# Vpp vs Vpn



## applepieordie (Jul 19, 2017)

I often find myself making the mistake of using Vpp rather than Vpn and vice versa. After I see the solution and realized the mistake I made, it seems so obvious from the start that I should have used Vpn.

Question 1: are there any techniques or rules to follow to be sure what voltage potential to use and minimize mistakes? or is it just trial and error learning process? I dont want to make such a simple mistake on exam day. 

Question 2:  When converting from Vpp to Vpn I usually follow the formula Vpn = Vpp/(sqrt(3)∠30°). In the example below, the ∠30° is left out of the denominator. Can someone explain to me why? When to include ∠30° and when to exclude?

 






Thanks!


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## rg1 (Jul 19, 2017)

applepieordie said:


> I often find myself making the mistake of using Vpp rather than Vpn and vice versa. After I see the solution and realized the mistake I made, it seems so obvious from the start that I should have used Vpn. .
> 
> Question 1: are there any techniques or rules to follow to be sure what voltage potential to use and minimize mistakes? or is it just trial and error learning process? I dont want to make such a simple mistake on exam day. Reading the question carefully is the only solution. I have a tip. We all make a few common mistakes. Write them down in bold letters, keep them in front of you  and look at it before attempting every question. This will remind one; of the common mistake and after practicing a few times, the bad habit may vanish. Not uncommon many do it, so I always take a margin of 6-7% loss on this account.
> 
> ...


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## TNPE (Jul 19, 2017)

This exam will, with 99.99% certainty, give you line values with a 3 phase problem.  If you're not told otherwise, assume line values.  Now, you could be given a 3 phase problem, and for simplicity and to answer what's being asked, you may have to convert the circuit and any associated voltages, currents and/or impedances appropriately.  Practice, Practice, Practice!!

Think of it this way:

P=_/3 VIcos(theta), regardless of connection

Why is that so?


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## cos90 (Jul 24, 2017)

The PF is defined as the relationship between the voltage and current phasor. Therefore when doing a source transformation you need to rotate the current phasor with the voltage phasor as rg1 said above. The way the practice test authors work these voltage problems they just don't rotate the voltage phasor because it doesn't affect the magnitude of the answer.

If the problem gave you absolute quantities like a resistance or an absolute current phasor then you would leave the angle of it alone.

Does anyone disagree with this? I need to check my understanding.


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## rg1 (Jul 24, 2017)

cos90 said:


> The PF is defined as the relationship between the voltage and current phasor.  Only with phase Voltage and phase current not Line V to Line I or Line V to Phase I  or any other combination.
> 
> Therefore when doing a source transformation you need to rotate the current phasor with the voltage phasor as rg1 said above. The way the practice test authors work these voltage problems they just don't rotate the voltage phasor because it doesn't affect the magnitude of the answer.
> 
> ...


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## cos90 (Jul 24, 2017)

rg1 said:


> Only with phase Voltage and phase current not Line V to Line I or Line V to Phase I  or any other combination.


Yowza. Made it through undergraduate education without noticing this! Now the problem makes more sense.


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## rg1 (Jul 24, 2017)

cos90 said:


> Yowza. Made it through undergraduate education without noticing this! Now the problem makes more sense.


Because PF is a characteristic of Load.


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## rg1 (Jul 24, 2017)

cos90 said:


> Yowza. Made it through undergraduate education without noticing this! Now the problem makes more sense.


Understanding this will be a great help for one to appreciate wattmeter questions when they are connected randomly (Voltage coil at one place and Current coil at some other place) . Its very interesting. I really love it.


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