126 



ALTERNATING CURRENTS 



these leads being the three pairs of terminals from the three 

 phases. The proper phase relations must be observed in making 

 the connections, whether they are to be connected in Y or in delta. 



Let aa', W and cc f , Fig. 132, be the three coil windings of a 

 three-phase machine. 



Assume first that these three windings are to be connected in 

 Y. First connect ends a and b together. Measure E a v, the 

 voltage across their open ends. This should equal \/3 t lines t he 

 coil voltage. It may be equal to the coil voltage, in which case 

 one coil should be reversed. Next tie the end c of coil cc' to 

 point ab. The voltages E b > c > and E a > e ' should each be \/3 times 

 the coil voltage. If not, the coil cc' should be reversed. 



FIG. 133. Connecting alternator coils in delta. 



If it be desired to connect the coils in delta, the ends a and b', 

 Fig. 133 (a), should first be connected. The voltage E a > b , across 

 their open ends, should be equal to the coil voltage. If not, one 

 of these two coils should be reversed. End c' of coil cc' should 

 then be connected to 6. The voltage E ca ', across the open ends 

 should be zero, as shown by the vector diagram in (b). (See 

 Par. 46, Page 83.) If this voltage is practically zero, the two ends 

 c and a' may then be closed. The voltage E ca f may be twice the 

 coil voltage, as shown in (c) . If this is found to be the case, coil 

 cc' should be reversed. 



60. Rating of Alternators. The rating of electric machinery 

 is determined in general by its temperature rise. This tempera- 

 ture rise is caused by the losses in the machine. The PR loss 

 in the armature, due to the load current, limits the output of a 

 machine. This loss depends upon the value of the armature 

 current and is independent of power-factor. For exam pie, 100 

 amp. in a single-phase, 200-volt generator will produce the same 



