I got a machine from canada that has a 4-20mA sensor signal going to two (2) VFD's... not in series, but in parallel. There seems to be some issues with it not working correctly...huh. does kirchoff's current law not apply in canada?...should I chastise the OEM before or after I rewire the signal? I thought engineers learned this stuff in 100 level courses.

 

The drafting and manufacturing was outsourced to the USA.

Chastise their quality control, not their engineering.

 

I can imagine a number of ways in which the error could have occurred. However, regardless of how the errors occurred or who made them, the machine OEM should have been responsible for ensuring that any errors were detected and corrected before the machine shipped to you.

I am not aware of your exact circumstances. However, for a new machine which is not yet in service I would normally recommend contacting the machine OEM before making any changes. I would assume that there is some sort of warranty on the machine. You would be in a better contractual position to deal with any further problems which may be discovered if it is clear that you don't make any changes without consulting them first. Once the machine has been in normal service for a while and we can assume that all the major problems have been found, this consideration becomes less important.

 

Small question, do the two VFDs use a voltage input or a current input as the signal input? If both use a current input then your question is perfectly justified.

The problem is, if the two VFDs have a voltage input as their signal input. In such a case one would parallel the two input signals and put a resistor on one of them only, typically a 250ohm resistor to obtain 1-5V voltage signal from a 4-20mA current signal. The VFD without the resistor will also see the same signal voltage range on its input.

There is also the possibility of wiring the two VFD's input signals in series but in such a case you would put a resistor on each VFD input terminals. In addition one should check that the sensor can source 20mA into a 500ohm load (if we use the above example for loading).

What is exactly you layout?