116 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



(b) If this load is removed from the line and the field excita- 

 tion of the generator is unchanged what is the high-tension line 

 voltage? 



57. A 1000-kw., 2400-volt, 3-phase generator delivers power 

 to a transmission line through three single-phase transformers 

 which are connected in delta on the low-tension and in Y on the 

 high-tension side. Each transformer has a ratio of transforma- 

 tion of 5.29 to 1. The resistances of the high- and low-tension 

 windings are 2.02 ohms and 0.072 ohm respectively. The effective 

 resistance of the generator is 0.067 ohm per phase. The armature 

 windings are connected in Y. With the high-tension windings 

 of the transformers short circuited and with a field excitation 

 of 100 amperes for. the generator the armature current is 458 

 amperes. With this same excitation the open-circuit terminal 

 voltage of the generator is 2220 volts. The rotation losses in the 

 generator are 31.2 kw. at normal voltage and the core losses 

 in each transformer are 1.8 kw. 



(a) What is the combined efficiency of the generator armature 

 and the transformers when a balanced load of 954 kw. at 0.91 

 power factor is delivered on the high-tension side of the trans- 

 formers at a line voltage of 22,400 volts? 



(b) What would be the high-tension line voltage if this load 

 were removed and the excitation of the generator unchanged? 



58. A 760-kw., 2200-volt, 3-phase generator delivers power to 

 a transmission line through three single-phase transformers which 

 have both their high and low-tension windings connected in Y. 

 With the high-tension windings of the transformers short-cir- 

 cuited the output of the generator is 37.8 kilowatts, the armature 

 current is 450 amperes when the terminal voltage is 133 volts. 

 With the transformers on open circuit and with the same field 

 excitation, the terminal voltage of the generator is 1780 volts. 

 The effective resistance of the armature is 0.172 ohm per phase. 

 The armature windings are connected in Y. The rotation losses 

 of the generator are 17.2 kw. at normal voltage and the core loss 

 in each transformer is 1670 watts. The transformers have a 

 ratio of transformation of 10 to 1. 



(a) What is the combined efficiency of the generator armature 

 and the transformers when a balanced load of 680 kw. at 0.9 

 power factor is delivered to the transmission line at 22,400 volts? 



(b) What would be the high-tension line voltage if this load 

 were removed and the excitation of the generator unchanged? 



