Evaluation of methods for estimating motor efficiency without removing motor from service

摘要

This paper provides a brief survey of methods for evaluating the efficiency of an operating motor. In general, these methods estimate the motor's efficiency by measuring some combination of the current, voltage, power in and speed. The motor's efficiency is calculated using an equivalent circuit model or other mathematical representation of the motor. There is a need for an efficiency estimating tool that can be used easily and with a reasonable level of confidence so that motors can be evaluated for replacement with energy efficient motors in a simple cost benefit analysis. The report provides an overview of various methods for estimating the operating efficiency of a motor without actually removing it from service and testing it on a dynamometer. Dynamometer testing, while accurate, is expensive and highly intrusive to the operating process. The efficiency estimation tool needed for the cost benefit analysis must be easy to use, without disrupting the operating process, and must provide a reasonable accuracy. The study reports on several efficiency estimation methods and compares them with actual dynamometer measurements of efficiency. It is found that reasonable estimates can be made without a high level of cost and disruption of the process. For example, if the motor can be disconnected from its load and operated at no load condition, and if a measurement of stator resistance may be taken, several of its losses can be reasonably approximated as in Method E of IEEE Standard 112 using a segregated loss method. This method can then be used when the motor is operated at its normal load condition to evaluate the losses in the motor and estimate motor operating efficiency. The method has been found to provide a reasonable estimate (perhaps 3% accuracy) when compared with the dynamometer method in the laboratory. However, disconnecting the motor from the load does require a short interruption in the process.