Техническое чтение для энергетиков. Бухарова Г.П. - 69 стр.

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71
DIRECT-CURRENT MOTORS
Construction. – A direct-current motor consists of the same essential parts as a
direct-current generator, namely, field magnet, armature with its commutator, and
brush gear. The armature and commutator are constructed on exactly the same
principles as the armature and commutator of a dynamo, and any difference in
external appearances of dynamos and motors is due to a modification in the
mechanical arrangement of the field magnets and frame, designed to give the motor
the maximum amount of protection. Dynamos are employed mostly in a central
power station where they are not exposed to any mechanical danger, such as the risk
of heavy bodies falling on them, and as a result they can be of open construction.
This is a great advantage since they are accessible for repairs, and also they are easily
ventilated.
Motors, on the other hand, often work in very exposed situations thus
necessitating partial or complete enclosure of the working parts. The type of duty to
be performed also has an influence on the construction of the motor. The motor must,
of course, be totally enclosed, but at the same time must be capable of rapid
dismantling for inspection.
General Principles.– It is often thought that the principle of operation of a
dynamo is quite unconnected with that of a motor; actually the two cannot be
separated, since dynamo and motor actions go on at the same time in both dynamos
and motors. Any direct-current dynamo will run as a motor, that is, convert electrical
power to mechanical power, if its field and armature are connected to a suitable
electric supply. Also any direct-current motor will function as a dynamo
provided that the conditions for self-excitation are fulfilled. In order to realize the
essential connection between the two modes of operation consider the diagrams in
Fig. 23. The first diagram shows one armature conductor of a dynamo rotating in a
clockwise direction under
a N. pole. Fleming's right-hand rule
shows that the e. m. f. induced in the
conductor acts inwards, and this also
will be the direction of the current in
the conductor, since, in the case of a
dynamo, the current flows under the
influence of the e. m. f. induced in the
armature. Now whenever a current
flows through a straight conductor a
magnetic field is set up, the lines of force of which are concentric circles having their
centre in the conductor. The direction, or sense, of these lines of force is given by the
corkscrew rule, which states that if the current through a straight conductor is in the
same direction as the bodily motion of a corkscrew*, then the direction in which the
handle of the corkscrew has to be rotated gives the direction of the circular lines of
force. Applying this rule to A, Fig. 23, we should have to rotate a corkscrew in a
clockwise direction to drive it into the paper,** i. e. in the direction of the current in
Fig. 23. To Illustrate the Relation between
Dynamo and Motor Actions

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