PLC control

I am planning to convert our plant to PLC control, at present there are 35 conveyors 26 elevators 7 magnet seperators and 14 seperating screen plus other sundry items, so say roughly 100 items, so there are 100 ON PBs 100 OFF PBs 100 on lights and 100 fault lights. also for certain product runs only certain sections of the plant are run. the plant is a mess electricaly as when an upgrade has taken place another box is added with rows of switches and lights, so idealy we should just start again, but the powers that be say no too expensive!!! frustration!!!
So my questions are:

1.How is the best way to go about it?

2.Can both systems be run in parallel?.

3.How do you switch a contactor using both systems in parallel?

Only way I can figure is for the PLC to supply a brief pulse to switch the contactor and a brief pulse to open the latch circuit.as in the attached drawing.



Is this the normal way its done?
Also is it possible to switch a relay with a CT ?
I'm not an electrition but have a fair understanding of electrical and electronic circuits.

It seems to me that you need some expert help, for one thing if you are not suitably qualified you should not be doing this.
Current legislation requires you & your company to ensure that any modifications bring the plant upto the current requirements.
Under any country H&S rules you & the company will be liable in the case of serious injury or death.
I suggest you involve the help of a competent company or induvidual to design a control system, you would not need all those start buttons for example design a complete system with some form of HMI/Scada or descreet starts/selectors (selectors used for selecting a route and/or plant to run for example if you had 10 route selections based on type of product, then a ten way selector or HMI to configure the route & one start/stop button is all that is required, the software would then start the required plant in sequence & stop it in reverse sequence), then write the code & test it on a simulator, integrate a small part at a time.
I have been involved in a number of projects with the requirement the plant be running 5 out of the 7 day week, this required careful planning, some interim temporary wiring, updating e-stop safety circuits & all this had to be done during the weekends.

I'm going to agree with Parky about this being a project too big for a novice, unless you are willing to do it one conveyor at a time and your boss will let you use the first one as a learning experience. Even then I suggest you hire an experienced man to come in and review your first try before you hit the Start button. You don't want to be responsible for an injury, do you?

However, to answer your question, when I have manual and PLC control in parallel I always put a Hand/Off/Auto or Manual/Auto switch in the circuit so only one device (the PLC or the start push button) has power at a time.

Also, you should seriously consider using an HMI (Human Machine Interface, another term for display terminal or touchscreen) instead of all of those indicator lights. It will be less expensive in the long run, and permits a lot more information to be presented to the operators. This is true whether you do this yourself or go outside for the work.

As far as switching a coil with a CT, the answer is yes. I'm not sure what you are calling a CT, though. From your diagram I suspect this is a counter, and most of them have output contacts that trip when the count is reached.

CT is also used to mean Current Transformer, and in that case you need a limit alarm to trip a contact.

Tom is right about the hand/auto switch, this is exactly how I have done it in the past, I must re-iterate the need to ensure any safety circuit complies with current legislation.

RayK, This is not how it is normally done. For starters, 200 pushbuttons and 100 lamps will give you all sorts of problems eventually, have you investigated using an interface screen, these days there not that expensive and certainly beats rewiring your system. No wiring, just some programming. This only works if all your buttons are in the one location. Assuming your pushbuttons are not all in the same area and you need to re use the wiring then I would suggest that you install a PLC cabinet, terminate the PLC connection end and it is a relatively simple process to convert which can be done with the program already written and in small sections, perhaps a few drives at a time. To change the wiring you need to take the left hand wire at terminal one into the pLC as a "start PB" input, the right hand wire on terminal 1 to supply (to supply the stop PB input.Take the left hand wire at terminal 2 into the pLC as a "stop PB" input. The connect an output to terminal 2 to drive your contactor.
I just did a similar excerise for a saw mill. Its all a bit complicated in text but you can get the documentation done and program written up front and its not a difficult wiring process. If you want any more info email me [email protected]

RayK, I'm with Steve_D on this one. Work with someone and develop a written specification and then that someone (or others) can program your plant to run the way you want.

There are a number of devices available these days that will minimse the amount of effort required to upgrade to PLC control and provide heaps of functionality.

If you need any more info, email me: [email protected]

Thanks for the replies guys, yes I will be working with a qualified electrition on this thou it is in the early stages and may not eventuate!, for instance we are in the process of upgrading one section (2 more elevators 1 more conveyor and 1 new magnet)and I thought ideal opportunity to introduce a PLC into the equation i.e.1 button sequential start for that section instead of 10 more buttons 10 more lights!.so I wrote the program, selected the plc (a ML1500 which had the capacity to run the whole plant at a later date) then got knocked back because of management concerns on the reliability of a PLC !!!!!!.
Tom: the CT I was refering to was a Current transformer, and was just wondering about the easiest way(cheapest!)to get a "motor is running" signal, also these items are only stop start so no speed control needed, the programming excites me (get a life Ray !)
spose I should post this on the general forum.

The typical way to get a Motor OK signal is by adding an Aux. contact to the contactor. I think you can get CT's with a on/off output instead of analog. Look for a 'Hall' sensor. Driving a relay from the output of the contactor can be done, but it may be difficult mechanically.
A small gauge wire sharing a hole with the large gauge wire going to the motor. You need to be careful to make sure the small wire gets clamped properly.
Sounds like your boss trusts relays and fears computers. Your selling job should be how fast you'll be able to troubleshoot a failure. The reduced repair time and longer uptime will pay for the system in a short time. Looking up the history of PLC's may help. The first PLC's had to do the same selling job.

Justifying an HMI (aka. MMI, Man-Machine Interface) is easy on your system. Look at how many input and output modules you won't need by eliminating PB's and lamps.

Can your system be broken down to seperate sections? If so, multiple PLC's networked together with one HMI may be the best way to go. In your case, it looks like several of the lowest cost 'Bricks' networked would work fine. Wiring gets easier, service shuts down one section rather than the entire line, etc.

Make sure the control system is 24DC. In the US, I've seen control done at 120VAC. An electrician thinks 'AC' only - 120V on switches is no big deal to him. AC input modules cost more. Adding a DC supply is cheaper and safer.
If the contactors are old, consider using the new IEC types. Use DIN rail mounting. With IEC, you have several brands to choose from if you need to replace one. They follow a 'standard',so no need to drill new mounting holes, etc.

quote"The typical way to get a Motor OK signal is by adding an Aux. contact to the contactor."unquote

Keith; Yes I was thinking more on the lines of "is the motor drawing current?" rather than is the contactor engaged, thou idealy an analog input to the PLC with a window comparator in the PLC program would be good.
All of our control circuits are done at 240vac!

RayK said:

quote"The typical way to get a Motor OK signal is by adding an Aux. contact to the contactor."

Click to expand...

No

Please

Don't do it this way.... The Aux. contact will really only tell you the state of the contactor contacts.
If you need to check that a motor is running, you can use a CT that operates like a switch, and is 240VAC rated.

In Oz, contact your local NHP branch... The units we have used have been adjustable such that a broken drive belt could be detected, as the motor pulls much less current unloaded.

Hey guy's,

The best indication of motor running is to install under speed proximity switches located on the tail drum of the conveyors, or on the pulley shaft of the screens, this will give true indication of plant status, and can be used to debounce (DCAT) the starting sequence if the plant is bogged or motor shaft shears.

Yeah, you really need to speak to some control guy's and develop all the I/O and schematic and termination drawings before any hardware is purchased. This may sound like an unnecessary expense but it is the only way to go, if you continue in the same vein as the previous installation, you will only end up with a larger undocumented mess that will cost you 10 times more in down time relating to electrical fault finding.

How did you intend to test the PLC program? Install and program run? This method will put you under a lot of pressure if the field devices have not been point to point checked properly.

How much is plant down time currently costing you? I would be measuring some actual cost to build a justification for the PLC upgrade with an relevant pay back period, and look to tender the job to a third party.

Do you have any engineering resources available to you, like undergraduates from nearby UNI? These guy’s crave this sort of project to get brownies points toward their degree, you may be able to get some funding from your State development programs to fund 50% of development cost.

Anyway, sound like you’ll have your hands full, best of luck with it and feel free to ask for any help need.



Lance

The easiest way to get confirmation of motor load is by a current switch. I use these guys http://www.nktechnologies.com/ but I'm not sure they sell outside the US. M-Systems is another brand I've had good luck with.

One note of caution: a motor at no load may draw more current than it does at 25% load. If you motors are lightly loaded a lot you may need actual power detection and switches instead of just current: http://www.loadcontrols.com/products/products.html

Sounds like your boss trusts relays and fears computers.

Great expression and I am sure a lot of us have come across this! It is also fair to say that the intro of PLC's into a new environment can be troubled. Most forward thinking Maintenance Electricians will embrace PLC's and there benefits, however, all too often the Programmer or commissioning techs inherit the fault finding duty, as 'traditional' maintenance personnel fear (and blame) the PLC and will refer a basic I/O fault to the experts 'cos they just don't understand...

...bear this in mind during your transition and involve as many of your collegues as possible.

Regards

Darren

This is my start point! as you can see the usual upgrade path is to just add another box of switches and lights! also you may notice on the orange board the switches are on/off so in a minor power cut the lot goes off and then anything switched from the orange board thumps on together! major surge.

RayK,

Sorry for the delay, work has been hectic lately. I can see what you mean, it’s a little scary but I have seen worse.

To over come the problem with all the drives powering up after a power outage, you’ll need to install a self-latching hold in relay on the supply that controls these drives. This relay will need be re-activated with a reset button after the black out occurs. Place a label on the button to notify operators to turn off Drives 1,2,3… before resetting the control circuit.

As for the PLC interfacing this will require a little more work, the main gist of the job will be to mount your PLC in a box that will accommodate:

• Input terminal strip
• Output Interposing relay’s
• Output terminal strip
• E-Stop terminal strip
• Ballast regulator for ELV control (E-Stops and limits, usually 24Vac)
• DC power supply (optional depending on card arrangement, usually 24Vdc)
• Socket outlet for Laptop, and PLC supply
• Filtered Ventilation inlet and exhaust fan (axial computer fan)

On the front of the box you will need to fit as many Auto/Off/Manual 3 position selector switches as there are devices you wan to control with the PLC. Wire Multi-core cables from each control box back to the PLC panel. These cables will accommodate:

• Control circuit loops through E-Stop Relays
• Control circuit power return from Control Box’s to PLC interposing relays
• Control Circuit Manual selection from PLC to Control Box’s (connects to existing Stop/Start stations wiring)
• Control Circuit Auto selection from PLC to Control Box’s (connects to the TOL side of Start Station wiring)
• Input’s from Control Boxes to PLC for sequence fault indication (TOL, Belt Drift, Pull-wire’s Under-speed ext.)

This is what they call the distributed approach, there is a tendency now day’s to do away with the Multi-core interconnecting cable and use a propriety bus/network system to remote I/O. The hardware bill is a bit higher with remote I/O approach but the termination time is cut dramatically.



I hope this gives you a general idea on how to approach this job, as they say there is a 100 and 1 way’s to skin a cat. If you need further detail just ask.



Best of Luck

Lance