Scaling with analogue cards

[Spj]

We have a cooling line with multiple vfd's banked in groups in panel. All functioning well but a set of four which seem to to vary in speed in unison. When attempting to fix them by looking at scale they look good. But after a while they drop in hertz. When changing parameters not much changes?! What do you guys think? PID?

 

[ndzied1]

Can you use a communications bus (Ethernet I/P, Profi, Modbus) to send the speed command to the drives?

 

[boneless]

+1 For using bus.

Do you use a single analogue output to all drives in series? Maybe your analog output is not "powerful" enough? Perhaps you could try galvanic isolator. Or perhaps use the analog output option many drives have to send signal to second drive?

 

[Spj]

Thank you for your input guys

Boss ended up buying a new card, problem solved. I was trying to see if there was something I was missing. Production is not a friend of lengthy fixes so we will see. I am very appreciative of your time and knowledge, later guys