Blown PLC Outputs for bidirectional control valves

[funguy123]

Having numerous 24VDC (rated @ 2A continuous) output cards that are going bad over the past year (a point or two will remain ON/OFF.) Cards are 3 years old. 16 point in 2 groups of 8 with. The outputs are 24VDC Solenoid Control valves (rated @ 1.25A with built in surge suppression). They cycle ON once every 30 seconds and are on for about 15 and then turn OFF.

Have not been able to put a clamp current meter on it yet.

Anyone know what the make\break max amp. means? It is 1.2 Amps. Our load is 1.25A?

What if we have the valve energized on one side and de-energize it and energize the second side at the same time. Could that increase the arc to more than the SS diode could handle?

Just looking for ideas.

Thanks,

Funguy

 

[funguy123]

I forgot to mention, this is a relay contact output module.

 

[Skogsgurra]

8760 hours times 90 = 700 000 cycles each year. Common life is at 1000 000 cycles (sometimes 100 000) at rated load. So it is only normal that your contacts wear down.

The 2 A rated current is most probable resistive load. For inductive loads it is usually a lot less. Your surge protection is probably an RC combination. That is not good enough for a DC solenoid. Try putting a free-wheeling diode there instead. That should help. But you are still operating close to what the contacts can take.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Your voltage and current might just be within the reach of a solid state (transistor) PLC output card, but it would make long term economic sense to use a small solid state relay to drive the load, and use the solid state output card to drive the relay. SSR's don't have the mechnical life problems of mechanical relays. By using a cheap external SSR you protect your expensive I/O card.

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I don't suffer from insanity. I enjoy it...

 

[itsmoked]

Skogsgurra and Scotty have excellent points. You are flat using up your relays. They are actually lasting pretty well! I'm not sure any further diodes will help if you are mechanically wearing out the relays anyway.

You should probably consider the solid-state solution or just plan to replace the modules once a year as a standard maintenance project.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[funguy123]

Thanks for the input guys. I agree as well.

With an inductive load the contacts are derated quite a bit right? So with a 1.25 A load on 2 A contacts derated
by X amount with this amount of cycles this could be why
we are seeing the failures? How many cycles are PLC relay contacts typically rated for? I heard as high as 20 million?

I really appreciate the help and am going to look at SSR cards.

Thanks,

Funguy

 

[dpc]

Agree with answers given, but let me just emphasize that switching dc solenoid valves is tough duty for relay contacts. Those solenoids can have an amazingly huge inductance.

 

[Ashereng]

ScottyUK,

In that line of thought, I think I recall an optical relay of some sort? The I/O card switched the optical relay, which then turned the current on/off to the solenoid?

Or, I could be mistaken.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[funguy123]

Thanks fellas, I think we are going to go the SSR route. We will replace the cards with SSR cards and then use another set of SSRs powered by the cards to drive the loads.

Dumb question: There are no gotchas when it comes to SSRs like derating them for inductive loads or anything right. I found some that are up to 3 A continuous and so I don't derate that do I?

Thanks in advance,

Funguy

 

[Skogsgurra]

Yes, there are a few gotchas. First: use DC SSRs. But that is understood. Second: Use free-wheeling diodes. You may need a series resistor if the drop-out delay gets excessive. Or use MOVs. You do not need to derate if you do that.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

But do look at the SSR's specs and do follow Gunnars point about "DC" as the norm is "AC" so if you just ask someone for an SSR you will probably get the AC variety.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[ScottyUK]

Hi AsherEng,

I can't quite picture what product you are describing - could almost be a standard SSR because most of them use an optocoupler for the front end.

funguy123,

You might save some money by using an open collector transistor output card to drive the SSR's rather than an SSR card in the PLC. The input of an SSR is a very friendly load and could be driven by almost any of the open collector cards available. They only draw a few mA at, say, 12V.

Pay heed to skogurra's comments about a diode in antiparallel with the relay (i.e. diode reverse biased in normal operation).

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[Ashereng]

ScottyUK,

Could be. I am going on recollection.

Thanks.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[AGomaa]

I did application(s) for bag filters where the outputs for the solenoids are “on” for 80 ms and cycled each one or three minutes. I use interposing relays (normally 5 to 10 amps contact). All are running well for many years (I forget how many some six some ten) the PLC output card is a ssr/transistor type. The benefit is isolating the PLC from the field, replacing the relays is cheap than replacing the PLC card. But the initial cost may be high. (and I do not sell my client any spare!! But they like it and keep asking for new systems)