Figure 2-28.-Solenoid action.

SOLENOIDS

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

current).

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

ampere-turns.

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

passes.

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

rapidly.

USES OF SOLENOIDS

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.

MAINTENANCE OF SOLENOIDS

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.

2-18