OF THE EXCHANGE AT WEBB
CITY, MISSOURI]

As an alternative to the converting device made up of a motor coupled to
a generator, both motor and generator windings may be combined on the
same core and rotate within the same field. Such a rotary converter has
been called a _dynamotor_. As a rule the dynamotor is only suitable for
small power-plant work. It has the following objectionable features:
(_a_) It is difficult to regulate its output, since the same field
serves for both the motor and the dynamo windings. For this reason its
main use is as a ringing machine where the regulation of the output is
not an important factor. (_b_) Furthermore, the fact that the motor and
dynamo armature windings are on the same core makes it difficult to
guard against breakdowns of the insulation between the two windings,
especially when the driving current is of high voltage.

_Charging Dynamos._ The dynamo for charging the storage battery is, of
course, a direct-current machine and may be a part of a motor generator
or it may derive its power from some other than an electric motor, such
as a gas or steam engine. It should be able to develop a voltage
slightly above that of the voltage of the storage battery when at its
maximum charge, so as always to be able to deliver current to the
charging battery regardless of the state of charge. A 30-volt generator,
for example, can charge eleven cells in series economically; a 60-volt
generator can charge twenty-five cells in series economically.

Battery-charging generators are controlled as to their output by varying
a resistance in series with their fields. Such machines are usually
shunt-wound. Sometimes they are compound-wound, but compounding is less
important in telephone generators than in some other uses. A feature of
great importance in the design of charging generators is smoothness of
current. If it were possible to design generators to produce absolutely
even or smooth current, the storage battery would not be such an
essential feature to common-battery exchanges, because then the
generator might deliver its current directly to the bus bars of the
office without any storage-battery connection and without causing noise
on the lines. Such generators have been built in small units. Even if
these smooth current generators were commercially developed to a degree
to produce absolutely no noise on the lines, the storage battery would
still be used, since its action as a reservoir for elec