VFD problem with 485 Comm. Ports Filter Needed

I am new to the Forum,

I am an Automation Tech, the works for an Oil Company in Texas.
We have ROD PUMP oil Wells with Baker 8800 PUMP Off Controllers on them.
They sense when their is no Fluid in the Well and Turn the Pumping Unit Off.
Most of the pumps run with 20-40 HP Electric Motors that are 3 phase.

Our Company Electrical Dept. started to put VFD on 480 vac single phase power locations converting
it to 3 phase output.
They were using static phase converters before.

One problem with the Electric Motors on an oil well rod pump well application is the load changes some.
The Pumping Units have to be balanced to try and balance the load.

The Problem we are starting to see is when the Pump off Control has the pumping unit running we
see trash on out 485 comm ports and the Pump off units reboots every few minutes.
( The POC are small RTU with CPU in them )
Everything is OK when the Unit is on but the Pumping Unit is not running.

The ? is what filter is needed to control the interference from the VFD?
Our Electrical Dept. is not using any special motor or wire to hookup the VFD to the Motor.
I don't know if the problem is harmonics or RF ?

If I need to find out more Info to answer the ? better let me know.

Thanks,
Leland