Is the Expense of Equipment and Testing Time of Calorimetry Truly Justified? Stray Load Loss Efficiency Connections

INDUCTION MOTORS ARE THE most commonly employed electrical machines, in industry and elsewhere, throughout the world today. During its lifetime, an induction motor will convert such large quantities of energy that its “fuel bill” far exceeds its “first cost.” Consequently, investment in the quality of a motor that enables a reduction of its losses even by only a small percent that is frequently neglected is usually a financially sound practice. When examining the losses of an induction machine, it is possible to precisely identify the so-called “conventional losses,” which include stator and rotor conductor losses, magnetic core loss, and friction and windage loss. However, induction motors also have an additional power loss component termed “stray load loss” (SLL) that is caused by the nonideal nature of a practical machine. Also, because the SLL represents a small fraction of the total power, it is very difficult to calculate analytically. SLL has many components, and direct measurement is a very involved process. This has led to the adoption of allocations for SLL in efficiency standards. Unfortunately, many of the standards employed in manufacturing nations make different allocations for SLL. In an era of global commodity manufacturing, this makes for a difficult comparison of products. Usually, SLL can be evaluated by special experimental methods. Thus, one of the reasons for the differences in efficiency values from different national standards is the variation of SLL evaluation methods. However, SLL is a very significant part of overall losses that may have a big influence on the temperature situation of the motor. For some motors, the level of SLL can reach several percent of the input power and can seriously affect the efficiency. All of the above reasons make the evaluation of SLL in induction motors an important part of the efficiency estimation process. The use of calorimetry to precisely determine SLL is being pursued in several laboratories worldwide.