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Page Title: Figure 2-28.-Solenoid action.
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through the coil. If the current is expressed in amperes,
the mmf is expressed in ampere-turns.
Solenoids are electromagnets formed by a
From this it can be seen that a prescribed mmf can
conductor wound in a series of loops in the shape of a
be produced by using either a few turns of large wire
helix (spiral), Inserted within this spiral or coil is a
(high current) or many turns of small wire (low
soft-iron core and a movable plunger. The soft-iron
core is pinned or held in position and therefore is not
The soft-iron core will also influence the strength
movable. The movable plunger (also soft iron) is held
of the magnetic flux produced by the coil. The strength
away from the core by a spring when the solenoid is
of the field is greatly increased by the use of a soft-iron
de-energized (fig. 2-28).
core due to the greater permeability of iron in respect
to air. Consequently, by using an iron core, a greater
When current flows through the conductor, a
flux density can be produced for a given number of
magnetic field is produced. This field acts in every
respect like a permanent magnet having both a north
The magnetic flux produced by the coil will result
and a south pole. The total magnetic flux density
in establishing north and south poles in both the core
produced is the result of the generated mmf and the
and the plunger. These poles have such a relationship
permeability of the medium through which the field
that the plunger is attracted along the lines of force to
a position of equilibrium when the plunger is at the
In much the same way that electromotive force
center of the coil. As shown in figure 2-28, the
de-energized position of the plunger is partially out of
(emf) is responsible for current in a circuit, mmf is
the coil due to the action of the spring. When voltage
responsible for external magnetic effects. The mmf
is applied, the current through the coil produces a
that produces the magnetic flux in a solenoid is the
magnetic field that draws the plunger within the coil,
product of the number of turns of wire and the current
thereby resulting in mechanical motion. When the coil
is de-energized, the plunger returns to its normal
position by the spring action. It is interesting to note
that the effective strength of the magnetic field on the
plunger varies with the distance between the two. For
short distances, the strength of the field is strong; and
as distances increase, the strength drops off quite
Solenoids are used for electrically operating
hydraulic valve actuators, carbon pile voltage
regulators, power relays, and mechanical clutches.
They are also used for many other purposes where
only small movements are required. One of the distinct
advantages in the use of solenoids is that a mechanical
movement can be accomplished at a considerable
distance from the control. The only link necessary
between the control and the solenoid is the electrical
wiring for the coil current.
The first step to be taken in checking an
improperly operating solenoid is a good visual
inspection. The connections should be checked for
Figure 2-28.-Solenoid action.
poor soldering, loose connections, or broken wires.

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