# why does capacitor supply reactive power?

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• #10612

We all know that capacitor supplies reactive power and Inductor consumes it.

How does this happen?

Why capacitor can't consume reactive power?

#12497

it is interesting to know that Capacitor consumes and inductor supplies.

I have understood negetive power iR-jIXc is due to fact that it is consuming power,but

phase angle of voltage is lagging behind current by 90degree.ie(sqrt(r2+x2) and phase

angle is -90 degree.

So it is just about phase angle,not touching amplitude.

#12505

To try to understand what is happening, I suggest that you stop thinking of reactive power as being consumed or supplied. Nett power is neither consumed or supplied by a  capacitor or inductor. It is supplied in one half cycle of the AC wave and consumed in the other meaning that no net power is consumed. This is the reactive nature of capacitors or reactors as distinct to the passive resistor which consumes power on both the positive and negative half waves.

Now go back to your basic DC theory. When a step voltage is applied to a reactor it opposes the build up of current through it so the current builds up slowly to steady state governed by the RL time constant. When placed into an AC circuit it causes the current to lag the voltage. In contrast the capacitor resists change in voltage across it. You cannot instantaneously change the voltage across a capacitor without infinite current flow. When a voltage is applied to a series RC circuit the voltage instantly appears over the resistor and current =V/R flows. This charges the capacitor and the voltage across it builds up slowly. As the voltage on the capacitor rises, that across the resistor falls and as does the current. The rise in voltage (fall in current) is controlled bt the RC time constant. Thus when put in an AC circuit the current leads the voltage.

You can simply look at this in that the current slowly rises through an inductor when voltage is applied and current slowly falls through a capacitor. They perform in exactly the opposite way to oneanother. Thus application of either can be used to counteract the effect of the other but neither consume of supply real power. Saying that inductors consume reactive power and capacitors deliver it is only another way of saying the they do the opposite to each other.

#12507

Vidhyut S. said:

We all know that capacitor supplies reactive power and Inductor consumes it.

How does this happen?

Why capacitor can't consume reactive power?

Dear ;

First, we know all the Capacitors are ” Loads “ that consume a reactive current, so, the Capacitors are not generator to supply any kind of power.

Assume that we have a pure inductive load and a Capacitor installed to the network, in this case we have the following currents :

– An Inductive current for the inductive load, where this current is ” leading ” by 90°

– A Reactive current for the capacitor load, where this current is ” lagging ” by 90°

so, the vectorial sum for these 2 currents is depending on the value of each one, if the value of the Inductive current is bigger than the Reactive current, the sum will be smaller than the inductive current, that seems as the capacitor supplies the reactive power to the inductive load.

Regards.

#12527

ABRAAHM said:

it is interesting to know that Capacitor consumes and inductor supplies.

I have understood negetive power iR-jIXc is due to fact that it is consuming power,but

phase angle of voltage is lagging behind current by 90degree.ie(sqrt(r2+x2) and phase

angle is -90 degree.

So it is just about phase angle,not touching amplitude.

laalini said

i think reactive power is negligible  because of its complex nature .and it's all about a tendency but physically it may not have any applications.

#12529

As reactive power consumed is given as Q=VIsin(angle betn them)

In pure capacitor in which no resistance,angle between v and i is 90(clockwise angle +).

sin 90=1.That is reacive power is positive.and in power tringle we take it as suplying reactive power.

#12534

Vidhyut's question is as I undertand that Capacitor when used in power factor improving applications,it supplies reactive power and so

compensates for gneration of reactive power by Inductors.

In Q=VISin(phi)–>when phi=-90 ie from if you can remember vector diagram of I & Ic –>it is (-Q )–>so again negative power.so capacitor is supplying reactive power,not consuming.

I dont think reactive power is an imaginary net power zero thing,it cause secondary heat losses for eg while considering a current

pasing through a cable whose reactance is high.

#12537

Thanks…..

I asked this question in relation to power factor improvement.

My doubt is very clear now.It is like Capacitor consumes negative reactive power and so we can

say it supplies reactive power..

#12576

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