214 



ELECTRICAL MACHINERY 



Form of Inductance Reaction Curve. It can be shown 

 both geometrically and analytically that when the current 

 is a sine wave the rate of change of current is a wave of 

 similar shape but displaced 90 in phase. In Fig. 128 

 are shown two curves, the full line curve being that for 

 an alternating current, and the dotted one that for the rate 

 of change of this current. The inductance reaction is equal 

 to L, the coefficient of self-induction of the coil, multiplied 

 by the rate of change of current and is just opposite in phase 

 to the latter curve. Hence the inductance reaction is 

 similar in form to the rate of change in current curve and 



'Rate of chance 

 of current 



FIG. 128. Curve Diagram of Inductance Reaction. 



opposite to it in phase and it is shown in Fig. 128 by the 

 curve marked " inductance reaction." 



Inductance Reaction as a Vector. As this is a sine curve 

 in form it may be represented by a rotating vector. Hence 

 in the coil of wire connected to the 100- volt alternating 

 current line, there are offered two reactions to balance the 

 impressed voltage, the resistance reaction and the inductance 

 reaction, both of which may be represented by vectors. 

 We have seen before that the resistance reaction is in phase 

 opposition to the current and Fig. 128 shows the inductance 

 reaction to be 90 behind the current Fig. 129 shows 

 these two reactions at OA and OB and their vector resultant 

 atOC. 



