NCEES Sample problem ?

[kduff70]

[SIZE=10pt]Good Afternoon[/SIZE]

[SIZE=10pt]I’ve attach this practices problem (118) from NCEES book but I’m stuck on why the current coming from the generator is 760 instead of 1320 amps shouldn’t the current on the delta side of the transformer increase by sqrt of 3?[/SIZE]

ncees.pdf

 

[Ship Wreck PE]

This is actually a give me free points problem.

After you study and work transformer problems for several months, the answer will be apparent.

In this system, the current is passing thru the transformer and they are not asking for phase current in the transformer, so the current is just multiplied by the ratio of the transformer.

 

[kduff70]

[SIZE=10.5pt]Thank you so much for your quick response that where I was a little hazy at. So I should only assume they wanted the current not unless they specify the phase current? [/SIZE]

 

[kduff70]

I got it now it took me a mintue to see it but now i understand :thumbs:

 

[Swiftman25]

I got the correct answer but in a different way than the book.

This is how I looked at this problem: generator side as primary and line side as secondary. a = 13.2/(132/root(3)) = 13.2/76.21 = 0.17321.

Know equation: Is/Ip = a; therefore Ip=Is/a = 75.93/0.17321 = 438.38A. HOWEVER, this is amperage in the phases of the delta. LINE current into the delta is root(3) larger,

Therefore Igenerator=root(3)*438.38 = 759.3A. This is close to 760 or answer (A).