VFDs, Noise, and RS-485

I have a setup that consists of a variable frequency drive (VFD) and a PC.
The two talk to each other using RS-485. (The VFD output is 3-phase, with a
high frequency square wave riding on a 100 Hz sinusoid carrier wave.)

I'm running into some major VFD noise problems, and I don't know how to
solve them.

1. The noise problem is so bad that once the VFD starts running, it kills
the RS-485 signal.

2. The noise problem also kills our ability to use a hall current probe to
sense the current through one of the 3-phase output lines.

I've tried grounding everything. I'm using shielded cables and toroids for
the communication and measurement lines.

On the RS-485 end, we've tried isolated and non-isolated devices (with
proper termination resistors) on the PC.

The noise won't go away! Argh!

Any tips would be appreciated.

- Nobody


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