Vpp vs Vpn

[applepieordie]

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!

 

[rg1]

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. 

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? You try to draw phasors of Vpp. Vpn and I, do this question by assuming Vpp or Vpn as reference and see for your self. This is game of reference of phasors. In the example given below reference angle is not given for Voltage and answer also does not have an angle, now you are free to choose your angle; the question has choosen zero angle for Vpn; its okay;  choose anything as reference and you will have same answer ( But keep it in mind if you have chosen Vpp as reference then in this question you should have Vpn<-30 and I with angle<-30-36.86). Generally this 30 degree angle is required only when a reference angle is given and/or answer also has an angle. I get it by seeing the question. Think over why angle should be taken. There is no thumb rule for this. Like I say, in wattmeter questions you should play with reference, specially when they are connected randomly. If something else comes to my mind, I will let you know. If you ask me more similar questions where you have confusion, I will be able to tell you why a particular reference has been taken or why it is not required to be taken. May be that is the way to learn it.



Thanks!

 

[TNPE]

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?  

 

[cos90]

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.

 

[rg1]

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.

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.

 

[cos90]

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.

 

[rg1]

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

Because PF is a characteristic of Load. 

 

[rg1]

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.