перевести In general, DC motors are similar to DC generators in construction. They may
be described as generators “run backwards.” When current is passed through the
armature of a DC motor, a torque is generated by magnetic reaction, and the
armature revolves. The action of the commutator and the connections of the field
coils of motors are precisely the same as those used for generators. The revolution
of the armature induces a voltage in the armature windings. This induced voltage is
opposite in direction to the outside voltage applied to the armature, and hence is
called back voltage. As the motor rotates more rapidly, the back voltage rises until
it is almost equal to the applied voltage. The current is then small, and the speed of
the motor will remain constant as long as the motor is not under load and is
performing no mechanical work except that required to turn the armature. Under
load the armature turns more slowly, reducing the back voltage and permitting a
larger current to flow in the armature. The motor is thus able to receive more
electric power from the source supplying it and to do more mechanical work.
Because the speed of rotation controls the flow of current in the armature,
special devices must be used for starting DC motors. When the armature is at rest,
it has virtually no resistance, and if the normal working voltage is applied, a large
current will flow, which may damage the commutator or the armature windings.
The usual means of preventing such damage is the use of a starting resistance in
series with the armature to lower the current until the motor begins to develop an
adequate back voltage. As the motor picks up speed, the resistance is gradually
reduced, either manually or automatically.
The speed at which a DC motor operates depends on the strength of the
magnetic field acting on the armature, as well as on the armature current. The
stronger the field, the slower is the rate of rotation needed to generate a back
voltage large enough to counteract the applied voltage. For this reason the speed of
DC motors can be controlled by varying the field current.

nlogvinova14    1   20.05.2021 21:27    2