this is intended to be a new thread arising from http://www.control.com/thread/1308072497#1308227978

Phil, can you please tell me where you disagree with the following?

1. A capacitor stores energy - the amount of energy in joules is proportional to the square of voltage.

2. If an alternating voltage is applied to the capacitor, the energy in the capacitor will vary from 0 at 0 V to a maximum at the positive peak (90°); to 0 at 180° and back to maximum at the negative peak (270°) and returning to 0 at the start of the next cycle.

3. Since the energy stored in the capacitor is varying with time, there must be a change of energy over the cycle, and since power is defined as the rate of change of energy, this change in energy can occur only if there is a power flow.

4. The power flow will start at 0 at 0°, reach a maximum flow into the capacitor at 45°, and fall back to 0 at 90°. It will then reverse and flow out of the capacitor over the next 90°, then again fall back to 0. This cycle will then repeat.

5. The power flow into the capacitor is therefore alternating from positive to negative and back again, twice per cycle of applied voltage.

6. Similarly, an inductor stores energy - the amount of energy being proportional to the square of current.

7. If an alternating voltage is applied to an inductor, the current flowing will be such that its rate of change is proportional to the applied voltage. While the applied voltage is positive, the current into the inductor is increasing. Zero voltage will coincide with a current peak, and peak voltage will occur at 0 current.

8. Since the energy stored in the inductor is varying with time, there must be a change in energy over the cycle, and since power is defined as the rate of change of energy, this change in energy can occur only if there is power flow.]

9. For the inductor, zero voltage occurs at peak current from 7) above. Therefore the energy stored in the inductor is at a peak when the applied voltage is zero. At a voltage peak, the inductive current is 0, and the energy stored in the inductor is also 0.

10. Power therefore flows out of the inductor while supply voltage is increasing from 0 to a peak, and into the inductor while voltage is falling from a peak to 0.

11. If the capacitor and inductor are connected in parallel, the polarities of power flow at any instant are opposite, so the power absorbed by the capacitor while it is charging can be partly supplied by the inductor as its current decays.

My definition of reactive power is that it is the power flowing in a circuit to allow changes in the energy contained in capacitive and inductive elements as the voltage and current vary over a cycle. The average value of reactive power over a cycle is zero, as the energy that flows into a capacitor while voltage magnitude is increasing from 0 to a peak will flow out again as the voltage returns to 0. However, to say that reactive power therefore does not exist would seem to imply that in fact an AC voltage does not exist - after all, it too has an average value of zero.

I look forward to your reply..
Cheers,

Bruce.

 

Bruce... are you defining "reactive power" as the power loss in the resistive elements between the source and the load?

Regards, Phil

 

No - as I stated, reactive power is the power that flows into a reactive element for part of a cycle, and flows out again for the remainder. A possible analogy is with tidal energy. Energy stored in a tidal basin depends on the height difference compared to mean sea level. Changes in level in the basis require water to flow in and out. Since the average amount of water in the basin does not change over a day, the average inflow is zero, but there is certainly sufficient power in the flow of water in and out to make tidal power a good practical source of energy.

Bruce.

 

Bruce, I propose putting the subject to rest with a simple vote of the forum members! Anyone expressing an interest in stopping the constant bickering (remember my hermit story) need only consider the following question:

Should the entity Volt-Amperes, reactive, be addressed as "VAr" or "reactive power"? Those in favor of "VAr" use the thumbs-up icon associated with this thread; those in favor of "reactive power" use the thumbs-down icon!

Sincerely, Phil Corso

 

I am trying to get my head around why you feel that reactive power is fiction. If you can let me know where you disagree with the points I made, I can possibly start to see where you are coming from.

I do not think that a unilateral decision by the participants in this forum will make a lot of difference - most of the questions come from newbies who just want some more detailed explanations of what has probably been glossed over in their basic training (if they've had any at all, to judge from some recent posts!).

Bruce.

 

There's this well-accepted thing called a power triangle, and I believe you've referred to it many times.

One side is Watts, real power.

The hypotenuse is VA, apparent power.

And the other side is VAr, imaginary power.

If there is a power triangle, Phil Corso, how can one side not be power if the other two sides are power?

 

Thomas Edison would have agreed with you whole heartedly...