SLOW RELEASE RELAY DESIGN 213 



wliicli varies directly with {NI)c, and enters the pull relation (16) 

 directl^^ as a major term. If high coercive material were used to increase 

 {NI)c and thus reduce the release ampere turns NI, the effect of the 

 latter on the time would be offset by the increase in (po . The latter varies 

 also with the reluctance (R, , and if (po/<p" is not small, the time for a given 

 value of NI is no longer independent of the reluctance variation, and the 

 release ampere turn value is unsatisfactory as a criterion of adjustment. 

 Thus a small stable value of {NI)c , as given by low coercive material, 

 is preferable when the release current is used as the criterion of adjust- 

 ment. 



The pole face chmensions are determined bj' the need for a low stable 

 value of gap reluctance. As shown in Section 4, maximum release sensi- 

 tivity is obtained bj^ using as large a dome radius as will assure a con- 

 sistent configuration, together with a projected area optimum with 

 respect to the reluctance in series with the gap. Fig. 11 shows this op- 

 timum to be broad, allowing considerable latitude in the choice of the 

 projected area with reference to other design considerations. 



The finish applied to the magnetic parts at the pole face is critical w ith 

 respect to wear and stability, as well as to the danger of mechanical ad- 

 hesion which would affect the release load. A nickel chrome finish is 

 used in the Y and AG relays. The effective air gap thus introduced is 

 the dimension x of Figs. 9 and 10. As these figures show, this has a rela- 

 tiveh' large effect on the pull relation, and any change in this dimension 

 as a result of wear tends to increase the pull and the release time. The 

 analysis of Section 4 shows that maximum sensiti\'ity is attained with as 

 small a finish separation as can be provided, which is fortunately in 

 agreement \\'ith the manufacturing considerations for the finish. 



The adjustment means used with a fixed pole face, e.g., the dome type, 

 is some form of spring adjustment for controlling the operated load. The 

 normal operated load of a relay is the product of the contact force and 

 the number of contacts, plus the back tension and the force required for 

 spring flexure. Adjustment of the back tension is used, but is limited by 

 the requirements for operation. Hence a buffer spring is employed in 

 .1 G relays, which is picked up in the last few mils of armature tra\'el and 

 affords control of the operated load. This spring is adjusted by bending, 

 and a tolerance range of 50 gm wt is allowed in this adjustment. 



Performance Variations 



Performance variations fall into two categories: the initial differences 

 among individual relays that are nominally identical, and the further 

 differences that develop in service as a result of wear and other changes. 



