VFD with Computer Equipment

[mls1]

We are installing a 50kw VFD in a laboratory environment that also includes a couple dozen computers and IEDs. The lab is a SCADA lab where we regularly test RS232 and RS485 networks. The VFD is 480V but ultimately originates from the same padmount transformer as the 120V supply (I'm not sure of the size of the padmount).

Should we be concerned with harmonic issues? Specifically, is a 50kw VFD sufficiently large to cause voltage distortion over 5%? What about RFI in the communication networks? I know the answers to these questions depend greatly on the transformer size, cable lengths, etc. but I'm interested to know if anyone else has run into problems with a similar configuration. Thanks!

 

[dpc]

I wouldn't expect too many problems from power line harmonics at 120V - the 480V - 208/120V stepdown transformer represents a substantial buffering impedance.

This link has some software that migh give you a first approximation.

http://www.transcoil.com/analyzer/Analyzer.htm

But another potent source of trouble is radiated RF coming from the drive itself and especially the cable running from the drive to the motor. I would keep this feeder well away from any of your electronic gear. The motor feeder should be run in steel conduit and using shielded triplex cable. Okonite's C-L-X metal-clad cable is a good option.

 

[Electic]

dpc's advise about radiated signal is real. Sometimes VFD's will cross talk to each other and cannot be independently controlled when there are several units with medium length control and power cables.

Per your original question: Yes a 50kW VFD can cause over 5% distortion, easily. Distortion will be a function of current demand waveform x system impedance, so you need to know both. VFD's come in many flavors; 6 pulse/12 pulse with filtered incoming sections etc. You need to know what you need.