Notes on induction motor losses

Abstract

The data obtained from numerous tests on induction motors, check the losses as given by the Standardization Rules with the exception of the fixed and stray losses. The tests show that both losses are less than would be obtained by the Institute method. The true fixed loss should be taken as the power input, when the motor is running at rated voltage and without load, minus the stator copper loss produced by the magnetizing current. This copper loss is appreciable in slow speed motors. With stator cores built up of well varnished punchings and without drifting or filing of the slots, the stray loss in the magnetic materials is negligible. The stray or eddy current loss in the stator copper is proportional to the stator copper loss, the percentage loss depending upon the degree of lamination of the conductor. The average stray loss of the test for both 25-and 60-cycle machines is approximately 6 per cent of the stator copper loss. The writer is not aware of any satisfactory commercial method of determining the stray loss in induction motor stators. It therefore seems desirable that an average value of the stray loss in per cent of the stator copper loss be adopted as standard. In squirrel cage motors and small wound-rotor motors, where the slip is usually greater than 2 per cent, the total rotor loss should be determined from the slip reading when operating at full load. The rotor loss should be taken as the copper I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> r loss from resistance measurements in large wound-rotor motors where the slip of less than 2 per cent and a full load slip reading cannot be readily obtained.