Single Phase Loads on Three Phase Transformers

[JustSumGuy]

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.

 

[davidbeach]

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?

 

[waross]

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

 

[jghrist]

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.

 

[JustSumGuy]

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

 

[JustSumGuy]

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?

 

[davidbeach]

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?

 

[JustSumGuy]

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.

 

[jghrist]

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.

 

[waross]

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

 

[NAZ55]

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) )