THE ALTERNATING CURRENT GENERATOR 251 



the magnitude of the maximum reaction of one phase of 

 the winding. The direction of this reaction depends upon 

 the power factor of the load, a lagging load demagnetiz- 

 ing the field and a leading one magnetizing it. 



Previous Analyses for Balanced Loads Only. The pre- 

 vious analyses have been carried out on the assumption 

 of a balanced polyphase load, i.e. all phases carrying cur- 

 rents of the same magnitude. Summing up the results for 

 a polyphase machine, we see that, when the load is balanced, 

 the armature reaction is constant in magnitude and direction, 

 the same as in a continuous current machine; the magnitude 

 of the reaction depends upon the value of the armature 

 current and the direction depends upon the power factor of 

 the load. 



61. Rating of A-C. Machinery. The rating of alter- 

 nating current machines, generators, transformers, etc. 

 is practically never given in kilowatts but in kilovolt- 

 amperes. A generator having a safe current capacity of 

 10 amperes and giving an e.m.f. of 100 volts would be rated 

 as having a capacity of one kilovolt-ampere or abbre- 

 viated, 1 kv-a. 



To get the rating of a single-phase generator or trans- 

 former, the safe capacity in amperes is multiplied by the 

 terminal voltage and this product is divided by one thousand. 

 For example a 2300-volt alternator having a safe current 



2300X100 



capacity of 100 amps, would have a rating of 



1000 



= 230 kv-a. 



Capacity of Polyphase Machines. If it were a two- 

 phase alternator having a capacity of 100 amperes per 

 phase and giving 2300 volts in each phase, the kv-a. rating 

 would be twice that of one phase, or 460 kv-a. If it were 

 a three-phase alternator, giving 2300 volts between any 

 two lines and having a capacity of 100 amperes per line the 



2300 X 100 X\/3 

 kv-a. rating would be equal to = 398 kv-a. 



1000 



