306 ELEMENTS OF ELECTRICAL ENGINEERING. 



142. The neutralizing of armature inductance. A winding of 

 wire on an iron core has considerable inductance, that is, a cur- 

 rent in the winding magnetizes the iron core and causes a consid- 

 erable amount of magnetic flux through the core. The inductance 

 of such a winding may be annulled by anything which counter- 

 acts the magnetizing action of the winding on the iron core. 

 Thus, Fig. 26 1 shows a layer of wire, WW, wound on an iron 

 rod, the magnetic flux due to the current in the winding is indi- 

 cated by fine lines ; and Fig. 262 shows a second layer of wire, 

 aa, wound over WW and so connected in series with WW 

 that the current flows in opposite directions around the rod in aa 

 and in WW, as indicated by the dots and crosses. The magnet- 

 izing action of the winding WW is neutralized by the opposite 



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Fie. 261. 



magnetizing action of the auxiliary winding aa, the only magnetic 

 flux produced is the very small amount of flux between W and 

 a, and therefore the inductance of WW and aa combined is 

 nearly zero. 



If an alternating current is made to flow through the winding 

 WW, Fig. 262, and if the auxiliary winding aa is short-cir- 

 cuited, then the magnetizing action of the current in WW will 

 be very nearly. neutralized by the current which is induced in the 

 short-circuited winding aa ; in this case the number of turns in 

 the auxiliary coil aa need not be the same as the number of 

 turns of wire in the winding WW. 



The auxiliary winding aa, Fig. 262, which is used to annul 

 the inductance of the given winding WW is called a neutraliz- 

 ing or compensating winding. When the auxiliary winding is 

 connected in series with the given winding, we have what is called 

 forced neutralisation or forced compensation, and when the auxil- 



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