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What is Time Delay Relay (TDR) and how does it Work?

In the previous article – (What is a Star-Delta circuit and How does it work?) – We know that one of the essential components used in the course is the Time Delay Relay (TDR). This component regulates the power supply transfer to the induction electric motor from the STAR (Wye) to the Delta connection. So, what exactly is a time delay relay and how does it work in detail?

What is Time Delay Relay (TDR) and how does it Work?

Understanding the time delay relay (TDR)

Time delay relay is a component that is used to cause a delay in the operation of an electrical circuit. TDR is usually used to control the timing of events or actions in various industrial and automation applications. One example is the Star-delta circuit.

There are two main types of time delay relays. The first is the ON delay timer and the second is the OFF delay. The following is the explanation.

On-Delay (Delay-On-Make) Relay

In TDR On delay, the delay occurs after we provide a power supply to the coil. If we activate the control input, the relay contacts do not switch immediately, but after a time delay that we have determined. We usually use TDR ON delay in the Star-delta circuit, namely during the process of transferring the power supply to the motor from the Star to Delta connection.

Off-Delay (Delay-On-Break) Relay

In the off-delay relay, the delay time occurs immediately after we supply power to the TDR coil. When the delay time setting is reached, the relay contact will return to its original position. We can use off-delay relays in applications that require a delay to turn off the load after a control input is given.

Time delay relays can have adjustable or fixed time settings, and the delay time is usually specified in seconds, minutes, or hours. These relays can be digital or analogue, and we can generally use them in applications that require precise timing, such as in HVAC systems, industrial automation, lighting control, and many other fields.

How does the Time Delay Relay work?

Above, we already know what Time Delay Relay is and its types. Now we will study in detail how it works. In this article, I will give an example of how TDR works from one of the well-known brands, OMRON.

Omron released one of the TDRs with the H3CR model. Based on experience in the field, the Omron H3CR is widely used, both for starter circuits and other purposes.

The Omron H3CR TDR actually has several more versions, H3CR-A, H3CR-F, H3CR-G and H3CR-H. But in this article, we will focus on the first model, namely the H3CR-A, which also has several other series, namely the H3CR-A8, H3CR-A8S and H3CR-A8E. We will discuss the H3CR-A8 series.

Internal wiring of Omron H3CR-A8 TDR

What is Time Delay Relay (TDR) and how does it Work? (Internal wiring of Omron H3CR-A8 TDR)
Omron H3CR-A8 TDR internal wiring

In the wiring diagram above, you can witness the presence of the Omron H3CR-A8, showcasing its 8 terminal connections that encompass a multitude of practical functionalities. I can explain these functions as follows:

  • Terminals 2 and 7 act as voltage inputs to the coil. This means that when we apply voltage to the TDR coil via terminals 2 and 7, the TDR will turn ON and start working.
  • Terminals 1 and 8 are incoming. Omron H3CR-A8 has 2 control lines that we can use in the circuit. Firstly, we connect COM 1 to contacts NC (4) and NO (3). The second is the COM terminal 8 to the NC (5) and NO (6) contacts.

Omron H3CR-A8 interface

Omron H3CR TDR interface
Omron H3CR-A8 TDR interface

In order to use the Omron TDR well, we need to understand its interface. The following is an explanation of the Omron H3CR-A8 interface.

Interface functionDescription
Power IndicatorGreen when the timer is ON
Blinks green when delaying contact switching from NC to NO
Operating Mode display windowShows the current mode setting position
Operating mode selectorMode A (On Delay). In mode A, switching contacts from NC to NO waits for a time according to user settings. If the TDR time setting is 5 seconds, then when it starts to turn ON, the TDR will count for 5 seconds and then the contact will move from NC to NO. Once the TDR contact moves to NO, the contact will not return to NC until the timer is OFF.

B2 (Flip Flop) mode. In B2 mode, switching from NC to NO contacts occurs alternately according to user settings. If the TDR time setting is 5 seconds, the TDR contact will change position from NC to NO every 5 seconds).

Mode E (Off Delay). In mode E, the TDR contact is immediately in the NO position when the Timer is ON. Shortly after the TDR time setting is reached, the contact moves to the NC position. If the time setting in mode E is 5 seconds, then when it starts to turn ON, the TDR will count for 5 seconds and then the contact will move to NC.

Mode J (One-shot output). In J mode, the TDR contact is initially in the NC position. As soon as the time setting is reached, the TDR contact will move to the NO position in just 1 second, after that the contact will move back to NC. And so on. This process will repeat itself over and over again.
Scale range display windowShows the time range currently used on the timer (TDR)
Time unit display windowShows the time unit currently used in TDR. Whether seconds, minutes or hours.
Time unit selectorSelector to move the time unit currently used on the timer, starting from S (seconds), M (minutes), H (hours), 10 H (x10 hours).
Time setting KnobKnob to change the position of the TDR needle according to the time unit display.
Time range selectorSee the description below for details
Output indicatorWill be ON (orange) if the TDR output is active
Omron H3CR-A8 interface description
Timer range selector details

Selector to change the time range currently used. On the Omron H3CR-A8, the time range is as in the following table.

  • In S, M, and H modes, the operating time range will match the range shown by the display.
  • In 10h mode, the operating time range is a time unit display x 10. If the needle on the scale range display shows 1.2, then the current operating time is 12 hours. 1.2 x 10h = 12 hours. Likewise for other ranges.

Working principle of the Omron H3CR-A8 TDR

Working principle of the Omron H3CR-A8 TDR
Contact position in OFF-state
Working principle of the Omron H3CR-A8 TDR
Contact position after power ON
  • NC contact: 1-4 and 3-8
  • NO contact: 1-3 and 6-8

The image above shows how the contacts on the TDR change position. When the power is OFF, we see the contact position is in the NC (1-4, 3-8) position. Then after TDR ON, the contact position moves to the NO position (1-3, 6-8).

Please note, that each mode in TDR works in a different way. I explain each mode in the table below so that you can understand it easily.

In the table below, I demonstrate the interface positions according to the first column (Setting mode).

Setting ModeTDR when power OFFTDR when power ON
Time unit selector: S (Second)
Time setting knobe: 5
Time Time range selector: 30
Terminals 1-4 and 3-8 are closed. Terminals 1-3 and 6-8 are opened.Terminals 1-4 and 3-8 are opened. Terminals 1-3 and 6-8 are closed.
TDR with setting mode

Working principle in mode A

As I explained in the selector mode above, Mode A is delay ON mode. This means, with the mode setting that we made in this case when the TDR power is ON, the relay contact which was previously in the NC position, will move to NO within 5 seconds. By setting the time range to 30, this means we can change the maximum range by 30 seconds.

We can use this mode setting on a star-delta circuit. NC as STAR and NO as DELTA.

Working principle in mode B2

In B2 mode, the contacts on the TDR will change position every 5 seconds. With a range of 30, we can change it to a maximum of 30 seconds.

B2 mode would be good if used for sirens in alarm systems. Setting this mode will make your siren turn ON and OFF every few seconds.

Working principle in mode E

Mode E is the opposite of mode A. In mode A, when the timer is ON, the TDR contact will move from NC to NO and will continue to stay at NO after a count of 5 seconds. Meanwhile, in mode E, when the timer is ON, the TDR contact will immediately move from NC to NO for 5 seconds, then return to the NC position.

Similar to mode A, we can also use mode E for Star-delta circuits. Contact NO as STAR and NC as Delta. Compared to mode A, I prefer to use mode E in the Star-delta case.

The working principle in mode J

In J mode, when the TDR is first supplied with power to the Coil, the switch on the NC terminal does not immediately move to NO. The timer will count for 5 seconds according to the settings above, then the switch will move from NC to NO position in just 1 second. After that, the switch will return to the NC position, count again for 5 seconds, move again for 1 second and return to the NC position. And so on over and over again.

You can use this mode on applications that you think are suitable for the action of this TDR in mode J.

That’s an explanation of what time delay relay is and how it works. If you find this article useful, please share it with your relatives. If anything makes you confused, don’t hesitate to contact me on the contact provided.

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I am a dedicated and results-oriented Electrical, Instrumentation and Automation Engineer with a strong educational background and experience working in the Sugar and oil and chemical industry. My academic foundation from Unram University has provided me with a solid understanding of electrical engineering principles, and my professional experience has allowed me to apply this knowledge in a practical setting.

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