THE REGULATION OF TRANSFORMERS 99 



In 1896 I devised a method of estimating the reactance 

 dr.p of a core-type transformer with concentric windings and 

 an elongated winding-space, and I have used it with good 

 r. suits during the last fourteen years. The method i* base<j! 

 on the, following formula : 



percentage reactance drop = / 1 ; 



i> x <* 



/ = a factor depending upon the width of the winding window, 

 tli. depth of the windings, the number of concentric windings 

 employed and the thickness of the insulation or of the air or 

 oil space between tin- primary and secondary windings. 

 n = virtual (/.<., rms) value of the primary ampere-turns at 



1 load. 



b = height of winding space in cm. 

 c = core density in kilolines per sq cm. 



For a single concentric winding, /.- .. tor a winding of the 

 employ. (1 in our 20-kva transformer (see also Fig. 80) 

 the values for/ range from 0,05 to 0,12, being higher the 

 greater the depth of the coils. For triple-concentric windings 

 (see Fig. 31) the range of values for / is from 0,03 to 0,07. 

 l-'.-r -till givatrr subdivision of the winding, lower values 

 must lie used for/. For more definite values, each designer 

 will acquire his own experience, recording the results obser\ed 

 on test and working hack to the factor /. For single-coii- 

 rt-ntric windings the curve in Fig. 58 may be found useful in 

 obtaining values for /. Our -JO-kva de-sign has d vidcdly 

 low coils and a narrow winding window. It is for only 

 moderate pressure (5000 volts), and the primarv and secondary 

 (MiU come quite rinse up to mir another (as shown in \-"\\f. 50, 

 on p. 83). It will he appropriate in this ea-e to take 



/ = 0,080. 



The primary winding has 3100 turns, and tin- input at rat. .1 

 is 1.1 'I amp.-res. < 'oiiseqiient lv for " a," the virtual 

 vain.' of tin primary ampere-tin n, we have 

 a = 4,12 X 3100 = 12800. 



n "2 



