SYNCHRONOUS GENERATORS 327 



split, this can be done either lengthwise or crosswise to the shaft. 

 The former method is used when the diameter of the rotor is very 

 large and the latter method when the length is large (see Fig. 190). 

 The sections should be securely held together by heavy bolts and 

 link keys, and when the field is split crosswise to the shaft one set 

 of arms should preferably be provided for each section so as to 

 insure a rigid construction. 



Field Poles. The pole pieces are built up of laminated sheet 

 steel punchings, spreading at the pole face so as to secure not only 

 a wide polar arc for the proper distribution for the magnetic flux, 

 but also for holding the field coils in place. These punchings are 

 either riveted or bolted together and reinforced by two stiff end 

 plates. For machines of moderate speed the poles are simply 

 bolted to the rim, while for machines of higher speeds they are 

 solidly mounted on the spider by means of dovetail slots in the 

 rim (see Fig. 190). These dovetailed grooves should be made 

 somewhat larger than the corresponding part of the punchings 

 and a tight fit is obtained by means of steel wedges, which are 

 guarded from falling out by two bolted end rings. For high-speed 

 water-wheel-driven generators which must be designed for a run- 

 away speed of twice normal, it often becomes necessary to pro- 

 vide additional precautions against the increased centrifugal 

 stresses at such occasions. Solid steel rings as shown in Fig. 191, 

 are then often provided at each end of the rotor, these rings being 

 securely bolted both to the rim and each pole piece. On some 

 very high-speed machines, a design as shown in Fig. 192, is often 

 used. The field centers are here constructed of rolled steel plates 

 and the pole pieces are securely dovetailed thereto, thus making 

 a very substantial construction. 



The revolving parts of water-wheel-driven generators should 

 be designed so as to keep the stresses due to centrifugal force, 

 well within the elastic limit of all the material at the run-away 

 speed of the water wheel. This speed varies with different 

 types of wheels and different conditions of installation; but the 

 general practice is to design the rotors with a 100 per cent over- 

 speed in view. 



Flywheel Effect. This problem should be considered when the 

 design of the rotor is decided on, as well as when a comparison 

 between different proposed generators is made. This is really a 

 hydraulic problem, and where additional flywheel effect is required 



