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Estimating Motor Efficiency in the Field

August 6, 2007
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Some utility companies and public agencies have rebate programs in place to encourage customers to upgrade their existing standard-efficiency motors to NEMA Premium™ efficiency motors. Yet, to accurately estimate energy savings and determine annual dollar savings requires knowing the efficiency of the existing motor.

Efficiency is output power divided by input power, yet most of the methods and devices attempt to assess losses to circumvent the difficult task of measuring shaft output power. Efficiency needs to be measured accurately because, as shown in Table 1, a single percentage point of improved efficiency is worth significant dollar savings - even for motors as small as 25 horsepower (hp). A good electric power meter can provide an accuracy of 1 percent, but an inexpensive, portable way to measure shaft output power of a coupled motor does not exist. A further complication is that motor efficiency is dependent upon loading, power quality, and ambient temperature.

Credible efficiency ratings are normally obtained in a laboratory, following carefully controlled dynamometer testing procedures as described in IEEE Standard 112(b). Field measurements for determining motor efficiency pose challenges that require developing various methods and devices.

Motor losses fall into several categories that can be measured in various ways:

• Stator electric power (I2R) losses.

• Rotor electric power (I2R) losses.

• Friction and windage losses (including bearing losses, wind resistance, and cooling fan load).

• Stator and rotor core losses.

• Stray load losses (miscellaneous other losses).

The most direct and credible methods of measuring these losses involve considerable labor, equipment, and the availability of electrical power. Power readings must be taken with the motor running under load, then uncoupled and running unloaded. Winding resistance must be measured. Temperature corrections must be performed.

Some of the available field motor-efficiency estimation methods include:

Loss accounting methods. These measure most of the above losses using either special dedicated “lab-in-a-box” devices or very accurate conventional instruments, for example, power meters, thermometers, and micro-ohmmeters. These methods have the potential of being accurate within 1 percent to 3 percent if carefully applied. The necessary instruments are costly and the process is very time and labor consuming. Power meters must be accurate at very low power factors that occur when motors operate unloaded.

Slip method. The slip method has largely been discredited as a viable technique for estimating motor efficiency. This method computes shaft output power as the rated horsepower multiplied by the ratio of measured slip to the slip implied by the nameplate. Slip is the difference between synchronous and shaft speed.

Current signature predictive maintenance devices. A number of sophisticated devices are marketed for analyzing motor condition, particularly current harmonics, based upon electrical measurements of an operating motor. While the accuracy of these devices has not been verified, the marginal cost and labor of using these devices is small if they are already deployed for predictive maintenance uses.

MotorMaster+ 4.0. The U.S. Department of Energy’s (DOE’s) MotorMaster+ 4.0 software incorporates several methods for determining motor load. These involve the use of motor nameplate data in conjunction with selected combinations of input power, voltage, current, and/or operating speed. With the percent load known, the software determines as-loaded efficiency from default tables based on the motor type, condition, and horsepower. MotorMaster+ automatically chooses the best available method based upon the data it is given.

Suggested Actions

Conduct predictive maintenance tests to reveal whether efficiency is below the original or nameplate level. Decreased efficiency may be due to:

• Higher winding resistance compared to manufacturer specifications or an earlier measurement. This may be caused by winding being at a higher temperature than that of the manufacturer’s resistance specifications or by rewinding with smaller diameter wire. A low resistance ohmmeter is often required for winding resistance tests.

• Increase in no-load power or core losses. Core loss testing requires motor disassembly and is per­formed in a motor service center.

• Significant current unbalance when voltage is balanced.

• Evidence of cage damage.

Resources

U.S. Department of Energy (DOE) - For additional information on motor and motor-driven system efficiency measures, to obtain the DOE’s MotorMaster+ software, or learn more about training, visit the BestPractices Website at www.eere.energy.gov/industry/bestpractices.

Reprinted from Motor Systems Tip Sheet #2, “Estimating Motor Efficiency in the Field,” from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. For more information, visit www.eere.energy.gov.

Publication date: 08/06/2007

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