SYNCHRONOUS GENERATORS 307 



very dangerous, owing to the possibilities of a resonance voltage 

 rise. 



If three auto-transformers, Y-connected with the neutral 

 grounded as the only ground, are used to step up the generator 

 voltage, abnormal potentials to ground may result, due to the 

 presence of high harmonics. The distortion does not appear in 

 the voltage between the lines because the distortion between one 

 line and the neutral is canceled by that of the other two lines. 

 The voltage distortion may be eliminated by providing a path 

 for the triple-frequency exciting current which is required for the 

 magnetization of the transformer. This is done by connecting 

 the transformer neutral to the generator neutral. 



Rating. Synchronous generators should be rated by the 

 electrical output, and this should be expressed in kilo-volt- 

 amperes (Kv.A.) and not in kilowatts (Kw.) unless the power 

 factor of the load is also given. Preferably both should be given, 

 so as to avoid any misunderstanding whether Kv.A. or Kw. is 

 meant, for example 2000 Kv.A. (1600 Kw.-.8 P.F.). 



Most water- wheel-driven generators are now given a maximum 

 continuous rating, without any overload provision, except that 

 they must be able to carry momentary loads of 150 per cent of 

 the amperes corresponding to the continuous rating, keeping the 

 rheostat set for load excitation. 



The rated full-load current is that current which, with rated 

 voltage, gives the rated kilowatts or rated kilo-volt-amperes. 

 In machines in which the rated voltage differs from the no-load 

 voltage, the rated current should refer to the former. The rated 

 output may be determined as follows: 



If # = full load terminal voltage and 7 = rated current, then 



El 



for a single-phase generator Kv.A.= -. 



For a two-phase generator the total output is equal to the out- 

 put of the two single-phase circuits, and if /, in this case, is the 

 rated current per circuit, the output for a two-phase generator is 



2EI 



Kv - A= loo6- 



For a three-phase generator there are three circuits to be con- 

 sidered, whether the machine is star or delta connected. If E is 

 the terminal voltage and / the line current, then for a three-phase 



T, A 3 El V3EI 



generator Kv. A. = 



