action of the spring. When voltage is applied, the
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
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
One of the principal uses of relays is the remote
control of circuits. Circuits may be 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
4-52 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.
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 electrical control system is protected from
overloading by fuses and circuit breakers.
RELAY COIL TERMINALS
Figure 4-52.Relay construction.