3-Phase Power

[Redskinsdb21]

I asked a question similar to this earlier..but it was w/o a pf....I have a 19KVA load supplie by a 208/120, 4-wire, 3-phase, wye connected system....with a 0.85 pf....what is the current delivered to the load?

I know w/o a power factor....I = 19kva/(208*(sqrt(3)))= 52.7 amps..but how is it determined when a pf is involved?

Is it = 19Kva/(208*0.85*(sqrt(3))) ?

 

[Wael]

I think you have to learn the power triangle. If you review that, you will figure out that:

these formulas for 1 phase for simplicity. Multiply by sqrt 3 for 3 phase:

S(KVA)=V x I*

P(KW)= S x PF = V x I* x PF

it is crucial to have this concept cleared out prior the exam.

All the best.

 

[Wildsoldier PE]

Wael is right you need to learn what they are asking you for in this case the pf was given as extra info that you dont need. Suggest to have a review on 3 phase power. You need to have that very clear in order to succeed. A college book covering this would do the trick.

 

[Redskinsdb21]

I think you have to learn the power triangle. If you review that, you will figure out that:

these formulas for 1 phase for simplicity. Multiply by sqrt 3 for 3 phase:

S(KVA)=V x I*

P(KW)= S x PF = V x I* x PF

it is crucial to have this concept cleared out prior the exam.

All the best.

For 3 phase then:

S = V x I* x sqrt(3)

P(KW)= S x PF = V x I* x PF x sqrt(3)

?

 

[Wael]

thats right !

 

[Redskinsdb21]

thats right !

In the above example, it says the load is 19 KVA.....is there a way to calculate the transformer KVA from the given information?

Also, for 3 phase transformers, can you calculate its S value from : S= 3 x phase voltage x phase current and for single phase for both WYE and DELTA systems?