ungrounded 480v delta

[ibew501ed]

If a building is supplied 120/208 grounded wye , and a transformer is then used to provide 480v delta for motor load , would this transformer be considered ungrounded?
would it need ground detectors?

 

[Smart $]

If a building is supplied 120/208 grounded wye , and a transformer is then used to provide 480v delta for motor load , would this transformer be considered ungrounded?

Only if it is in fact ungrounded :grin:

would it need ground detectors?

I'll go out on a limb without looking and say yes

Is this an existing installation?

Are you sure the xfmr's secondary is delta configured?

There are a couple options other than the design mentioned... er-r-r-r implied. You could corner ground a delta secondary, or you could use a wye-wye transformer, ground the secondary neutral and simply not use it.

PS: Welcome to this forum!!!

 

[ibew501ed]

I needed to feed a drive thru an isolation transformer , delta delta, new installation, I question if this is ungrounded or is there an internal reference to ground.

 

[ibew501ed]

isn't there problems with a ground loop when you connect wye / wye

 

[jumper]

I do not know the answer, but here is a site for future reference, lots of good info:
http://www.3phasepower.org/

 

[chris kennedy]

You could corner ground a delta secondary

Is there a reason this would not be the wiring method of choice?

 

[Smart $]

I needed to feed a drive thru an isolation transformer , delta delta, new installation, I question if this is ungrounded or is there an internal reference to ground.

Typically no internal reference with dry-type xfmrs.

isn't there problems with a ground loop when you connect wye / wye

Not that I'm aware of... you wouldn't connect H0 and X0 only connects to local GES and nothing else.

By drive, do you mean you are feeding a VFD, and not a motor contactor/controller?

Aside from that, I forgot to mention you can just as easily go delta-wye which I believe would be the preferred configuration if feeding a VFD.

 

[Smart $]

Is there a reason this would not be the wiring method of choice?

I can think of two off hand:

1. Grounded wye secondary limits voltage to ground to less than that of a corner-grounded delta secondary. That is 277V vs. 480V to ground.
2. A corner grounded system is not a typical system and can be confusing to those inadequately equipped.

 

[ibew501ed]

the transformer fed a 100hp dc drive.
does "inductive coupling" give you a voltage reading to ground even if you dont groung the transformer

 

[jim dungar]

the transformer fed a 100hp dc drive.
does "inductive coupling" give you a voltage reading to ground even if you dont groung the transformer

Yes it will.

Is this an existing transformer or are you going to purchase one?

Many present generation VFD's do not like ungrounded or corner-grounded systems. To meet CE requirements many of them have line-ground connected capacitors as part of their input sections so their preference is to be fed by a solidly grounded wye source.

 

[ibew501ed]

The transformer came from an extruder manufacturer as part of a complete system. I fed it 3 phase in 3 phase out and bonded the casing to building steel. the nameplate showed delta delta but when I measured the secondary it was 277v to gnd is this possible if it is delta (or some phantom voltage)

 

[Smart $]

...277v to gnd is this possible if it is delta...

Yes it is. Reactive coupling to ground will give you a 277V to ground reading on a high-input-impedance DMM. If you measure with a low-input-impedance VM, your reading line-to-ground voltage may change as it is dependent on the reactive coupling impedance.

And of course if you have a low-impedance, line-to-ground fault you'll get 480, 480, and 0 volt readings... same as if you corner-grounded the secondary.