Comparison of induction motor field efficiency evaluation methods

Unlike testing motor efficiency in a laboratory, certain methods given in IEEE Standard 112 cannot be used for motor efficiency evaluations in the field. For example, it is difficult to load a motor in the field with a dynamometer when the motor is already coupled to driven equipment. The motor efficiency field evaluation faces a different environment from that for which IEEE Standard 112 is chiefly written. A field evaluation method consists of one or several basic methods. This paper separates and compares the basic methods according to their physical natures. Their intrusivenesses and accuracies are also discussed. This paper is useful for field engineers to select or to establish a proper efficiency evaluation method by understanding the theories and error sources of the methods. The basic methods discussed are: nameplate method; slip method; current method; statistical method; equivalent circuit method; segregated loss method; airgap torque method; and shaft torque method.