300 



ELECTRICAL EQUIPMENT 



FIG. 176. 



coils in which no current is flowing, and so serve to carry away the 

 heat from the two-thirds of the stator in which 

 there is current. 



With two-phase alternators, single-phase 

 current may be taken off from two of the termi- 

 nals, and assuming the same limiting current / 

 per coil and a coil e.m.f. Ei, we get the single- 

 phase capacity for Fig. 169, V2EiX2I, or equal 

 to 2,828#i7 which is 70.7 per cent of the cor- 

 responding two-phase rating 4EiI. , 



For the arrangements shown in diagrams 

 Figs. 170 to 172, the single-phase rating will be 

 equal to 2EJ, while for Figs. 176 and 177 it 

 will equal 2.828#i7. 



A comparison of the two-phase and three-phase capacities 

 both with respect to each other and 

 to the single-phase ratings obtained 

 is readily made. As E\ is equal to 

 V2XE, the two-phase ratings 4#i7, 

 when put in terms of three-phase, 

 will be 4XN/2X-EX7 or equal to 

 5.656^7. When comparing this with 

 the ratings obtained from the various 

 arrangements given on page 297, it is 

 seen that the three-phase closed-coil 

 arrangement gives a less output than 

 for two-phase, while the other three- 

 phase arrangements give an increased 

 rating. 



The best single-phase rating ob- 

 tained from a three-phase winding 

 occurred with the closed-coil arrange- 

 ment, Fig. 174 and was equal to 4EI. 

 For a two-phase winding, on the other 

 hand, the best single-phase rating was 

 shown to be equal to 2.82&EiI. As E\ is equal to ^/2E, this 

 equals 2.828 X\/2X#X/, or 4E'7; thus the same as with closed- 

 coil winding, shown in Fig. 174. 



The above capacities have, as previously stated, only refer- 

 ence to machines which can be adapted to both polyphase and 



FIG. 177. 



