180 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
ture current and field strength, doubling the input in an 
induction motor nearly quadruples the torque. This is 
well shown in Fig. 98, which gives the relation between 
current and starting torque in the motor referred to in 
Fig. 97. The maximum torque was obtained with a very 
small resistance in the armature circuit, which resistance 
was gradually raised to obtain the other points in the 
curve. ‘The torque was truly static and the power factor 
of the machine under this condition was lower than when 
running normally, as shown by the larger current than in 
Fig. 97. | 
The maximum starting torque, more than four times 
the full load running torque, was obtained at normal volt- 
age by the use of about 224 times the normal full load 
current. 
Four times the normal drawbar pull is enough for 
ordinary starting purposes even in severe street railway 
service, but even this can be still further increased if nec- 
essary, by raising the voltage. The torque, so long as the 
field is unsaturated, then increases nearly in proportion to 
the square of the applied voltage. Thus, if the field coils 
of the motor are in the star connection for normal opera- 
tion, and are thrown over to the mesh connection as an ex- 
treme measure, the applied E. M. F. per coil 1is increased 
in the ratio of 1.73:1, and the resulting torque is three 
times the normal. ‘This in combination with the changes 
of armature resistance indicated in Fig. 98, is enough to in- 
crease the torque enormously in spite of increasing satura- 
tion of the field. In fact one can ohtain from an induction 
motor more starting torque than is ever called for in prac- 
tical work. 
Fig. 99 shows the results obtained in testing a pair of 
three phase induction motors specially arranged for rail- 
way work. Fach motor was designed to produce a normal 
drawbar pull of 8oo lbs., equivalent at full car speed to 
about twenty-five horse power. These machines were 
wound for 110 volts between lines, weighed substantially 
the same as standard railway motors of the same output