A SOLENOID is a control device that uses electromagnetism to convert electrical energy into a
mechanical motion. The magnetic field of the coil and core will attract the plunger of a solenoid when
current flows through the coil. When current is removed, the spring attached to the plunger will cause the
plunger to return to its original position.
If a solenoid fails to operate, check the terminal connections, the plunger and attached mechanism
for smooth operation, the energizing voltage, and the coil of the solenoid.
A RELAY is an electromagnetic control device that differs from the solenoid in that the solenoid
uses a movable core (plunger) while the relay has fixed core. Relays are classified as CONTROL
RELAYS, which control low power COMMON circuits and POWER RELAYS or CONTACTORS
which control high power circuits.