THE ROTARY CONVERTER. 



323 



is due to the same causes as those producing the drop in a 

 separately excited generator, although the armature reac- 

 tions and loss due to armature resistance will both be 

 smaller in the present case in proportion to the current. 



Fig. 162, which shows the relation between the primary 

 direct current and the output alternating current, is also 

 a straight line, but does not pass through zero. The 

 ratio of current transformation appears consequently not 

 to be a constant. 



68 

 67 I 



66 

 65 

 64 

 63 

 62 



10 



12 



A.C. Amperes, Load. 



Fiu. 161. EXTERNAL CHARACTERISTIC OF ROTARY CONVERTER DRIVEN 

 FROM D.C. SIDE. 

 B.C. VOLTS = 100. 



The explanation of this apparent want of propor- 

 tionality in the ratio of transformation is that the machine 

 required 2'5 amps, to drive it when the alternating output 

 as zero, as indicated by the point at which the curved 

 cuts the vertical axis. As the power required to drive 

 the converter when giving out current on the alternating 

 side will be at least equal to the current taken to drive it 

 at no load, there will always be at least 2'5 amps, of the 

 direct current supplied which are not converted into 

 alternating current, but which pass through the armature 

 from brush to brush on the commutator, and are spent in 

 overcoming the frictional losses, so as to maintain 

 the revolution of the machine. Thus, if the power 

 spent in driving the machine were a constant, the 

 true ratio of current transformation would be obtained 



