×
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!
  • Students Click Here

*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.

Students Click Here

Jobs

3 phase open delta?

3 phase open delta?

3 phase open delta?

(OP)
When measuring line voltage at the secondary taps of a 3 phase open delta 4 wire(2 pot transformer configuration) wired for 120v/240v. Why wouldn't you read 480v instead of 240v when voltage is measured across the two outside taps? When measuring the voltage from the high leg to the grounded tap you read a higher voltage because your measuring all of one winding and half of the other. Wouldn't you be reading the voltage of two windings or is it because the two phases are out of sync and they are canceling out each other at point? Thanks for any clarification.

RE: 3 phase open delta?

Suggestion: The voltages are added vectorially on the open delta transformer connection. To obtain 480V the transformer windings would have to be connected in parallel on the primary and in series on the secondary. The series connection would have to be additive such that 480V would materialize.

RE: 3 phase open delta?

Because it is the vector summation. The voltage from the midpoint to one end is 120 degree apart from the voltage from the midpoint to the other end.

V/_0 + V /_120 = V /_ -120.

Please refer to my response to Thread238-67436 started by ossie for 'unbalanced three phase loads'.  Although it is for current, the math is still applicable for the voltage summation.

Removing one of the tranformers of a delta system (creating an open delta) does not change the voltage and current relationhips, only capcity of the system is affected.

RE: 3 phase open delta?

(OP)
Thanks rbulsara, I have another question, is this really 3 phase?

RE: 3 phase open delta?


Visualize the situation as a right triangle with 60° between adjacent {1 per-unit length or ½ coil} and hypotenuse {2 per-unit length or full coil.}  The opposite length is 1.732 per unit.  This holds for unloaded open-delta versus delta transformer-secondary configurations.  Reference last two illustrations at http://home.att.net/~benmiller/elecsys.htm
  

RE: 3 phase open delta?

Whipit:

Yes. I presume you are talking about a two transformer VT (PT) connected to a 3 phase supply on the primary side, and 120V secondary(or 240V).   It is not equivalent to a trasformer with a mid-tap.


If an open delta were not a 3 phase system, it would not be used as emergency (at reduced capacity) two trasnformer system to feed 3 phase loads when one of the 3 phase bank unit fails.

RE: 3 phase open delta?

Suggestion: The open delta is a three phase source; however, the power rating is given by two transformer power ratings that are added. Therefore, it is not the same as close delta formed by three single phase transformers. Also, the open delta voltage source triangle is not very symmetrical, when under the load.

RE: 3 phase open delta?

Just for infromation:
Power rating of a Open delta system will be 57.7% of the completed delta system with 3 transformers of same rating.

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