using armature-reaction-excited machines, e.g. metadyne, amplidyne, rototrol

Metadyne Amplidyne The 'Metadyne' and the 'Amplidyne' are special-purpose DC generators historically used as high power electro-mechanical amplifiers in control systems. In use, such machines are driven at constant speed by a motor. The electrical output is varied by control of field excitation, as in a Ward-Leonard system. The Metadyne and the Amplidyne include an arrangement of cross-connected brushes on one axis and a further set of brushes on a perpendicular axis. This arrangement allows the machine to provide very high gain, that is, large changes of output may be controlled by small changes in the controlling field current A rototrol The 'Rototrol' is a two-stage machine with static and dynamic characteristics similar to those of the 'Amplidyne'. The 'Rototrol' may also be operated as a three-stage machine (also known as a 'Magnicon') in which the output is further used to excite a pole winding.