third harmonic voltage distortion

[fg45]

I have carried out a power survey on a site with a 11KV to 400V transformer feeding several non-linear loads (which are VFDs). They are the most significant non-linear loads. The analyzer provided a graph showing third harmonic voltage distortion on two phases, but none on the other phase. Has any body encountered this before? Any ideas what could give this result? Thanks

 

[DRWeig]

FG45,

Try searching this site for unbalanced VFD current or unbalanced VFD input voltage, or even single-phase input to a three-phase VFD. There is good discussion there. Here's what I got out of it.

Basically, if you have one phase voltage lower than the others, and the VFD is lightly loaded, the higher two phases will charge the DC bus to full level and the lower phase doesn't get a chance to pitch in -- so very little current is drawn from the lower-voltage phase.

Make sense? I urge you to search out those other threads.

Others here might remember better search terms -- or they might pitch in more, so stay tuned.

Good on ya,

Goober Dave

 

[waross]

How is your voltage balance.

Bill
--------------------
"Why not the best?"
Jimmy Carter