An on delay relay is an electrical switching device that needs a certain amount of time to elapse before its output contact will switch on. When the relay is powered up a certain amount of preset time will count down before the contact of the relay turns on.
How It Is Used
Take a look at this electrical diagram of an on delay relay used in a motor control circuit, you can see the T1 “relay” and the T1 “contact”. Although these two things are drawn separately in the diagram they are actually physically located within the same module. In the drawing if the start button was pressed this would provide power to the T1 relay and start the time delay process, but the T1 output contact would not switch on yet. After the predetermined time delay had elapsed, the T1 contact would close. Then the K1 contactor coil would be energised and power would be sent to the motor via the K1 contacts. If the finger was then taken away from the start button, the T1 relay would be de-energised and the T1 contact would open instantly.
Quick Tip
You can see that the T1 contact looks like a parachute! It is handy to remember that the parachute drawn on any contact will show that the contact will have a delay in the direction of the parachute. In this case it will have an on delay effect.
To Recap
An on delay relay is an electrical component. Once it is powered up, its output contact will switch on after a certain time delay. Once the relay module is de-energised its output contact will turn off too.
Learn more
Fundamentals of Electrical Controls
3 Phase Motor Control Bootcamp
Learn Variable Frequency Drives