Seal in run circuit.

When wiring a motor starter the normal practice to to seal the run circuit by running a wire from an overload contact to the starter coil. This way if the overloads trip or energy is removed from the machine the motor won't start up when energy is restored. As you probably already know this same seal can be programmed in the PLC.

My question is if I use a seal within the PLC do I still need to wire the starter to seal as well?

Thanks.

I may be misinterpreting your question so let me know if I am.

There are multiple ways to wire a starter to a PLC and each with its own advantages.

If you are using a starter with conventional heater style overloads, the safer method is to wire from the PLC output through the overloads to the coil. That way if the PLC malfunctions or there is a problem in the program, the starter will still drop out in the event of an overload.

You will also want to wire a set of auxiliary contacts on the starter to the PLC for feedback to let the program know the overloads have tripped. In the program, this will need to be done with a timer activated using the coil output to give enough time for the contactor to pull in before assuming the OL has tripped.

When using a Motor Starter Protector(MSP) in combination with a contactor, you can wire the PLC output directly to contactor's coil since the MSP will open the circuit in the event of an overload or short. (similar to a circuit breaker)

You will want to wire a set of auxilary contacts from the MSP to a PLC input for status of the MSP. This will not require a timer since the instant the input changes state, it indicates an overload has tripped.

I do not use the seal on the starter if it is controled by a PLC. What I do is use the N.O. contact on the starter to a input on the PLC. This way I know if the starter is pulled in.

Archie,

Thank you for replying to my question. But, I'm not sure I explained myself well enough.

If the seal is within the program so that an output is on until a stop button is pushed AND that output is wired to a starter do I still need to wire the starter so that it is sealed?

My initial thought on this was to say, "No." another seal is not required. But I do second guess myself and would like the input of those more experienced than myself.

jseavers,

Thanks for your opinion on this. I am of a similar mind but I want to make sure.

Do Not use Starter Seal In:

Philip:

A seal is not required on the Contactor/Starter when you are using a PLC. It would serve no purpose. Your Seal In will be in the PLC Rung. It is good practice to route the PLC Output through the Overload Contact, and then to the Starter Coil. Then use the Normally Open Contact that would usually be used as the Seal In for a 3 Wire Start/Stop as an Input to your PLC. This will tell you if there is a problem. Normally this will be a tripped Overload, but there are other things that can go wrong. A burned out Coil for instance. I have seen this and many other things prevent a Starter from pulling in properly. I realize this is pretty much what other posters have advised. It is good advice.

Hope that helps.

Stu.....

Yes Stu, it helps very much.

Thank you.

EDIT: As others have suggested, but for clarification...

IMHOO, You want the hardwired maintained OL contact in hardwiring...

If the PLC output to the coil is wired directly from an output, then also monitor the auxillary contact of the contactor as proof that the contactor is closed.

Then your PLC logic can monitor the contact closing and opening delays a indicators of the health of the unit, and when a certain delay time is exceeded, it will be a nearly sure indication that the overload contact is tripped...

I would keep the hardwired OL contact wired in series with the neutral of the contractor coil(s), and only monitor with the PLC. Often the output can go through the OL contact first, but for reversing starters you may only have one contact to work with, so keep 'em all the same...

Some prefer to monitor the OL relay with an auxillary from it, but that won't always prove that the motor starter pulled in...if the coil fails, fuse blows, etc, etc, but monitoring the motor contacts with an auxillary will.

Paul

Hey Philip

I guess it depends on the application. If your safety department want's a cat 3 safety circuit then I would recommend a hard wired seal in circuit. This way the seal circuit has to go through the positive guided relays and when the safety circuit is interuppted it is broken. Right now I'm working on a vision application that uses a small conveyor to transport small cartons. Here I would use a programmed seal in circuit. ( As always safety was the first thing considered for this application but after an assement a cat 3 circuit was not needed )

That's my 2 cents

Steve

Paul and Steve,

Thank you both for your opinions on this -- they really are important and helpful. I worked as a maintenance mechanic at a plant near here for 15 years and was laid off this past Feb. Safety standards there were very high and management was strict on enforcement. This past June I found a maintenance position with another company but in the same industry. They have NO safety rules; anything goes. We received a machine from the home plant wired for 240. We run 480 so I opened up the motors to change the leads. There were 4 GREEN wires used to power the motor. One of the green wires (I presume to be for ground) wasn't even attached.

That means I am my own safety department. I want to make sure I do stuff the correct and safe way. The answers provided by you and the rest of the respondents go a long way in helping me with my decisions.

Thank you.

Hey Philip

Sorry to hear about your layoff, good to see you landed on your feet.
I think everybody has horror stories similar to yours, I know I got a few with everything from safety to panelbuilding.

I just wanted to clarify my post. I think the message I was trying to convey was that I would use either hardwired OR programmed, I don't think I've ever used both at the same time.

Good post and good question keep them coming...

Steve

PhilipD said:

Paul and Steve,

Thank you both for your opinions on this -- they really are important and helpful. I worked as a maintenance mechanic at a plant near here for 15 years and was laid off this past Feb. Safety standards there were very high and management was strict on enforcement. This past June I found a maintenance position with another company but in the same industry. They have NO safety rules; anything goes. We received a machine from the home plant wired for 240. We run 480 so I opened up the motors to change the leads. There were 4 GREEN wires used to power the motor. One of the green wires (I presume to be for ground) wasn't even attached.

That means I am my own safety department. I want to make sure I do stuff the correct and safe way. The answers provided by you and the rest of the respondents go a long way in helping me with my decisions. Thank you.

Click to expand...

Sounds like the last place I worked. Grounds - we dont need em and dont have the time - and the list goes on and on.
It is hard to do good ethical and safe work in those places.
Dan Bentler

Steve,

Thanks for the clarification; we are on the same page. As to the layoff I really consider myself to be the fortunate one. The company, Smurfit-Stone Container Corp. filed bankruptcy and closed several plants. All the other plants had a mandatory 10% workforce reduction. I say I was fortunate because I of my maintenance background. The other people who had to look for new jobs were unskilled labor. The company has emerged from bankruptcy and my hope is that they can prosper and get those who were laid off back to work..

Me? I'm gonna stay where I am because when I repair the simplest of things the production manager thinks I am a pure genius. Oh, if he only knew! Things have been patched and welded so much that there are patches on the welds and welds on the patches. Burnt PLC output cards that have been jumpered out just to get the machine running, regular household light switches hanging out of cabinets so the operators can control various devices on the machine, wiring that looks like a rat's nest, and on and on and on.

Most of this work was done by someone who knew better but didn't care. The moto was, "Get the machine running no matter how!" My motto is, "Get the machine running but do it correctly and take pride in my work." So in the areas where I am "fuzzy" I do some research to get a better grasp. That is the reason for this thread and I'm sure I will have more to come.

Anyway, sorry for the long reply here and thanks again.

Dan,

I know what you mean. I am truly surprised that someone has not been seriously injured from some of the things I have seen. It's a long row to hoe but one-by-one I'm getting these things corrected. If this company stays in business I should have a job for life. LOL

Further reading on wiring overloads

http://www.plctalk.net/qanda/showthread.php?t=51447&highlight=overload