Notes on stray losses in synchronous machines

The present Standardization Rules recommend that one-third of the “short-circuit core loss” be taken as stray loss for the computation of efficiency of synchronous machines. An analysis of short-circuit tests of quite a variety of machines indicates that this is too small an amount and it is recommended that the Standardization Rules be revised to include a greater part of the “short-circuit core loss” as stray loss. The tests show that in the case of slow or moderate-speed machines practically all of the short-circuit loss can be accounted for by armature resistance loss plus eddy current loss, and that there is very little short-circuit loss in the armature core in case it is well built. In the case of high-speed turbogenerators there is a large loss appearing on short circuit which cannot be accounted for by eddy currents in the conductors. A large portion of this is due to eddy current and hysteresis loss in the un-laminated parts of the machine. An additional reason for taking more of the “short-circuit core loss” as stray loss in the case of a turbogenerator is that there is probably an appreciable additional loss with actual load on account of increased magnetic leakage from the field resulting from the increased excitation required.