Relay outputs and inductive loads
Relay outputs and inductive loads
(OP)
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!
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!





RE: Relay outputs and inductive loads
I'm afraid an interposing relay is probably the way to go here.
RE: Relay outputs and inductive loads
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
RE: Relay outputs and inductive loads
Keith Cress
kcress - http://www.flaminsystems.com
RE: Relay outputs and inductive loads
If relay contact rating is 2A or less..
the .42A < 2A = Good
carry on.
RE: Relay outputs and inductive loads
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.
RE: Relay outputs and inductive loads
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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RE: Relay outputs and inductive loads
RE: Relay outputs and inductive loads
Best to you,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Relay outputs and inductive loads
RE: Relay outputs and inductive loads
RE: Relay outputs and inductive loads
RE: Relay outputs and inductive loads
RE: Relay outputs and inductive loads
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
RE: Relay outputs and inductive loads
http://www.phoenixcontact.com/global/news/248_7874...