Evaluation of stray load losses in induction motors at different supply frequencies

Increasing use of induction motors at other than a single frequency, and the importance of their operating efficiency, has renewed interest in losses and the methods by which they can be measured and segregated. In particular this is true of the stray load loss, which is probably less well understood/quantified than other loss components. This paper describes the use of an adjustable frequency source, of low total harmonic distortion, to power several commercial induction motors over a frequency range from 46 Hz to 65 Hz over load ranges from 25% to 120% of rating. The data is analyzed in accordance with IEEE Std 112 B and thus stray load loss is quantified by loss segregation, as a function of both frequency and load. Through this analysis, it is determined that stray load loss is directly and linearly proportional to the frequency, and directly proportional to the square of the load. The value of IEEE Std112 E-E1 as an alternative approach is also discussed.