80 ELECTRICAL ENGINEERING PROBLEMS 



what changed loss would result from the combined changes in 

 frequency and density? (6 min.) 



15. If 4750 be the highest density (maximum) allowable in a 

 5-kilovolt-ampere, 60-period, 10 to 1 transformer on 1150 volts 

 primary, how many secondary turns must be used if the area of 

 the iron is 112 square centimeters? (4 min.) 



16. A 20-kilovolt-ampere transformer, designed for a 60-period 

 circuit, and having losses as follows: Hysteresis, 110; eddy current, 

 19, and copper loss, 272, is to be used on a 140-period circuit. 

 If, at the density employed, the hysteresis loss per cycle varies as 

 B 1 - 7 , what will be the efficiency in each case (neglect the change 

 in copper loss). (5 min.) 



17. A 1500-watt transformer used on 1000 volts has losses as 

 follows: 28 watts hysteresis, 4.5 watts eddy current, and 40.6 

 watts copper. It is also run on a 600-volt circuit; with the same 

 current output and neglecting the change in the primary current, 

 required the efficiency in each case. (4 min.) 



18. A 30 kv-a., 1100 to 110-volt transformer has a no load loss 

 of 165 watts, and primary resistance of 0.25; find the resistance 

 of the secondary and determine the efficiency of the transformer 

 at full load, f , \ and J load. Plot these results. Neglect the effect 

 of the exciting current. (10 min.) 



19. A 2400 to 120 volt, 20-kilovolt-ampere transformer has a 

 primary resistance of 1.9 ohms, and an iron loss of 129 watts; 

 required the all-day efficiency, if the load is as follows: 



Number of hours 16 J J 2 3 2 



Per cent of full load 100 75 60 50 25 (10 min.) 



20. A 2500-watt transformer has an iron loss of 32 and a copper 

 loss of 53 watts at full load; the primary and secondary losses are 

 equal; required the efficiencies at f , f, J and J load; also the all- 

 day efficiency if the transformer runs 2 hours at full load, 4 hours 

 on \ load and 18 hours on no load. (10 min.) 



21. The total volume of iron in a certain transformer is 12,400 

 cubic centimeters. The iron used is a silicon steel, having a 

 constant of 7 X 10~ n for watts hysteresis loss per cycle per cubic 

 centimeter and an exponent of 1.7. The eddy current loss coeffi- 

 cient may be taken as 4 X 10~ 15 and the exponent as 2. What will 

 be the hysteresis, eddy and combined iron loss for B = 10,000 per 

 square centimeter and 60 cycles per second? (5 min.) 



