Apparent Power

Polymorph

Joined Feb 6, 2009
6
I am in the USA so my House Voltage is 120 Volts.
So when I take my VoltMeter to my Wall Outlet and it Reads 120 Volts this is the RMS Voltage.

Now if I Plug a Fan in this is an Inductor and will have some Loss of Energy.

This Loss is Reactive and is V/A witch is a Wattless Watt.
As several have pointed out, it is volts times amps, usually just written as VA and pronounced "volt-amp". Or in the case of purely reactive, VAR for volt-amp-reactive.

No, a fan is a partly inductive load. VAR is not loss of energy. It is energy that is supplied during part of the cycle, and returned during another part of the cycle. It is Watts aka real power that is the "loss" of energy, although I'd prefer to call it consumed energy as it does actual work moving air.

Apparent Power or VA is the whole enchilada. Watts and VAR are mathematically lifting out what part of the power is not returned (in the case of Watts aka real power) and what part is returned without being consumed (VAR or reactive power).
 

Thread Starter

biferi

Joined Apr 14, 2017
342
I think I have it?

Real Power is in Watts and Reactive Power is in V.A.R and Apparent Power is in V.A. am I Right?
 

dcbingaman

Joined Jun 30, 2021
798
I think I have it?

Real Power is in Watts and Reactive Power is in V.A.R and Apparent Power is in V.A. am I Right?
According to Polymorph that is correct. I was not aware they had different nomenclature. Looks like I also get to learn a few things about proper nomenclature.
 

Thread Starter

biferi

Joined Apr 14, 2017
342
I Live in the USA.
So if I Take a Reading from my Outlet I get 120 Volts.
But if I do the Math I will get 170 Volts Peak Voltage.

And Yes I know what this is.

And I know the 120 Volts is the RMS Voltage.
I know the RMS Voltage is the Effective Voltage.

So am I Right RMS Voltage is Real Voltage?
 

dcbingaman

Joined Jun 30, 2021
798
I Live in the USA.
So if I Take a Reading from my Outlet I get 120 Volts.
But if I do the Math I will get 170 Volts Peak Voltage.

And Yes I know what this is.

And I know the 120 Volts is the RMS Voltage.
I know the RMS Voltage is the Effective Voltage.

So am I Right RMS Voltage is Real Voltage?
It is the 'average' of the square of the voltage because power if V^2/R. To get it you normalize a sine wave to unit 1 peak and take the integral for 1/2 cycle or the square of the sine waveform for a normalized waveform of 1 unit for 1/2 a cycle.
You end up with 1/sqrt(2).
 
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sparky 1

Joined Nov 3, 2018
732
In Addressing the original question it is common initially to approach this and get confused but improves with review.
The terminology verbally has a tendency to be confusing but more so when not used often, in a mathematical expression it may become more straightforward in an introduction to power factor, keeping the terms straight, making a circuit diagram takes a little practice but it does have a repeatable solution.
 
Am I Right if I take RMS Voltage Times RMS Current = Real Power?
Not always when either v(t) or i(t) are not sinusoidal.

Another way of looking at RMS voltage and current is if you had a resistive load the RMS voltage and the RMS current woud be equal to the DC voltage and current for the same amount of heat.
 

crutschow

Joined Mar 14, 2008
31,116
Am I Right if I take RMS Voltage Times RMS Current = Real Power?
Only if the two are in phase.
If they are out of phase then you have reactive power.
That is why measuring voltage and current separately with an RMS meter cannot tell you how much of the power is real and how much is reactive. You need to also include the phase, which is what a power meter does.
 

Thread Starter

biferi

Joined Apr 14, 2017
342
Ok Tell me if I have this Right?

If you have an all Resistive Circuit RMS Voltage X RMS Current = Real Power in Watts.

If you have an all Reactive Circuit RMS Voltage X RMS Current = Volts Amps Reactance in VAR.

Do I have just these Right?
 

Thread Starter

biferi

Joined Apr 14, 2017
342
Now I know this may not happen but let Say you have an all Reactive Circuit what is the Formula to Find the VAR?
 

Thread Starter

biferi

Joined Apr 14, 2017
342
Just Tell me if I have this Right?

If I have an AC Circuit that has both Real Power and Reactive Power and I want to Find the Apparent Power I do this?

I take Real Power to the power of 2. Plus Reactive Power to the power of 2. then take this Answer and get the Square Root of it?
 

dcbingaman

Joined Jun 30, 2021
798
Just Tell me if I have this Right?

If I have an AC Circuit that has both Real Power and Reactive Power and I want to Find the Apparent Power I do this?

I take Real Power to the power of 2. Plus Reactive Power to the power of 2. then take this Answer and get the Square Root of it?
Yes, that is correct. It is the absolute magnitude of the vector or |P|
 

Thread Starter

biferi

Joined Apr 14, 2017
342
Thanks for getting back to me.

Now in the Formula I know Reactive Power is a Wattless Watt so it has no RMS.

But in the Formula I take Real Power to the power of 2. then Add Reactive Power to the power of 2. then take this Answer and get the Square Root of it should I use the RMS of the Real Power?
 

dcbingaman

Joined Jun 30, 2021
798
Well, from a dimensional analysis perspective you have watts as the unit. The square root of a sum of squares returns to the original units (in this case watts). So from that perspective it has to be the same units you started with. Otherwise the idea of 'dimensional analysis' would have an exception to its own rules and be false.
The interesting thing about the square root, it has two answers! One positive and one negative. So much for all math making sense. We naturally throw out the negative answer, but on what formal basis? None, except common sense from what I can see. There is one answer people rarely consider: Any physical process can theoretically run in the other direction. No laws of physics are violated when you run time backwards. Thus the negative answer is simply what you get going from 0 to -t time!
 
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