True Power-- Math Question
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 18th October 2010, 06:45 AM #11 wdcw   diyAudio Member   Join Date: Jul 2010 Hi again Irakli, In your equations you get IT = 46.06A. Using the following calculation I seem to get a different number. (I1= 23A, I2 = 30.9A, I3 = 13.52A) IT = sqrt I1^2 + (I2 - I3)^2 = sqrt 23^2 + 30.9-13.52^2 = sqrt 529 + 302 = sqrt 831 = 28.8A Could you have a look & see what you think, I think this may be correct because IL Lags being Inductive where Ic Leads being Capactive so you have to have (IL - Ic) or (Ic - IL). I just edited this as I had some silly errors! Cheers Last edited by wdcw; 18th October 2010 at 07:12 AM. Reason: Typo Error
Elvee
diyAudio Member

Join Date: Sep 2006
LTspice lets you quickly measure and calculate currents and (active) powers, so that you can easily check your calculations.

Here for instance, the total active power is 10.141KW (Alt + left click on V1, then Ctrl + left-click on the waveform name) (the display window has to be a multiple of the period)
Attached Images
 PFcalc.GIF (34.0 KB, 64 views)

 18th October 2010, 11:04 AM #13 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Elvee, can LTspice be asked for the two power values? the 10.5kW & the 10.2kW. I think your first question and answer are the accurate value of 10.14kW for the lower power of 10.2kW. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
wdcw
diyAudio Member

Join Date: Jul 2010
Irakli,

I have just drawn a Phasor diagram which I will attach to see if firstly if I could & then to see how things added up by using this method.

Your math is correct by the look of it, my calculation above would be correct only with a Purely Inductive & Capacitive Impedance or maybe I should say Inductive Reactance & Capacitive Reactance in that case XL & XC ?.
Then there would be a 90deg current Lag for the Inductive Part & A 90deg Lead for the Capacitive part. You would have to subtract one from the other in that case only.

From the Phasor diagram I get 46.16A which is very close to what you said & what LTSpice says.

I think I have the Phasor diagram drawn correctly if anyone can see I haven't done it correctly could they let me know please.

Elvee,

Yes LTSpice is a great Tool, I use it a lot but am still just learning about all this so any hints etc are greatly appreciated.

Cheers
Attached Images
 PHASOR.gif (8.3 KB, 55 views)

Last edited by wdcw; 18th October 2010 at 12:10 PM.

Elvee
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by AndrewT Elvee, can LTspice be asked for the two power values? the 10.5kW & the 10.2kW. I think your first question and answer are the accurate value of 10.14kW for the lower power of 10.2kW.
Basically, LTspice can provide directly actual physical quantities, and P, the active power is one of them (10.141KW).
But S, the apparent power has no physical existence: it is just a convention that we, humans, find convenient to use, and it isn't available as a native function.
However, LTspice can also do maths, and any arbitrary function could be programmed if needed.

Irakli
diyAudio Member

Join Date: Aug 2004
Location: washington, DC
Quote:
 Originally Posted by wdcw Irakli, I have just drawn a Phasor diagram which I will attach to see if firstly if I could & then to see how things added up by using this method. Your math is correct by the look of it, my calculation above would be correct only with a Purely Inductive & Capacitive Impedance or maybe I should say Inductive Reactance & Capacitive Reactance in that case XL & XC ?. Then there would be a 90deg current Lag for the Inductive Part & A 90deg Lead for the Capacitive part. You would have to subtract one from the other in that case only. From the Phasor diagram I get 46.16A which is very close to what you said & what LTSpice says. I think I have the Phasor diagram drawn correctly if anyone can see I haven't done it correctly could they let me know please. Elvee, Yes LTSpice is a great Tool, I use it a lot but am still just learning about all this so any hints etc are greatly appreciated. Cheers
Hi Again,

Your vector (phasor) diagram is correct. That's the right way to do things. By the way, complex numbers are nothing more (for our purposes) then another representation of these vectors.

I strongly recommend not to use simulators at all at this stage and do everything with paper, pencil and simple scientific calculator.

first, with simple circuits the math is not complicated. Most importantly, you will learn how things really work (good example is right here spending time with vector diagrams and understanding phase relationships). Also you will learn how to break big problem to smaller ones and isolate the essential things. All this is called good engineering and you can't learn it relaying on simulators.
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 18th October 2010, 09:35 PM #17 wdcw   diyAudio Member   Join Date: Jul 2010 Hi Irakli, Yes, I understand what you mean about learning the math etc, this is a good example, I didn't quite understand until I drew the Phasor-Vector diagram. I am happy I got it correct as it helped me a lot, I am trying to learn some complex math along the way & using the phasor diagrams may help me with this also. I also understand that using programs to work things out for you can hinder the full understanding of things, that's why I thought it best to ask for some help to work this out properly. Thanks for your help with this, I learnt a lot from this, I also learnt how much I don't know Thanks again Elvee, Yes I understand what your saying about LTSpice, I think I also have to learn how to do the math etc to be able to use this correctly as well. It's all very interesting, I only wish I had learnt more some years ago. Thanks everyone, I actually learnt something Cheers
Elvee
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by wdcw Elvee, Yes I understand what your saying about LTSpice, I think I also have to learn how to do the math etc to be able to use this correctly as well. It's all very interesting, I only wish I had learnt more some years ago.
Remember what I wrote:

Quote:
 LTspice lets you quickly measure and calculate currents and (active) powers, so that you can easily check your calculations.
I suggest you use it as a means of control after you've done the maths.

Just playing aimlessly with it won't be of any good.

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