For many years special difficulties have been experienced with sliding contacts, especially commutators, in aircraft generators. These difficulties have been attributable chiefly to the high running speeds, the dry rarified air at high altitudes and the wide range of temperatures encountered. Improvements in commutators and brushes have been made at considerable cost, but at a rate which, if continued, would not keep pace with contemporary advances in aircraft performances.Fortunately the silicon rectifier has now been developed to the point at which it can be used as a static commutation device. The characteristics of silicon rectifiers and their application to the commutation of polyphase alternating currents generated in rotating machines are discussed and some of the fundamental limitations are indicated.The known types of brushless generator are then described; they may be divided into three main groups. The first includes permanent-magnet and induction generators, whose inherent charateristics are unacceptable for aircraft applications.The second, and at present most important, group comprises a.c. and d.c. generators with revolving field windings which are energized through shaft-mounted (rotating) rectifiers from the armature of an a.c. exciter. These generators have been fully developed and proved in aircraft service, and can be cooled with air at an inlet temperature up to 150°C. With further development and improved rectifiers it is believed that this temperature can be raised to at least 200° C. These inlet air temperatures are normally associated with supersonic aircraft speeds and a substantial drag penalty for air cooling, and in very-high-speed aircraft of the future liquid cooling may be preferred, if not inevitable. Liquid cooling does, however, add to the complexity of a generating system. Partly for this reason much attention has been paid in recent years to generators with stationary field windings which do not require rotating rectifiers and which, it is hoped, can be cooled with air at inlet temperatures up to 250° or 300° C. The various types of generator of this kind constitute the third main group. Careful studies have shown that they are all about 40–50% heavier than equivalent generators of the rotating-rectifier type, and prototype solid-rotor generators have revealed difficulties of construction which, although not insuperable, have yet to be resolved. Nevertheless, these generators hold promise of simplicity with attendant reliability for severe operating conditions.