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I need to control a 3A 24V DC motor via a relay. The relay is not the primary means of control, its purpose is to temporarily disable the motor under an unusual condition. One that almost never occurs, think once a year. Should I be worried about the relay becoming unable to open after a long period of not being exercised? Is there a type of relay best suited to this application?

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    \$\begingroup\$ Welcome! It will be down to your accepted level of risk that the relay is stuck and the consequences of it. Is it safety or mission critical? \$\endgroup\$
    – winny
    Aug 20, 2023 at 14:05
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    \$\begingroup\$ Does the motor run continuously 24/7? Or is it powered off on a regular basis. Even though the relay is not the primary means of control, if the motor is powered off, it might be wise to use a normally open relay, and power it down when the motor is powered down. That way, it will cycle regularly. \$\endgroup\$ Aug 20, 2023 at 14:09
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    \$\begingroup\$ There are high-reliability, sealed, relays for critical applications, e.g., rockwellautomation.com/en-us/products/hardware/allen-bradley/… . That said, would the intended use justify the cost? \$\endgroup\$ Aug 20, 2023 at 22:29

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You are referring to a 'safety relay' that actually consists of three relays - two to perform the dual-redundant output function and one to monitor the status of the output contacts.

Should either of the contacts get stuck during operation, start of the next cycle is prevented.

It is to be emphasized that it's the redundant 'NO' contacts that perform the switching on / off function.

Here's a classic 'safety relay' schematic.

enter image description here

Off-the-shelf 'safety relay' modules are available for various applications.

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This isn't usually how you design such applications. You'd power the motor through the NO contacts of a relay which gets turned on at power up, so that each time you power up, you get a relay toggle by whatever is driving the coil. This is good against oxidation and you get a check that the relay is working. Also gold-plated contacts would be mandatory for this kind of application.

If the function is safety related, then you would typically use a safety relay with multiple contracts that are forcibly guided (if one of the hangs/weld, then both do at once), then use one of the contacts for supervision. These are also of generally higher quality than regular relays.

If you don't want to build the supervision of your own, vendors who have a lot of industry automation products will have DIN rail mounted safety relay modules with built-in supervision.

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    \$\begingroup\$ A note on gold plating: there are different qualities and the cheap one is to have just a very thing gold plating which will prevent oxidation during storage but will be gone after the first cycles, especially when used with higher loads. \$\endgroup\$
    – Arsenal
    Aug 21, 2023 at 11:49

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