3-22
will cause less attraction between the plunger and the core of the coil. This will result in improper
operation similar to that caused by low voltage. If the coil is open, shorted, or has changed in resistance,
the solenoid should be replaced.
Finally, you should check the solenoid to determine if the coil is shorted to ground. If a short to
ground is found, the short should be removed to restore the solenoid to proper operation.
Q21. What is the operating principle of a solenoid?
Q22. What is one example of the use of a solenoid?
Q23. If a solenoid is not operating properly, what items should be checked?
RELAYS
The RELAY is a device that acts upon the same fundamental principle as the solenoid. The
difference between a relay and a solenoid is that a relay does not have a movable core (plunger) while the
solenoid does. Where multipole relays are used, several circuits may be controlled at once.
Relays are electrically operated control switches, and are classified according to their use as POWER
RELAYS or CONTROL RELAYS. Power relays are called CONTACTORS; control relays are usually
known simply as relays.
The function of a contactor is to use a relatively small amount of electrical power to control the
switching of a large amount of power. The contactor permits you to control power at other locations in the
equipment, and the heavy power cables need be run only through the power relay contacts.
Only lightweight control wires are connected from the control switches to the relay coil. Safety is
also an important reason for using power relays, since high power circuits can be switched remotely
without danger to the operator.
Control relays, as their name implies, are frequently used in the control of low power circuits or
other relays, although they also have many other uses. In automatic relay circuits, a small electric signal
may set off a chain reaction of successively acting relays, which then perform various functions.
In general, a relay consists of a magnetic core and its associated coil, contacts, springs, armature, and
the mounting. Figure 3-19 illustrates the construction of a relay. When the coil is energized, the flow of
current through the coil creates a strong magnetic field which pulls the armature downward to contact C1,
completing the circuit from the common terminal to C1. At the same time, the circuit to contact C2, is
opened.
