Principles of Slow Release Relay Design 



By R. L. PEEK, Jr. 



(Manuscript received Scpteinher 25, 1953) 



This article presents an analytical treatment of the relations controlling 

 the release time of slow release relays, in which field decay is delayed by the 

 currents induced in a conducting sleeve or slug. An hyperbolic relation 

 between flux and magnetomotive force, fitting the decreasing magnetization 

 curve, is used for the relation between induced voltage and current in the 

 sleeve circuit in determining the rate of field decay. Methods arc given for 

 estimating and measuring the magnetization constants and those appearing 

 in the relation between pull and field flux. 



These relations are used as a basis for a disciission of the design of slow 

 release relays and of the adjustment procedures employed to meet timing 

 requirements. 



1 INTRODUCTION 



Slow release relays are built and adjusted to provide a time delay be- 

 tween the opening of the coil circuit and the release motion which restores 

 the contacts to their unoperated condition. They are used to assure a 

 desired sequence of circuit operation, as, for example, in maintaining a 

 closed path through the slow release relay's contacts during the pulses 

 sent in dialing a digit, and opening this path during the much longer 

 interval between digits. Slow release relays constitute a minor but sig- 

 nificant part of the relay population in an automatic central office: 

 about ten per cent of the total. 



For economy in manufacture, installation, and use, slow release relays 

 are made as similar to the ordinary or general purpose relays with which 

 they are used as their special reciuirements permit. Each general struc- 

 tural relay developed for telephone work has had a variant form for 

 slow release use. Thus the Y type^ relay is the slow release form of the U 

 type relay^ widely used in the Bell System, while the AG relay is the slow 

 release variant of the recently developed wire spring relay. 



The circuit functions of most slow release relays permit considerable 

 \'ariation in release time if the minimum delay specified is assured. This 



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