Apparent current with complex conjugate : industry notation vs. college notation

[sweber]

In my advanced classes on alternating current we're asked to do research why there's a difference in notation in the industry and in the classrooms when it comes to apparent current with a complex conjuge.

Industry notation: S=U^*I
University notation: S=UI^*

I assume that the difference in notation is done because of didactic reasons but that there's no 'real' difference. Having said that, I cannot figure out why the industry uses a different notation.

So can anyone answer is there a difference in practice? If so:
- what is the difference?
- if it does not follow from the first question: why?

It can of course be the case that they are similar. But then still needs the question be answer: why the difference?

 

[GoldDigger]

In my advanced classes on alternating current we're asked to do research why there's a difference in notation in the industry and in the classrooms when it comes to apparent current with a complex conjuge.

Industry notation: S=U^*I
University notation: S=UI^*

I assume that the difference in notation is done because of didactic reasons but that there's no 'real' difference. Having said that, I cannot figure out why the industry uses a different notation.

So can anyone answer is there a difference in practice? If so:
- what is the difference?
- if it does not follow from the first question: why?

It can of course be the case that they are similar. But then still needs the question be answer: why the difference?

The answer, taken in part from Quora knowledge sharing, is that the real part will be the same in either form but the change in which term you apply the complex conjugate too will change the sign of the reactive part of the result.
Either way will give you the magnitude numbers you want to work with, but the sign will be reversed when comparing capacitive reactive power to inductive reactive power.

well... both the expressions for power is good but more frequently the expression p=vi* is used because if you expand the expression in terms of real and imaginary components you will get a positive imaginary component (for power of course) for lagging current i.e +ve reactive cmponent which fits in with our frequently used sign convention.

But if you are comfortable with using -ve sign(reactive power) it good to use the 2nd exoression.

That still does not explain why the two working groups adopted opposite standards, and the best answer is probably that they each started off independently and went in different directions since they did not talk to each other much.

There is a tendency to set things up so that the numbers you use most often have a positive sign. In industry, with motors and transformers playing such an important role, it seems natural to have inductive reactive power being positive.
In the university they are more likely to be working with capacitors than inductors, so they choose the form where capacitive reactive power is positive. (Just a wild speculation.)

 

[sweber]

The answer, taken in part from Quora knowledge sharing, is that the real part will be the same in either form but the change in which term you apply the complex conjugate too will change the sign of the reactive part of the result.
Either way will give you the magnitude numbers you want to work with, but the sign will be reversed when comparing capacitive reactive power to inductive reactive power.



That still does not explain why the two working groups adopted opposite standards, and the best answer is probably that they each started off independently and went in different directions since they did not talk to each other much.

There is a tendency to set things up so that the numbers you use most often have a positive sign. In industry, with motors and transformers playing such an important role, it seems natural to have inductive reactive power being positive.
In the university they are more likely to be working with capacitors than inductors, so they choose the form where capacitive reactive power is positive. (Just a wild speculation.)

Thanks for your response. I'll run the numbers, see if they add up. Wanting to have a positive sign makes sense for the industry, although it almost sounds too simple...
I distinctly remember the professor saying that the reverse is done for didactic reasons. I'll have to check though if we work more with capacitors, it doesn't seem that way.

So I guess the first part is covered: thanks for that. I'll continue my quest regarding the last bit!

 

[GoldDigger]

I'll have to check though if we work more with capacitors, it doesn't seem that way.

It may not seem that way now, but the usage convention probably goes back to academic/scientific early history.

 

[steve66]

There is a tendency to set things up so that the numbers you use most often have a positive sign. In industry, with motors and transformers playing such an important role, it seems natural to have inductive reactive power being positive.
In the university they are more likely to be working with capacitors than inductors, so they choose the form where capacitive reactive power is positive. (Just a wild speculation.)

I really have no idea - I don't think I've ever seen the U*I form.

However, your post has me wondering if its the difference between the utility providing power (which they probably would want to see as a positive number), and consumers using power (which they would also want to see as a positive number).

My text book compares the "load convention" and the "generator convention" almost as soon as they introduce complex power. But I haven't looked close enough to see if that would explain the difference between U*I and UI*. (I'm not sure I remember enough complex math to look at it any closer).

 

[sweber]

It may not seem that way now, but the usage convention probably goes back to academic/scientific early history.

I got feedback today on your idea: the professor liked the idea, but he said that it was not a good answer... Unfortunately.

 

[jumper]

I got feedback today on your idea: the professor liked the idea, but he said that it was not a good answer... Unfortunately.

Darn academic eggheads.

 

[sweber]

I really have no idea - I don't think I've ever seen the U*I form.

However, your post has me wondering if its the difference between the utility providing power (which they probably would want to see as a positive number), and consumers using power (which they would also want to see as a positive number).

My text book compares the "load convention" and the "generator convention" almost as soon as they introduce complex power. But I haven't looked close enough to see if that would explain the difference between U*I and UI*. (I'm not sure I remember enough complex math to look at it any closer).

Interesting suggestion, but I think that's not the case. With industy I didn't mean the supplier side but the 'consumer' side of the industry.

 

[sweber]

Darn academic eggheads.

How they come up with this stuff, you can never know. I'd like to suggest: stop making a difference. Just stop...

 

[junkhound]

FWIW to others that are bewildered by U*I and UI*: or, is that just my computer doing strange things?

As an EE in the poco and aerospace business for 60 years, have never seen that notation in either form. Have always use V and I with phase angle notation.


Further, no idea if it is simply a notation for reactive current or pos/neg/zero sequence, or shorthand for phase angle difference of V and I? Similarly, 55 years ago in a circuits class, we were taught an ABCD method of solving loop equations, have NEVER used it in real life.

Guess is it was some prof's wet dream used to sell textbooks?

Corollary question: When did the "*" notation become common - who started it and when ?

 

[GoldDigger]

If you represent a vector as a + bj where j = sqrt(-1) you can add and subtract without needing to use angles. In fact a = magnitude x cos (theta) and b = magnitude x cos(theta).
There is also a rule for multiplying complex numbers. If you use this rule to multiply a complex voltage times a complex current you get nothing particularly useful. But if you change the sign of the imaginary part of one of the two numbers ( called taking the complex conjugate and represented by a * after the letter) then when you multiply the two it is useful. The real part of the product is the real power and the imaginary part is the reactive power.
The two equations represent taking the conjugate of the first term or the second term.

It was in math before it got to physics and electronics and became "practical".

 

[junkhound]

Very seldom use math anymore, nearly everything I've done in the last 25 or 30 years has been on PSpice. Probably why I missed seeing the I* notations.

One grandson is taking calculus and circuit analysis with Laplace transforms in school this year, told him I have not used Laplace for over 25 years for sure except to put the equation for a filter or network response into an evalue expression in PSpice.

Sure have gotten spoiled with FEA programs.

 

[sweber]

The real part of the product is the real power and the imaginary part is the reactive power.
The two equations represent taking the conjugate of the first term or the second term.
".

Thanks for your reply. This was the reasoning I was looking for!

 

[ggunn]

I had to look up "didactic".

 

[Phil Corso]

Sweber... You probably misread the original text.

While either UI* or U*I will yield the correct magnitudes for the both Real (P) and Reactive (Q) component, only UI* will have the correct sign for the Q term. Conversely U*I produces the opposite, obviously incorrect, sign for Q!

Need an example?

Regards, PhilCorso

 

[GoldDigger]

What is the origin of the convention that inductive reactive power is positive and capacitive reactive power is negative? Or rather, where is this officially defined?

 

[iceworm]

In school in the 70s, complex power was defined as S = VI*.

Taking a masters class in 80s, complex power was defined as S = VI*

(Hummmm .... not much change)

So I ask the instructor (the one in the 80s) Paphrased response:
Inst: That's the meaning of 'definitation'. One picks convenient directions, assigns values, applies consistently.

Me: So Tesla, Steinmetz wanted inductive reactance to point up, cause that was the normal load.
I: Yes

Suggestion: Check IEEE 100 for definition of Complex Power

(edit to add) GD - Short answr is: I don't know

ice

 

[iceworm]

Sweber... You probably misread the original text.

While either UI* or U*I will yield the correct magnitudes for the both Real (P) and Reactive (Q) component, only UI* will have the correct sign for the Q term. Conversely U*I produces the opposite, obviously incorrect, sign for Q!

Need an example? ...

Phil -
With all due respect, if you read his posts it should be clear, sw is not looking for what it means, he is looking for why the convention was chosen. I doubt he needs an example.

ice

 

[sweber]

Sweber... You probably misread the original text.

While either UI* or U*I will yield the correct magnitudes for the both Real (P) and Reactive (Q) component, only UI* will have the correct sign for the Q term. Conversely U*I produces the opposite, obviously incorrect, sign for Q!

Need an example?

Regards, PhilCorso

Thanks for your response. It is indeed true what Iceworm says: I know the how, not the why. So thank you for the offer, but an example (unless it clears up the difference) won't be necessary.