Motor control · lesson 8 of 12

On-delay and off-delay timers

A timer relay is a relay whose contacts wait. An on-delay timer waits before acting when you energize it. An off-delay timer acts at once and waits when you de-energize it. Say "the delay is on the way ON" or "the delay is on the way OFF" and you will never mix them up.

On-delay: warn, then start

A conveyor that starts without warning is a hazard, so this circuit sounds a horn for 5 seconds first. Start latches a run relay CR. CR energizes the timer TR and, through TR's timed-open contact, the horn. When the time is up both timed contacts change together: the horn stops and the conveyor starts.

Retardo al conectar: bocina de aviso, luego el transportador

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Presione Arranque: la bocina avisa durante 5 segundos y luego el transportador se mueve.

Off-delay: run on, then stop

A heater that shuts off with the fan still cold-starting would leave the elements glowing in still air. Here the fan runs off an off-delay timer's contact: it starts the instant the heat is called for, and keeps going for 6 seconds after the call ends.

Retardo al desconectar: el ventilador sigue al cortar el calor

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Un calefactor con un ventilador que sigue funcionando después de que se apaga la calefacción, para sacar el calor.

Reading timed contacts on a print

A timed contact is drawn like any contact with a small arc — the "parachute" — over it. The arc shows which way the delay acts, and the letters under it say where the contact ends up: T.C., timed closed, or T.O., timed open. The lab labels each one. The preset (5 s, 6 s) is set on the timer itself and usually noted next to its coil.

En la obra

Timers are where a print stops being obvious, because a timed contact looks almost like a plain one and behaves nothing like it. Read the arrow before you read anything else.

Common mistakes

Reading a timed contact as an instantaneous one. The little arc on the symbol says which way the delay works. Get that backwards and your prediction of the circuit is inverted.
Using an on-delay where the design wants off-delay. "Warn then start" and "run on then stop" are different devices, not a setting — though many electronic timers can do both, chosen by a dial you must actually set.
Forgetting a timer needs its own supply. An electronic timer has a coil circuit like anything else; lose it and the contacts sit wherever they rest.
Setting the time and never writing it down. The next person finds a dial at "6" with no unit and no reason.
Leaving instantaneous contacts and timed contacts mixed up on a multi-contact timer. They are different terminals on the same device.

On a real panel

The classic on-delay job is a warning horn or beacon before a conveyor starts. The classic off-delay is a fan that runs on to clear heat after the process stops.
A timer that "does not time" is usually a timer whose reset condition never goes away — check what drops it, not what starts it.
Electronic timers have a minimum load on their output contacts on some models. If a contact chatters on a tiny load, that is worth checking on the sheet.

Prove it before you leave

Time it with a watch, not with the dial. A dial marked in one unit and set in another is a real fault.
Interrupt the circuit halfway through the delay and confirm the timer resets the way the design expects.
Run the whole sequence twice in a row. A timer that works once and not again is usually not resetting.
Write the setting and its unit on the panel schedule and on the print.

Preguntas frecuentes

What is the difference between on-delay and off-delay?

An on-delay timer waits after its coil is energized before its timed contacts change, and resets instantly when the coil drops. An off-delay timer changes its contacts instantly when energized, and waits after the coil drops before letting them go back.

What do T.C. and T.O. mean?

Timed Closed and Timed Open — which way the contact moves when the time is up. An on-delay timer has NOTC (normally open, timed closed) and NCTO (normally closed, timed open) contacts; an off-delay timer has NOTO and NCTC.

Do timers have instantaneous contacts too?

Many do — contacts that move the moment the coil energizes, like a plain relay. On a print they are drawn without the timing arc.

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