346 



ELECTRICAL EQUIPMENT 



This method of ventilation is also readily adopted with vertical 

 units. The fresh air is drawn from the pit and forced through 

 holes in the spider between the pole pieces through the ducts in 

 the stator and then out in the station through the opening in the 

 top. In case it is objectionable to let the heated air out in the 

 station, as in the summer time, it may instead be piped to the out- 

 side. The top of the generator may be covered with a sheet- 

 steel hood to which a duct leading to the outside may be attached. 



FIG. 206. Horizontal Water-wheel-driven Generator, Showing Hoods Pro- 

 vided for Ventilation. 



During the winter months it is, of course, advisable to discharge 

 the heated air into the station in order to heat the same. 



With the advent of very slow-speed machines and low periph- 

 eral velocities where fans attached to the rotor cannot be effec- 

 tively used, it may become necessary to resort to forced ventila- 

 tion by providing motor-driven fans, as shown in Fig. 207. The 

 ventilating system should preferably be sectionalized, each section 

 being provided with' at least two fans one for spare. Where 

 three fans are provided, the combined capacity of two must be 



