ESTIMATION AKD COXTKOL OF OPEKATE TIAIE OF KELAYb 133 



\'elocity attainalile with a neutral electromagnet. This ujoper limit is of 

 the order of 100 cm/sec. Thus, for example, an operate time less than 

 0.25 millisec cannot be attained with an armature travel of 10 mil-in, 

 no matter how laroe the stead^^ state power applied. 



7 Release Waiting Time 



Like operation, release is made up of an initial stage of flux change 

 with the armature at rest, followed by a stage of armature motion, and 

 the total time is the sum of the waiting time and the motion time. In 

 release, the waiting time is usually larger than the motion time. 



There are three distinct circuit conditions inider which release occurs. 

 These are: 



Normal, or unprotected release, in which the coil circuit is open, and 

 the only magnetomotive force maintaining the field is that of the eddy 

 currents. 



Protected release, in which the coil circuit is closed through a protec- 

 tive shunt, usually comprising a condenser and a resistance in series. 

 The magnetomotive force comprises that of the eddy currents and that 

 of the coil circuit transient. 



Slow release, in which a sleeve or short circuited winding is used to 

 maintain the field and delay release. The magnetomotive force is pre- 

 dominantly that of the sleeve or winding current. 



The slow release case is the only one of the three for which the flux 

 decay relation is accurately represented by equation (2). In protected 

 release, the coil current transient is controlled by the condenser, as dis- 

 cussed below. In normal release, the variation in the field linked by 

 different eddy current paths results in the changes in the field pattern 

 discussed in Section 1. As noted there, equation (2) applies to this 

 case only as a very crude first approximation. 



In all three cases, the relation between tp and ^ applying is that for 

 decreasing magnetization, as illustrated by the curve for x = in Fig. 7. 

 Unlike the linear relation applying in operate, corresponding to the 

 constant reluctance given by (4), the decreasing magnetization curve 

 has a hyperbolic character, and is asymptotic to the saturation flux (p". 

 Residual magnetism results in a residual flux (^o , the intercept of the 

 decreasing magnetization curve on the (p axis. An analytical treatment 

 of the decreasing magnetization curve is given in a companion article,^ 

 where it is shown to provide a satisfactory basis for predicting the release 

 time of slow release relays, the third case above. The other two cases, to 

 w liich the following discussion is confined, involve both non-linear mag- 



