INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

(OP)
We have some 2-bushing 277/480V overhead transformers in stock and would like to use them to install a 480V open delta bank (versus using 3-bushing transformers that have a center tap, which we don't have in stock). I'm not a transformer connection expert, but I'm thinking I can delta-connect two 2-busing 277/480V transformers and run the 3 legs for a 3-wire 480V service. I took a stab at the attached electrical diagram, but I'm wondering if the winding polarity will give me the correct voltage.

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

Your output from this type of transformer is 277 V. Connect 3 or 2 of them in wye or open wye, and you will have 480 line to line. Connect them in delta or open delta, and you will have 277 V line to line.

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

(OP)
Thanks. I thought so. If a customer is on a 480V open delta, is there any reason not to give them a 277/480V wye service and just run the 3-480V legs?

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

A reason NOT to? Cost. If all of their loads are simple 480V motors running full speed, or they are feeding a step-down transformer for single phase loads, then there is no reason to add more transformers.

But if ANYTHING in their facility is using 480V 3 phase input to a power conversion device, such as a VFD, servo, UPS, large 3 phase power supply etc., then open delta is a disaster waiting to happen, and the preventative medicine for that is to create a solidly grounded wye system, even if you never use a Line to Neutral connection on it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

(OP)
We're trying to phase out the open delta banks, so when an open-delta transformer fails the linemen are replacing them with 480V wye banks and not running the neutral. However, we're told this is a safety issue and could burn up the customers' equipment.

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

I don't see an issue with changing open delta to grounded wye. Any equipment that has an issue is probably faulty.
Jeff, I understand that he is going from delta to wye. From that perspective, your answer will be correct in that safety issues will be with the existing delta system, not the proposed wye system.
The safety issue may be if a customer has a ground fault on an ungrounded delta system that he has neglected to repair.
The first warning of the failure will be the ground detector system.
If the customer chooses to ignore that indication of a failure, the second indication of failure will be more dramatic when the grounded wye system is connected.
You may want to have the customer sign a disclaimer before energizing the new transformer bank.
Sign now and the power will be on in a few minutes.
Decline to sign and the proper transformers are on back order and the power will be on in about 6 weeks when the new transformers arrive.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

You will need to run a neutral. First of all you will need it for metering. You don't need it if all loads are connected line to line, but free power can be obtained once they start illegally connecting loads line to ground. I believe code requires running a neutral even if it ends at the service without any further connection to load.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close