Electric motors are the largest end use of electricity in the U.S. industrial sector. According to the U.S. Department of Energy (DOE), an estimated 40 million motors use 70% of the electricity used by industry at a cost of $30 billion a year. In addition, DOE predicts that up to 18% of industrial motor energy would be reduced by applying proven efficiency technologies. DOE recommends motor efficiency upgrades and application improvements.
Only an estimated 10% of all motors in use today meet the minimum efficiency targets set by the Energy Policy Act of 1992 (EPAct). EPAct established minimum efficiency standards for industrial electric motors, whether manufactured or imported, sold separately or as a component of another piece of equipment.
Measuring Motor Efficiency
As with other machines, the efficiency of electric motors is measured as output divided by input. For motors, the output is the amount of usable mechanical power produced, and the input is the total energy required to produce that power. For example, a motor with an efficiency rating of 93% will provide $93 of output for every $100 of energy input-$7 would be wasted energy in the form of heat loss.
Precision laboratory tests are required to measure the efficiency of specific electric motors. The most complete and accurate test standard is the IEEE Standard 112, Test Method B.
Why Upgrade?
Upgrading electric motors reduces operating cost. A 50-horsepower (hp) motor, for example, costs $25,000 per year to operate full time. However, the best way to evaluate what a motor really costs is to compare the initial cost with the cost of energy used over the life of the motor. A common mistake is to place too much emphasis on initial cost. With electricity accounting for 98% of the lifetime cost of motors, energy-efficient models save money every minute they operate. Increasing the efficiency just a few points can yield fast savings. A 30-hp energy-efficient motor, operating continuously, can save more than $1200 per year.
Although high-efficiency motors cost more than standard motors, the premium cost can quickly be recovered. After that, the savings flow directly to the bottom line. Exact savings depend on motor size, operating hours, and the electric rate. A DOE program called the Motor Challenge offers computer programs that help determine precise savings. Some motor companies offer a similar service. In addition, about 30 electric utilities in the U.S. offer rebates as high as 15% to customers who purchase high-efficiency motors.
Motor Challenge
Motor Challenge, administered by the DOE's Office of Industrial Technologies (OIT), is a voluntary partnership program with U.S. industry to promote the use of energy-efficient electric motor systems. The program is designed to help industry achieve significant electrical energy cost savings. Participation in the program helps companies manage their motor systems, resulting in improved reliability, reduced downtime, and lower operating costs.
The Motor Challenge Clearinghouse provides motor handbooks, data fact sheets, case studies, technical reports, MotorMaster+ software and other analytical software, education and training material, workshops and conferences, and a bulletin board system. To learn more about Motor Challenge visit: www. oit.doe/gov/bestpractices/motors.
MotorMaster+ 3.0 is downloadable software that provides energy-efficient motor selection and management; it includes a catalog of over 20,000 motors. In addition, the program features maintenance log tracking, efficiency analysis, savings evaluation, energy accounting, and environmental reporting capabilities.
Loss of Energy
Unbalanced three-phase power and underloaded or oversized motors are the major causes of lack of motor efficiency.
Unbalanced three-phase power supplied to industrial motors causes motor efficiency to decrease. Seemingly small unbalanced voltage can cause extremely high current unbalance that will also cause motors to run hotter and shorten motor life. The tendency for efficiency reduction with increased voltage unbalance can be observed for all motors at all load conditions.
When voltage unbalance is suspected, a comprehensive investigation should be made to determine the cause. There are numerous causes for voltage unbalance in a facility. These causes and details of voltage unbalance and its effect on motor efficiency can be found in the Energy Tips section of the Motor Challenge website.
A few years ago, common energy management knowledge said that all motors operating below 50% of rated load were not efficient and should be replaced with appropriately sized energy-efficient models. In truth, a more comprehensive investigation is required to determine energy savings. These include motor load, operating efficiency (at the load point), full-load speed (rpm) of the motor to be replaced, and full-load speed of the replacement motor.
Motor load is usually determined from part-load input power measurements compared to full-load value or from an operating speed to full-load slip relationship. The power technique is used whenever input kilowatt measurements are available. The slip technique is used when a strobe tachometer measurement can be made and power values are not available. Full-load (synchronous speed) for the existing motor can be taken from the nameplate data, and the speed characteristics for the new motor can be found in manufacturers' catalogs. Motor load estimation calculations can also be found in the fact sheet section of the Motor Challenge website.
Motors rotate slower as loads on the motor increase. At the full-load point, the motor operates at the full-load speed. This is why oversized and lightly loaded motors tend to operate at speeds that approach synchronous speed. Fully loaded energy-efficient and appropriately sized smaller motors (with higher full-load speed than the motor to be replaced) may actually operate at a slower speed than the original oversized motor. These shifts in speed and load can be substantial and have to be taken into account when computing both energy and demand savings.
In addition, the National Electrical Manufacturers Association (NEMA) does not require exact nameplate full-load revolution per minute values. For example, for 1800 rpm motors, errors of I20% in slip are allowed, leading to I10% variation in rpm for both the existing and new motors. This introduces a significant uncertainty, and even greater deviations can occur in the full-load rpm if the existing motor has been rewound.
The MotorMaster software contains a speed-correction algorithm that automatically adjusts the load for the replacement motor when the nameplate full-load speed of the motor to be replaced is entered. The software includes data at four preset load points (full, 75%, 50%, and 25%) and power factor data for most motors. An oversized motor replacement analysis is easily made as the software interpolates to determine the efficiency at the appropriate internally computed load point for the new downsized motor. Equipment and installation cost data for the replacement motor are automatically entered into the analysis with speed correction automatically accounted for. This is done in the MotorMaster Compare Section to provide a true comparison and a reliable calculation of savings.
