S7-200's - Relay output contact

I was checking the manual, and found that the Relay type module can stand up to 2Amp of load, 24VDCv and upto 220VAC.

What I am confused about, I have to use some existing 440VAC contactors.

Now when I check the manual, I found nothing helpful, more confusingly, under the ISOLATION heading of the tech blah blah, It says it says "750VAC for 1 minute".

My doubt is still there, can it work if 1Phase is given direct to the contactor and the other 220VAC passes thru the Relay contact of the S7-226 module? would it matter if this is done for longer periods, supposing 5 minutes or so?

is there a formula or something to calculate this? or perhaps someone can share thier personal experience?

Thanks and Kindest Regards,

I wouldn't run 440VAC through a PLC output module, even if it was rated for it. For that matter, I would run 220VAC either...

Use an external relay or contactor, driven by the PLC output at a lower voltage (like 24VDC), to switch your 440VAC load. Better yet, get a contactor with a 24VDC coil if you'd like to eliminate the middleman... :nodi:

Your "1Phase is given direct to the contactor and the other 220VAC passes thru the Relay contact" statement scares me. Are you sure you know what you're doing?... utoh

beerchug

-Eric

thanks Eric, 4 yr advise.

If I use DC Relays to turn on Contactors, this would mean twice the price! for instance in this particular case I need about say 20 contactors.

apart from above, someone was telling me I can operate contactors directly from the PLC's relay outputs, and to extend the life of the relay contact, he suggested I use condensors as well, so that there is not much load on the contact?

this may seem an odd question, but since I am more in programming side than the elec. wiz. kid, I do appreciate help from friends.

** Now, regarding the operation of 440VAC contactor, as I mentioned, what is there that scares you? could you be specific, because I have never done it, but ppl say it can be done, subject the RELAY CONTACT has enuf strength to withstand the $^@%#^@#%....

Now, regarding the operation of 440VAC contactor, as I mentioned, what is there that scares you? could you be specific, because I have never done it, but ppl say it can be done, subject the RELAY CONTACT has enuf strength to withstand the $^@%#^@#%....

Click to expand...

OK go ahead and proceed. But replacing the PLC WILL cost you FAR MORE than the price of say 20 interface relays. And you WILL have the need to replace. My advice: take an electronics course and an electrics course as well. You'll need both of them. And so do the people who told you it can be done. But I guess THEY won't have to pay for YOUR replacements.

Kind regards,

PS One thing I forgot about: if you do connect 440VAC directly to the relay outputs of the PLC, be sure the fire brigade is not only notified, but already present on the spot. And make sure your fire insurance policy does cover this kind of mayhem.

Kind regards,

I agree STRONGLY with Eric and Jean Pierre. There is a really good reason why output cards on PLCs are rated 120VAC or LESS. That reason is so no one pulls a stunt like connecting 220VAC or higher, multiphase loads directly to the card. If this is what you are going to do you might as well go ahead, now, and bolt one of those wires to your a$$. You might know that there is a multiphase situation but what about the poor guy comin' in behind you. Do the right thing, spend the extra money on relays and sockets. Then go home that night and sleep well, knowing you did it the right way.

Thanks for the replies!

I too was not sure about this, so I was asking you guys.

but still the question remains there, why not?

b'coz, when you need to energize a 440V contact (or something) you connect one of the phases (i.e., 220V in this country) and the other phase (220V) comes thru, by well, many ways.

since the PLCS output is RELAY type, it has a Common for the bus of X outputs, and this 220V goes in the Common and comes out of the output Qx.x, so what is there which is technically wrong.

and RUBE, the module I mentioned, supports 220V, and I have been using it to switch many things here.
and as I mentioned in my first post, the tech. side of the manual says I can use 220V in the RELAY OUTPUT with no problemo, obviously this means the Two wires I'll b using is a NEUTRAL and a 220V, but since I need to turn on a 440V coil, this means I need to use another 220V instead of a NEUTRAL.

again later on in the tech specs. it says ISOLATION for this unit is 750V (for one minute)

This is what confuses me, you see. or maybe there is a problem with me in explaining, (as usuall|)

Actually Im not the regular guy who handles the electrics, I was just curious. (looks like I didnt learn any lesson from the cat!)

take care

Thank you, Rube, for reminding me about this thread by replying.

Noman Jamil said:

If I use DC Relays to turn on Contactors, this would mean twice the price! for instance in this particular case I need about say 20 contactors.

Click to expand...

Twice the price? Either you're using cheap contactors, or expensive relays. And I thought you already HAD the contactors? If you are buying NEW contactors, simply specify 24VDC coils and you won't need the interposing relays.

Noman Jamil said:

apart from above, someone was telling me I can operate contactors directly from the PLC's relay outputs, and to extend the life of the relay contact, he suggested I use condensors as well, so that there is not much load on the contact?

Click to expand...

Don't believe everything you hear...

Noman Jamil said:

this may seem an odd question, but since I am more in programming side than the elec. wiz. kid, I do appreciate help from friends.

Click to expand...

Not an odd question at all. Always good to get other's opinions instead of relying on a single 'someone' for suggestions... :nodi:

Noman Jamil said:

** Now, regarding the operation of 440VAC contactor, as I mentioned, what is there that scares you? could you be specific, because I have never done it, but ppl say it can be done, subject the RELAY CONTACT has enuf strength to withstand the $^@%#^@#%....

Click to expand...

When you say 440VAC contactor, I assume you mean it has a 440VAC coil, right? But then you talk about "1Phase" and "the other 220VAC". Where did you come up with the 220 number? THAT'S the part that scares me... utoh

beerchug

-Eric

I see you snuck in while I was typing, but my response is still valid.

beerchug

-Eric

Noman J said:

when you need to energize a 440V contact (or something) you connect one of the phases (i.e., 220V in this country) and the other phase (220V) comes thru, by well, many ways.

Click to expand...

Now I see where the confusion lies. You don't have half the voltage on each wire (to gound maybe, but that has nothing to do with your application). You must HAVE two wires to have voltage. In your case, the voltage that the contact is switching is 440V, NOT 220V. If I try to explain this, you will only become more confused...

Anyone want to attempt an explanation?...

beerchug

-Eric

OK, I suspect Nomanjamil is not in the US, so normal husehold single phase is 220 line to neutral and not 220 line to line/120 line to neutral like the US single phase.

I also suspect that the 440 is three phase, and Nomanjamil is trying to switch one phase of the 440 to the coil on his contactor.

Nomanjamil, is that right? Can you post a drawing of your circuit?

Voltage ratings of contacts are generally based on dielectric resistance. That means that above a certain rated voltage the gap between the contacts is small enough to let the voltage arc and jump between the contacts and close the circuit.

The 750 VAC for one minute confuses me - I'm not sure where the time aspect comes in.

Can you do what you want - probably. Will it work - probably, at least for a while. Is it good design practice - probably not. We need better info to know for sure.

Maybe it's just my schooling (the school of real world) that my way of thinking is--polyphase on large loads only; motors, heat elements, etc. Controls, yes, a contactor coil is a load, but it's a small load, should not be polyphase. You have a contactor with two wires going to it. Most everyone in the US would think you have one of those wires "volt-free" (neutral for 120vac or Negative for 24volt). If you have a polyphase load, removing one leg of voltage from the PLC cards output DOES NOT remove voltage from the contactor coil or the screw on the output card--basically it shifts the potential to the output card itself instead of the contactor coil. This can cause confusion and shocks to personnel who think the danger is removed when the output card is not sending voltage to the device. We normally think an output that is not on has no voltage anywhere along the circuit. This is not the case when using polyphase loads.

Relays and bases (sockets) are cheap compared to most other controls. We use them all over our plant and I don't have to worry about whether the coil has voltage when the output is off. This is confirmed when we check with a VOM.

Since you didn't register yet, where are you doing all this?

electric diagram

Thanks for all the advises! I wish I cud understand all of it

Yes, Tom, I'm not in USA, I'm work for a firm with projects in Pakistan and in UAE (Dubai as most callit), To turn on a bulb here, we need a 220V and a Neutral, not 110V as in US.

I've tried to attach a drawing according to what I was thinking.


I have decided to go for 24VDC contactors, for motors. But still I prefer to receive a better understanding of this topic, for future.

one more thing, do I still need to use Diodes on the coil of this contactors? if yes, how do I calculate the rating of diode to use?

Kindest Regards,

Noman Jamil

now why cant I display my image!

I see you remembered your username and password!...

I don't know why your attachment isn't showing up. What type of file are you trying to attach?

Yes, installing a diode across a DC inductive load (i.e. a contactor coil) to protect the relay is always a good idea. You can use a standard 1N400x diode, but since you're buying new contactors anyway, it would make for a cleaner installation if you can get it as part of your contactor. Most manufacturers offer this option as built in, or a snap-on module. HERE'S an example.

beerchug

-Eric