Техническое чтение для энергетиков. Бухарова Г.П. - 70 стр.

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72
the conductor, and this clockwise direction, therefore, gives the direction of the lines
of force set up by the current. For simplicity only one of these lines of force is shown,
and it is represented by the dotted circle. Now the lines of force of the main field
from the N. pole cross the air-gap from the pole to the armature, i. e. downwards in
the figure and, therefore, on comparing the directions of the lines of force of the two
fields, we see that the armature field acts in the same direction as the main field on
the right-hand side of the conductor, and in opposition to the main field on the left-
hand side. As a result there is a strong field on the right-hand side and a weakened
field on the left-hand side. It is impossible for two sets of lines of force to intersect
one another as shown in A, the actual arrangement resulting from the combination of
the two fields being as shown in B. It will be seen that some of the lines of force are
bent round the conductor. Now magnetic lines of force are always in a state of
tension and therefore, the bent lines of force will set up a mechanical force on the
conductor much in the same way that the bent elastic of a catapult produces a
mechanical force on the stone. In the case of the conductor in the figure this force
obviously acts from right to left, i. e. opposite to the motion of the conductor. This
applies to all the conductors on the armature of a generator delivering current, and it
follows that the steam engine or other prime mover has to drag the armature round
against this opposing force. For this reason the force is called the "magnetic drag."
For a current of J amperes flowing through a conductor of length l cms., placed
at right angles to the lines of force of a magnetic field of strength В lines per sq. cm.,
the magnitude of the drag is given by the expression
dynesf
BJl
10
=
the denominator 10 being introduced because the practical unit of cur-tent, the
ampere, is one-tenth of the C.G.S. unit of current. The direction of the force is
opposite to the direction of motion in the case of the generator shown in A and В and,
therefore, since the right-hand rule gives the relationship between the directions of
field, current and motion, a similar left-hand rule will give the relationship between
field current and force. The rule is therefore as follows: hold the thumb and first
finger of the left hand at right angles, and bend the second finger so as to point at
right angles to the plane of these two. Then if the first finger is pointed in the
direction of the field, and the second finger in the direction of the current the thumb
will point in the direction of the force.
This is illustrated by Fig. 24.
Back E.M.F. – Suppose the
machine, instead of generating its own
electrical power, is uncoupled from the
prime mover and is connected to an
external supply, and current sent through
the armature and field windings in the
same directions as before. Then it is
obvious that the magnetic fields
produced by the field magnet and by the armature will be the same as before and
therefore the shape of the lines of force will still be as shown in В Fig. 23. Therefore,
Fig. 24. To Illustrate the Left-hand Rule

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