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How to calculate active power from waveform.

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Dec 4, 2010
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I am simulating a reactive load in LTspice. I got a voltage and current waveform with some phase delay due to reactive nature of the circuit. I want to calculate the total power given to the load. I have attached a voltage and current waveform measured across load.


Compute integrate V*I over all the sampled data and Pd is the net result of dissipated power, which mostly but not all cancels out, as phase is not exactly a phase shift is -76 deg. rather than -90 so pf=0.24 . My guess VAR = 35V*0.35A= 10VAR-pk or 7.07VAR (rms pk) *0.25p.f. = 1.77W over 10us or 17uJ.

Since the VA product oscillates + and - the VI product is mainly reactive VAR.

However the dampened energy is dissipated, it is not all done for this component as it is mainly reactive. (C) But you can extract the data and compute in a spreadsheet to compute exact power absorbed. I just did a rough calc.

Compare the ratio of real VI product power to apparent |VI| power to reduce to dissipation factor or calc p.f.


Compute integrate V*I over all the sampled data and Pd is the net result of dissipated power, which mostly if not all cancels out.
View attachment 121405

I think i should compute V*I over all the sample data and than i should calculate the average of all sampled multiplication which gives average power. Am i right? Because if i integrate V*I than i should get energy instead of power. Is my understanding correct?

In Ltspice, there is a power probe we can use to measure dissipited power within device. It gives average power. What kind of power it indicates? Active, reactive or apperant power?

Sure use the power probe if it includes both elements of C,Rs.
Instantaneous power lost requires knowing the phase or p.f. over a cycle for each instant in time, thus extracting separating real power (t) from apparent |power (t)| requires computation of energy then assuming linear components and factoring power factor * apparent power.

There may be more elegant but simple ways.

e,g, compute the VA product over 1 complete cycle using VA product where polarity is negative if either V or I is negative but not both.
Then VA²= W² + VAR² where Real Power = W = Apparent Power x cosΦ
power factor=p.f.= cosΦ where Φ= ΦV-ΦI from peaks or zero crossings of same polarity.
RMS is the root sum of each sample ² over a cycle or multiple cycles but with polarity retained as j²=-1
It's been 4 decades since I did this.
I see Φ difference between V=0 I=0 Did you compute Pd(t)? and Energy?

Yes, actually in LTspice the power probe can directly compute P(t) by multiplying V(t)*I(t), It also shows average power over entire period of time and Energy in terms of joule. The waveform is attached. I also have three text vector files containing data for voltage, current and power but don't know how to upload here. VI_Power.png

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