SYNCHRONOUS GENERATORS 345 



this auxiliary system at the end of the room can immediately be 

 thrown on to any one of the generators. 



While there are many modifications of the systems described 

 above, they will serve to indicate the general types of oiling and 

 filtering systems now in vogue. The design of these oiling sys- 

 tems is a highly specialized branch of engineering because in laying 

 them out and determining the pipe sizes it is necessary to take into 

 consideration the kind of oil to be used, and especially its viscosity, 

 the flow of oil being dependent upon the viscosity of the oil, which, 

 in turn, varies with the temperature of the plant. These factors 

 all have to be considered in laying out the piping, calculating the 

 quantity of filtering surface and designing the pumps. 



Ventilation. With large generating units the question of 

 ventilation becomes of great importance; and modern machines 

 are, therefore, being designed to control the path and utilize the 

 cooling effect of the moving air to the greatest extent. Such a 

 machine is shown in Fig. 194. The frame is provided with ven- 

 tilating holes only above the base line, no outlets being provided 

 toward the pit. The end-shields are so designed that they enclose 

 the end of the rotor; and all of the air for ventilating the machines 

 is forced by means of fans on the rotor into the end shields where 

 it is put under pressure, thus ventilating the end windings. The 

 air which passes through the core and windings below the base is 

 forced out of the large openings in the feet of the armature frame. 

 This will prevent the collection of heated air in the pit, which may 

 again be returned to the field, and so used over and over, and 

 become more and more heated. In certain instances no fans 

 need be provided, the field poles themselves providing the required 

 fan action. Another very noticeable feature of this construction 

 is the quiet running of the machines. 



For machines requiring a large amount of cooling air it is 

 becoming general practice to provide ducts whereby fresh air may 

 be taken directly from the outside to the generator pit. With 

 moderate and high-speed machines, which have a sufficient fan- 

 ning action in themselves, it is only necessary to provide hoods 

 for enclosing the ends of the machine over the pit, as shown in 

 Fig. 206. The air is then drawn directly from the outside and 

 enters both ends of the generator and is forced through the stator 

 and out in the station. The bottom of the frame has no holes so 

 as to prevent the heated air from re-entering the pit. 



