ESTIMATION AND CONTROL OF OPERATE TIME OF RELAYS 181 



cross-section. The residual flux is directly proportional to the coercive 

 force times the length of the magnetic material, and inversely propo- 

 lional to the closed gap reluctance. The two largest factors affecting 

 the closed gap reluctance are magnetic permeability, and fit of the joints, 

 including variations in stop pin height. By measuring these factors on 

 the test relay and estimating the corresponding values for the desired 

 reference condition, the measured data can be corrected to the reference 

 ('(indition. Then having the release pull curves after magnetic soak for 

 the same reference condition, the release ampere turns for any load 

 under consideration and hence its release waiting time can be deter- 

 mined. These release pull curves are also shown as part of Fig. 14 for 

 the wire spring relay. The ordinate scale is marked for both contact 

 spring load in grams and releasing time in milliseconds. The particular 

 chart shown is for a constant initial number of ampere turns. For other 

 initial values, a correction chart is provided. If not as desired, the time 

 can be adjusted either upward or downward by changing to a different 

 sleeve, shunting resistor, or both. Of course, in no shunting conductance 

 case can the release time be less than the open circuit time. 



BC Shunt 



The above considerations all related to shunting conductances. These 

 serve the purpose of increasing the release time. The shunting resistor 

 also greatly reduces the transient peak voltage developed when the 

 winding circuit contact is opened. For contact protection reasons, RC 

 networks are frequently used across the winding or contact for this 

 same purpose. Such a network also has no power drain when the relay 

 winding is energized. By a suitable choice of capacitance the network 

 can reduce the release time to a value less than the open circuit time. 

 It does this by developing a hea\'ily damped oscillation of winding cur- 

 rent. The frequency must be of the order of the reciprocal of the un- 

 protected release time for a release time reduction. This fixes the choice 

 of capacitance to a small range i.e., there is a best capacitor, one for 

 each winding. Smaller values cause the time to increase toward the 

 unshunted case. Larger values also cause an increase but for this case, 

 if too large, an increased time beyond tlic unshunted case can result. 



For speed windings, where timing is important, a network is designed 

 for each. For the slower higher resistance windings, because time is not 

 as important, the closest to the best of the available networks is chosen. 

 This results in a time penalty but furthers standardization. 



To evaluate a network, the transformations developed and shown in 



