Computerized Load and Efficiency Estimation Techniques

There are several sophisticated methods for determining motor efficiency. These fall into three categories: special devices, software methods, and analytical methods. The special devices package all or most of the required instrumentation in a portable box. Software and analytical methods require generic portable instruments for measuring watts, vars, resistance, volts, amps, and speed. These need to be instruments of premium accuracy, especially the wattmeter that must have a broad range including good accuracy at low power and low power factor.

Washington State University Cooperative Extension Energy Program, in partnership with the Oregon State University Motor Systems Resource Facility, recently conducted lab testing of several efficiency-measuring methods. These included three special devices: the Vogelsang and Benning Motor-Check, the ECNZ Vectron Motor Monitor, and the Niagara Instruments MAS-1000. Their efficiency readings were carefully compared to “true” efficiency, measured by a dynamometer and precision lab instruments per IEEE testing standards. From 25% load
to 150% load the special devices tended to hold an accuracy within 3%, even in adverse conditions of voltage deviation and unbalance on old, damaged, or rewound motors. In less challenging test conditions, they tended to operate within 2% accuracy. These instruments require a skilled electrician or other personnel trained in the safe connection of electrical equipment in industrial power systems plus about a day of training and practice. The motors must be temporarily unpowered for a resistance test and temporarily uncoupled for a no-load test, i.e., running at normal voltage unloaded. Uncoupling in-situ is rarely convenient, but the no-load test can be run at times such as receiving inspection or following service at the shop. No-load performance does not tend to change significantly over time in the absence of a failure/repair event.

Software and analytical methods were also tested in the lab research described above. When measurement of input data was made with precision lab instruments, the accuracy of methods requiring a no-load test approached that of the special devices’ performance.

The Oak Ridge National Laboratory has developed ORMEL96 (Oak Ridge Motor Efficiency and Load, 1996), a software program that uses an equivalent circuit method to estimate the load and efficiency of an in-service motor. Only nameplate data and a measurement of rotor speed are required to compute both the motor efficiency and load factor. The program allows the user to enter optional measured data, such as stator resistance, to improve accuracy of the efficiency estimate. Future refinements of ORMEL96 are expected to create a more user-friendly product.

Finally, motor load and efficiency values are automatically determined when measured values are entered into MotorMaster+ software’s motor inventory module. MotorMaster+ contains a database of new motor price and performance, and features many motor energy management capabilities including replacement analysis, maintenance logging, inventory control, energy and dollar savings tracking, and life cycle cost analysis. MotorMaster+ is available at no cost to Motor Challenge Partners.

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