22. Motors.- The operation of a motor is the same as that of a generator except that the current of
a motor flows in the opposite direction through the machine from that in which it flows when the
machine runs as a generator; that is, the current flows in one direction with respect to the generated
voltage in a generator, and in the opposite direction with respect to the generated voltage in a motor.
The current flows into the motor through the positive brush and out at the negative brush. The
current of a generator flows out of the positive brush and in at the negative brush. The reason for this
is the fact that the generated voltage of a motor is always in opposition to the applied voltage, but
never quite as great. Therefore, the applied voltage forces a current through the motor against the
electrical pressure offered by the generated voltage.
When a current is passed through the armature winding of a motor, a magnetic field is created in
it which has a north and south pole. These poles are attracted by one pole of the motor field and
repelled by the other, causing the armature to rotate. In the case of an alternating current machine the
field rotates according to the frequency of the alternating current, and the windings follow this field
In the case of a DC machine, as the winding rotates the commutator rotates, and the windings
through which the current flows is continually changing. The magnetic poles of the revolving
windings are always kept at a fixed angle with respect to the field poles and a steady rotating force is
maintained. As each portion of the armature winding reaches a position where the field poles produce
no turning effect, the commutator action is such as to disconnect these coils, and to connect others
which are in a position where a turning effect may be had.
23. Commercial Motors and Generators.- Motors and generators, as they are built for use, do not
look anything like the illustrations shown in Figures 11 and 12. These illustrations show the action of
the machines, but the machines themselves are built in a considerably different form. The field poles
of a commercial machine are not "permanent" magnets, but are "electro-magnets" consisting of a field-winding on an iron core. The current for the field-winding is supplied from some source of direct
current. In the case of a DC generator, this current is usually supplied by the generator itself,' and in
the case of the DC motor the field current is supplied from the same source as the current that runs
the motor. AC motors are more complicated in their action although the same principles are involved.
The explanation of the process is very involved and will not be taken up here.