Determination of load loss correction factors for rotating electric machines

The paper describes tests which have been made to determine load loss correction factors for direct-current motors and generators, alternating-current generators and synchronous motors, and synchronous converters. The term “load loss” is understood to mean the difference between the total losses under load and the sum of the separate losses as determined from no-load measurements. A series of very careful loss measurements, made with conditions equivalent to those normally existing under load, is described. The losses thus measured are the sum total of the individual losses under load. These values are compared with the sum of the separate losses as determined from no load measurements. The difference between the readings is the “load loss” or “stray loss.” This load loss is due, first, to the increased core loss resulting from field distortion, second, to eddy current losses in the armature conductors caused by fields set up by useful currents flowing in them. All the other losses can be accurately determined from no load measurements. Therefore the proposed correcting factors are not to be applied to the sum of the separate losses but to the sum of the calculated armature I²r loss and the no load core loss. A few laboratory tests on the different types indicate certain fairly uniform ratios existing between the sum of these losses under actual load conditions and their sum as determined from no load measurements. A large number of commercial input-output tests is then examined and checked by means of the correcting factors determined from these laboratory tests. As the averages of the commercial tests agree quite closely with the laboratory tests the authors feel justified in recommending the following values for the proposed correction factors.