Re: VFDs, Noise, and RS-485
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 28 Jul 2005 01:34:54 GMT
Hello,
Can you give us a first name?
1. The noise problem is so bad that once the VFD starts running, it kills the RS-485 signal.
As Terry said it is probably a PWM signal that actually runs the drive and not just controls it. The ripple you see would then be the stuff that didn't get smoothed out in the output filters of the converter unit. One thing you could look at is filtering some more at the source but you may no be able or allowed to modify that, let alone get inside the converter.
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 usually prefer current transformers. You can filter the output of these since you are then dealing with isolated and low level signals. Running the output of the current transformer differentially to the system where there would be another transformer may also be an option. But don't ever move the resistor away from the current transformer. Else havoc can happen should somebody disconnect the cable.
I've tried grounding everything. I'm using shielded cables and toroids for the communication and measurement lines.
At these power levels you'd have to be a real RF expert to get it running as is. Or hire one ;-)
On the RS-485 end, we've tried isolated and non-isolated devices (with proper termination resistors) on the PC.
This situation really calls for transformer isolated transmission, not just differential receivers/drivers. The required transformers would be pretty small and in a pinch can be made by hand. Toroids, no pot cores and the like. Making your own transformers instead of relying on some "isolation device" or transformer in a can has the advantage that you know how good and symmetrical it is.
Regards, Joerg
http://www.analogconsultants.com .
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