AC Power Analysis: Part 1, Basics - Comments

[anorlunda]

anorlunda submitted a new PF Insights post

AC Power Analysis: Part 1, Basics

Continue reading the Original PF Insights Post.

 

[cpscdave]

I never could quite wrap my brain around AC power in school.
I got decent enough to be able to do the problems but never really had a good conceptual picture in my head.

Your first sidebar there addresses the root of my issue. I always understood DC circuits with the water in the pipe analogy.

In the end I took just enough Power classes to realize I didn't want to be an power engineer :)

 

[Jeff Rosenbury]

One reason for using other approaches is that the complex (phasor) math is the result of the general wave equation when the frequency is held constant.

While that's true for typical power applications and can be stretched for harmonics analysis, it is less true for communications theory where frequencies can vary a lot. When one tries phasors on some such problems aliasing errors abound.

Still, this insight is about power, and phasor math is great for power analysis.

 

[jim hardy]

I missed the definition of Left and Right as describing direction of VARS first few times through... My bad not yours. That hung me up...
i see you clearly stated it in thge text just above graphic... Maybe a red and a green arrow on the graphic for dummies like me?
Instantaneous vars must represent energy that goes somewhere. I think of them as stored alternately in capacitive or inductive components of the power system.
Neither Inductance nor capacitance generates heat , so imaginary power is a good name for the energy hiding there .

For those who think in equations,
observe Power = VI
if both V and I are sines (as you established)
sin^2(x) = 1/2 - 1/2 cos(2x)
observe it gained both the DC offset and frequency doubling shown in your graphic.
Which i really like.

 

[jim hardy]

This paragraph confuses me

" Next, think once again of the pictures from above with the red-green areas depicting V*I. Instead of time-varying instantaneous V*I, we will focus on just the whole cycle averages, P (as measured by an AC Watt meter) and Q (as measured by an AC VARs meter). P and Q will be constant in time, but they will vary as we change the phase shift . The meter readings versus ϕ are shown in the table."
Perhaps this verbage would lead the mind more directly ?
"Volts and amps will remain constant in amplitude but their phase will shift. So power P which is VIcostheta , and Q which is VIsintheta, will vary as we change the phase shift."

?

old jim


"

 

[jim hardy]

Those two minor adjustments make part 1 flow for me.

 

[anorlunda]

Thanks Jim, I'll review that paragraph tomorrow. But you have to let go of "instantaneous VARS". they don't exist. The article says that more than once. VARs exist only in whole cycle averages.

 

[mfb]

"The water analogy does not work for AC."
It does work, but you have to switch the direction of water flow (I) and the direction of height difference (U) at the same time.

“this plant supplies enough power for 1000 homes per year.”
Oh, I love it. So after 50 years it can power 50,000 homes.

"I am baffled that 123 years after Steinmetz, tedious methods are still being taught to students, including ), and forms."
Is there something missing? I'm not sure if students always know about complex numbers at the point where AC is introduced. I had some basic AC stuff in school, but no complex numbers as far as I remember (and even if we had them, then certainly later).

 

[anorlunda]

Glad you enjoyed it. Just yesterday, I saw it again in the press -- "this wind farm makes up to 100 MW per year."

"The water analogy does not work for AC."
It does work, but you have to switch the direction of water flow (I) and the direction of height difference (U) at the same time.

Well yes, but you have to embellish the water analogy a lot. Reservoirs on each end instead of a hose with nozzle, something like surge tanks to analogize capacitors, and something like a fluid with intertia to analogize inductors.

 

[Jony130]

Water analogy (just for fun)

As you can see Inductor water analogy is a turbine with a flywheel. The mass of the flywheel determines the value of a inductor inductance.

And the bipolar transistor water analogy

 

[anorlunda]

That's amazing. It does look like fun, thanks for sharing Jony130.

The surge tank is there for C1, and inertia is there in the turbine's flywheel.

Actually, my biggest objection to the water analogy is that based on the questions I see, some water analogy students never learn pressure*flow is power. They think the flow is power, and that electrons in a circuit are like little energy capsules that deliver discrete bits of energy to the destination. Even Jony130's clever water circuit does not help anyone visualize power versus time, only water flow versus time.

The whole thing is exacerbated because teaching the water analogy is often 100% verbal, when pictures are really needed to clarify. Jony130's is an example of a very clever circuit, but too difficult to explain without a picture.

 

[makemoneyu]

Thanks for sharing your knowledge

 

[jim hardy]

Just found an interesting article about HVDC
http://www.lead-central.com/AssetManager/02427e68-6f15-4f3a-9749-d37abf613741/Documents/ABBReview/ABB-1259-WPO_60_years_of_HVDC.pdf
good introduction for those of us who never worked with it

looks like it's coming our way