Mechanical Relays for 24 VDC Solenoid Valves

We have a new system here at our facility that is using Mechanical Relays to control 24 VDC solenoid valves in a hydraulic system. Our relays are failing very frequently. When looking at the relays the contact does appear to be charred. And during operation I notice sparking. Do you think that the sparking is from the inductive kick back of the voltage being stored in the coil? And, if so, would a diode be the answer to increasing the life of these relays?



Or, do you think that the spark is being generated during the charging up of the solenoid valve coil? If this is the case, I may need to use a solid state relay to solve the issue.


Please, I'd like to hear any advice you have to offer.


The relays that we are using are AB 700-HK36Z24-4.
The solenoid valves that are being controlled through these relays are located about 20 to 50 feet from the relays.

We control a number of hydraulic pressure regulating valves using 24VDC solenoids. We had the same issue as you. I had some improvement with fitting a freewheeling diode across the coil. We still were switching some quite often so we switched to solid state relays about 6 months ago. No issues so far.

psychosunshine said:

Do you think that the sparking is from the inductive kick back of the voltage being stored in the coil?

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Yes.

psychosunshine said:

And, if so, would a diode be the answer to increasing the life of these relays?

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A freewheeling diode, preferably at the coil of the solenoid, is one way. A disadvantage is that a freewheeling diode delays the turn-off of the solenoid.
Alternative is a RC-surge supressor.

psychosunshine said:

Or, do you think that the spark is being generated during the charging up of the solenoid valve coil?

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No.

psychosunshine said:

The solenoid valves that are being controlled through these relays are located about 20 to 50 feet from the relays.

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Long leads without surge suppression can lead to excessive noise.

If the DC coils are very large, and/or are switched frequently it may not be enough with a freewheeling diode or RC suppressor.
For large currents, consider using contactors instead of simple relays.
For frequent switching, consider using solid state relays.

I agree with solid state to drive 24v DC coils, however, a lot can depend on the relay manufacturer, we had an IMO PLC with relay outputs, about every 4-5 weeks a relay would fail on the PLC unit, these relays were available form RS so we kept a good stock, I put forward a proposal to replace the PLC with a Mitsubishi (the standard for the site) ordered a unit with Solid state outputs.
We replaced the unit & to my horror the buyer had purchased a relay output. We had similar systems on site that were Mitsubishi doing thousands of operations every shift using relay outputs. The replacement was left in place with a proposal to order a solid state output unit to replace in the future, over a five year period it did not fail so even though the calculated operations was about five times the rated operations for the relays. So it can sometimes be down to component reliability.

Unless the switching is very frequent, SSRs may be overkill. A contactor will do the job and is inexpensive.

Instead of a PLC with integrated SSR outputs, simply drive the external contactor/SSR from the regular PLC output.

You can look at some of the Phoenix DIN SSR's, The entire module is about $45, but the relay module isn't all that expensive for replacement.

I just checked my solenoid valves. They are Parker 24VDC at 1.5 amps. The spark on the relays occurs during the turning off. Which confirms that it is from the inductive kick back.



From what you guys are saying. A diode will extend the life. But, maybe an SSR would be the best choice here. Or a contactor.



Have any of you ever used the Automation Direct DIN rail mount SSRs?


I will check out the Phoenix ones that you suggested.


Surely, people still aren't using the Crydom ones that have to screw mount to the panel.

Even if you add an SSR or a contactor to drive the solenoids, you should also add a freewheeling diode or an RC suppressor directly at the solenoid.
Especially with the relatively long distance between relay/ssr/contactor and the solenoid, the solenoid without suppressor acts as a noise generator.

This may help


https://m.littelfuse.com/~/media/el...load_arc_suppression_application_note.pdf.pdf

Some DC relays have surge protection built into coil circuit...

if your solenoid draws 1.5 amps, the inrush can be up to 10 times that amount. to break that current, you would need a relay contact that will handle that much current. I got into that several years ago and ended up using a contactor. you may also want to look at mercury contactors. we switched one company to them on an application and it worked great, saved the company a ton every year.
james

What type of freewheeling diode or an RC suppressor do you recommend? Would a 1N4007 diode be good enough? Last time I used an RC network, it was across the coil of a 120V relay.

psychosunshine said:

What type of freewheeling diode or an RC suppressor do you recommend? Would a 1N4007 diode be good enough? Last time I used an RC network, it was across the coil of a 120V relay.

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I would make that decision largely based on the physical properties of the connections at the solenoid. You want the flyback diode or surge suppressor installed as close to the solenoid coil as possible. If the connections at the coils are some sort of plug-in connector, you might be able to find a package that fits inside the connector, a new connector that includes suppression, or you might be forced to locate the protection further away than desirable in order to be practical. Sometimes the only realistic place to do this is on a terminal strip in the main control panel. It really depends on the solenoids and how they are terminated.

There were comments about adding suppression to the relay coils, but that is not quite right for your application. Your arcing is across the relay contacts caused by a coil which is located in the solenoid...same principle, different electrical location. Your most preferred solution would involve flyback diodes at the coil terminals of the solenoids. I am confident that you can do this and extend the life of even the cheapest relays to well beyond reasonable expectations...stepping up to SSRs for infrequent switching of a 1.5A DC inductive load would be overkill in my opinion.