TURBO-ALTERNATOR 87 



approximation to a sine wave, the Oerlikon Co. employs laminated 

 pole-shoes, consisting of a number of sets of core-plates displaced 

 relatively to each other, or " staggered," so as to form a series of 

 steps, as shown in Fig. 72. 



42. Construction of Turbine-driven Alternator 



Steam turbine-driven alternators present many constructional 

 difficulties. Owing to their higli speed,* the number of field-poles 

 for the usual frequencies (25, 30, 50, and 60) is small, and as a result 

 each field-pole is relatively very large, and the difficulty of securing 

 it satisfactorily correspondingly great. Where the speed is moderately 

 high, the poles may be dove-tailed into the field-ring as in Fig. 70. 

 But for very high speeds special forms of construction must be used. 



P ' .ROUE-SHOE 



'IRON /^/V^T^x- .BRASS SUPPORTS 

 FOR COILS 



FIG. 73. Field Construction for Turbo-generator. 



One of these, used by the Oerlikon Co., is shown in Fig. 73. The 

 entire field structure is laminated, the core-plates being directly 

 threaded on the shaft. The edge-strip winding is prevented from 

 bulging sideways by means of wedge-shaped brass supporting frames. 

 In order to prevent noise always a troublesome feature in turbine- 

 driven machines the field is surrounded by a laminated ring, built 

 up of alternate segments of brass and iron made to overlap and 

 riveted together, the iron portions of the ring forming the pole-shoes. 

 This ring presents a perfectly smooth outer surface, which is conducive 

 to silent running. 



* The peripheral velocity may be as high aa 18,000 feet per inin. 



