Some relays are constructed with a kind of “shock absorber” mechanism attached to the armature which prevents immediate, full motion when the coil is either energized or de-energized. This addition
gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay armature motion on coil energization, de-energization, or both. Time-delay relay contacts must be specied not only as either normally-open or normally-closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts. First we have the normally-open, timed-closed (NOTC) contact. This type of contact is normally open when the coil is unpowered (de-energized). The contact is closed by the application of power to the relay coil, but only after the coil has been continuously powered for the speci¯ed amount of time. In other words, the direction of the contact’s motion (either to close or to open) is identical to a regular NO contact, but there is a delay in closing direction. Because the delay occurs in the direction of coil nergization, this type of contact is alternatively known as a normally-open, on-delay:
Time-delay relays are very important for use in industrial control logic circuits. Some examples of their use include:
² Flashing light control (time on, time o®): two time-delay relays are used in conjunction with one another to provide a constant-frequency on/o® pulsing of contacts for sending intermittent power to a lamp.
² Engine autostart control: Engines that are used to power emergency generators are often equipped with “autostart” controls that allow for automatic start-up if the main electric power fails. To properly start a large engine, certain auxiliary devices must be started ¯rst and allowed
some brief time to stabilize (fuel pumps, pre-lubrication oil pumps) before the engine’s starter motor is energized. Time-delay relays help sequence these events for proper start-up of the engine.
² Furnace safety purge control: Before a combustion-type furnace can be safely lit, the air fan must be run for a speci¯ed amount of time to “purge” the furnace chamber of any potentially °ammable or explosive vapors. A time-delay relay provides the furnace control logic with this
necessary time element.
² Motor soft-start delay control: Instead of starting large electric motors by switching full power from a dead stop condition, reduced voltage can be switched for a “softer” start and less inrush
current. After a prescribed time delay (provided by a time-delay relay), full power is applied.
² Conveyor belt sequence delay: when multiple conveyor belts are arranged to transport material, the conveyor belts must be started in reverse sequence (the last one ¯rst and the ¯rst one last)
so that material doesn’t get piled on to a stopped or slow-moving conveyor. In order to get large belts up to full speed, some time may be needed (especially if soft-start motor controls are used). For this reason, there is usually a time-delay circuit arranged on each conveyor
to give it adequate time to attain full belt speed before the next conveyor belt feeding it is started.
