DYNAMIC MEASUREMENTS ON ELECTROMAGNETIC DEVICES 1 110 



sometimes the use of a search coil is iiicoiiveiiient, or for other reiwons 

 it is desired to measure the flux hiikages in the magnetizing winding itst>lf . 

 The test set includes a bridge circuit shown in Fig. 3 for such measure- 

 ments. With the contacts .1 and B ch)sed and the bridge in the flu.x rise 

 arrangement, the bridge is first bahuiced for no current in the instrument, 

 by adjustment of Rd. The same instrument is used for setting the balance 

 as later used for the flux measurement. The dampinj^ resistance is next 

 set to the value facing the instrument, namely, 



Re = j^^ {R. + R.i\ (0) 



If then the contacts are operated with the period 7\ the instniment 

 will read a current /o, and the flux-turns at time h during rise will be 



N^ X 10-' = RmhT Tl + A' ^ -f |i -f ^S\ - />,{, 



(10) 



where A'' is the number of turns of the winding, ^ is the average flux per 

 turn, and the other quantities are indicated on the drawing. The term 

 Ll^■ is a correction term, usually negligible, involving the self inductance 

 of the primary of the air core current transformer and i the instantaneous 



KR 



E=riii xio-« + L,i)± Rd 



(b) 



E = (Mx,o-*L,i)i 



(d) 



K(Ro+Rm) 



Fig 3 _ Bridge circuit for the measurement of flux— (a) and (b), rise; (c) 

 and (d), decay. 



