ALTERNATING MOTORS FOR RAILWAY WORK. L7
The simplicity of the structure has already been set
forth. The nature of the field winding is well shown in Fig.
96, the field of a slow speed, two phase motor of one hundred
horse power output, and the winding is for 2000 volts. In
ordinary American practice the field coils are in open slots
so that they can be the more readily repaired or replaced.
The armature winding is usually of massive bars with
heavy end connections and is well exhibited in Fig. o4.
The matter of durability is best settled by experience.
During the past three years there have been put in opera-
tion in this country polyphase induction motors aggregat-
ing more than 12,00ch. p. in output; and from the author’s
own personal knowledge it may be said that the repairs
upon these have been almost negligible, far smaller than
in any other class of moving electrical machinery. ‘This
is a strong statement, but it is fully borne out by the
facts.
Speed regulation in polyphase induction motors is ef-
fected by means not unlike those used for continuous cur-
rent motors. A common shunt motor may have its speed
varied in two very simple ways. First, the field strength
may be changed; second, the armature current may be cut
down by a rheostat. A series wound motor may be simi-
larly governed by changing the field strength or changing
the voltage.
In an induction motor the same devices are used in a
somewhat different way. Weakening the field of such a
motor by reducing the voltage of supply causes the arma-
ture to run slower, but since the armature current is sup-
plied by the field as a transformer the armature is also
greatly weakened and, hence, the torque falls off very rap-
idly as the voltage is lowered. Modifying the armature
strength by a rheostat in circuit, however, cuts down the
speed until the added transformer effect of the field sup-
plies current enough to handle the load at the new rate of
speed. By varying the resistance in the armature circuit
the speed can be varied to any desired extent, the torque
remaining constant throughout. Fig. 97 shows the speed
