jraef
Electrical
- May 29, 2002
- 11,342
I have been heavily promoting the use of VFD shielded power cable in my installations, and most of the problems I have experienced in the past have been diminishing to near extinction. But today, I went to a site with MULTIPLE motor failures connected to a VFD system that I designed, but was altered before purchase and installed by neanderthals. So in my investigation, I found that ALL of the repeated motor failures are happening in the exact same type of application, wherein they took the one single drive I had put into an MCC, and used it to run 4 smaller motors (12A drive running 4 motors that are 2.7A each) on evaporator coils in a cold room. The motors are supposedly "inverter duty" (but I find that is often about as reliable as a fox telling the farmer that the chickens are safe), I could not see the actual motors. So at first glance it SHOULD have been OK in theory. They did install a Load Reactor on each drive, then they used VFD shielded cable, one that has 3 power leads, 3 symmetrical ground leads nested in them, an overall braided shield over those 6 and a separate EGC (Electrical Grounding Conductor, a term used in the US similar to a "PE" conductor). The typical linear distance from MCC to evaporator set is between 300 and 400 ft, through open cable tray. The VFDs did not show faults in the fault logs, other than Power Loss faults typical of being powered down from opening the MCC cubicle doors.
The only big problem I can find with the installation is that the neanderthal electricians just routed the cables haphazardly (as in a bowl of spaghetti) in cable tray along with other full voltage motor leads and the incoming power leads, and then when they ran the shielded cable to the motors, they cut and taped off the shields at the motor connection box end, so they are shielded only at the VFD end. The motor lead cable goes from the MCC to a fiberglass box on the side of each evaporator, and inside is a local disconnect switch, a power splitter block, and a ground bar, to which the EGC is attached, along with 14ga ground wires from the motors. Motor leads from this box to the motors themselves are NOT shielded any longer, they used what appears to be non-shielded flexible wire through gland fittings on the box. I always tell people to ground both ends of shields on POWER cables, because the intent is to form a sort of Faraday cage. But what does / could happen if only one end is grounded? Could this possibly explain the multiple motor failures?
More relevant info:
(1) The motor re-winder that has repaired these has told the user that the motors are failing on "single phasing", which makes no sense to me. The VFDs in question should trip out on loss (or severe imbalance) of output phase current. I have provided the end user with EASA photos of typical single phase motor failures and typical turn-to-turn insulation failures due to standing waves, they have yet to share that with the re-winder to confirm his findings.
(2) To my horror, the user deliberately CHOSE to remove the separate Manual Motor Starters at the evaporators which were to be protecting the individual motors. So there is NO realistic motor overload protection for these 2.7A motors, other than the OL setting of the VFD, which is set at 10.8A. I have told my contact on the job site that this violates the NEC, but the decision came from a manager, so he said he does not want to challenge him and has asked that my report de-emphasize that issue (not likely). Still, these are evaporator coil fans, it is virtually impossible to overload them, and they are only 3 months old so no time for normal bearing wear. While I was there today the VFDs were at 55-60Hz and the 4 motors were drawing only 4.5 - 4.8A collectively for all 4 motors.
My theories at this point are:
1) The lack of shield grounding at one end rendered the shield ineffective, so standing wave generation caused first-turn insulation failure, which the motor re-winder was unfamiliar with and misidentified as single phase damage.
2) The lack of shielding is causing rapid bearing EDM damage and failure, which causes the motors to overload, which again, the re-winder misidentified as damage from single phasing.
3) None of this has any importance whatsoever, and all of it was just caused by the neanderthals who did the motor hookups likely not making good connections at the splitter blocks. Still, I would have expected the VFD to trip on that though.
Please share your thoughts.
"Will work for (the memory of) salami"
The only big problem I can find with the installation is that the neanderthal electricians just routed the cables haphazardly (as in a bowl of spaghetti) in cable tray along with other full voltage motor leads and the incoming power leads, and then when they ran the shielded cable to the motors, they cut and taped off the shields at the motor connection box end, so they are shielded only at the VFD end. The motor lead cable goes from the MCC to a fiberglass box on the side of each evaporator, and inside is a local disconnect switch, a power splitter block, and a ground bar, to which the EGC is attached, along with 14ga ground wires from the motors. Motor leads from this box to the motors themselves are NOT shielded any longer, they used what appears to be non-shielded flexible wire through gland fittings on the box. I always tell people to ground both ends of shields on POWER cables, because the intent is to form a sort of Faraday cage. But what does / could happen if only one end is grounded? Could this possibly explain the multiple motor failures?
More relevant info:
(1) The motor re-winder that has repaired these has told the user that the motors are failing on "single phasing", which makes no sense to me. The VFDs in question should trip out on loss (or severe imbalance) of output phase current. I have provided the end user with EASA photos of typical single phase motor failures and typical turn-to-turn insulation failures due to standing waves, they have yet to share that with the re-winder to confirm his findings.
(2) To my horror, the user deliberately CHOSE to remove the separate Manual Motor Starters at the evaporators which were to be protecting the individual motors. So there is NO realistic motor overload protection for these 2.7A motors, other than the OL setting of the VFD, which is set at 10.8A. I have told my contact on the job site that this violates the NEC, but the decision came from a manager, so he said he does not want to challenge him and has asked that my report de-emphasize that issue (not likely). Still, these are evaporator coil fans, it is virtually impossible to overload them, and they are only 3 months old so no time for normal bearing wear. While I was there today the VFDs were at 55-60Hz and the 4 motors were drawing only 4.5 - 4.8A collectively for all 4 motors.
My theories at this point are:
1) The lack of shield grounding at one end rendered the shield ineffective, so standing wave generation caused first-turn insulation failure, which the motor re-winder was unfamiliar with and misidentified as single phase damage.
2) The lack of shielding is causing rapid bearing EDM damage and failure, which causes the motors to overload, which again, the re-winder misidentified as damage from single phasing.
3) None of this has any importance whatsoever, and all of it was just caused by the neanderthals who did the motor hookups likely not making good connections at the splitter blocks. Still, I would have expected the VFD to trip on that though.
Please share your thoughts.
"Will work for (the memory of) salami"