Re: VFDs, Noise, and RS-485
- From: Terry Given <my_name@xxxxxxxx>
- Date: Thu, 28 Jul 2005 12:48:22 +1200
Nobody wrote:
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.)
No its not, its a high-frequency variable duty cycle "square" wave, whose duty cycle is modulated such that the average output voltage is a 100Hz sinusoid. Who's VFD? (Danfoss is my guess, they like the term VFD)
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
Gidday,
Drives are nasty. Firstly, ensure you have some form of a co-axial feed from the drive to the machine such as neutral screened cable, with the screen well bonded to the drive *AND* the machine. This is exactly wrong for an instrumentation cable, but right for the drive. Herewith an excerpt from a report I wrote for a customer experiencing this sort of problem in Nov 2003 with a 400kW rock crusher:
"The existing hardware operates in an extremely adverse environment, both physically and electromagnetically. The physical environmental hazards are primarily temperature, vibration and fine particles, which should be assumed to be conductive due to crusher plate abrasion. Electromagnetic hazards are primarily caused by the motor drives used in the rock crusher itself; drives generate significant EMI, and are responsible for the bulk of EMI problems in industry. Ideally drives and motors should be connected using “screened” cable, typically steel-wire sheathed. The drive cable screen should be firmly bonded to both the drive and the motor – note that this is exactly the wrong thing to do for an instrumentation cable.
Motor drives generate huge high-frequency (HF) current spikes which flow through the motor winding capacitance to the motor chassis. Both the drive and the motor chassis are earthed, so these HF currents flow through the earth connection and back to the drive. The amount of this HF energy which radiates away is directly proportional to the size of the physical loop the current flows in. Connecting the cable screen as above ensures a good high-frequency connection between the drive and motor – basically it minimises the physical size of the loop the HF currents flow through, thus greatly reducing the amount of HF energy which radiates away"
Once you have done this (minimised the source of emissions) you can then reduce the susceptibility of your setup. Areas to look at are:
- optically isolate the RS485 link. This is necessary to break the earth loop from PC to drive (which, by Norton, carries some of the HF currents). A moot point if you are running the PC from batteries, but of vital importance when mains powered
- keep wiring loops small. twist, twist, twist. ground plane, ground plane, ground plane. Drives spew out vast quantities of H-field, which your loops will happily receive. Technically this reduces the coupling, rather than the susceptibility, but who cares as long as it works right?
- correctly terminate the RS485 cable
- use high permeability common-mode chokes on the various dangly wire bundles.
As to why your hall effect sensors dont work, I suspect its poor layout/decoupling, or perhaps ratshit DCCTs. The VFD has 2-3 inside, and they work fine. The problems I've come across with current transducers are:
- capacitive coupling from the drive output. the fix is simple, a grounded electrostatic shield around the cable as it passes thru the DCCT. The ground connection must be low inductance. In practice drive manufacturers sample in the middle of each pulse, thereby avoiding the switching edge noise.
- variation of current as a function of cable position, and/or adjacent metalwork. sign of a crappy DCCT....
Make sure your DCCT supply (supplies) are well decoupled right at the DCCT. Again, twist the wires (stranded CAT5 cable is great for this sort of job, esp. STP). Also look at how you measure the DCCT results, and reduce/remove loop area - if with a scope, use a coax connection to the scope/probe rather than a 6" ground clip.
Read Linear Technologies AN47.
HTH
Cheers Terry .
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