188 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



tolerance in the timing requirements permits the use of more economical 

 practices in the construction and use of slow release relays than would 

 be needed for closer control. As these wide tolerances apply to the great 

 majority of applications, over-all economy is attained by developing 

 slow release relays with reference to them, using other devices for the 

 special applications requiring close timing control. 



The magnitude of time delay desired is fixed by the operate and release 

 times of the associated general purpose relays. The latter times lie in the 

 range from 5 to 50 milliseconds, with the majority in the lower part of 

 the range. The delays rec^uired to assure sec^uences of events each reciuir- 

 ing time intervals of this order conseciuently cover the range from 50 to 

 500 milliseconds (one twentieth to one half a second). Delays of less 

 than 100 milliseconds can generally be provided by ordinary general 

 purpose relays having shorted secondary windings or sleeves (single 

 turn conductors) to delay their release. Special slow release relays are 

 used for delays in excess of 100 milliseconds. 



Factors Controlling Release Delay 



In terms of circuit operation, release time is the interval from the 

 opening of the coil circuit to the completion of contact actuation during 

 the return motion of the armature. This time is the sum of (a) the time 

 for the magnetic field to decay to the level at which the pull just equals 

 the operated spring load and (b) the motion time for contact actuation. 

 The motion time is never more than a few milliseconds, and is therefore 

 a trivial increment to the long delays of slow release relays. The release 

 time of such relays is therefore, for practical purposes, simply the time 

 of field decay. 



When the coil circuit is opened, the field decay induces currents in 

 any circuit linking the field. These currents tend to maintain the field 

 and to delay its decay. A short circuited winding or the single high con- 

 ductivity turn provided by a copper sleeve gives a high magneto- 

 motive force for a low induced voltage, resulting in a relatively slow 

 decline in field strength. The time for the flux to reach a given level de- 

 pends upon the conductance of the sleeve, and upon the reluctance of 

 the electromagnet: the ratio of magnetomotive force to flux. The flux 

 level which determines the end of the delay is that for which the pull 

 equals the spring load. The delay is therefore prolonged by a pull char- 

 acteristic such that the load is held until the flux drops to a minor frac- 

 tion of its initial value. 



Thus the essential features of a slow release relay are a shorted winding 



