SLOW RELEASE RELAY DESIGN 



209 



o 

 o 

 OJ 0.10 



I 



H 



UJ 0.08 



tu 



-I 0.06 



LU 0.04 



5 



Fig. 14 

 relaj'. 



30 20 10 10 20 30 40 50 60 70 80 90 100 



COIL CONSTANT, Gc, IN KILOMHOS 



- E.xpcrimpntal evaluation of ofldj- rurront conductance for release of 



Edd]) Current Conductance 



In one of the companion articles it is shown that when the eddy cur- 

 rent conckictance Ge is a minor term in G, as with slow release relays, it 

 is given by the equation: 



G^E = ^ — ' 

 oirp 



(22) 



where p is the resistivity of the material, and ( is the length of the mag- 

 netic path. For iron, p = 11 X 10~ ohm-cm, so for ( = b cm, the value 

 of Ge given by (21) is 17 X 10^ mhos. The equation applies to a path of 

 uniform circular cross section, so that the effective value of t for most 

 relay structures is intermediate between that of the core and that for 

 the complete path. 



The eddy current conductance of a specific model may be experi- 

 mentally determined by measuring the release time with a winding 

 shorted through an external resistance. A series of measurements are 

 made in which this resistance is varied, while the initial current, which 

 determines the "soak NI" value, is kept constant. The different values 

 of the resistance correspond to different values of the coil constant Gc 

 or N /R. From (6), the time t in this series of measurements varies 

 directly as G, or Gc -\- Ge ■ Then a plot of t versus Gc , as illustrated in 

 Fig. 14, is linear, and has a negative intercept giving the value of Ge . 



