power factor in electronics

hello to all


I am an electrical and electronics engineer and I am here with one basic question if you guys appreciate it please reply to it in detail.

the query is that in electrical field with the Synchronous generator, Induction Motor and transmission line there is on most important factor related to the
power flow in this field and that is the power factor. In electrical field where we deal with the high voltages the power factor is very crucial term.

But in electronics where we have capacitors and inductors and resistors in awesome amount we don't talk about the power factor we simply calculate the power by the formula P=VI and not P=VIcos(phi).

So guys can you people tell me that why we don't consider the power factor term in the electronics field.


Thank you

P=VI is correct for DC and instantaneous quantities, but not for complex AC magnitudes. Your question doesn't refer to the possibly different meanings. In fact, power factor is applied in electronics where appropriate.

In electronics the power levels are much lower than in the case big electrical motors. It is an economic consideration

klystron can you elaborate your point in more detail rather than just typing a single line please.

I think you should consider this link too:

**broken link removed**


here the components are all of very small values in the electronic ranges and still the power factor angle is quite considerable.


Thank you

---------- Post added at 17:30 ---------- Previous post was at 17:28 ----------

FvM said:

P=VI is correct for DC and instantaneous quantities, but not for complex AC magnitudes. Your question doesn't refer to the possibly different meanings. In fact, power factor is applied in electronics where appropriate.

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I think I have got your point FvM but would you like to shed some more light in detail on your point I am getting your point but I have not got of it that much so I need pretty good expalanation on it.


Thank you

---------- Post added at 17:31 ---------- Previous post was at 17:30 ----------

I think I have got your point FvM but would you like to shed some more light in detail on your point I am getting your point but I have not got of it that much so I need pretty good expalanation on it.


Thank you

piyushpandey,

I am an electrical and electronics engineer and I am here with one basic question if you guys appreciate it please reply to it in detail.

Click to expand...

If you are what you say you are, why are you asking such a basic question?

But in electronics where we have capacitors and inductors and resistors in awesome amount we don't talk about the power factor we simply calculate the power by the formula P=VI and not P=VIcos(phi).

Click to expand...

That is completely false. The first formula you quoted is only good for DC.

So guys can you people tell me that why we don't consider the power factor term in the electronics field.

Click to expand...

Can you prove the above statement? What about resonance and antenna tuning?

Ratch

you are right ratch


you now let me to discover more about my question, So I am first of all thankful to you and yeah I am electrical and electronics engineer , and being an electrical or electronics engineer does not mean that you know everything, SO it is possible in many cases that people just don't notice small things and when they encouter that thing regularly than they notice that ..... oooohhhhhh ............. I missed that , and how I missed that and they start thinking.

SO same thing happened to me , and I started thinking about it and therefore alongwith thinking about the solution I also thought that why not ask you people too, after all we all are here to cooperate and help each other for the problems which I think does not matter on the size of someone's designation or knowledge or question.


right



Thank you

The theory of the power factor is the same for small and large load currents. The economic impact of the power factor is major in high power systems. Electricity is sold based on the virtual power usage (VA) but the user only gets the benefit of the real power (W).

klystron,

Electricity is sold based on the virtual power usage (VA) but the user only gets the benefit of the real power (W).

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What is virtual power? The consumer is billed by the amount of electrical energy they use. They are not charged by how fast they consume that energy (power). Energy companies do not like low power factors because they have to supply higher currents to maintain the same power delivery as compared with a power factor of one. That causes IR losses in the delivery lines that they cannot bill to the customer.

Ratch

The consumer is billed by the amount of electrical energy they use. They are not charged by how fast they consume that energy (power).

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This applies at least to private consumers. Industrial consumers are possibly additionally charged for exceeding a certain power limit or sinking/sourcing reactive power beyond an previously agreed amount.

Hello

Power Factor, as used today is defined as:

PF = Preal/(Irms*Vrms).

Preal = (1/T)*integral( V(t)*I(t)*dt ) for one period. So this is the real power that is supplied to the load.

In a system with only sinusoidal voltage and current (that is a linear load), PF = cos(phi). In a system with many harmonic currents (bridge rectifier with electrolytic capacitor), you need the full definition.

As mentioned by Ratch also, the energy companies don't like to have current in their infrastructure that doesn't carry power to the customer. Especially the higher harmonics introduce significant power loss in transformers (proximity loss).

Regarding small signal AC analysis, we use the impedance concept to specify phase. You can specify an impedance via magnitude ( |Z| ) and phase (Arg(Z) ). So in our calculations we take phase shift into account, but we don't call it power factor.

Resuming...

Power factor - the relation between the real power and reactive power...

real power is the power consumed when both Voltage and Current are constant in time domain

the reactive power the power consumed in capacitive or/and Inductive circuit , when you have variations of Current in time (i.e. inductors) and when you have variation of voltage in time (i.e. capacitors)...

With this said,
In AC both current and voltage have variation in time
In DC you have voltage and current variations in transitions.. like when you turn on or off the coil of a relay, or when you wave a PWM, a digital signal etc....

Reactive Inductive power is bigger as the relation dI/dt grows....
Reactive Capacitive power is bigger as the relation dV/dt grows...

when a circuit inductive reactance and the capacitive reactance are of equal magnitude ωL = 1/ωC,, you have resonance...

If a circuit with no resistance (i.e. LC circuit) is in resonance , you have a shorted circuit...

mgate,

Power factor - the relation between the real power and reactive power...

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What relation is that?

real power is the power consumed when both Voltage and Current are constant in time domain

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In AC circuits, voltage and current are not constant, yet energy is lost from the circuit. How does that square with the above statement? By the way, power is not consumed, energy is. Power is a rate, and is not consumable.

the reactive power the power consumed in capacitive or/and Inductive circuit , when you have variations of Current in time (i.e. inductors) and when you have variation of voltage in time (i.e. capacitors)...

Click to expand...

The energy of reactive power is never consumed. It is given back to the circuit. Reactive power is the rate at which energy is stored and released. Real power is the rate at which energy is lost from the circuit.

In AC both current and voltage have variation in time

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So does a turn on/off transient that is not AC.

If a circuit with no resistance (i.e. LC circuit) is in resonance , you have a shorted circuit... .

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If it is a parallel circuit, you have a open circuit.

Ratch

mgate said:

Resuming...

Power factor - the relation between the real power and reactive power...

Click to expand...

To be honest, I don't think this is true, instead of reactive power, you should use the apparent power (sinusoidal wave forms).

With the current electronic equipment, it is better to use the product of Vrms and Irms instead of apparent power, as this definition incorporates harmonics also.

Ratch... I agree with almost everything that you said... I'm sorry for being lazy one the descriptions I've made...

Yes ... Power is energy per unit of time... and its the energy that is being consumed....

About AC ... AC general calculations are made in permanent regime:

voltage and current variation in time are considered to be sinusoidal (witch is a simplification), with a time constant phase shift \[\varphi\]...

\[ v(t) = V_{max} . \cos{(\omega{} t + \theta{}_v)}\]
\[ i(t) = I_{max} . \cos{(\omega{} t + \theta{}_i)}\]

or

\[ v(t) = V_{max} . \cos{(\omega{} t + \varphi{})}\]
\[ i(t) = I_{max} . \cos{(\omega{} t )}\]

with

\[\varphi{} = \theta{}_v - \theta{}_i\]

So the power in permanent sinusoidal regime is given by

\[p(t) = v(t) . i(t) = V_{max} I_{max}. \cos{(\omega{}t + \varphi{}) . \cos{(\omega{}t)\]


considering this trigonometric relation,
\[\cos{(\alpha{})}\cos{(\beta{})} = \frac{1}{2}\cos{(\alpha{}- \beta{})}+ \frac{1}{2}\cos{(\alpha{} + \beta{})}\]

the instantaneous power in permanent sinusoidal regime can be write in:
\[p(t) = \frac{ V_{max} I_{max}}{2} . \cos{(2\omega{}t +\varphi{})} + \frac{ V_{max} I_{max}}{2} . \cos{(\varphi{})}\]

and considering this trigonometric relation,
\[\cos{(\alpha{}+\beta{})} = \cos{(\alpha{})}\cos{(\beta{})} - \sin{(\alpha{})}\sin{(\beta{})}\]

the instantaneous power in permanent sinusoidal regime can be write in:
\[p(t) = \frac{ V_{max} I_{max}}{2} . \cos{(\varphi{})} + \frac{ V_{max} I_{max}}{2} . \cos{(\varphi{})}. \cos{(2\omega{}t)}- \frac{ V_{max} I_{max}}{2} . \sin{(\varphi{})}. \sin{(2\omega{}t)}\]


the medium active power is given by the integration in time of the instantaneous power, in a period duration, divided by its duration..
\[ P= \frac{\int_0^T {p(t)}dt}{T} = \frac{ V_{max} I_{max}}{2} . \cos{(\varphi{})}\]

Considering the previous equation, the instantaneous power in permanent sinusoidal regime can be write in terms of the medium active power by:
\[p(t) = P + P. \cos{(2\omega{}t)}- \frac{ V_{max} I_{max}}{2} . \sin{(\varphi{})}. \sin{(2\omega{}t)}\]

.. with this said, if you consider a pure resistive circuit, where current is always in phase with the voltage, the phase shift angle will be zero, meaning
that the instantaneous power in permanent sinusoidal regime in a pure resistive circuit will be given by:
\[p(t) = P + P. \cos{(2\omega{}t)}\]
witch is called the real instantaneous power..

the reactive instantaneous power is this way given by
\[p(t) = - \frac{ V_{max} I_{max}}{2} . \sin{(\varphi{})}. \sin{(2\omega{}t)} = - Q . \sin{(2\omega{}t)}\]

where Reactive power Q is
\[ Q = \frac{ V_{max} I_{max}}{2} . \sin{(\varphi{})}\]


the complex power S is given by:
\[ S = P + jQ \]


the aparent power witch is the absolute value of complex power is given by
\[ |S| = \sqrt{ P^2 + Q^2 }\]

___________________________________________
About the power factor..
\[ P_f = \cos{(\varphi{})} \]

You are right.. Is the relation between half the instant active power wave amplitude and the medium active power


DC... In my opinion, if we use Fourier Series to describe the voltage and current of a dc signal, we will end up with similar conclusions... I just don't know If we can call the same names... for example the kickback current of a coil is a reactive current for me...

---------- Post added at 20:23 ---------- Previous post was at 20:08 ----------

WimRFP said:

To be honest, I don't think this is true, instead of reactive power, you should use the apparent power (sinusoidal wave forms).

With the current electronic equipment, it is better to use the product of Vrms and Irms instead of apparent power, as this definition incorporates harmonics also.

Click to expand...

I totally agree with you

piyushpandey said:

................

So guys can you people tell me that why we don't consider the power factor term in the electronics field.

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Not sure if your question was adequately answered, but the convention is that the term "power factor (PF)" is generally only used with regard to mains power, since that is where it can have a significant effect on the power losses in the generating system.

For electronic circuits we do deal with phase shifts due to reactive components but it is seldom referred to as a power factor, only as phase shift between voltage and current, or real and reactive power or current. In RF transmission lines the phase-shift is related to the SWR.

piyushpandey said:

So guys can you people tell me that why we don't consider the power factor term in the electronics field.

Thank you

Click to expand...

With line electronics, the power supply is typically responsible for presenting the line with what appears to be a resistive load. In the past, this wasn't done at all -- a simple diode+capacitor would be used. This ends up having very poor power factor because current doesn't flow until near the peak of each cycle. This leads to a high peak current, and a "high ripple current" into the energy storage capacitor. More modern supplies use an "active PFC" scheme to supply the capacitor with a current that follows a sine wave (abs(sin(x)) better, allowing for a PF of near 1.0. While the power factor might not be directly discussed, the ripple voltages and currents are discussed. These are similar concepts -- the supply only provides a DC voltage, so only DC (average) current can contribute to power. any AC (ripple) current will only increase stress on components (eg capacitors that have ripple current limits).

Past this point, there isn't much that can be done. The circuits will tend to draw the amount of power they draw, and will do so when appropriate. Its generally better to draw low current vs low ripple current in modern times. However, some designs do stagger the turn on/off times of high power circuits in order to prevent sudden changes in output current.