The Basics of Time Delay Relays

Aug. 1, 2010
Application requirements for time delay relays (TDRs)

No matter what the application is, when a definite-purpose solution is required, time delay relays (TDRs) can provide simple, reliable, and economical control. Adjusting the delay time is often as simple as turning a knob. Providing time-delayed switching to start a motor, control a load, or affect a process, TDRs are typically used in industrial applications and OEM equipment. Additionally, they play an important role for targeted logic needs, such as in a small panel or in sub-panels. They have a variety of features and operating characteristics, such as compactness, economy, simplicity, and ease-of-use.

In a standard control relay, contacts close immediately when voltage is applied to the coil, and open immediately when voltage is removed. In a variety of applications, it’s desirable to have the operation of the contacts delayed following application or removal of voltage. A TDR solves the problem handily. However, some TDRs postpone closing of the contacts after voltage is applied while others close the contacts — and then reopen them after a delay.

TDRs are available as plug-in devices, much like plug-in control relays. However, they are also available in a range of other forms, including base-mounted devices and direct IEC DIN-mounted controls. For instance, a TDR can be fixed on a motor starter. In this application, energizing the motor starter causes the timing function to begin; contacts within the device operate when timing is complete. Electronic, starter-mounted TDRs are also available. Some TDRs have solid-state outputs instead of relay outputs.

Traditionally, TDRs were available only as single-function, single-time-range devices. These devices are still available and are typically used in applications where the timing needs to be locked in. Today, many TDRs are also available with multiple timing ranges and functions. Costing little more than single-function devices, these TDRs also have wide control voltage ranges. In addition, newer multifunction IEC-style timers allow for reduced inventories. Let’s take a closer look at a few of the more common types of TDRs.

On-delay timers

With an on-delay timer, timing begins when voltage is applied. When the time has expired, the contacts close — and remain closed until voltage is removed from the coil. If voltage is removed before time-out, the time delay resets (click here to see Fig. 1).

Off-delay timers

When using an off-delay timer, nothing happens when voltage is applied. Closing the control input (SW) causes the contacts to transfer (click here to see Fig. 2). Opening the control input causes timing to begin, and the contacts remain closed. On time-out, the contacts transfer. Closing the control input prior to time-out causes timing to reset. Removing voltage prior to time-out resets the timing and opens the contacts. In addition, true off-delay timers provide this functionality (keeping contacts closed) after input voltage is lost. They have capacitors to keep contacts closed even if the timer loses power.

There are special contact symbols for the on-delay and off-delay timers. In fact, they are the only TDRs for which special contact symbols have been assigned. Other types of TDRs simply use the same contact symbols as those for relays. Often, a note is made near the relay symbol to denote the operating condition. The state of relay contacts is always shown with voltage removed.

Single-shot timers

A single-shot TDR has voltage and control inputs similar to the off-delay TDR. Nothing happens when voltage is applied. On closure of the control input, the contacts transfer and timing begins. While timing, the control input can be left open, closed, or opened and closed; in each case, timing continues, and the contacts remain closed. Only at time-out will the contacts transfer. The TDR is reset at this time, ready for another cycle. The only means to interrupt the operation is to remove voltage.

Real-world applications

Let’s put our basic TDR knowledge to practical use. In the three examples below, learn how you can use TDRs effectively to manage processes in various manufacturing facilities.

Crusher and conveyor line — Figure 3 (click here to see Fig. 3) illustrates a simple circuit for a crusher and its feeder conveyor. The circuit is meant purely to illustrate the operation of the TDRs discussed in this article; additional circuitry is required to complete a functioning crusher circuit. Furthermore, familiar voltages are referenced throughout to lend an air of familiarity to the circuitry.

The following sequence of events takes place to start the crusher:

  • When the start button (line 1) is depressed, relay CR closes.
  • CR’s contact on line 2 seals in across the start button. Simultaneously, CR’s contact on line 3 closes, causing 1TD to begin timing.
  • CR’s contact on line 4 closes, causing the warning horn (line 5) to sound through 1TD’s NCTO contact.
  • The CR contact on line 6 closes, causing 2TD to energize its NOTO contact on line 8.
  • When 1TD times out after 10 sec, its NCTO contact on line 5 opens, silencing the warning horn.
  • Simultaneously, 1TD’s NOTC contact on line 8 closes, energizing 2M, starting the crusher, and causing 3TD to begin timing; 2M’s contact on line 9 seals in across 1TD on line 8.
  • When 3TD times out after 10 sec, the feeder conveyor starter, 1M, energizes. 3TD serves to delay starting of the feeder conveyor until the crusher is operating at full speed.

It’s important to note that 3TD does not have a coil but is pneumatic and mounted on top of 2M. 3TD’s timing begins when 2M energizes.

Two methods for stopping are available: normal (labeled cleanout stop) and emergency stop. To stop the crusher under normal circumstances, the button labeled “cleanout stop” is depressed; relay CR de-energizes, causing its seal-in contact on line 2 to open. Additionally, CR’s contacts open simultaneously on all the other lines. The conveyor starter 1M (line 4) de-energizes and stops the feeder conveyor. On line 3, 1TD de-energizes, opening 1TD NOTC on line 8; because 2M is sealed in across 1TD NOTC, 2M stays energized. Timer 2TD allows the crusher to run until all material is out to prevent clogging the crusher — 60 sec in this example. When 2TD times out, its NOTO contact on line 8 opens, de-energizing 2M and stopping the crusher. Note that 3TD’s NOTC contact (line 4) will also stop the feeder conveyor if the crusher starter’s overload relay de-energizes 2M to prevent overfilling of the crusher.

While the cleanout stop is used to assure that the crusher is cleared of material during normal operation, the emergency stop is for real emergencies — when personnel are in danger or a malfunction occurs. The emergency stop button is a large, palm-operated device (hence, the special symbol), shown as a latching button. When depressed, it opens, stopping all motors immediately, and remains latched in the open position. To restart the system, the emergency stop button must first be pulled out and the start button depressed.

Window manufacturing process — At times, it’s imperative to have a momentary input trigger in an operation. For example, a paint drying line for windows uses infrared heaters to dry the paint. Because the material flow is irregular and energy conservation is important, the owner wants the heaters to remain off between windows. A method is required to trigger the heaters to remain on as long as a window is in the oven. For this example, assume the windows are of equal lengths but of varying widths. Because lengths are the same, a uniform drying time can be selected for all windows.

A single-shot TDR can be used in this manufacturing process nicely (click here to see Fig. 4). A photosensor (line 3) is positioned to detect the leading edge of the window. When the photosensor’s output contact closes (line 4), it triggers timer TD. TD’s output contact closes (line 6), causing the infrared heater’s contactor to close. Time delay TD also begins its timing cycle. Although other parts of the window, such as the muntin strips separating panes, may retrigger TD, the output contacts remain closed and TD continues to time. Only when TD has timed out does its output contact open. After time-out, the single-shot is ready to be retriggered for the next cycle. The M contacts are interlocked with TD to prevent overheating the window should M trip-out due to an overload.

Belt conveyors and bucket elevators — Many bucket elevators have buckets attached to a wide rubber belt, lift grain, and other materials vertically — depositing it onto other conveyors. Drive motors for conveyors are located on the head pulley (to pull the belt). Conveyor drives consist of a motor coupled to a gear reducer, which drives the head pulley. Should the head pulley slip, because the conveyor belt is not tensioned properly, the head pulley will slip and burn through the belt. In this event, the best case scenario is that the belt may break and spill the contents, requiring an outage — or worse, the belt and its contents may erupt in flames.

A watchdog timer can serve to prevent such a calamity. An inductive (metal-sensing) proximity sensor detects a metal protrusion (like a key) on the tail pulley. As long as the pulley is rotating, the watchdog timer resets (retriggers) and allows the starter to run. When the conveyor’s main belt goes slack, the tail pulley fails to rotate, the proximity switch fails to retrigger the timer, and the watchdog timer times out, stopping the conveyor motor.

A watchdog timer in conjunction with a conveyor starter is illustrated in Fig. 5 (click here to see Fig. 5). Again, note that additional circuitry may be required in an operational system. The start button (line 1) is depressed and held until the conveyor reaches sufficient velocity to allow TD’s contact on line 2 to close and seal across the start button; the contact is normally open and times open (NOTO). Should the tail pulley fail to rotate, TD will time out, causing the starter to de-energize, stopping the conveyor before damage occurs. In practice, TD’s time delay is set for a few seconds.

The examples demonstrated above are simply meant to provide an introduction into the myriad of uses for TDRs. Actual applications for TDRs are quite expansive.

Bredhold is an application engineer with Eaton Corp., Louisville, Ky. He can be reached at [email protected].

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David Bredhold

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