18 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



(a) The regular transformer is rated to deliver 100 kw.-a. at 

 110 volts. If the losses at full load are the same for both 

 transformers, what is the kilovolt-ampere rating of the auto- 

 transformer? 



(b) What voltage should be impressed on the high-tension 

 winding of the auto-transformer so that there will be full-load 

 current in the short-circuited low-tension winding? 



(c) What are the regulation and efficiency of each transformer 

 for full-load current at 0.8 power factor? 



82. A lO-kv.-a. and a 25-kv.-a. transformer, each of which has 

 a ratio of transformation of 5:1, have their primaries connected in 

 parallel across an 1100-volt circuit. Their secondaries are 

 also connected in parallel and supply 152 amperes at 0.84 power 

 factor to an induction motor. Referred to the secondary sides 

 the equivalent resistances are 0.0865 ohm and 0.0272 ohm 

 respectively and the equivalent reactances are 0.143 ohm and 

 0.0856 ohm respectively. 



What current does each transformer take from the circuit? 

 Compare these currents with their rated full-load values. 



83. The following short-circuit data are given on two 100- 

 kv.-a., ll,000:460-volt transformers: 



Type Amperes 1 Voltage Watts 



These transformers are connected in parallel on both the high- 

 and low-tension sides and supply a load of 186 kw. at 0.93 power 

 factor. What percentage of its full-load current does each trans- 

 former supply? 



84. The following short-circuit data are given on two trans- 

 formers which have a ratio of transformation of 11,000:2300 

 volts: 



These transformers are connected in parallel on both the high- 

 and low-tension sides and supply a total current of 293 amperes 

 at 0.92 power factor on the low-tension side. What is the current 



1 Full-load current. 



