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4-20mA dueling 6

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itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
1) I have two PLCs running separate refer systems.
2) Each system has 3 zones.
3) For redundancy the two PLCs handle everything for each cooling/heating system. (one goes down the other half runs normally)
4) I have 3 zone VFDs to be controlled via their 4-20mA inputs.
5) Each VFD runs a zone blower.
6) The blowers are the only common element between the two separate refrigeration systems.
7) The blowers are to run at slow speed unless cooling is active.
8) Either or both PLCs could go to cooling.

Yes, there is a question here...:

Since either system could call for cooling while the other isn't what's a workable technique for tying the PLC blower 4-20mA signals from these PLCs together so if either is in cooling it can override the VFD blower signal with the higher speed? That includes if one PLC dies completely.


Keith Cress
kcress -
 
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Microcontroller, two inputs and one output, and simple SW in between.
 
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Configure for voltage control. Use a pair of diodes so that the higher voltage overrides.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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There are commercially-available high/low select units that work with 4-20 mA inputs and have 4-20 mA outputs. A quick internet search turns up several. You could also run the 4-20 mA AO signal from one PLC into an AI on the other PLC (you may need an isolator or splitter) and use logic in the PLCs to deal with the high select.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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VE1BLL; In a word: Brutal!

Bill; That might work. The biggest issue might be packaging. No clean way to have resistors and diodes mixing in a typical field installation.

X; Not finding many. Only one Chinese one and one that I can't seem to make a real part number out of. Like the idea though.

Keith Cress
kcress -
 
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Bill; I'm starting to like the couple of 249Ω resistors and a couple of Schottkys with the VFD set for 1-5V then calibrated to 0.5 to 4.5V.

Keith Cress
kcress -
 
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Keith, take a look for these: D1052Y, M2SES, Action I/O Q498

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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The other option, if available, is to use digital outputs in parallel, ie: on/off. and switch the VFD between two preset speeds.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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To add to waross' suggestion (the simplicity of it is awesome), there are likely to be some terminal manufacturers that offer diode modules built into the terminals. It makes the install a lot cleaner, and reduces the issue of having to connect such equipment in a field installation. I think one reason for using the terminals is a lamp test arrangment, hence you're likely to be able to find them.

Its possible, although I've not seen them, that you can also get 250 ohm resistors in terminals, given that 4-20mA loops are quite common, and at least a certain proportion of equipment would need such an arrangement.

EDMS Australia
 
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Analog circuit concept: Two lamps or LEDs optically combined (AND-ed? OR-ed? A bit of both?) into a phototransistor. The 4-20mA range is pretty convenient for many LEDs. Then the phototransistor would control the 4-20mA output. The circuit almost designs itself. :)

 
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Allen Bradley makes "internal component" terminal blocks, this one with 249 ohm resistor, 1492-JD3RC001, this one with a diode, 1492-JD3DF, you can find them here,
Phoenix also makes a plug in module for some of their terminal blocks, this one is a diode block, they also have the block without out the resistor or diode, you just install your owm,
And also internal component as well,
 
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Thanks for those part numbers Nuke!

Bill I thought of that but it actually turns out I need to do this to 3 VFDs... That means six total outputs which hurts on this type PLC (limited module count allowed). Though that could still work using the relay outputs not being used on the CPU modules..

Freddy; Got a link? Terminal with integrated diodes?? That would be pretty cool. I could maybe get the resistors and diodes cleanly wired using really narrow DIN mounted terminal blocks. It would provide a more formalized and robust result.


VE1BLL; You're hell bent on getting me to design a solution! LOL. I wish I had the time for the 12bit in micro running a PWM 4-20mA out the back. Just about a one or two chip solution and a circuit board, and layout, and software development, and DIN rail packaging..
URL]


Cat; WoW! That is the cat's meow. Those things are under $10 even. Thanks. I think we have a winner.

Keith Cress
kcress -
 
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Hi Keith.
With those Phoenix devices you may also need these:
Link
Still under $11.00
I was hoping that the VFDs could be programed for a base speed and jump to a second preset speed on a contact closure. One set of output contacts per PLC, all in parallel, driving a three pole relay.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Bill: Good point with the fuse blocks. I hear you but these blowers actually need to be shut down too. It's a requirement to suspend circulation when transiting long tunnels. I guess seventeen minutes in a tube with two locomotives with your blowers running can be... unpleasant.

Keith Cress
kcress -
 
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plc card owner feature
I am wondering if the card can be in a shared rack between processors then there is some fancy features that some plc manufactureres have where the output can be owned by one processor and the other one is just monitoring. And when a fault or power goes down on the 1st processor the 2nd processor takes over ownership

plc rack owner feature, (redundant media)
Or you could have a rack that uses the feature of both own the rack but one is a backup, so when the 1st owner goes down the 2nd plc takes over ownership and does its thing. This is called redundant media.

This is probably more complicated then the other solutions.
 
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What did we do before PLCs?
We used a High select relay.
 
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Dan that's a little too close to the ground for this application. LOL

DrWeig; Fascinating product! What a world we are operating in now.

Control; hmmm. Sounds expensive.

Roy; Those high select thingys are great.

Funny/crazy/annoying thing. At the control space we opened hatches moved insulated ducts and got to the "VFDs" standing on a ladder so I could make sure they'd take 4-20mA. Someone reported six wires connected to one of them heading out to the motor. I told them 'don't be ridiculous, a VFD only has three wires heading to the motor'. We headed to the other end of the car to open the blower hatches to look at the motors on these "VFD controlled blowers" and in shock realized they're all single phase motors!! They have six leads coming out of them! Back to the 'VFDs' and we discover they're some sort of tap changer that simply sends power down different pairs to a multi-speed single phase motor...

beating-head-against-the-wall_d6z0lm.gif


Now I have to figure out what they are and how they're controlled.


Keith Cress
kcress -
 
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