TRANSFORMERS 9 



with full-load current. This transformer is connected to the 

 supply circuit continuously. During 7 hours the load is 60 kw- 

 at 0.75 power factor, and for the remaining 3 hours of the work- 

 ing day the load is 80 kw. at 0.83 power factor. The rest of the 

 time no load is supplied. What is the all-day efficiency? 



41. A oOO-kv.-a. power transformer is connected to the supply 

 circuit for 12 hours each day. The iron loss is 3330 watts and 

 the copper loss is 4680 watts with full-load current. During 5 

 hours it delivers 400 kw. at 0.80 power factor and during 4 hours, 

 250 kw. at 0.75 power factor. The remaining hours it is idle. 

 What is the all-day efficiency? 



42. l The magnetic core of a transformer has a mean length of 

 77.3 in. and a cross-section of 34.4 sq. in. How many turns 

 should there be in the high-tension winding if the applied voltage 

 is 22,500 volts at 60 cycles? The maximum flux density to be 

 used is 63,000 lines per square inch, at which the permeability of 

 t he iron is 1600. The core loss per cubic inch of iron at this flux 

 den-ity and frequency is 0.364 watts. What are the no-load 

 hitfh-t<-nsion current and power factor? 



43. l The magnetic core of a 25-kv.-a. transformer has a cross- 

 section of 17.5 sq. in. and an average length of 57.5 in. The 

 high-tension winding is designed for an impressed voltage of 

 22,000 volts at 60 cycles and for a maximum flux density in the 

 magnetic circuit of 70,000 lines per square inch. At this flux 

 density the permeability is 1860 and the iron loss is 0.37 watts 

 per cubic inch. How many turns should there be in the hi^h- 

 t en-inn winding? What are the no-load current and power 

 factor' 



44. ! A small experimental transformer has a magnetic circuit 

 with a net cross-section of 3.4 sq. in. and a mean length of 22.5 

 Assume that the permeability of t he iron is 1800. There are 

 four coils, each <>f which has 232 turns, and a resistance of 0.23 

 <>hrn. (a) If all of the mils are connected for a 2:1 ratio of trans- 

 formation what is the mamieti/ing component of the no-load 

 current when 220 volts at 60 cycles is impressed on the primary? 

 (b) If the eddy current and hyst< -n-sis losses are 50 watts what 

 is the no-load current? (c) What is the efhciency <.f this tran-- 

 former when the secondary delivers 20 amperes at 110 volts and 

 100 per \ver factor? 



1 Assume that at this flux density 75 a<Miti<m.il t mrr<l 



because of the joints in the magnetic < m uit 



