Has anyone ever worked on a transformer with the following: primary 120/208v 3ph 4w/ secondary 120v 3ph delta? 120v phase to phase and nothing phase to ground. The problem is on one receptacle circuit, when the load is energized (lights, vacuum, etc..)the voltage goes down proportionally to the load. All the fuses check ok.

from the hip, this sounds like an isolation x-former in reverse...

~S~

It is possibly for operation of {military?} shipboard equipment on shore power, so may be acceptable as 'grandfathered.' 99NEC250-21 allows ungrounded-secondary operation under very restrictive circumstances.

[Note 99NEC Table 430-150 Full-Load Current Three-Phase Alternating-Current Motors has a '115-volt' column.]

I don't know why you'd need a neutral on the primary side.?

This transformer feeds an old open face stage lighting board.

I agree with Redsy, the X0 (neutral) terminal on the primary side is not required and should be left "open".

How long has this transformer been installed?
When did the problem begin?
Is the voltage drop occuring at the load or at the transformer terminals?
Does the voltage drop occur on all of the 120V combinations A-B, B-C, C-A?

The transformer was installed in the late 70's.., part of a building renovation. The problem started after a underused performance stage got used heavily. It's part of a Masonic lodge. I wish I could show you a picture.

“Odd” transformer configurations are my favorite kind. It is likely that the receptacle circuit fed from the described transformer is a misapplication and/or afterthought, and should likely be reconnected to a “normal” grounded branch circuit. I’ll bet the 120V-∆ secondary originally was intended to be reserved solely for stage-lighting load. The installation for theatrical lighting is sort of a stopgap arrangement, but could have been acceptable by code or local AHJ at the initial time of installation. [Current versions of NEC Article 520 have a number of non-mainstream mandates.]

The transformer was probably specified initially as a means of safety/shock protection, considering that conductors {presumably in an ancient live-front lighting board} will be limited to 120V between conductors, while 208Y/120V service has potentially 208V between conductors. Relying on that for shock protection is a poor idea in that the inherent capacitance between live conductors and ground is not zero, although in many cases may be somewhat limited. {It can also be higher than 120V if a resonant or transient condition occurs in the natural phase-to-ground capacitance in the ungrounded system.}

The problem with the winding wye-point grounded {probably “HO” in this case} in a wye/delta transformer is that voltage on an open wye-side phase will be ‘artificially’ supported {kept energized} by the transformer [acting as a zero-sequence source.] If the winding is correctly overcurrent protected, it should operate to prevent transformer damage. The wye-side imbalance will cause circulating current in the delta winding, regardless of other things connected to it.

It is a novel and interesting application for the given voltages and end use.




[This message has been edited by Bjarney (edited 12-02-2003).]

Of course, the first thought was this is a "Reversed" Transformer (Primary and Secondary input / output points reversed).
This is due to the reference of 3Ø 4 Wire Wye, not to the 120/208 VAC.

With this aside (mention later), the 120 VAC 3Ø 3 Wire Delta is a "Non-Normal" setup - but not something unheard of.

Bjarney's mentioning of NEC's reference to 120 VAC three phase motors is why this system is not weird (to me).

The Transformer is obviously (to me, again) built to supply power for the above mentioned scenario, along with a few others which may or may not be "obvious" - such as:
* System with maximum of 120 VAC on any L-L circuit, or L-G fault,
* To deal with Harmonic problems - similar to what is achieved with using a 1Ø 3 wire system off a 4 wire wye (via transformer),
* To eliminate the ground loop noise involved with the use of an active grounded conductor in normal circuitry (like the 60/120 VAC 1Ø 2 Wire systems do).

In this case, the system may be grounded or ungrounded. Grounding the system will make the Voltage to ground more stable, so this would be my choice overall (if used for non-critical loads!!!).
Grounding may be done in one of tow ways:
<OL TYPE=1>

[*] Ground the "Center Tap" of one coil, and run this conductor out to be bonded to all the metallic equipment ony - not an active conductor; this would be "my best choice option",
or

[*] Ground one active "Phase" line, and use as an active conductor; this would be "my last choice option".
</OL>

The Wye / Delta setup is really not very odd to me, but as said before, the "4 Wire" reference is kind of strange for a Primary of such low voltage!
Maybe the reference is "Either 120 VAC or 208 VAC" for the Primary input, or just that the Primary is planned to be used on a 4 wire wye system. Either way it's simply a guess to me!

As to the voltage drop problem - it sounds like the Transformer (likely a single coil of the Secondary) is being overloaded temporarily; or the circuit affected is very loaded and the heaviest loads are far away from the source.

Check the KVA of the Transformer, and the loads of the branch circuits.
If the KVA is, for example: 30 KVA, then figure only 10 KVA will be available per coil.
If most loads are across only "One L-L Circuit", such as "A-B" only, this will load up one single coil and result in the problem you are experiencing.

Keep us informed to the outcome!!!

Scott35

Just curious to the outcome of this scenario!

Like the concept of a 120 VAC 3Ø 3 Wire Delta!

Scott35

No resolution yet, but you guys have given me much to think about.