120/208 to 480 wye-delta

[Melanconryan]

Ok so I'm new here and I have a question I recently went on a call where the customer had a transformer feeding an air conditioner with a Vfd in it's controls to regulate head pressure he kept burning the vfds in my investigation I started with the transformer it was a wye-delta withthe delta being corner grounded I read 117 on the primary on all 3 phases and on the secondary it's a=467 b=26 and c=437 all are tapped in the same location on tap 3 which should be 480 for he output I need to know why I'm getting such weird readings and how to correct it...now anOther thing there is no physical ground connected to the center of the delta and I'm not sure if there should be? Everything else looks fine as far as I can see any help would be appprciated thanks...Ryan

 

[raider1]

Welcome to the forum.

It sounds like the secondary of the transformer is not grounded and bonded properly and that is why you are getting the strange voltages.

For a properly grounded corner grounded system the grounded phase should have a voltage of 0 in reference to ground.

It also seems like someone reverse fed a delta/wye transformer to get 480 volts. Is anything connected to the XO on the primary side?

Chris

 

[Phil Corso]

Current Flow in GroundING -Conductor

Current Flow in GroundING -Conductor

Melanconryan... the 'b' voltage, 26V, measurement suggests current-flow between the delta-corner and ground. The fact that there is surrent current could be indicative of a rather large cacacitance-to-ground parameter in the 3-phase, 3-wire, 480V system.

Can you determine, by means of a clamp-on ammeter, if its frequency is 60Hz, 180-Hz, or other?

Regards. Phil Corso

 

[Melanconryan]

Yes they did reverse the delta-wye to output the higher voltage now Ive never worked on this type of system before.. So do I take a ground wire off x0 and put it on x2 since the x is my delta side would that be the way to bond the corner?

 

[raider1]

Yes they did reverse the delta-wye to output the higher voltage now Ive never worked on this type of system before.. So do I take a ground wire off x0 and put it on x2 since the x is my delta side would that be the way to bond the corner?

Since the transformer is being fed in reverse there should be NO system bonding jumper installed on the XO. The system bonding jumper would need to be connected to the B phase of the delta side of the transformer.

Chris

 

[Melanconryan]

Ok it's starting to make a little more sence but just to make sure... There is a ground coming in landed on the case also a jumper going to xo I I'm right your saying I should take the jumper off xo and land it on my b phase so it should be case ground to b sorry for asking so much I'm just tryin to understand it a little better

 

[Joethemechanic]

117 on the primary? I take it that's phase to ground? What is it phase to phase on the primary?

And that weird voltage on the secondary 26 or whatever it was, was that phase to phase? Are you sure?

Were these readings taken under load or with an open secondary circuit?

 

[ActionDave]

It is important to distinguish between Grounded and Equipment Grounding Conductors. The case of the x-former should be Equipment Grounded from both sides.<br>The x-former is reverse fed so the X-O is left open. If H-2 is your B phase then it gets bonded the same way you would normally treat the X-O and becomes your Grounded conductor.

 

[Joethemechanic]

If that 26 volt reading was phase to ground on the secondary, and this thing is a corner grounded 480 delta secondary, you have to have a missing "Phase to Ground" bonding jumper.

And I say "phase to ground" only because there is really no "Neutral" in a corner grounded delta secondary.

I kind of wonder if your wye primary has been mistakenly N-G bonded, and your delta secondary doesn't have any grounded conductor

 

[Melanconryan]

Joe it's 117 on the primary phase to ground phase to phase 208 it's 467 a phase to b on the secondary and 137 b to c a to ground is 167,b26, and c137 I think I got it figured out thanks to you guys help and some of my books Im gonna try to take the ground jumper off xo and land it on the b phase on the secondary this is just the first time I've ever physically messed with a corner grounded delta I've heard horror stories about it again thanks to everyone for the help I'm sure I'll be back one day

 

[augie47]

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.

 

[Joethemechanic]

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.

 

[Melanconryan]

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

 

[Jraef]

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.