find transformer size when power factor is known

[O'donisR]

Hi everyone I'm new here, and I need help with the question below. I know the formula to get KVA for single phases transf KVA=(E x I / 1000) but i don't know what to do with the power factor data ? please help and thank you!!

:?

Question: What zise of transformer is requiered for a 100A, 240V, single phase noncontinuos load that has a power factor of 85%

1. 15KVA
2. 25KVA
3. 37.50KVA
4. 50KVA

 

[charlie b]

Your formula needs to include power factor, as follows: KVA=(E x I) / (pf x 1000)
In your example, KVA = (240 x 100) / (0.85 x 1000) = 28.2.
The next higher standard size (or so I believe) is 30 KVA. But the next higher choice in the list they gave you is 37.5 KVA. So I would select choice #3.

Welcome to the forum.

 

[O'donisR]

Thank you charlie b. !!!!!!!!!!!!!

 

[markstg]

Your formula needs to include power factor, as follows: KVA=(E x I) / (pf x 1000)
In your example, KVA = (240 x 100) / (0.85 x 1000) = 28.2.
The next higher standard size (or so I believe) is 30 KVA. But the next higher choice in the list they gave you is 37.5 KVA. So I would select choice #3.

Welcome to the forum.

Charlie,
KVA = V*I/1000
KW=V*I*PF/1000

pf is not needed to answer the question. 25KVA transformer....100A * 240V/1000 =24 KVA

 

[Besoeker]

pf is not needed to answer the question. 25KVA transformer....100A * 240V/1000 =24 KVA

A bit of a woopsie there from our PE...
Apologies, Mr Beck....

The basis of the 24kVA calculation is obvious. But does the "noncontinuos load" need to be considered?

 

[rbalex]

A bit of a woopsie there from our PE...
Apologies, Mr Beck....

The basis of the 24kVA calculation is obvious. But does the "noncontinuos load" need to be considered?

If it were an exam question as stated in the OP with respect to transformers, both power factor and "noncontinuous" loads are essentially red herrings.

 

[charlie b]

OK, memo to self: No posting on the forum allowed before the second cup of coffee.

Memo to O'donisR: Please ignore post #2, and pay attention instead to posts 4-6. Mea culpa.

 

[Besoeker]

OK, memo to self: No posting on the forum allowed before the second cup of coffee.

Memo to O'donisR: Please ignore post #2, and pay attention instead to posts 4-6. Mea culpa.

Beautifully and graciously put, that man!
:thumbsup:

 

[Besoeker]

If it were an exam question as stated in the OP with respect to transformers, both power factor and "noncontinuous" loads are essentially red herrings.

You're probably right. Certainly PF is irrelevant and thus a red herring.
And, if the question contained no information on duty cycle there would be no way to factor it in.

 

[Hv&Lv]

Well... The measurements are all fine and dandy of course, but every one of you know when I drive up I am going to hang a 15, but only because we don't hang 10's anymore...:lol: