×
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

Single Phase Loads on Three Phase Transformers

Single Phase Loads on Three Phase Transformers

Single Phase Loads on Three Phase Transformers

(OP)
This is a question that has sparked some debate around here so any input will appreciated. How does one go about calculating how much single phase current is available from a three phase transformer?  For example, if I have a 75kVA 230V 3 Phase transformer, what is the maximum single phase load (amps) that I can connect to each phase?  A 75kVA @ 230V transformer should yeild 188A of 3 phase current, so would it not be that I have 1/3 of this available at each phase for single phase loads?  Thanks for the help.

RE: Single Phase Loads on Three Phase Transformers

If you can get 188A on each of three phases for a three-phase load, why do you think that you could only get 62.67A on each phase for single-phase loads?

RE: Single Phase Loads on Three Phase Transformers

Line to neutral loads on a wye transformer may be considered as single phase loads.
Three phase at 230 volts is a legacy delta voltage so the following remarks pertain to a delta transformer.
CAUTIO: Some three phase transformers do not like single phase loads.
The maximum stand alone single phase load will be 2/3 of the rating of the bank, or 50 kVA. If you add a single phase load on each phase, they become a three phase load. For equal single phase loads, the load equals a three phase load, three times the rating of one single phase load. eg:, Three 10 kVA single phase loads equal one 30 kVA three phase load. The calculations for adding single phase loads to a transformer that is partially loaded with three phase loads are more challenging, particularly when the loads have differing power factors.  

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

RE: Single Phase Loads on Three Phase Transformers

Quote:

230 volts is a legacy delta voltage
Not if you're outside North America, where 230 volts is the Ø-N voltage of a 400 volt system.  JustSumGuy needs to tell us what the connection is.
 

RE: Single Phase Loads on Three Phase Transformers

(OP)
Actually, the specs I have is 75kVA 460V Delta Primary, 230V Wye Secondary

RE: Single Phase Loads on Three Phase Transformers

(OP)
Is not a valid assumption that a 75kVA 3 Phase transformer is really just three 25kVA single phase transformers connected together?  I know, for example, that my utility company will set three 50kVA single phase transformers (cans) on a pole to deliver 150kVA of power to me.  

In the case of a 75kVA transformer running single phase loads, wouldn't each phase be limited to the maximum of 25kVA or run the risk of overloading that phase core of the transformer?

RE: Single Phase Loads on Three Phase Transformers

So your secondary voltages are 230 for 3-phase and 132.8 (133) for single phase.  What are the currents associated with 75kVA, 230V, 3-phase and 25kVA, 133V, 1-phase?

RE: Single Phase Loads on Three Phase Transformers

(OP)
Single phase device voltage on the secondary side is still 230V, just 230V single phase (phase to phase, not phase to neutral) Sorry, I should have made that clear to start with.

RE: Single Phase Loads on Three Phase Transformers

So each secondary winding can provide 188A (75/sqrt(3)/0.23).  This is the limit of your Ø-Ø connected single phase load since the entire current will flow through each of two phase windings.

RE: Single Phase Loads on Three Phase Transformers

Hello jghrist;
Thanks for the heads up on the 230/400V.
I agree with you completely on the loading.

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

RE: Single Phase Loads on Three Phase Transformers

I would like to make a commet here. It took me a while in school to understand the significance of the equation jghrist wrote, but reading from JustSumGuys original post where he is asking if current in single phase would be 1/3 of I(3Ø), I think we need to go back to the fundamentals of this equation:

3 phase power = 3* single phase power

single phase power = V(Ø-N)* I(Ø)

therefore

3 phase power = 3 * V (Ø-N) * I(Ø)

or

3 phase power = SQRT(3) * V (Ø-Ø) * I (Ø)

therefore

I (Ø) = 3 phase power / (  SQRT(3) * V(Ø-Ø)  )

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