Техническое чтение для энергетиков. Бухарова Г.П. - 71 стр.

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73
the magnetic drag will be set up in the same direction as before, and since there is
now no engine to force the armature round against this drag, the armature will rotate
in the same direction as the drag, as shown in C. The machine will now be running as
a motor. We therefore see that for the same direction of the armature currents and the
same polarity of the field-magnets, the direction of rotation of a machine when
running as a motor is opposite to its direction when running as a dynamo. On the
other hand, if the directions of rotation are the same and the polarities the same, then
the directions of the armature currents will be different for the two modes of
operation.
Now when a machine is running as a motor, the conductors on the armature cut
the lines of force of the magnetic field just as they do when the machine is acting as a
dynamo. As a result they have e.m.f.'s induced in them. The direction of one such
induced e.m.f. in an individual conductor is, of course, given by the right-hand rule,
and applying this rule to the conductor shown in C, we see that the induced e.m.f.
acts outwards, that is, in opposition to the current. This induced e.m.f. in the case of a
motor is, therefore, called the "back e.m.f."
Application of the Principle of Work.– It is interesting to look at the above
problems from another point of view. We know that an electric motor does
mechanical work, and we also know that in order that any machine may do work, an
equal amount of work (plus the losses in the machine) has to be put into it. Again,
when the work is done some force has to be overcome. Now, it is the supply e. m. f.
which puts work into the motor by driving the current through the armature, and since
work is only done when some force is overcome, we see that in order that the motor
may perform mechanical work, the supply e.m.f. must have some opposition. This
opposition must obviously come from a force of the same nature, namely an e.m.f.,
from which it follows that the armature must set up a back e.m.f. A similar process of
reasoning shows that a magnetic drag must be set up on the armature of a dynamo
delivering current.
It will thus be seen that the motor action and the dynamo action, which for the
sake of convenience are studied separately, cannot, as a matter of fact, have separate
existences. They are inextricably bound up together, and one cannot come into
operation without the other. As soon as a dynamo delivers current, the motor action
comes into play and sets up the resistance to motion called the magnetic drag; and
when a motor is made to perform work the dynamo action immediately comes into
play and sets up the back e.m.f.
* the bodily motion of a corkscrew движение самого штопора; bodily – наречие в функции
определения к сущ. motion.
** to drive it into the paper чтобы он (штопор) как бы ввинчивался в бумагу
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