What is a time delay relay?
Some relays are constructed with a "shock absorber" mechanism attached to the armature that prevents immediate full movement when the coil is energized or de-energized. This addition gives the relay a delayed drive characteristic.
Delay relays can be constructed to delay armature movement of coil energization, de-energization, or both. Delay relay contacts must not only be specified as normally open or normally closed, but also whether the delay is in the closing or opening direction.
The four basic types of time delay relay contacts are described below.
Normally open, timed closed contacts
First, we have the normally open, timed close (NOTC) contacts. This type of contact is usually open when the coil is not energized (de-energized). The contacts close when the relay coil is energized, but only after the coil is continuously energized for a specified time.
In other words, the contact moves in the same direction (closed or open) as a regular normally open contact, but there is a delay in the closing direction. Since the delay occurs in the coil energization direction, this type of contact is also known as a normally open on-delay:
Delay Relays in Industrial Control Logic Circuits
Delay relays are very important for the use of industrial control logic circuits. Some examples of their use include:
Flash control (timed on, timed off):
Two time delay relays are used in conjunction with each other to provide constant frequency contact open/close pulses to send intermittent power to the lamp.
Engine Auto Start Control:
Engines used to power emergency generators are often equipped with "auto-start" controls that automatically start if the mains supply fails.
To start a large engine properly, it is necessary to start certain auxiliary equipment first and allow a short stabilization time (fuel pump, pre-lube pump) before the engine starter motor is energized.
A time delay relay helps sequence these events to properly start the engine.
Furnace Safety Purge Control:
Before a combustion-type furnace can be safely ignited, the fan must run for a specified time to "purge" the furnace chamber of any potentially flammable or explosive vapors.
Delay relays provide this necessary time element for the furnace control logic.
Motor soft start delay control:
Instead of starting a large motor by switching full power from a dead stop, a reduced voltage can be switched for a "softer" start and less inrush current. Full power is applied after the specified time delay (provided by the time delay relay).
Conveyor sequence delay:
When arranging multiple conveyor belts to transport material, the conveyor belts must be started in reverse order (last one first, first one behind) to prevent material from accumulating on stopped or slow-moving conveyors. It may take some time for the large belt to reach full speed (especially when using soft start motor control). For this reason, a delay circuit is usually placed on each conveyor to give it enough time to reach full speed before the next conveyor starts to feed.
Advanced timer function
Older mechanical time delay relays used pneumatic bumpers or fluid-filled piston/cylinder arrangements to provide the "shock-shock" needed to delay armature movement.
Newer time delay relay designs use an electronic circuit with a resistor-capacitor (RC) network to create a time delay and then use the output of the electronic circuit to energize a normal (instantaneous) electromechanical relay coil.
Electronic timer relays are more versatile than older mechanical models and less prone to failure.
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