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  1. #1
    Join Date
    Oct 2010
    Location
    oklahoma city
    Posts
    74
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    VFD on side-grounded delta

    I recently had 2 ABB drives fail within weeks of each other at a wealthy residence. I replaced this first one with a Honeywell hvac 5hp drive. The pump will run up to 40hz, but going over that caused a fault 10 trip- input phase loss. Upon further checks the c phase does not draw any current. Move it to any terminal on the drive input, no current while A and B phases (B is the 208) both draw amps, verified with 2 meters. Upon further investigation (honeywell tech support, checked by electricians) I learned that honeywell does not support unbalanced ground systems, whatever that means. ABB does allow it with the removal of a screw, so I ordered the same ach-550 that was previously installed. Same problem, no current on the C leg. So now I have 2 different drives on two different pumps with the same problem. The other shoe, is that on the same panel is a 15 ton unit that draws amps on all 3 legs.I will entertain any thuoughts you may have, cause I'm almost out of them.

  2. #2
    Join Date
    Jan 2015
    Location
    OKC
    Posts
    1
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    I am curious if you resolved this as I have encountered a very similar situation except it is on a 120/208Y system.

  3. #3
    Join Date
    Jul 2006
    Posts
    14
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    When you have no current flow in one incoming phase, the VFD will still work, because all it does with the line power is rectify the AC to DC, everything else happens off of the DC bus from there on. That is why VFDs can be used as phase converters, to run 3 phase motors from a single phase source. But when you only feed single phase in, the rectifier conducts to the DC bus less often, so there is more DC ripple on the bus, plus the diodes in the rectifier have to pull 173% more current through them (because there are only 4 diodes instead of 6 that are sharing the load). The VFD may not even know that there is a phase loss on the input directly, but it will see the excess DC bus ripple. The ripple gets worse with loading because there is insufficient cacitance to smooth it, so if using the VFD for phase conversion, you must at least double the size of the drive compared to the motor size in order to get that added capacity. When you size it for 3 phase and lose a phase however, that over sizing is not there. So when the VFD sees the excess ripple, it has to shut down to avoid damaging the transistors in the inverter section. below 40Hz, the load on the drive is low enough that it can still handle it, but at 40Hz and above, it can't take it any more, that's why it trips. It's also likely trying to limit the speed to 40Hz for a while, but if you keep demanding it run higher, the ripple effect builds up and causes the trip anyway.

    Reading zero current on one phase means something is wrong up stream, you need to find and fix that.

    The issue of feeding a VFD with delta power is a different issue. Removing the screw is just removing the ground reference point for the built-in surge protectors so that they don't try to become a Wye point for your entire service and destroy themselves. It's important, but not related to this.

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