The object of this paper is to indicate some of the problems which still await satisfactory solution in the application of the automatic rotary-convertor substation to heavy electric traction.The need for overhauling present designs, with a view to decreasing maintenance, is pointed out.The phenomena which occur on short-circuiting a rotary convertor are discussed.The high-speed breaker and its advantages are described, and the difficulty now found in securing accurate calibration is referred to, coupled with a suggestion for overcoming this difficulty by the use of stabilizing resistances.The importance of sufficiently insulating series windings, whether connected on the earthed side of the machine or not, is pointed out.The effect of high ambient temperature on machine output is discussed, with special reference to sets operated through thermal relays, as is the case in automatic substations, and the importance of ventilating the machinery with fresh air from outside the substation is emphasized.The use of air filters, and the type of filter most suitable for the conditions, are discussed.A type of opening which gives free egress for the air and prevents the entrance of rain is described.Suggestions are made regarding the method of ventilating the armature core when forced ventilation is adopted.A vertical design of rotary convertor which seems to offer advantages is described.The present design of thermal relay is discussed and suggestions are made for its modification, so as to imitate more closely the heating which occurs in the rotary-convertor armature under the varying conditions of operation. An alternative method, depending upon the actual hot-spot temperature and eliminating an independent thermal relay, is described.The available means of obtaining control current for the operation of the automatic equipment are considered, particularly from the point of view of the complete or partial elimination of batteries and, if they must be retained, of the most suitable type to employ.Fire risk and the means available for fire quenching in the event of sustained conflagration are discussed.