Instrumentation questions 2

[Lc85]

The client has requested that transmitters be 2 wire loop powered so taking. Am I correct in assuming that if the transmitter is powered (24VDc) from the power supply in the panel then it is not loop powered? Also would severe modifications be required for them to work on a 2 wire loop powered AI card?

As it stands for these transmitters we have
+24VDc going to terminal U+ in the transmitter
SP1 from the transmitter is wired to a termnal for the client to connect to
-ve from 24VDc supply going to a terminal for the client to connect to.


Also we have a fan start signal which is an open contact for the client. The clients PLC Digital output is 24VDC how can they close this N/O contact? Would a relay be needed?

Finally a general question not related to the above project.
There is a pressure transmitter wired to an Omron K3HB-X display uni. The output then goes to terminals in a panel. My questions are
Is this loop powered or powered from the Omron display unit?
would this arrangement as it stands be able to work on a Mitsubishi AI：Q68ADI?

Thanks Very much

 

[waross]

A typical loop powered transmitter may be configured as follows;
One wire from the power supply to the transmitter.
One wire from the transmitter to the PID controller.
One wire from the PID controller returning to the power supply.
Other devices such as indicators, transducers, relays, etc. may be added in series in the loop. The sum of the voltage drop ratings of all devices in a loop must not exceed the voltage of the power supply.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ceac]

Hello,

Some devices need a separate power, others can work with or without (although they are not common).


I don't know the final application so this advice may not be valid (This cannot be done in Ex environments, for example).

If you have a 2 wire transmitter, but no "active" AI card that can supply the transmitter, you can always place a power supply in the loop.

Power Supply + -> instrument +
Instrument - -> PLC input +
PLC Input - -> 0V or power supply -

This will power the loop and provide a mA reading to your input card.

A relay would be the best way to isolate your system from the client's. However, some more information about the contact would be useful.

Do you have the pressure transducer manual? Wihtout that and number of wires in the transmitter, it's impossible to say for sure.

 

[danw2]

The diagram shows connections for the power supply for generic 2-wire and 3-wire connections.

I can't tell from your description whether your transmitter is 2-wire or 3-wire.

>Am I correct in assuming that if the transmitter is powered (24VDc) from the power supply in the panel then it is not loop powered?

Ans: The physical location of the power supply does not define 2-wire/3-wire, the number of wires running to the transmitter define 2-wire/3-wire. Conventional use of the term "2-wire" is associated with "loop-powered", meaning only two wires run out to the transmitter, even though the complete loop circuit includes the power supply. In conventional useage, "3-wire" is NOT loop powered.

>Also would severe modifications be required for them to work on a 2 wire loop powered AI card?

AI cards are powered by the device in which they reside, indicator/controller/PLC/whatever.

An AI card might supply 'loop power' to a field instrument, but the card itself is not powered by a power supply intended for powering field transmitters.

The field instrument might be loop powered (meaning 2-wire) or it might be a 3-wire or 4-wire device, neither of which is loop powered.

A field instrument is either 2-wire or 3-wire and cannot be converted. Some 4 instruments offer the choice of an isolated, loop powered analog output or an active, powered output but the instrument itself is 4 wire.

With regard to the Omron display unit, look at the pressure transmitter and count the number of wires wired to the terminals. 2 is 2-wire, 3 is 3-wire.

Digital indicators might or might not have a DC power supply for loop power. You need to look up the model number and decode it, or compare field wiring to the installation sheet's wiring diagram.

 

[Lc85]

My transmitter going to the Omron Display has three wires (+,- and o/p) also the data sheet I was given for the transmitter is for a sick PBT. Out of the display we have two wires (o/p and-) these are going to terminals for the client along with a screen.

so does that make this loop powered and suitable for the Q68ADI?

Thanks

 

[Lc85]

Just to get this clear and make sure I'm not being daft. If an instrument is 2 wire loop powered it doesn't matter where this power comes from? It can either come from the dcs/plc analogue input card or from a 24v psu in a panel?

Thanks

 

[itsmoked]

That's correct though attention to polarity needs to be included.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

You might want to limit common-mode voltage too. ;-)

 

[Lc85]

Thanks for the replies. Our client is using an Allen Bradley Plc and I assume the analogue input module will be 1756 series because the digital input modules are this series. So for this pressure transmitter we have,the loop power can come from either this analogue input module or from the power supply in the panel and the loop will be able to work in any configuration?

Thanks again

 

[ScottyUK]

Check whether the I/O card channels are fully isolated from each other or whether they share some common point. If you're using external power then you really want fully isolated channels because you have little if any control over what is done outside of the panel.

 

[Lc85]

Rereading my previous posts, I've not fully explained my question properly.

We have the same supplier for two separate projects.
One client is using an Allen Bradley Plc with, I assume, a 1756 series AI card
And one client is using a Mitsubishi plc with AI card q68ADI.

So if we have a pressure transmitter wired to the omron display module as explained above with a 24VDC power supply entering the display module from the panel and the outputs from the display module also explained above. Then this is also a loop powered situation?

Really thanks very much

 

[Compositepro]

"Loop powered" simply means that the instrument gets its power from the same two wires that are carrying the signal. There must be a power source somewhere (anywhere) else in the loop.

 

[Sparky328]

I'll second what compositepro said... a loop powered transmitter just means the transmitter is powered from the analog input loop. Depending on the type of AI card the power will come from the AI card itself or from an external power supply.

If a transmitter is not loop powered that means it requires a separate power supply and then the transmitter ends up powering the loop. These are normally referred to as 4-wire transmitters because they usually have 4 wires ran to the transmitter... two for power and two for the loop (4-20 mA, etc). One downside to 4 wire transmitters is installation cost especially if the power supply is 110V AC (separate conduit). The other problem is some analog input modules (DCS, PLC, etc) don't accept active inputs (active means loop is powered from transmitter) or they are not easily switched between the two types of inputs (internal vs external loop power). Also some 4 wire transmitters have the ability (via configuration or jumpers) to generate an active or passive loop even though they have their own power supply.

Concerning the pressure transmitter and omron display the simplest way to tell if the transmitter is loop powered is to see how many wires it has... if only 2 then it is loop powered. For what it's worth it's rare to see a pressure transmitter that is not loop powered.

Hope this helps