Stray losses in squirrel-cage induction motors. Validity of the reverse-rotation test method

The paper describes the sources and components of stray load losses in squirrel-cage induction motors and examines the validity of the reverse-rotation test method which has previously been recommended as the best available method for measuring the loss. It is shown that this method of test is based on assumptions which are fundamentally incorrect, and that the consequent errors in measured loss will be small only if the slot harmonics produce the major proportion of the total high-frequency stray loss. In other cases, with the particular design features detailed in the paper, the discrepancies can be significant. It is concluded that the validity of the test when applied to large motors is open to some doubt.