118 



ALTERNATING CURRENTS 



dove-tailed to the spider in the manner indicated. The slots 

 in the pole faces of this rotor should be noted. Damping or 

 amortisseur windings are placed in these slots as will be described 

 later. (See page 319, Fig. 295.) 



Salient poles cannot be used for high-speed turbo-generators, 

 owing to the large centrifugal forces developed and to the 

 excessive windage. Therefore, a non-salient-pole rotor is used. 

 There are two common types of such a rotor, the parallel-slot 

 type shown in Fig. 125 and the radial-slot type shown in Fig. 126. 



FIG. 125. Parallel-slot, 2-pole rotor for a turbo-alternator. 



The winding in the parallel-slot type is of strip copper, wound 

 by hand in the slots. The wires are held in the slots by means 

 of non-magnetic metallic wedges. There is not sufficient space 

 to run the shaft through the center of the rotor, so it is boltod to 

 the ends by phosphor-bronze flanges, Fig. 125. These flanges 

 must be non-magnetic or they would short-circuit the magnetic 

 poles. The fact that they are of phosphor-bronze makes them ex- 

 pensive. This construction gives a smooth rotor, with little wind- 

 age loss and strong mechanically, especially as regards the support 

 of the coil ends. Parallel-slot rotors are seldom used except for 

 two-pole rotors in small machines. The metal back of the slots 

 becomes too small in cross-section to withstand the centrifugal 



