testing a 4-20 mA output with a meter alone? 2

[PaulKraemer]

Hi,

I have a drive board (drive #1) that outputs a 4-20 mA signal that goes to a ABB signal converter, which converts the 4-20 mA to a 0-10 VDC signal that is used as a speed reference by another drive (drive #2). When I put my meter in series with the 4-20 mA circuit from drive #1 to the converter, I measure 4 mA when drive #1 is stopped (which is what I would expect). When I start drive #1 at a slow speed, I measure 5.25 mA. The fact that measured current increases from zero speed to a slow speed is also what I expect. What is causing me some confusion is that when I increase the drive #1 speed, my measured current stays at 5.25 mA, when I would expect that it would increase.

I have reason to believe that something is wrong with the converter. The reason I have this suspicion is because I removed power to the converter and removed all field wiring to it. I then measured the resistance between the terminals that the 4-20 mA signal goes to. This resistance measured 0.26 mA. I contacted ABB technical support and they informed me that this is a tell-tale sign the the converter is faulty and needs to be replaced.

I have no problem replacing the converter, but I would like to also (if possible) come up with a test that would prove whether the drive #1 4-20 mA output is working correctly.

I am wondering if I remove the converter from the circuit, would my meter (typical fluke digital multi-meter) alone be enough of a load to test whether the 4-20 mA output from drive #1 is behaving as it is supposed to? Or will I have to put some other load (to replace the converter) in the circuit?

I haven't mentioned the manufacturer/model number of drive #1 because I believe the 4-20 mA output is typical of what most drives and other automation components use to transfer analog values to other components. If it will be useful, I can certainly provide manufacturer and model numbers of all components.

Thanks and best regards,
Paul

 

[IRstuff]

Hypothetically, but typically, you load it with around 250 ohms

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[waross]

Beware of total loop resistance. I had a problem with a loop that was loaded with several devices that took it close to the maximum resistance.
It looked OK on paper and should have worked, but one device was off spec. It worked perfectly but the internal reistance was more than stated. As a result, the power supply could not drive the loop to a full 20 ma.
It was also incapable of reaching the final emergency shutdown set point.
The failure to shut down resulted in melting a stainless steel thermo-well.
This is probably not your problem, but with a lot of devices in a loop it is sometimes well to check the actual resistance values of the devices rather than accepting catalogue values.

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

 

[waross]

By the way:
"This resistance measured 0.26 mA."?????

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

 

[PaulKraemer]

Hi Bill and IRStuff,

Thank you for your responses. Yes, when I had the loop in tact (both conductors terminated at drive #1) and both conductors terminated at the converter, I measured 0.26 ohms between the two conductors (when I measured it across the drive #1 terminals and when I measured it across the converter terminals). This seemed very close to a dead-short to me, so I wanted to see if the short was in drive #1, the converter, or in the conductors as they go from one location to the other (through terminal blocks and one multi-pin connector).

I first disconnected the conductors at drive #1, and measured 270 kΩ across the drive #1 4-20 mA output terminals (with power off). At the drive #1 location, with the conductors disconnected, I measured 1.32 Ω across the conductors. At the converter location (with the conductors still landed at this end), I measured 0.26 Ω across the terminals that receive the 4-20 mA signal. Next, I disconnected the conductors from the converter. At this point, I measured nearly infinite resistance (my meter showed over-load) across the conductors, but still 0.26Ω across the terminals on the converter. This is what made me suspect that something might be wrong with the converter. I contacted ABB and their tech support confirmed that this low resistance is in fact an indication that the converter is faulty.

At the moment, I have pretty much accepted the fact that I need a new converter, but I'd like to come up with an easy way that I can make sure that the 4-20 mA output on drive #1 is working correctly without the faulty component (the converter) in the circuit.

I was hesitant to try this with just my meter in the circuit because I was afraid my meter might not present enough of a load. IRStuff's response makes sense to me - that adding a 250Ω resistor might add the (expected) or required load. I suppose I just have to be careful that I do not add too much of a load (not knowing the specs on drive #1's 4-20 mA output). This is on quite an old (Yaskawa) drive, so I expect that I will find it difficult to find much detailed info on it. I am hoping that most of these 4-20 mA drive outputs are more or less standard.

If either of you have any suggestions or concerns about me giving this a try with a 250 ohm resistor, I will greatly appreciate your input.

Thanks again,
Paul

 

[danw2]

Does the drive board have a manual mode that allows you to force the output? So that you can start at some low output (5.2mA) and then gradually increase the output to check what the mA output is?

If you put your mA meter and a 250 ohm resistor in series with the current output of the drive #1 board and you force the output and it doesn't go to a full 20mA then 250 ohms is too high a load.

If the load resistance is too high for the output, it won't damage the output, it just won't drive a full 20mA. If that happens, try a 100 ohm resistor.

 

[crshears]

If you put your mA meter and a 250 ohm resistor in series with the current output of the drive #1 board and you force the output and it doesn't go to a full 20mA then 250 ohms is too high a load.

...You mean too high RESISTANCE a load, correct? That's what the next sentence seems to suggest, but I'm just making sure.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Check the specs on the convertor or ask tech support;
What is the normal resistance across the convertor?
Use that value of resistor in place of the convertor for testing.
However that may not be the complete problem.
If the drive is unable to force more than 5.25 mA through 1.32 Ohms I suspect the drive may have a problem.
Testing the drive output.
1. Test the open circuit voltage of the drive.
Use the formula E/I = R
For instance with 24 Volts output on an open circuit E/I = 24V/0.02 A = 1200 Ohms. A 24 Volt source should be able to force 20 mA through a maximum of 1200 Ohms.
Use your measured open circuit voltage to determine the maximum resistance allowable and use about 1/2 of that for testing the drive output current.

2.Use an available power source, and calculate the resistance required to drive 20 mA through the resistance.
A 12 Volt source would require a 600 Ohm resistor.
With your available power source select a resistor that will drive about 10 mA through the resistor at your available voltage.
Connect it and test the current with your meter. You should see 10 mA within the accuracy limit of the resistor.
Now use this power supply and the limiting resistor to force current through the convertor.
Given the resistance values that you have stated you should see close to 10 mA through the convertor.
BTW has this circuit ever worked or is this a first time problem?

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

 

[PaulKraemer]

Thank you danw2 and Bill for your follow-up responses,

I am not sure if the drive has a manual or diagnostic mode that would allow me to manually enter what I want from the 4-20 mA output rather than relying on the drive's configuration and internal scaling that is supposed to result in a linear relationship between the 4-20 mA output and drive/motor speed. This would be a nice feature, and I have seen this feature on quite a few drives I have dealt with in the past. I do not know if the particular Yaskawa drive in this case has such a feature, but I will see if I can find some documentation on it.

To answer Bill's question, this circuit was working fine (as far as I know) for many years. The reason I say "as far as I know" is because all I can say is that the machine's operators have not (until now) reported any problems with the particular machine feature that depends on this circuit. (The feature is a master/follower relationship in which a follower drive/motor is supposed to follow a master drive/motor). I was able to get the machine up and running yesterday by installing a potentiometer that allows them to manually adjust the follower speed without relying on the 4-20 mA circuit through the converter.

Bill, thank you for the thorough explanation of recommended test procedures. I have to buy myself some resistors, but I now feel like I have a good game-plan. Now that they are at least able to run the machine, I have to move on to a more time sensitive project for a while, but I will report back when I have a chance to test these things and narrow down the problem (drive output, converter, or both).

I really appreciate your help!

best regards,
Paul