SYNCHRONOUS GENERATORS 281 



A cycle is a complete wave of two alternations and the frequency 

 is equal to the product of the number of pairs of poles and the 

 speed of the machine in revolutions per second; it is, therefore, 

 strictly proportional to the speed of the machine. 



The wave shape of the e.m.f. depends on the distribution of 

 the magnetic flux at the armature surface, and the total e.m.f. 

 is the sum of the e.m.f. waves in the different armature conductors, 

 added in the proper phase relation. The instantaneous values of 

 the e.m.f. and current are constantly changing from maximum 

 positive to maximum negative and the specified or effective value 

 is equal to the square root of the average value of the square of 

 the instantaneous values. For a true sine wave shape it is equal to 

 the maximum value divided by V2. 



The phase relation differs symmetrically for polyphase sys- 

 tems. In the two-phase system the terminal voltages of the two 

 circuits differ in phase by 90 electrical degrees, Fig. 151, and in the 



180 \270 /360" 



FIG. 151. Two-phase Alternating Current. 



three-phase system, the terminal voltages of the three circuits 

 differ in phase by 120 electrical degrees, Fig. 152. The terminal 

 voltage of two-phase generators is equal to the e.m.f. of the arma- 

 ture circuits and the line current equal to the current in these cir- 

 cuits. For three-phase machines, however, the armature winding 

 can be connected either Y or A, which will be discussed more fully 

 later. If the winding is Y-connected, then the terminal voltage 

 is equal to V5 times the e.m.f. per armature circuit and the line 

 current equal to the armature current. If the winding is A-con- 

 nected, then the terminal voltage is equal to the e.m.f. per circuit 

 and the line current equal to V~3 times the current in the armature 

 circuit. In general, when speaking of current and voltage in 



