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