ESTIMATION' AXD COXTKOL OF OI'KIt ATI') Tl.MH OF UKLA'^S 



133 



where (e = 4x6'^ '(R(0). T'nless(^ Vo is near unity, ^p^) may be omitted from 

 this expi'ession. It" this is done, the expression may be re-written in terms 

 of jMiIl F in the form: 



, te , Fi 



(26) 



where F is the pull at tim(^ t, and Fi the initial pull. This expression 

 follows from (25), for c^o = 0, because of the proportionality between F 

 and (f'. If F is the spring load or tension for the operated position, and 

 Fi the steady state pull there, (26) is an approximate expression for the 

 release time. 



PROTECTED RELEASE 



The commonly employed method of contact protection is to use a 

 condenser-resistance shunt connected either across the coil, as in Fig. 8, 

 or across the contact. Except for the steady state voltage of the con- 

 denser, the circuit relations are the same for the two cases. As a first 

 approximation, eddy currents may be represented by the current in a 

 short circuited secondary, as in Fig. 8. The effect of such a secondary is 

 determined by the value of Ie representing the ratio of its inductance 

 to its resistance, as this determines the ratio of its contribution to the 

 flux to the time rate of change of the total field. 



If perfect coupling and linear magnetization are assumed to appl}^, 

 the circuit ecjuations for Fig. 8 can be solved and expressions obtained 

 for the flux, current and condenser voltage as functions of time after 

 the opening of the contact. These expressions are similar to those for 

 the simple C-R-L circuit without a secondary, except that they involve 

 the time constant (e as well as CR and L/R. The discharge may be either 

 exponential or a damped oscillation, but while the discharge of the 

 simple C-R-L circuit is always oscillatory for small values of C, the 

 discharge in the circuit of Fig. 8 is only oscillatoiy for an intermediate 



Fig. 8 — Coil ciifuit with capacitative contact protection. 



