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Thread: 120/208 to 480 wye-delta

  1. #11
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    That should solve your questionable voltage, however, if I'm not mistaken, you still need to meet NEC 250.30 in regard to a grounding electrode system, etc.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  2. #12
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    just thinking you really should meg this transformer before you connect a corner ground. If it's had an ungrounded secondary and there is a fault to ground, lots of times nothing happens. Now if you ground one of the phases and there is already a fault to ground someplace else,,,,,,, POOF and the magic smoke all comes out.
    Proverbs 31;10


  3. #13
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    Yeah Joe I already meged it everything checked out now after hookin it up the voltage straightened up 480 between all phases also 480 on a&c to ground zero on c everything was good but there was a burning smell coming from it it was light no smoke would anythig have happend being that it was hooked up wrong? Other than the slight smell everything is good no heat other than normal

  4. #14
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    All of that said, there is about a 99% chance that your VFD, assuming it is on the 480V side, is NOT supposed to be connected to a Delta power system, and ESPECIALLY not a corner grounded delta system, without paying very close attention to the installation manual with regards to ground references in the surge protection / MOV systems on the front end of the drive.

    Years ago when men were men and drives sold in the US were designed for the US, this wasn't a big deal. But in the "global economy" we live in now, drives have to be designed for international standards, some of which require common mode noise reduction. As a result, since elsewhere in the world they NEVER use delta systems like we do, they designed in a set of common mode caps on the DC bus with a ground reference, as well as a set of MOVs on the AC side of diode bridge, both of which have a reference to ground. In a delta system, that ground reference on the filter caps can cause the voltage on the DC bus to raise beyond limits even if the drive is not running, causing nuisance Over Voltage tripping. You almost always need to remove the reference to ground on that filter cap system.

    But worse yet is the fact that because the VFDs are designed for grounded wye systems, the MOVs selected for the front-end are often NOT designed for line-to-line voltages. So depending on the manufacturer, you must REMOVE the MOVs altogether. Some mfrs are a little smarter in that they use MOVs rated for line voltage, but simply instruct you to remove a jumper for the reference to ground. If the original installer did not RTFM, and they RARELY do if they are HVAC technicians, then any time there are even slight voltage surges, your delta connection is frying those MOVs. At the same time, that reference to ground and the fact that your ground is also carrying a voltage on it is letting any common mode noise cause a serious voltage ripple and OV situation on the DC bus.

    That is likely what is behind your VFD failures. It's quite common actually. So first fix your installation issues, then before you energize it again, RTFM on the drives and follow the instructions for installing the drives on a delta or HRG system. If there are none, then it's likely that the VFD cannot tolerate a delta supply at all and will need replacement with one that does. this is often the case with cheap drives used in HVAC systems.
    Last edited by Jraef; 05-26-12 at 03:00 AM.
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