488 ALTERNATING CURRENTS 



analyze the reactions which cause the transformer to maintain the current 

 constant. Why is the power-factor of this type of transformer usually 

 low? 



39. Sketch the connections of a constant-current transformer as used 

 with a mercury-arc tube to obtain uni-directional current for magnetite 

 arcs. Why is reactance necessary? 



40. For what reasons is it necessary to use instrument transformers for 

 measuring power on high-voltage, alternating-current circuits? 



41. Describe potential transformers. What is the usual voltage rating 

 of their secondaries? Why should the secondaries always be well-grounded 

 at one point? 



42. Describe the construction of a current transformer. What prevents 

 it from giving a ratio of transformation that is exactly proportional to the 

 ratio of secondary to primary turns? Why should the secondary always 

 be kept closed? In what ways does a current transformer differ from a 

 constant-potential transformer? 



PROBLEMS ON CHAPTER VII 



98. A transformer, rated at 2,200/220 volts, has 2,400 turns on the pri- 

 mary or high-side winding. How many secondary turns are there if the 

 secondary no-load voltage must be increased 4 per cent, to allow for the 4 per 

 cent, voltage drop in the transformer when under load? 



99. A 2,000/110-volt transformer has 80 turns in the low-side winding 

 How many turns are there in the high-side winding if compensation is made 

 for a 4 per cent, voltage drop in the transformer when load is applied to the 

 low side? 



100. What voltage is induced in a transformer winding of 1,200 turns if 

 the frequency is 60 cycles per second and the maximum value of the flux 

 is 1,880,000 lines? (Assume sine wave.) 



101. A certain 60-cycle transformer is to have a primary winding of 4,000 

 turns and a secondary winding of 200 turns. What must be the cross- 

 section of the core if the maximum fliix density in the iron is 60,000 lines 

 per square inch? What is the voltage rating of each winding? The ratio 

 of net iron to the total volume of the core is 0.9. 



102. Repeat problem 101 for a 25-cycle transformer operating at 75,000 

 lines per square inch. Which transformer has the more iron and the 

 more copper? Why? Explain why the iron in a 25-cycle transformer can 

 be operated at a higher flux density than it can be in a 60-cycle transformer. 



103. A 2,200/550-volt transformer is rated at 20 kw. at unity power- 

 factor. What is the current rating of each winding? What is the current 

 rating of each winding at 0.8 power-factor, lagging current? 



104. A certain transformer at no load takes 12 amp. at 220 volts and 

 the powej-factor is 0.16. What is the energy component and what is the 

 magnetizing component of this no-load exciting current? If a load is 

 applied to the transformer so that an additional 30 amp., substantially in 

 phase with the line voltage, is taken from the line, what does the total 

 primary current become? What is the power-factor of the primary? If 



