Happened to see this while doing some work at a municipal water pumping station. The conductors in the picture are on the load side of a VSD feeding a 400HP 600V pump motor. The circuit consists of two parallel runs of 350kcmil CU THHN per phase. Both conductors of phases B and C are in the left 2”EMT and both conductors of phase A are in the pipe on the right which also has a 4 AWG EGC. At the time pics were made each conductor was carrying about 125A. Wire was cool but pipe on left was noticeably very warm to the touch.
Would you expect the one on the left to be warmer since it had more net current in it. There wouldn't happen to be another EGC buried in those wires on the left? Can these wires simply be considered motor leads?
I'd also like to hear more about "circulating ground currents" with respect to being used on a VSD. I thought this type of problem wasn't an issue with a VSD.
Is it possible that the VFD is listed for this type of installation?
Just thought that I'd offer a bit of insight on this topic. I am a Power Engineer, not an EC, but this is my area of specialisation.
The VFD is likly not doing anything to cause the conduit heating (unless it is malfunctioning). The two conduits are carying the same net current:
Left Conduit: 125<120 A + 125<-120 A = 125<180 A
Right Condit: 125<0 A
If you were to total the two current you would get 0, this is why we combine all 3 phases (and neutral if aplicable) in the same conduit.
The metalic conduit is acting as a curent transformer, the net current in the conductors is inducing a current in hte conduit and because there is a second ground path (the EGC) this current can flow and heats the conduit by P=I^2R.
As you all know the correct way to do this is to split the conductors into two ABC groups. I hope some one will be fixing this.
The only reasons that I can come up with for one being hotter is either a bad bond an one end of the cold counduit or the EGC is carying current.