Figure 4-60.—Solenoid action.
current through the coil produces a magnetic field,
which draws the plunger within the coil, thereby
resulting in mechanical motion. When the coil is
de-energized, the plunger returns to its normal position
by the spring action.
Solenoids are used in steam catapult systems for
electrically
operating bridle tensioning valves,
lubrication valves, engine retraction valves, and relays,
and for various other mechanisms 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 station. The only link necessary between the
control and the solenoid is the electrical wiring for the
coil current.
RELAYS.—One of the principal uses of relays is
the remote control of circuits. Circuits maybe energized
by control relays from one or more stations simply by
closing a switch. Switches used to energize relays
require lightweight wire only, and may thereby
eliminate the necessity of running heavy power cable to
the various control points. An additional advantage
resulting from relay control is the removal of safety
hazards, since high-voltage equipment can be switched
remotely without danger to the operator.
In general, a relay consists of the following
components: a magnetic core and associated coil, the
contacts, springs, armature, and the mounting. Figure
4-61 illustrates the fundamental construction of a relay.
When the circuit is energized, the flow of current
through the coil creates a strong magnetic field, which
pulls the armature to a position that closes the contacts.
When the coil is energized, it moves the armature to
contact C1, which completes the circuit from the
common terminal to C1. At the same time, it has
opened the circuit to contact C2.
The relay is one of the most dependable
electromechanical devices in use; but like any other
mechanical or electrical equipment, relays occasionally
wear out or become inoperative for one reason or
another. Should inspection determine that a relay has
exceeded its safe life, the relay should be removed
immediately and replaced with one of the same type.
FUSES AND CIRCUIT BREAKERS.— The elec-
trical control system is protected from overloading by
fuses and circuit breakers.
The fuse is the simplest protective device. A fuse is
merely a short length of wire or metal ribbon within a
suitable container. This wire or metal ribbon is usually
made of an alloy that has a low melting point and is
designed to carry a given amount of current
indefinitely. A larger current causes the metal to heat
and melt, opening the circuit to be protected. In
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