Plc & Computer Communication

[smfula]

Thanks everyone for your advice, I think if I read your posts a few more times and research this a bit more I will be ready to begin my PLC building.

I have to go to work now, but I am planning to read each post more closely later this week. I will not ask more questions until I have done something with what has already been suggested.

 

[smfula]

TURN ON A 1/2 HP MOTOR

I am wondering why anyone would use high voltage output cards.

This question probably exposes my inexperience. I think if you have a 12v DC output card, it should be enough to energize a relay coil and the contacts of that relay coil can pass whatever current/voltage is necessary to run a large motor.

Of course this means purchasing solid state or hard contact relays, but it sure beats running PLC output current through the motor coils. Most output cards could not handle the current requirements of a large 120 volt 1/2 hp motor.

Someone explain why you would need more than a 12v DC output card? I think even 5VDC for some PLC's is enough for an "on" state and could energize an output relay.


THANKS

 

[mgvol]

This is typically driven by the voltage of the device(s) to be controlled by the PLC. Here, most industrial devices operate at either 120 VAC or 24 VDC. And the PLC output is not typically used to directly control a high-current device (such as a single-phase motor), but a secondary device like a motor-starter or relay. While other operating voltages may be available (5vdc, 24vac, etc.), selection can also be driven by economics. The least number of different output module types I have to keep in spare parts, the better for my bottom line. And if I buy the most commonly used voltage range devices, these are the ones that my supplier is most likely to have the best stock and possibly the best prices on.

 

[elevmike]

If your using descrete I/O and have a 120vac power supply I would consider choosing AC inputs (120vac), and relay outputs for the PLC.

The motor shoud be run by an aux relay that would be controlled via a PLC output. Your motor would draw 5-7 amps at 120 volts. A small contactor should do the trick.

 

[rsdoran]

If you are learning and "need" to start with AB then look at Micrologix http://www.ab.com/plclogic/micrologix/

You can purchase a Micrologix 1000 for $99 and get a "FREE" downloadable version of RSLogix500..note it will only work with that one ML1000 but will familiarize you with the software. They have an RS232 port that works with DF1 so an adapter isnt needed to program.

You can always upgrade the software and hardware as you learn.

Automationdirect also has a DL05 for $99 with software for the DL05, 06 and 105 costing $149 http://web6.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_05/PLC_Units

 

[smfula]

I need to research a bit about automationdirect "relay output cards."

I assume the PLC program tells the output relay coil to energize, and the contacts of that relay close. This allows complete isolation between the output card and the device you are trying to control. (In my case a few motors)

If you have complete isolation for each output (ex. o:0/00, o:0/01...o:0/16) with these "relay output cards" then what is the purpose of another "auxillary" relay?

Does a 120VAC motor require auxillary relays? Does a 240VAC (7.5A) motor require auxillary relays? What is the difference between a motor starter and an auxillary relay? Perhaps motor starters are bigger and handle more current.

Should I use motor starters or aux. relays for each motor? Maybe for the smaller 120 motors I can get away with aux relays, but for the one 240v motor I will have to use a big motor contactors.

Any help would be appreciated.

 

[jstolaruk]

The 1747-UIC's DB9 Dshell connection is RS232 level signals and works fine connecting to any of the AB processors that has a RS232 port including the Micrologix. I used it today to connect to a ML1000, a Panelview 600, and channel 0 of a 5/03.

 

[akreel]

If you have complete isolation for each output (ex. o:0/00, o:0/01...o:0/16) with these "relay output cards" then what is the purpose of another "auxillary" relay?

1. The relay outputs are typically rated for 2A, which may not be enough for a good-sized starter (NEMA rated starters come to mind).

2. The relays may isolate the card, but they're not as replacable as a nice DIN rail mounted ice cube.

For pilot lights, I wouldn't bother with an interposing relay. For motor starters, I think I would. Motor starters can deliver some bad ju-ju sometimes.

AK

 

[elevmike]

smfula

Judging from your questions, either you are way out of your leage, or are pulling our leg..

You posted eariler that you are using a 1/2 HP motor on a 120 volt power supply. I responded by telling you, YOU WILL REQUIRE a CONTACTOR. Not a big contactor as the motor will draw about 5-7 amps. The PLC realay output will energize the contactor, and the contactor will run the motor.

when your motor tage looks like this:

Volts: 220/110
Amps: 2.5/5.0

That means that if your using 220 volts to your motor it will draw 2.5 amps. If your using 110vac your motor will draw 5 amps.

Also you stated that the motor is 1/2 HP but then you stated that its a 220 volt motor rated at 7.5 amps?? Im presumeing that this is a single phase motor??? In either case your numbers dont make since.

A 1/2 hp motor (single phase) would more be like:

115 vac: 9.5 amps
230vac: 4.9 amps.

So what does the tag on the motor actually say??? What is the actual supply voltage for the motor?

 

[akreel]

akreel-

I appreciate your advice on the RS-232 port. I did not know 1)that it "bypasses" (?) firewalls and security programs for simple communication, 2) that it is used so much in industry.

I missed this comment!

1) The RS232 is a "bypass" in that it is a direct connection and IT folks tend to ignore anything that's not CAT5. MOST firewalls and security are geared toward TCP/IP messaging, too. RS232 is used to carry a different set of protocols. The easiest way to protect a PLC from unwanted intrusions on the RS232 port is to lock the cabinet.

2) RS232 is a good, generic programming media. It's been around a while and was very PC compatible, until they started building laptops without it. Notice the comments you've received on DH-485, etc. You need extra equipment because your off-the-shelf PC doesn't support them. I don't think USB is going to become the new media of choice.

Ethernet ports are popping up on equipment because it is easier to network. Now, instead of going over to the PLC to program it you can access it over the office LAN (if you chose not to isolate the control network). Unfortunately, this makes LAN security an important issue when you design your control network. While access is cool, anyone who's built an Ethernet control system has had to learn the black arts of firewalls, VLANs, switches, and subnets. That's more time spent on design. Depending on your scope, it could be a LOT of time!

AK

 

[MarkTTU]

For any motor (of any size anyway) you'll need a motor contactor, a motor overload isn't a bad idea either. For all but the largest motor contactors you should be able to drive the contactor's coil directly from the PLC's relay output.

Also note that on the 06 all the outputs are not completely isolated from each other. There is one common for each set of 4 outputs. As I recall (not checking the book at the moment) the total current the common is rated for is only 4A.

This means that for 4 outputs sharing a common (say C0 and Y0,1,2,3) you can only pull 4A combined through all 4 outputs, but no output can exceed 2A.

For example if you had 4 devices to connect each of which draws 1.5A then you can not connect all 4 to outputs using the same common
(1.5 x 4 = 6A!)... You could connect a device to Y0 another to Y1, then skip Y2 and Y3 and connect your last two devices to Y4 and Y5 (which share common C1).

 

[rsdoran]

Just in case some missed this

My dad has a barn that needs to be updated, and it contains a simple mill for grinding feed, a few sensors, limit switches, trip rods, (about 14 inputs) and 7 AC motors (outputs)

My idea is to prototype a PLC in my lab/bedroom, using little lights to simulate the operation of the milling system. There is 5 motors, all 120V and one 240V (7.5 A) motors.

14 inputs and 7 outputs kills the Micrologix free software thing. The DL06 may even be overkill but it covers your I/O needs easily.

Yes you will definitely need relays or contactors for the motors, what is used depends on several factors. Auxillary contacts are used for many reasons. This pictures shows a hardwired start/stop circuit, ma would be an auxillary contact.

With a plc you may use an auxillary contact to verify the contactor has energized.

Give exact details on the size of the 120v motors and the process and more help may be offered.

 

[elevmike]

Everybody be carefull!!

It's a feed mill. Covered under NFPA-61

Class II, Division I, Group G.

Given smfula's obivious lack of expierance, and the potential serious hazards involved, this project should be left up to a local professional.

Just MY opinion.

 

[rsdoran]

Many farmers or people that raise livestock have a small feed mill in a barn, its a fairly common thing to see down here.

Its funny, I am presently working in a feed mill type plant, dog and cat food, and didnt realize that NEC had a standard that applied. I will have to check that out.

I wont get too excited about that standard though, I quoted NFPA 79 several times and everyone more or less stated that its not "legal" and no authority enforces it so it means nothing.

The message I got was that the NFPA is a bunch of people that write papers as guidelines to fire prevention. They have no authority. Therefore any NFPA paper is not a standard because they are followed only because someone decides they want too, not because its law or they should.

 

[mgvol]

I ran into this same "attitude" towards the code at a pelletized feed plant in Illinois many years ago. "Nah, that dust ain't explosive anyway." What the people that make these statements fail to realize is, the local code enforcement body decides on whether or not to adopt NFPA codes in whole or in part, and which parts. And an even greater "wild-card" is what does their insurance underwriter expect them to adhere to? The bottom line for me is, regardless of the cutomer's insistance to do something a certain way due to ignorance of codes and standards or plain old profit motives, as a designer or engineer, I would ultimately be held accountable (in whole or in part) in a civil (ha!) trial.