more efficient motors to blow air over the condenser fan of an air conditioner or heat pump can provide additional savings with a relatively short payback time (typically six years).

Refrigerators and Freezers

        Residential refrigerators and freezers can have up to three motors.not counting the ice maker and defrost timer. The largest (typically hp) drives the refrigerant compressor. In frost-free units, two additional smaller motors drive fans that force air over the condenser and evaporator.
        The compressor motor is typically a 115V AC single phase, two-pole induction motor. The current NAECA standards have led to the use of more efficient induction motors. Even more stringent standards will take effect in 2001, reducing allowable refrigerator and freezer energy consumption by an average of 25% from current levels. Manufacturers are beginning to assess higher-cost ECMs, which would be used as either a high-efficiency constant speed motor or a variable speed motor. The federal Environmental Protection Agency (EPA) has estimated that using a variable-speed ECM-driven compressor in combination with variable-speed ECM evaporator and condenser fan motors would save one-quarter of the total motor energy use. EPA estimated that this would increase the retail price by $75; energy savings should pay the consumer back for this increase in about eight years. Variable-speed compressors also provide the benefits of very quiet, steady-state operation and rapid cooling when warm food is placed in the refrigerator.
        Replacing only the fan motors, not the compressor, with variable-speed ECMs should reduce the payback period to only five years. This shorter payback period is due partly to the fact that increasing the efficiency of the evaporator fan also reduces compressor energy. The evaporator fan and motor are located within the refrigerated cabinet, so their use adds to the refrigeration load.

NAECA Standards Not Universally Helpful

        There are a few appliances for which NAECA standards have not made much difference. In clothes washers and dryers, for example, the motors consume only a small part of the total energy input compared to the heat input for water and air heating.
        Although indoor blowers that are integral to air conditioner and heat pump systems are covered in the NAECA standards, these standards have not affected gas furnace indoor blowers. This is because the DOE gas furnace test procedure does not include the indoor blower power in the annualized fuel utilization efficiency measurement.
        Appliance standards have gone a long way toward improving motor efficiency. But as motor-using appliances and gadgets continue to proliferate in American households, further efficiency improvements are necessary to reduce energy use.

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