THE TRANSFORMER. 143 



will be negligible compared with the applied voltage. If 

 this is the case, column 4 need not be filled in, and the back 

 voltage may be considered equal to the applied voltage. If 

 an appreciable drop of voltage occurs in the windings, a 

 second curve should be plotted, comparing back volts 

 (the vector difference between V l and C l R^ and current. 



In addition to the total no-load current, the energy and 

 idle currents should be plotted separately. The energy 

 component is obtained by dividing the power supplied to 

 the transformer (as read on the wattmeter) by the voltage 

 of supply. The idle or magnetising current is then obtained 

 by calculation, or from a diagram in which the total no-load 

 current is drawn as the hypotenuse of a right-angle triangle, 

 and the energy and idle currents are represented by the sides. 

 The method of drawing this triangle is -to describe a semi- 

 circle on a line A B representing the total current to a scale 

 of amperes, and then to draw a circle from one end as centre, 

 with radius equal to the energy component. Joining the 

 intersection of the circles to A and B, the triangle is complete. 



Plotting both currents on a voltage base, we obtain curves 

 showing the dependence of both magnetising current and 

 energy loss current upon the saturation of the transformer 

 core, since the back voltage of the transformer at constant 

 frequency depends only on the magnetic flux, i.e., on the 

 density of magnetic lines in the core. 



Thus the curve of magnetising current gives a magnetisa- 

 tion curve for the transformer analogous to the magnetisation 

 curve which is of such great importance in the case of direct 

 and alternating-current generators. 



If the number of turns in the winding to which the voltage 

 is applied is known, the curve may be plotted with " Number 

 of lines " instead of " volts " as the base, since the lines can 

 be calculated from the formula already given. 



V = n t Z 4-44 x 10- 8 , 

 when V = voltage. 



n = periodicity. 



/ number of turns. 



Z = maximum number of lines. 



The power lost in the windings and recorded by the watt- 

 meter can be separated readily from the watts spent in over- 

 coming the iron losses, if the resistance of the winding is 

 known, since watts lost in winding = C\ ^R^ 



