SLOW RELEASE RELAY DESIGN 189 



or sleeve, a low reluctance magnetic circuit, and a high level of pull for 

 relati^'ely low values of field strength. In the following analysis of slow 

 release performance, expressions are developed for the time of field 

 decay for the decreasing magnetization characteristic, for the reluctance 

 of the electromagnet, and for its pull characteristics. These expressions 

 permit the estimation of release times, indicate the design conditions to 

 be satisfied to attain a desired level of delay, and indicate the effect on 

 the release time of variations in the spring load, in the dimensions of the 

 electromagnet, and in other design parameters. 



The notation used in this article conforms to the list that is gix'en 

 on page 257. 



2 FIELD DECAY RELATIONS 



If a closed circuit of resistance R and .V turns links a magnetic field 

 of flux (p, the voltage equation is : 



at 



where i is the circuit current. IMuItiplying bj^ 4x.¥ and dividing by R 

 this equation may be written as: 



^i + ^irGi ^ = 0, 

 at 



where ^i is the magnetomotive force of this circuit, and d is its value 

 of N~/R, which may be termed the equivalent single turn conductance. 

 If there are several such circuits linking the same magnetic field, a 

 similar expression applies to each, and these may be added to give the 

 equation : 



g: + 47r(? ^f = 0, (1) 



at 



where ;J = ^ 5; , and G = ^Gi . In the case of a slow release relay, one 

 linking circuit is usually a slee\'e, whose conductance may be designated 

 Gs . The applications are identical for a short circuited winding, if the 

 applicable value of N^/R is substituted for Gs . In either case, G also 

 includes a term Ge representing the net effect of the eddy current paths. 

 As the eddy currents at different distances from the center of the core 

 link different fractions of the total field, this representation by a single 

 term is an approximation. As shown in a companion article, however, 

 the approximation is satisfactory when Ge is a minor part of G, as in the 



