Considerations on in-field induction motor load estimation methods

Due to poor motor system design or excessive safety factors, it is common to find oversized three-phase, squirrel-cage, induction motors, i.e., with a rated power higher than that actually necessary by the driven equipment. In those cases, if the oversizing level is significant, besides the unnecessary additional capital investment, the motors operate with poor efficiency and power factor, being an undesirable situation for the user. In this paper, an overview on the in-field motor load estimation methods is offered, being its importance evidenced. The simplest, low-cost, in-field methods to indirectly estimate the motor load are analyzed, being the focus on methods based upon the motor current, slip, and input power. Such methods require simple measurements with relatively low-cost equipment and information from the motor nameplate, manufacturers, catalogues, and/or databases, having a fair accuracy if applied to specific motors and load ranges. Some improvements in those methods are proposed to increase their accuracy, including combined solutions. A new definition of ldquouseful accuracyrdquo is introduced, for evaluation of the effectiveness or adequacy of the motor load estimation methods.