TRANSMISSION OF POWER FOR SUBSTATIONS. 159 
the two machines are almost identical. In fact the earli- 
est rotary converters were made by adding rings to stand- 
ard generators, but of late some modifications have been 
found useful. ‘The three phase rotary converter has 
already been shown in connection with the Lowell plant. 
All rotary converters require alternating current of 
less voltage than the continuous current derived from it. 
  
  
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A7 Ry JOURNAL 
  
  
  
FIG. 9I. 
The exact voltage varies with the number of phases used 
and with the field excitation of the converter. In the two 
and three phase machines ordinarily used the alternating 
voltage ranges from 3ooto 375 for a continuous voltage of 
500 to 550. 
The efficiency of conversion by this means is very 
high, at least as great as would be obtained from standard 
railway dynamos of similar size and speed, and the con- 
verters as a rule work admirably. If the voltage per com- 
mutator segment is kept within conservative limits and the 
armature inductance is moderate, converters give no trouble