# 4-20mA question



## Cow (Jan 16, 2008)

Wireman415 said:


> I have a 4-20mA signal from a new analog control board (SCADA) at a pump house. i'm using this analog output to provide a pressure reference to 2 Danfoss VFDs
> 
> 
> The pressure setpoint has already been set in these VFDs and has been operating as such. (Until lightning destroyed the original analog output on another Scada system)
> ...



If the only difference is the voltage then you answered your own question.


For the record, I've been around a fair amount of process equipment, etc and have never seen a 4-20mA loop with anything other than a 24vdc supply.


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## Wireman415 (Aug 17, 2016)

It's an unusual SCADA called "Mission Control" it's controlled with a cell phone. It's customer provided crap. I've been able to get pressure transducers to work with the voltage. Also, I've seen voltage drop at a large chemical plant lower the 24VDC out to a device substantially And, that still worked.


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## MDShunk (Jan 7, 2007)

The 4-20 loop will ordinarily work fine down to 9 volts on a VFD analog input, but I'm sure there are exceptions. It's at these lower voltages that the resistance of the wire, particularly on long runs, comes into play. If you have a milliamp amp clamp, or interrupt it with your meter, measure the milliamp value observed at the VFD. I'm betting it's not what you think it is. Also, is there any possibility you have the polarity backwards? You other option is to run it through a signal conditioner right at your SCADA system and run a more conventional signal to the VFD from the output of the signal conditioner.


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## Wireman415 (Aug 17, 2016)

MDShunk said:


> The 4-20 loop will ordinarily work fine down to 9 volts on a VFD analog input, but I'm sure there are exceptions. It's at these lower voltages that the resistance of the wire, particularly on long runs, comes into play. If you have a milliamp amp clamp, or interrupt it with your meter, measure the milliamp value observed at the VFD. I'm betting it's not what you think it is. Also, is there any possibility you have the polarity backwards? You other option is to run it through a signal conditioner right at your SCADA system and run a more conventional signal to the VFD from the output of the signal conditioner.



In this installation the VFD is 5 feet away from the signal source. I did double check polarity, even reversed it just to be sure. I also used a mA clamp to be sure I was getting the appropriate current to the VFD.


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## paulengr (Oct 8, 2017)

It sounds like the polarity is backwards:

All analog to digital converter chips are voltage. The input has a precision resistor and the converter measures the voltage across the resistor. Virtually all inputs are 100, 250 (most common), or 500 ohm metal film resistors. If you measure the resistor with your multimeter you can then measure voltage and use Ohms law to figure current: This lets you check the electrical side of things. Start here because this sounds like your problem if it runs off a simulator. Usually this just means the receiver voltage is either 1-5 V or 2-10 V. Op amp circuits and “universal inputs” (also 0-10 V) work best on 10 V inputs, and ADC designs usually call for 5 V inputs so you can kind of see the choices being made in the internal circuits.

The input (excitation) voltage and line length aren’t supposed to matter with one exception. The transmitter (output) acts like a variable resistor and adjusts the output resistance to control the current in the loop precisely so the excitation voltage, line length, and number of receivers does not matter to a point.

The transmitter has a maximum voltage input so on many transmitters with an external voltage input you can jack up say a 24 V power supply to typically 28-32 V to overcome a marginal situation or often work off a really weak voltage.

Second and this is the exception every transmitter has a rated load that includes the line and receiver(s) combined. That’s the most it can handle and with two receivers on a weak transmitter you’re probably there. So check the manual for maximum load in ohms and measure what you have,

A final issue is that 4-20 mA receivers are supposed to be isolated and ungrounded. Most Allen Bradley input cards in particular are notoriously not isolated. But that doesn’t sound like your issue. If it is and it’s the last receiver (or the only one) frequently you can jumper the negative sides together to force floating inputs to a common ground and all your problems go away.

If you can’t jumper around loop isolation OR you have too much load you need a loop isolator. This is just a receiver and transmitter that electrically isolated the two usually with an optical isolator. Use it for nonisolated inputs or when you have long lines or a weak transmitter where you need a booster in the line. This is probably your solution if the SCADA isn’t defective.



Sent from my iPhone using Tapatalk


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## gpop (May 14, 2018)

Hook your meter in line with the analog input to get a true reading. If i remember correctly the danfoss does not have a true analog input. It uses a 250 ohm resistor to covert the analog signal to 1-5v (old ones used the same terminals and unlike new drives there was no switch or jumper on the input to choose 4-20 or 1-5v). 
Another thing with danfoss is to make sure that either voltage or analog input is disabled in the program. If both have been enabled it will do some strange stuff.

If the pressure sensor has a option for loop power output you can use the drive as your voltage source. or you may need additional hardware to get the voltage back up.


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## Kmurd (Jun 6, 2016)

Wireman415 said:


> In this installation the VFD is 5 feet away from the signal source. I did double check polarity, even reversed it just to be sure. I also used a mA clamp to be sure I was getting the appropriate current to the VFD.


Did you switch the polarity back to where it was?


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## Kmurd (Jun 6, 2016)

I’m interested in what you found. Did you get it figured out?


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## Kmurd (Jun 6, 2016)

I’m interested in what you found was the problem? Please do tell.


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## Wireman415 (Aug 17, 2016)

I apoligize, I thought I replied to this!


I found out 3 different things, all of them exacerbated my problem.


1: The existing transducer was 0 - 100 psi. The discharge pressure transducer was 0 - 300 psi. (My fault for not double checking)

2: The analog input of VFD #2 is damaged and won't accept the input. I measured the resistance of the input on both VFDs. #1 = 200ohms, #2 = ~ 1500ohms.


3: The SCADA output only updates every 15 minutes, or if there is a 5% chance. 
5% of a 300psi transducer is 15psi. So, with a setpoint of 75psi the VFD goes between 90psi and 60psi. This is an issue I'm still trying to resolve with the SCADA people.

And there you have it!


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