Wye / Delta XFMR - XO factroy bonded

[sandsnow]

Backfeeding a delta / wye XFMR has come up numerous times. Most know not to bonded or otherwis connect anything to XO when used this way.

Yesterday I came across this old transformer. I'm not sure of the vintage. Maybe 1950's or earlier? It is installed in one of the hangars at the old El Toro MCAS. It was used as shore power to planes.

Note how XO is factory bonded to the case on the primary side. Seems to go against everything I've learned up to this point. I'm hoping someone has an explanation.

It also has a rating of 50 cycles which is unsual to me. A lot of equipment is dual rated 50/60, but I have never seen strainght 50 cycles.

 

[jim dungar]

This is not a reverse fed transformer, it is designed for step-up.
Maybe the transformer was designed with a special core (i.e. a triplex core) instead of the 'standard' dry type three-legged core?

 

[winnie]

That nameplate sure looks to me like a wye _primary_ delta secondary. The taps for voltage adjustment are on the wye side, and normally these taps are on the primary side. On the other hand I've used transformers with secondary taps, and I agree that one normally wants to feed the delta side.

-Jon

 

[sandsnow]

I understand it is a purpose built wye delta. It is actually step down - 4160wye to 480delta.

I'm almost positive it had three coils inside just like any other dry type. One of theese days I can get back and check. Is that what you mean by three legged.

Doesn't the bonded XO on the primary strike anyone as strange? Why would you do that?

 

[jim dungar]

I was wrong, it definitely is a step-down.

Most likely this is a design for a utility MV system, which is one of the times where grounding the midpoint of the wye is acceptable

 

[quogueelectric]

50hz is usually european utility frequency as far as I know.