What is Time Delay Relay?

Some relays are constructed with a kind of “shock absorber” mechanism attached to the armature that prevents immediate, full motion when the coil is either energized or deenergized. 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 specified not only as either normally-open or normallyclosed, but also must specify 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:

  1. The NOTC (normally-open, timed-closed) contact is normally open when the coil is unpowered (de-energized). It is closed by the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time. The direction of the contact’s motion (either to close or to open) is identical to a regular normally-open contact, but there is a delay in closing direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally-open, on-delay.

  2. The NOTO (normally-open, timed-open) contact is like the NOTC contact, in that it is normally open when the coil is unpowered (de-energized), and closed by the application of power to the relay coil. Unlike the NOTC contact, the timing action occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in the direction of coil de-energization, this type of contact is alternatively known as a normally-open, off-delay.

  3. The NCTO (normally-closed, timed-open) contact is normally closed when the coil is unpowered (de-energized). The contact is opened with the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time. The direction of the contact’s motion (either to close or to open) is identical to a regular normally-closed contact, but there is a delay in the opening direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally-closed, on-delay.

  4. The NCTC (normally-closed, timed-closed) contact is like the NCTO contact, in that it is normally closed when the coil is unpowered (de-energized), and opened by the application of power to the relay coil. Unlike the NCTO contact, the timing action occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in the direction of coil de-energization, this type of contact is alternatively known as a normally-closed, off-delay.

Time-delay relays are very important for use in industrial control logic circuits. Some examples of their uses include the following:

 Flashing light control (time on, time off). Two time-delay relays are used in conjunction with one another to provide a constant-frequency on/off pulsing of contacts for sending intermittent power to a lamp.

 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 first 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.

 Furnace safety purge control. Before a combustion-type furnace can be safely lit, the air fan must be run for a specified amount of time to “purge” the furnace chamber of any potentially flammable or explosive vapors. A time-delay relay provides the furnace control logic with this necessary time element.

 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.

 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 first and the first 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.