Power factor questionwith contradicting answers (Exam Prep)

[Krist Midbrod]

Hi, Im preparing for an exam and working through Mike's exam prep course. I have a problem with a couple of question in Mike Holt's Basic Elec.theory book 3rd edition and also the same question in his Exam Prep book 2011. They contradict each other! Help! Looking for some clarification. The questions are as follows.

What size transformer is required for a 100A 240V single phase non-continuous load with a power factor of 85%?

Answer in Theory book: 240 X 100 / 1000 = 24kva transformer (note: power factor value given in question has nothing to do with determining the KVA of a load.

Answer to same question in Exam Prep book is as follows:

240 X 100 / 1000 = 24kva (Note: transformers are sized to the VA of the load, not the KW)
25,000W/0.85 = 29,412VA transformer size 37.5KVA

Not sure where the 25,000 comes from here, im assuming its a mistake but what I'm not sure about is DO I USE POWER FACTOR IN THIS CALC OR NOT?

Help please! I thank you in advance!

 

[mivey]

1st is correct. 2nd is a typo, probably from a question that was modified without completely modifying the answer.

add: since kW was not given, pf is not needed.

 

[Krist Midbrod]

Thanks for the response. I guess I would have thought the second was correct as it was my understanding that we would size the Tfmr to the VA or KVA and not the KW? Is this incorrect?

 

[Krist Midbrod]

I thought we sized transformers by the VA not the kW? In that case wouldn't power factor need to be considered seeing that it's given in the question?

 

[GoldDigger]

I thought we sized transformers by the VA not the kW? In that case wouldn't power factor need to be considered seeing that it's given in the question?

If you are given amps and volts for the load, as in this problem, that gives you VA directly. Additionally knowing the power factor just allows you to calculate the Watts. Which you don't need.

 

[mivey]

Thanks for the response. I guess I would have thought the second was correct as it was my understanding that we would size the Tfmr to the VA or KVA and not the KW? Is this incorrect?

It was correct on the 24 kVA part. The 25 kW was where it went haywire.

 

[Smart $]

The 25 kW was where it went haywire.

Not if it is a separate example where kW and power factor are the supplied variables.

 

[Besoeker]

Not if it is a separate example where kW and power factor are the supplied variables.

But they weren't given in the question.

 

[Smart $]

But they weren't given in the question.

Example to the note?

 

[Besoeker]

Example to the note?

The second answer doesn't answer the question that was set.

 

[Smart $]

The second answer doesn't answer the question that was set.

But you cannot say it is not an example to the note.

Also, we are relying on rewritten copy. Would have to see actual print version to say which way as fact, typo, or not.

 

[Besoeker]

But you cannot say it is not an example to the note.

Not sure what you mean by that. The note?

Also, we are relying on rewritten copy. Would have to see actual print version to say which way as fact, typo, or not.

One question.
Two versions of the answer. One is correct, the other isn't.

Of course, the actual question might have been transcribed incorrectly. Or maybe there were two different questions and the two answers reflect these. That gets into the realms of spectulation..........and is not very complimentary to the OP.

 

[don_resqcapt19]

Typically when they give you a current, voltage and PF, the current is nameplate current and you need to use the PF to get the required VA.

 

[mivey]

Typically when they give you a current, voltage and PF, the current is nameplate current and you need to use the PF to get the required VA.

How so? Given a current and voltage I have the VA. If you give the pf then we know the W.

What does knowing W have to do with the required VA?

 

[Smart $]

Hi, Im preparing for an exam ...
Answer to same question in Exam Prep book is as follows:

240 X 100 / 1000 = 24kva (Note: transformers are sized to the VA of the load, not the KW)
25,000W/0.85 = 29,412VA transformer size 37.5KVA
...

Not sure what you mean by that. The note?
...

See above.

 

[Besoeker]

See above.

OK. So where does the 25kW come from?

 

[don_resqcapt19]

How so? Given a current and voltage I have the VA. If you give the pf then we know the W.

What does knowing W have to do with the required VA?

You are correct, I have it backwards

 

[Smart $]

OK. So where does the 25kW come from?

That I cannot say with certainty. Off hand, my best guess is the author's imagination...

 

[ggunn]

OK. So where does the 25kW come from?

1.042 pf?

 

[Besoeker]

1.042 pf?

Yes, that would do it.................