The following investigation is concerned with the operation of a rotary convertor during conditions of abnormal loading, such as violent fluctuations occurring at definite intervals or during short-circuit conditions. In the former case the rotary convertor, being an oscillatory system having a natural period of its own, behaves just like any mechanical oscillatory system which is subjected to uniformly timed impulses. It is shown that when the natural and the forcing periods synchronize, resonance takes place and very large displacements may be set up, not only with the fundamental period of the disturbance but also by selective resonance with any harmonics which may be present. Methods of determining the natural period of the convertor and of calculating the amplitude of the oscillations set up under specified conditions are given, the methods being verified by experimental results.The second part of the investigation deals with the motion of the rotor when the machine is subjected to a short-circuit of short duration. The mechanical analogy is in this case a ballistic pendulum, and expressions are developed which enable the displacement and slip velocity to be determined at the moment the short-circuit is cleared, and also the motion subsequent to clearing. These expressions also are verified by experiment, and considerable light is thrown on the requirements of the quick-acting circuit breakers which are commonly used on the d.c. side of rotary convertors.The research has, of necessity, been conducted on a small experimental machine, but the theoretical and experimental investigations are found to be in reasonably close agreeement and, in the case of the operation under short-circuit conditions particularly, it is possible that some other investigator having the necessary facilities may be induced to repeat the investigation on a machine of commercial size.