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Power Calculation in DC

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seyyah

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With a pure voltage and current it's easy to calculate power. In AC also we can calculate active, reactive power etc. But what about DC signals that aren't pure? E.g. a rectified sine wave with a dc offset? What way do we follow to calculate power in these type of signals? Similar to AC? Do current and voltage lags or leads in this type of DC? Does phase angle exist? Please give me the necessary information to complete my knowledge. Thank you.
 

Hi.

Power density at a time is given by P = I * V, regardless of AC or DC. If you integrate the power density over an interval of time, you can get the energy spent over the interval of time. The average power is obtained by dividing the energy by the time interval. You can apply this procedure to any time of signals, e.g., AC or DC, with or without lags, signals of arbitrary shape.
 

You can calculate the rms voltage and current and calculate the average power using Vrms*Irms.
Or you could reduce the signal to its fourier components and then calculate the power.
 

What i really try to learn is; do reactive, active power apply in this condition? Because i'm trying to calculate the resistance of a circuit. I have the information of current and voltage and so power. If there are relations between impedances like in the ac circuits then i will easily calculate the resistance. Consequently, I'm trying to determine the relationship between the terms that can help me to calculate resistance.
 

seyyah said:
What i really try to learn is; do reactive, active power apply in this condition? Because i'm trying to calculate the resistance of a circuit. I have the information of current and voltage and so power. If there are relations between impedances like in the ac circuits then i will easily calculate the resistance. Consequently, I'm trying to determine the relationship between the terms that can help me to calculate resistance.
Click to expand...

Well in such a case you will have to calculate the fourier components of your signal.
For example in an inductor I=1/L(∫Vdt).
So if V has components of frequencies w1 and w2 then you can break down the V-I relationship into
I=1/L(a1∫sin(w1)dt +a2∫sin(w2)dt)
So you can analyze the circuit seperately for each of the frequencies. Remember that your reactances will be different for each of these frequency components i.e (jw1)L and (jw2)L respectively..
Then add up the currents that you can get from each of these seperate compnents.
 

usernam said:
Well in such a case you will have to calculate the fourier components of your signal.
For example in an inductor I=1/L(∫Vdt).
So if V has components of frequencies w1 and w2 then you can break down the V-I relationship into
I=1/L(a1∫sin(w1)dt +a2∫sin(w2)dt)
So you can analyze the circuit seperately for each of the frequencies. Remember that your reactances will be different for each of these frequency components i.e (jw1)L and (jw2)L respectively..
Then add up the currents that you can get from each of these seperate compnents.
Click to expand...

Sorry but i couldn't understand how can i calculate the resistance (R) ? I forgot to mention; i also know L.
 

seyyah said:
usernam said:
Well in such a case you will have to calculate the fourier components of your signal.
For example in an inductor I=1/L(∫Vdt).
So if V has components of frequencies w1 and w2 then you can break down the V-I relationship into
I=1/L(a1∫sin(w1)dt +a2∫sin(w2)dt)
So you can analyze the circuit seperately for each of the frequencies. Remember that your reactances will be different for each of these frequency components i.e (jw1)L and (jw2)L respectively..
Then add up the currents that you can get from each of these seperate compnents.
Click to expand...

Sorry but i couldn't understand how can i calculate the resistance (R) ? I forgot to mention; i also know L.
Click to expand...

Exactly what do you know?
Do you know the voltage across the resistor and the current through it?
If so Ohm's law and therefore R=V/I still holds.
 

Hi

one way to calculate the resistance is to measure the REAL (active) power and the RMS current through the load. then we have: R = P/I².

R = Load Resistance (Load is assumed to modeled such a resistance series with an inductance).
P = REAL (active) consumed in load.
I = RMS value of current.
V = RMS value of voltage.

Note that the REAL power consumed in load is NOT equal V×I. you must a watt-meter or make one to measure the REAL power.
 

Ok. i don't know real power(P). And i can't measure or calculate it. I only know the voltage across the L-R circuit. I know the current flowing through the circuit. I can calculate RMS values. When i multiply Irms and Vrms than i get apparent power, which doesn't help me to find R. I know the frequency of the voltage and current(or current ripple). Voltage is rectified sine wave(but it has harmonic components also due to pwm) and i know the frequency of the fundamental. But current is unknown since L is variable. It may be rectified sinus, it may have dc component etc. So what are my choices to calculate the resistance?
1) Calculate real power then pull out R from it, but how?
2) Find the voltage drops on the L then pull out the voltage drops on the R then find R. But how? How can i calculate voltage drop on the L? By this? VL=L(di1/dt) + L(di2/dt)... How can i know which harmonics are included? Also seems not practical.

May be i'm confused and can't see the point.
 

How can i know which harmonics are included?
Click to expand...
In fact ALL harmonics are included. The current will have the same harmonics as the voltage.If you break down the input voltage in its Fourier components (harmonics) for EVERY harmonic you can apply the usual circuit analysis in the frequency domain. In fact in your case it is enough if you know the voltage and current amplitude of the same harmonic (even dc). And if you find the current for every harmonic frequency, you can find the total current by summing. It all depends on the superposition principle, according to which you can think that each time only one voltage harmonic is the source and all other harmonic sources are short circuits (dead) and analyse the circuit as if only this harmonic existed. And in the end you sum up the results.
 

i think you should try to get real power. explain your circuit more to find a way to measyre real power.
 

As i said, circuit is a simple L-R circuit. How will i obtain the real power? I think i should go on this.
 

If for a frequency n*wo the real power is P(n) then the total real power is the sum of all the P(n)s from dc to infinite. I dont know if a wattmeter would show the total real power. But if you have the value of the current and the voltage and you can perform a Fourier analysis, I insist that you can find the R by analyzing the circuit only for a given frequency w1=k*w0. The simplest would be to measure the dc components of the current and voltage and divide them!
 

i think that you will not be able to find R, using this circuit except that you find real power dissipated in the load.
 

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