Drive fault as analogue Input below 4ma

Hi, I was wondering if you could help. I am using a 4-20ma to control the speed of a drive unit. At infrequent random stages the drive goes to fault and coasts to stop. Accuring to the drives HMI, the fault occurs due to the analogue signal falling below the set minimum limit i.e the 4ma.

Within my PLC, I am send the signal from an Allen Bradley current output card with an output coding of 0 - 32,764 for 0-21ma. From this my min digital output (For 4ma) within the PLC has been set to 6241 (4/21 x 32,764) and my max digital output (for 20ma) to 31204 (20/21 x 32,764).

What I was wondering is whether my min digital signal is set to low? Is it possible that the output on the PLC Output card could dip below 4ma to say 3.99, 3.98 etc and cause this fault (Maybe set digital signal to 6250 or so). Or is it possible that the output card signal is being interupted by a external source? Would anyone of experienced a similiar problem, appreciate if anyone has any suggestions in troubleshootting this or recommending a previous thread on a similiar topic. Many thanks

If you don't need to take the motor to exactly zero speed, you may be able to configure the drive analog input minimum for, say, 3.8ma, and get rid of the nuisance fault that way. Or, if the protection isn't important to you, turn that particular fault off in the software.

On many Allen Bradley analog cards the output isn't dead exact on the theoretical data. You may fix the problem by simpy measuring the analog signal with a meter and then kicking up your minimum output point until the signal is just over 4.0 mA.

Firstly many thanks for your replies. I will try to adjust the drive signal as yee have mentioned. DickDV, I noticed that the detection parameter can be switched to inactive.
In general, if this is done what will happen in the case of a signal disconnect i.e 0ma. Will the drive run at the minimum speed parameter or will it see a 0rpm speed (0 digital output)?. From looking through the manual, I get the impression that the minimum speed will kick in when the drive is switched on. However, is there anything that will prevent this from going unnoticed (i.e operator thinks he\she has correct speed) for quite a while since the action is not visible? Again thanks for the help.

In the drives I am familiar with, the motor speed would drop to whatever minimum speed you had programed for that analog input. So it would, in effect, be the same speed as if 4ma were present.

Note Tom Jenkins suggestion above, however. I'm not a PLC guy but if you can tweak the PLC output to be just above 4ma, you may preserve the protection and get rid of the nuisance trips.

DickDV said:

In the drives I am familiar with, the motor speed would drop to whatever minimum speed you had programed for that analog input. So it would, in effect, be the same speed as if 4ma were present.

Note Tom Jenkins suggestion above, however. I'm not a PLC guy but if you can tweak the PLC output to be just above 4ma, you may preserve the protection and get rid of the nuisance trips.

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I totally agree here. You really need to connect your meter to read the 4ma signal from the PLC to the drive. Then adjust your scale value until you get the 4ma. I have never seen a 4-20 output that did not need a little adjusting at setup.

BTW, is the drive input actually the 4-20 input? I have seen the input set for voltage and when you try to give it a current signal you get unusual happenings.

On the drive speed, if min speed is 0 rpm, anything less than 4 ma will give 0 rpm. If min speed is 6 hertz, the drive should run at 6 hertz if the signal drops below 4ma.

IMO, only disable the detection as a last resort. It is there for a reason.

Again thanks for replies. I think your right, i'll try and avoid disabiling the parameter and tune signal. The Drive rep is in-house towards the end of week so will try and find out more. Will report back if he has anything new on this topic. Leadfoot the drive input is 4-20ma. Thanks again

Does your drive have a parameter setting for loss of 4-20mA signal? Some drives have this feature, and if yours is set to stop on signal loss, it may be stopping the drive if and when the signal drops below 4mA.