Relay outputs and inductive loads 2

[akrus]

I have an application where among other things I need to control some solenoids and motors from a PLC. One of the project requirements is that I must keep leakage current as low as possible and therefore I must use relay outputs instead of solid state.

The datasheet for the module says that each output has a 2A switching capacity for inductive loads. Some of my loads are 10W 24VDC solenoids, 30W 230VAC fans, and 3W 230VAC motors. Is it ok to power these loads directly from the PLC outputs assuming that I provide the appropriate surge suppression?

I can see the argument for an intermediate relay because it’s easier to replace one relay than a whole module if something fails. However as always space is a concern and it would be convenient to use the relay outputs directly if possible.

Please share any comments or practical experience that you have. Thanks!

 

[LiteYear]

The AC loads will be fine since despite being quite inductive, they're low current and have zero-crossings every cycle which helps a bit. The 24Vdc solenoids will be a problem - that's 2.4Adc of mostly inductive current, exceeding your relay's specification without even considering in-rush. Suppression wont solve the problem since the relay may not be able to interrupt the current in the first place, so the suppression may never operate and the relay contacts will fuse.

I'm afraid an interposing relay is probably the way to go here.

 

[iceworm]

LiteYear -
I get .42A for the 24 DC. I think it would be okay. Most I have seen use a diode across the coil to eliminate arcing when the contacts open.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[itsmoked]

You need to check your PLC output ratings. Often they're limited to low values like 100mA.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[mcgyvr]

10W @ 24V = .42A
If relay contact rating is 2A or less..
the .42A < 2A = Good
carry on.

 

[akrus]

All- I appreciate the responses.

iceworm -
I agree that the current for the solenoids is 420mA.

itsmoked –
I believe 2A is the output rating of per contact and 10A per common. I think a low value like 100mA would be for solid state outputs.


Maybe I should reframe my question a little. I think all of the loads I mentioned should fall well below the 2A rating of the contact. I’m more interested in whether it is bad practice to power these types of loads directly from the PLC or if this is something that is common practice.

 

[DRWeig]

akrus,

With dry-contact outputs, we very often power relays, solenoids, and small motors such as yours. Whatever fits within the dry contact output rating. It saves an interposing relay and thus eliminates a point of failure (and its cost).

Solid state outputs are another story.



Best to you,

Goober Dave

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[ScottyUK]

I'd likely use a solid-state or high density / low current I/O card and fit external interface relays. The interface relay can be selected to suit the application, and if you wipe one out due to a fault then your expensive I/O card doesn't get taken with it and other loads served by the card will continue to operate normally. You can buy relays which are the same size as a standard DIN rail terminal, so space won't be a problem in most industrial applications.

 

[DRWeig]

Now you have me scared for my I/O boards, Scotty!

Best to you,

Goober Dave

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[ScottyUK]

Given enough time, operators and maintenance techs can break pretty much anything. Part of my job is to limit their ability to do so. :-D

 

[LiteYear]

My apologies, brain explosion on the initial calculation. Agree with the other commentators.

 

[LionelHutz]

The question should be if you're willing to replace output relay cards if/when the relay does fail vs using interposing relays to protect the relay cards. You will have to assess your connected loads and the likelyhood of each one damaging the output card. The decision to risk a $100 IO card is easy to make but it's more difficult for a $1000 IO card.

 

[blacksea]

The wreewheeling (flyback) diode must be strongly recommended for activation an inductive load from PLC Output - you can find such recommendation in PLC manual as well.

 

[skiier]

I dont know how many times I have seen the scam service call where only the company that the supplied the equipment can change and reassign a burned processor output because they have a password on their processor to protect their own intelectual property rights. Jerks couldnt design the system right in the first place and then you as a customed have to pay them to fix what should be a warranty claim. Are you going to be like that?
Put in relays and the maintenance electrician can fix the problem WITHOUT a laptop.
In my experience most solenoids eventually burn out the contact they are connected to.
A high cycle rate requires surge suppression and interposing relays, period. Dont let any salesmen or wannabe tell you any different. Go cheap and you will pay eventually.
Oh yes I have seen this problem many times with solenoid operated equipment. The plc output burns out and wont pass the required current to the load, the contact resistance is now so high that it effectively has the supply voltage across it and no vltage appears across the load.
So do I get out my laptop and reassign the output? No the first thing I do is hook an icecube relay to that output and see if operates. If the relay works I simply use that to redrive my solenoid valve. Done. No expensive service call.
Use the relays

 

[controlsdude]

Most manufactures make either solenoid interface type relay if your worried about current draws and inrush. See Example below. Just look for the Din rail slim type below if your worried about an IO card being destroyed. Only in the worse cases would you need something like below.

http://www.phoenixcontact.com/global/news/248_7874.htm