I'm having a problem with "noise" on some 4-20 MA control loops when using the following combination:

• 240V single-phase power to (2) 60 hp RPCs (Δ)to give us our 240V 3-phase power.
• 240-480 step-up transformer (I think it's γ-γ as I can only find 9 taps, 3 of which are bonded together) to run our machine.
• Dura-Pulse VFD (480V, 20 hp) running a regenerative blower (γ)

The motor is mounted on isolators, and blower is connected with rubber hose. As soon as I start the motor (drive), the 4-20mA loops start going crazy. If I disconnect the motor ground, "noise" disappears, but you get a poke from the motor case.

I'm pretty sure I don't have any ground loops going. All of the signal cables are shielded.

I've used similar combinations before, and not had (or maybe I haven't noticed) this problem. Any help would be greatly appreciated.

 

How is the noise manifesting itself? I mean, just what are you seeing?

In theory, power supply noise should be rejected but you might be making some fields near enough to the current loops to cause interference. Shielding and re-routing the wires might help. Just a speculation.