4-20 MA transmitter signal

[srinivasan]

Field tranmitter has 4-20 ma output which is connected to plc. plc output supply to transmitter is 24Vdc. but in field transmitter side receives only 16.5 Vdc. What will be the causes for voltage drop.

 

[gar]

130525-1508 EDT

Loop resistance of the wire.

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[Smart $]

Could also be 250 ohm series resistor in the loop. An older method, but sometimes local indicators with 1-5Vdc input are used in conjunction with the transmitter. The resistor creates a 1-5Vdc drop across it for input to the indicator.

 

[srinivasan]

Could also be 250 ohm series resistor in the loop. An older method, but sometimes local indicators with 1-5Vdc input are used in conjunction with the transmitter. The resistor creates a 1-5Vdc drop across it for input to the indicator.

yeah, used model is 3051 cg rosemount pressure transmitter with local indicator. my question is, if 16.5 voltage only occurs across the transmitters. will transmitter work properly. what are the test we have to do for transmitter working condition

 

[mike_kilroy]

put ammeter in series with 4-20ma line and measure it over full range?

just scale the plc input to match?

4-20ma circuits often have scaling adjustments called span & elevation or similar names.

 

[gar]

139526-0739 EDT

srinivasan:

Look on the transmitter data sheet for the minimum voltage across the transmitter. The purpose of the current loop is to allow accurate data measurement over wire loops with varying resistance. The transmitter is a constant current source down to some minimum voltage across the transmitter.

Experimentally adjust the transmitter to produce a 20 mA load. Increase the series resistance until the transmitter can not maintain a 20 mA load. Measure the voltage across the transmitter. That is your minimum transmitter voltage. Add some voltage to this as a safety factor.

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[Smart $]

yeah, used model is 3051 cg rosemount pressure transmitter with local indicator. my question is, if 16.5 voltage only occurs across the transmitters. will transmitter work properly. what are the test we have to do for transmitter working condition

Here's what the manual states...

Power Supply for 4-20 mA HART
Transmitter operates on 10.5 - 42.4 Vdc. The dc power supply should provide
power with less than two percent ripple.

NOTE
A minimum loop resistance of 250 ohms is required to communicate with a
Field Communicator. If a single power supply is used to power more than one
3051 transmitter, the power supply used, and circuitry common to the
transmitters, should not have more than 20 ohms of impedance at 1200 Hz.

Figure 2-19. Load Limitation
Maximum Loop Resistance = 43.5 * (Power Supply Voltage ? 10.5)
The Field Communicator requires a minimum loop resistance of 250Ω for communication

The total resistance load is the sum of the resistance of the signal leads and
the load resistance of the controller, indicator, and related pieces. Note that
the resistance of intrinsic safety barriers, if used, must be included.

If this is a new installation, recommed going through the entire commissioning and calibration procedure, Sections 3 and 4. Link to manual...

http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00809-0100-4001.pdf

 

[Smart $]

put ammeter in series with 4-20ma line and measure it over full range?

just scale the plc input to match?

4-20ma circuits often have scaling adjustments called span & elevation or similar names.

While that may work, that's not the proper way to do it, and should only be used as a last resort remedy to keep things running.

 

[srinivasan]

if wire sizing is not proper. what will be the effect. for 24 v dc and 4-20 ma, our selected cable size is 1.5 sq.mm and 2 core. is it reason for voltage drop

 

[gar]

130526-0829 EDT

srinivasan:

If your open circuit voltage is a regulated 24 VDC, and there is a 5 V drop at 20 mA in the receiver, then your wire loop resistance drop is only 24 - 5 - 16.5 = 2.5 V. That is not much drop for the wiring. Find out the minimum voltage that the transmitter will operate at over its full range. In other words at 20 mA.

Pay attention to Smart $'s information.

.

 

[Lost_RFTech]

Sri - your PLC (or a DCS) that is powering a field device is not a regulated DC voltage source, it is a DC power source that connects to a field device that acts as a constant current controller based on the value of a process variable.

The PLC can not be a firmly regulated and filtered voltage source like a bench-top DC power supply because most modern day 4-20mA systems must also be HART capable, which is in essence a modem tone (ac signal) superimpossed on the dc current signal. If you think in terms of a regulated and filtered DC supply, its output is a virtual ground that would immediately filter out or strip off any modem tones applied. This is why common bench top test set ups require something like a 240 ohm resistor in series with the test supply lead, the connection point for a HART communicator being isolated from the virtual ground by that series resistor.
The PLC or DCS must also provide this isolating impedence in the real world which helps explain an apparent voltage drop. Think of 4 mA through 240 ohms being equal to a voltage drop of 0.96 VDC and 20 mA being equal to 4.8 VDC. The PLC may not act as a pure resistance, but I hope this conveys the idea.

The 3051 should have a loop test feature that would allow you to step the current to any value within the range of 4 to 20 mA. If you are concerned about dropping below the specified supply range for the instrument you could monitor the voltage at the 3051 terminals while steppimg through the range of 4-20 mA. I doubt that you are going to have an issue.

Most 4-20 instrumentation that I have seen deployed in the field use 18 awg shielded twisted pair wiring - regardless of brand, commonly called a "Belden".

If I get an opportunity while this question is fresh in my mind I will try to perform the voltage monitoring test I just mentioned and report my results.

 

[gar]

130526-1250 EDT

Lost_RFTech:

I can describe any energy source that is stable as a constant voltage with an internal series impedance, or as a constant current with a parallel shunt internal impedance. A simple transformer with bridge rectifier and filter capacitor can be approximately represented as a constant (meaning zero change in voltage or regulated) voltage source with some internal impedance. The open circuit voltage of such a source is that theoretical constant voltage.

.

 

[Smart $]

if wire sizing is not proper. what will be the effect. for 24 v dc and 4-20 ma, our selected cable size is 1.5 sq.mm and 2 core. is it reason for voltage drop

As stated by others, voltage drop is dependent on factors other than just the wire. The manual states, "Use shielded twisted pairs to yield best results. To ensure proper communication, use 24 AWG or larger wire, and do not exceed 5000 feet (1500 meters).

FWIW, 1.5mm? is ~12ohms/km and approximately 2,960cmil, so slightly larger than our 16AWG (2,580cmil, 1.3mm?). 24AWG is 404cmil, 0.205mm?.

You haven't stated the loop length.

 

[Smart $]

Lost_RFTech,

PLC's are typically not used as the power source of a 4-20ma loop. Power comes from a regulated power supply. PLC modules are just another device in the loop.

Power supply voltage > device(1)vd + device(2)vd... + ...devicevd + wire-vd

 

[srinivasan]

Sri - your PLC (or a DCS) that is powering a field device is not a regulated DC voltage source, it is a DC power source that connects to a field device that acts as a constant current controller based on the value of a process variable.



If I get an opportunity while this question is fresh in my mind I will try to perform the voltage monitoring test I just mentioned and report my results.

we have used barriers for all transmitters just to regulate the 4-20 ma properly but what we shall do for regulating voltage?

 

[srinivasan]

As stated by others, voltage drop is dependent on factors other than just the wire. The manual states, "Use shielded twisted pairs to yield best results. To ensure proper communication, use 24 AWG or larger wire, and do not exceed 5000 feet (1500 meters).

FWIW, 1.5mm? is ~12ohms/km and approximately 2,960cmil, so slightly larger than our 16AWG (2,580cmil, 1.3mm?). 24AWG is 404cmil, 0.205mm?.

You haven't stated the loop length.

Transmitter is located in 400 meter from panel. all the instrument cables are armored type and single pair 1.5 sq.mm only.

 

[Smart $]

we have used barriers for all transmitters just to regulate the 4-20 ma properly but what we shall do for regulating voltage?

Do you mean intrinsic safety barriers? They also contribute to voltage drop.

So far all I can glean from your posts is that you're measuring 16.5Vdc at the transmitter's terminals. IMO, that's not low enough to warrant any concern, especially if output is 20ma while measuring voltage. If it's at 4ma output, I'd be a little concerned... but I'd just do a HART analog output trim while loop powered and measure the voltage when output is 20ma. If you drop below 10.5Vdc or can't get up to 20ma or better output, then you have a problem.

 

[Smart $]

Transmitter is located in 400 meter from panel. all the instrument cables are armored type and single pair 1.5 sq.mm only.

At 400 meters you have about 9-10 ohms (according to my noggin ) on the loop from the wire. As long as its integrity has not been compromised, that should only be about 0.2 V drop.

If the transmitter will operate at 10.5Vdc, and you started with 24Vdc, that leaves 13.3V drop potential across all other devices. So what all devices are on the loop other than PLC? You said earlier there was a local indicator. Did you mean integral or separate? Integral should not affect the VD as it is powered by the current throughput. A separate indicator will add to loop VD as will the PLC module, any resistors, barriers, and other devices.

 

[don_resqcapt19]

Most of the transmitters I have worked with are designed for a maximum of 750 ohms, total loop resistance. Many of the DCS and PLC inputs have an internal 250 ohm resistor that needs to be included in the loop resistance total. Even if you have two devices with 250 ohm resistors, you would have to have an extremely long loop to cause any problem. #18 has about 6.5 ohms/1000'. (13 ohms for both conductors in the cable). Even at 15,000' of cable, you should not have any problems with two devices in the loop.

 

[Smart $]

Most of the transmitters I have worked with are designed for a maximum of 750 ohms, total loop resistance. Many of the DCS and PLC inputs have an internal 250 ohm resistor that needs to be included in the loop resistance total. Even if you have two devices with 250 ohm resistors, you would have to have an extremely long loop to cause any problem. #18 has about 6.5 ohms/1000'. (13 ohms for both conductors in the cable). Even at 15,000' of cable, you should not have any problems with two devices in the loop.

I posted the 3051 formula for loop resistance earlier.

Maximum Loop Resistance = 43.5 * (Power Supply Voltage ? 10.5)

The -10.5 takes the transmitter out of the equation... fairly obvious if it needs at least 10.5V to achieve 20ma output, and that equates to 525 ohms. What's left using the equation is 587 ohms for other devices and wire, provided the supply voltage is a stiff 24Vdc. A remote indicator, a PLC connection, and IS barrier could easily make it borderline. May need a loop repeater.