High speed turbo-alternators — Designs and limitations

The paper discusses the design of turbo-alternators with reference to the limitations imposed by very high rotative speeds and large capacities. Bolted-on rotor shafts have superseded through-shafts for very high speeds, and both the radial-slot and the parallel-slot types of rotors have inherent advantages for machines of certain capacities. Ventilation is difficult on account of the large total loss which occurs in a limited space, requiring very large volumes of cooling air at very high velocities. Various ventilating systems are described, and the effects of temperature limitations upon the design are discussed. Insulating materials are considered with reference to their mechanical strength and resistance to heat. The use of mica in various forms has solved many difficulties due to high temperature and to static discharges. The copper and iron losses of these units are analyzed; the total losses are no greater than those of low-speed units of corresponding capacity. Protection against fire within the machine is most effectively provided by doors or valves, entirely cutting off the interior of the machine from the outside air. The regulation is sacrificed to some extent, in order to limit short circuit current, by making the armature reactance as high as the design will permit.