I60 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
from sparking and require very little attention. ‘They are 
usually for rather low f requency, twenty-five to thirty-five 
cycles per second, owing to the fact that a higher frequency 
necessitates a rather complicated commutator in order to 
keep the volts per bar sufficiently low, and this condition 
makes the design of the armature somewhat embarrassing, 
especially in large low speed machines. 
As in the case of synchronous alternating motors, the 
strength of the field in a rotary converter has a profound 
influence cn the voltage of the alternating line and can 
cause the current therein to lag or lead by a considerable 
amount. ‘The proper adjustment cf the field strength is a 
very important matter. It should be so arranged as to keep 
the line current as nearly in phase as possible, which is 
probably best accomplished by using generators and con- 
verters of low inductance and compounding them. 
The rotary converter is the best means at present 
available for obtaining continuous from alternating cur- 
rents. Its weak points are the close interdependence of 
the alternating and continuous voltages, and the hecessity 
of using quite low frequencies. For certain cases the com- 
bined synchronous motor and generator, in principle like 
Fig. 89, may be advantageous, but for all around working, 
the rotary converter is generally preferred. 
Power transmission lines for alternating current re- 
quire rather more care in computation than do continuous 
current lines, for one has to deal with the phenomena of 
inductance in line and load, and the resulting ‘‘ false cur- 
rent’’ which may compel the delivery of a current greater- 
than is indicated by the energy concerned. 
In the general problem of power transmission these 
considerations are most troublesome, but when the princi- 
pal work is the operation of substations for railway pur- 
poses, which is the case in hand, it becomes comparatively 
simple, 
For since changing the excitation of a synchronous 
alternating motor shifts the phase of the line current, this 
excitation can be adjusted so as to neutralize the induct-