SLOW RELEASE RELAY DESIGN 197 



release time is relatively independent of magnetic variations, jirovided 

 it is adjusted to release at a specified ampere turn value. 



The range in which / is inversely proportional to ;T is thai in which the 

 logarithmic plot of Fig. G has a slope near unity. The point of tangency 

 with a line of unit slope coincides with the value of z for which the 

 l)racketed function of z in (9) is a maximum. By equating the derivative 

 of this to zero, it is found that this maximum occurs for z = 3.09, cor- 

 responding, from (8), to a value of ().4() for \^/{(S{i{(p" — <po)). From (6), 

 the corresponding value of t.Gii/i-iirG) is 0.40. Thus by drawing a line of 

 unit slope tangent to the observed relation between / and J, and reading 

 the co-ordinates of the point of tangency, -iirG/iRi and TH,: ((p" — (po) may 

 be evaluated. If G is known, these suffice to determine the values of (R, 

 and (f" — (fo . 



Thus the release time is given by (6), and may be expressed in terms 

 of the flux (f at which release occurs by means of (7), or in terms of the 

 corresponding value of 3^, or 47r.V/, by means of (8). To relate the time 

 to the spring load determining release, expressions are required relating 

 the pull to (p or to JF. To relate both time and pull characteristics to the 

 design requires means for evaluating the magnetic constants and G in 

 terms of the dimensions and materials of the design. The magnetic 

 constants and the pull relations are discussed in the two following sec- 

 tions. 



3 DECREASING MAGNETIZATION RELATIONS 



To determine the decreasing magnetization relation experimentally, 

 the magnet is demagnetized, and a measurement made of the flux de- 

 veloped on applying a full ''soak," or high ampere turn value. The de- 

 creasing flux is then measured as the applied current is reduced, and 

 finally reversed to determine ;7c , the \'alue reriuired to restore the field 

 to zero. The relation between tp and ^ thus determined has the character 

 shown in Fig. 1. If this curve conforms to the empirical relation given 

 by (3) and (4), it is characterized by three constants: (H", ^", and either 

 JF(7 , as in (3), or <po , as in (4). These may be evaluated from measure- 

 ments or estimated in preliminary design by the procedures indicated 

 below. 



Experimental Determination of Magnetic Constants 

 E(iuation (3) may l)e written in the form: 



?l±l = ^" + ^S+I. (10) 



