What is a Time-Delay Relay?
A time-delay relay (or timer relay) is a switching device that performs actions after a set time delay. The relay switches on or off after a preset time has elapsed and is often used for starting, stopping or protecting industrial processes.
In OT environments, a time-delay relay is used for starting motors (such as in starโdelta), preventing inrush peaks, fan run-on times or delaying alarm signals.
๐ง Basic operation
A time-delay relay consists of:
- An input signal (trigger, voltage or contact)
- An internal timer (adjustable: seconds, minutes, hours)
- One or more switching contacts (NO/NC)
- A delay on energising or de-energising
Time-delay relays are available in electromechanical, electronic or digital versions.
๐ Common functions
| Function code | Name | Description |
|---|---|---|
| TON | On-delay | Contact switches on after the set time |
| TOFF | Off-delay | Contact remains on for X seconds after the trigger signal |
| TP | Pulse function | Contact switches for a fixed duration, regardless of input |
| Flasher | Periodic on/off | Alternating pattern, e.g. 1s on, 1s off |
| Star-Delta | Specific function for motor start | Starts in star, then automatically switches to delta (ฮ) |
โ๏ธ Typical applications
| Application | Time-delay relay function |
|---|---|
| Starโdelta motor start | Start delay before changeover |
| Fan run-on time | TOFF (off-delay) |
| Delayed alarm on fault | TON (on-delay) |
| Periodic pump control | Flasher / cyclic relay |
| Machine safety | Off-delay during emergencies |
โ Selection considerations
- Time setting range (e.g. 0.1 โ 30s, 1 โ 60 min, etc.)
- Supply voltage (AC/DC, 12V, 24V, 230V)
- Mounting type (DIN-rail, front panel, PCB)
- Number of contacts (1 CO, 2 CO, etc.)
- Reset behaviour (automatic, manual, on voltage drop)
- Standards compliance (e.g. IEC 60947)
๐ Alternatives and integration
| Alternative | When to use? |
|---|---|
| PLC with timer function | For complex or flexible timing logic |
| Soft starter | For controlled run-up with timing parameters |
| Variable frequency drive | Built-in timers for motor scenarios |
๐ In summary
Time-delay relays provide a simple, robust way to implement time-dependent switching functions. In OT systems, they are indispensable for motor control, ventilation, alarm systems and protection functions.