ESTIMATIOX AND roXTIJOL OF OPERATK TTMI'; OF 1{KT,AVS 121 



(9) for r = 'Jo/tfs • While this is a crude approximation, it i)ro\i<l('s 

 satisfactory estimates of the tim(> for slow o]~)eration. 



EDDY CUUUKNT CONDUCTANCE DKTEUMINATION 



Equation (9) is used in one experimental method for the evaluation 

 of the eddy current conductance (/£ . In this, measurements are made of 

 I lie time at which motion starts for various values of the coil constant Gc . 

 The latter may be varied by adding series resistance, and the applied 

 \()ltage adjusted to maintain the current and hence ^s constant in suc- 

 cessive measiu'ements. The time / for motion to start may be determined 

 from shadowgraph measurements. P'or a constant l)ack tension, the 

 pull and hence the value of v or 3^/Js when motion starts is constant in 

 these measurements. From (9), t is then directly proportional to Gc + 

 Ge , so that a plot of t vs. Gc should be linear, with a negative intercept 

 on the Gc axis numerically equal to Ge . Experimentally, an approxi- 

 mately linear relation is obtained in this way, provided the values of 

 Gc covered are in excess of 5Ge , in agreement with the discussion of 

 Section 1. Values of Ge thus determined are consistent with those obtained 

 from other measurements, and in approximate agreement with esti- 

 mates obtained from (7) or (8). 



COIL CONSTANT 



In eciuation (9) Ge and (Ri are determined by the magnetic design. 

 The latter, together with the load, determines Jo , the just operate mmf, 

 or 47r(XI)o . As V equals 3^o/3^s , or {NI)o/{NI), the only quantities not 

 fixed by the design and the load requirement are Gc and the steady 

 state ampere turn \'alue NI. As the square of this latter quantity is 

 equal to Gc-I'R, equation (9) may be written in the form: 



, = i'(e, + (^»), 1 . (10) 



(Ri \ v-PR / I — V 



The steady state power I'R is either determined bj' circuit require- 

 ments, or chosen with reference to economy of power consumption. 

 With I'R fixed, the only independent quantity in (10) is v, which is 

 determined by the choice of the coil constant Gc . Neglecting Ge , the 

 time is proportional to 



1 , 1 



— In 



v~ I — V 

 wiiich is shown plotted against v in Fig. 5. This figure includes a cur^•e 



