DESIGN FACTORS INFLUENCING RELIABILITY OF RELAYS 981 



COIL CONSTRUCTION 



If, for any reason, the winding of a relay should become open during 

 its life, usefulness of the relay ceases. One of the most prevalent causes 

 for failure of this kind can be due to so-called corrosion of the wire. This 

 is not corrosion in the ordinary sense of the word, but is caused by an 

 electrolytic action within the coil when an electrical potential is applied 

 to the winding. This action can take place only in the presence of mois- 

 ture. If there are any active impurities in the insulating materials in- 

 timately associated with the copper wire, an electrolyte is formed and 

 disintegration of the wire proceeds to the point where failure may occur. 

 This trouble is accentuated with coils using small diameter wires, be- 

 cause with the smaller cross-section of copper, failure of the section will 

 occur in a shorter period of time. 



There are two methods of approach to minimize corrosion failure. One 

 is to thoroughly dry the coils, and in this condition seal them in a potting 

 compound, which prevents the entrance of any moisture into the coil, 

 or enclose the coil in a hermetically sealed chamber. For relays this is 

 an expensive and cumbersome way to overcome corrosion troubles. The 

 second and more practical method is to use, in the construction of the 

 coil, insulating materials that are chemically inert and free from corro- 

 sion promoting impurities. 



For many years, studies were made with a view towards eliminating 

 the occasional corrosion failures of fine wire windings which occurred 

 under unfavorable atmospheric and circuit conditions. Although im- 

 provements were effected by the use of the better grades of phenol fibre 

 for spoolheads and waxed varnished papers for core and winding insula- 

 tion, an entirely satisfactory coil was not achieved until the use of cellu- 

 lose acetate insulation was adopted. This material, in thin sheet form, 

 can be applied to spool-wound coils, where the coils are woinid indi\'id- 

 ually, and the so-called "filled" coils, where a multiple number are wound 

 simultaneously on automatic winding machines. The coils are wound on 

 a mandrel, as many as twelve individual coils per mandrel, with a 

 thin sheet of insulation between the layers of ware. The "stick" of 

 coils is wound so as to leave a small separation between coils to provide 

 insulation at the ends of the coils and to permit cutting the stick into 

 individual coils, after which they are assembled to relay cores using 

 conveyor assembly methods. This results in not only a moi-e economical 

 coil, but in a higher (luality coil as well. The thin sheet of insulation 

 between the layers of wire, generally not provided on the spool wound 

 type of construction, eliminates the occurrence of short-circuited turns. 



