8 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



third greater than the iron losses of the first. The same size of 

 wire is used in winding both transformers but the mean length 

 of one turn is 5 per cent, greater in the second on account of the 

 larger magnetic core. Assume that the iron losses vary as the 

 1.7 power of the flux density. Both transformers have the same 

 voltage rating. The first transformer has a full-load capacity of 

 100 kw. and a full-load efficiency of 97.9 per cent, at unit power 

 factor. On the basis of the same total losses what should be the 

 full-load rating of the second transformer? What is the ratio 

 of the iron losses to the copper losses at full load in the second 

 transformer? 



36. Three equal 10 to 1 transformers, each of 100 kv.-a. 

 capacity, are arranged with both the primaries and secondaries 

 in Y to receive power from a 3-phase, 2200-volt circuit. The 

 losses in each transformer at full load are 1960 watts of which 940 

 are core losses. If these transformers are connected in delta to 

 this circuit, what power will they deliver without exceeding their 

 full-load heating losses? Assume that the iron losses vary as 1.7 

 power of the flux density. 



37. The name plate of a transformer gives the following 

 data: 100 kv.-a., 6600 : 220 volts, 25 cycles. The percentage 

 distribution of the losses at full load is: copper loss, 1.0 per cent.; 

 eddy current loss, 0.3 per cent.; hysteresis loss 0.8 per cent. If 

 the insulation of the transformer will safely stand double volt- 

 age what should be its rating on the basis of the same total 

 heating losses when taking power from an 11,000-volt, 50-cycle 

 circuit? 



38. The name plate of a transformer gives the following data : 

 500 kv.-a., 13,200 : 400 volts, 50 cycles. At full load the copper 

 loss is 4600 watts and the iron loss is 2800 watts. Of the latter 22 

 per cent, is due to eddy currents. If this transformer is to receive 

 power from an 11,000-volt, 60-cycle circuit what should be its 

 full-load rating on the basis of the same total heating losses? 



39. A 10-kw. lighting transformer takes 122 watts on open 

 circuit and 178 watts on short circuit with full-load current in the 

 windings. This transformer is connected to the supply circuit 

 continuously. It delivers its rated load, however, during but 6 

 hours each day and is idle the remaining 18 hours. What is the 

 all-day efficiency? 



40. A lOO-kv.-a. transformer supplies a lighting and power 

 load. The iron loss is 1010 watts and the copper loss is 1004 watts 



