The paper presents a theoretical analysis of the asynchronous operation of turbo-generators, and equations are developed for predicting performance characteristics for two important cases in which the field circuit is either open-circuited or short-circuited through a resistor. The basic problem is that of calculating the effects of slip-frequency eddy currents induced in the surface of the highly saturated rotor forging, and the results of an approximate non-linear analysis are used for this purpose.Results of asynchronous tests conducted on a 60MW generator at Marchwood power station in March, 1960, are presented, and the accuracy and validity of the theoretical treatment are examined by comparison with these and earlier test results. The agreement obtained is considered to be sufficiently encouraging to warrant the use of the method of calculation as a basis for assessing the permissible asynchronous loading of turbo-generators.