The real power P is the average power in watts delivered to a load; it is the only useful power. It is the actual power dissipated by the load. The reactive power Q is a measure of the energy exchange between the source and the reactive part of the load. The unit of Q is the volt-ampere reactive (VAR) to distinguish it from the real power, whose unit is the watt. We know that energy storage elements neither dissipate nor supply power, but exchange power back and forth with the rest of the network. In the same way, the reactive power is being transferred back and forth between the load and the source. It represents a lossless interchange between the load and the source.
Would the electric utility meter give zero reading no matter if the load is run for hours? I assume it will give zero reading. I believe the electric utility meter measures the amount of energy consumed on basis of current passing through it in either direction. Even though the load retransmits the absorbed energy but how does the meter know that? Why would it give zero reading?
In what way the retransmitted energy of the load is used by the source? The source is not some kind of rechargeable battery which will get charged up by absorbing the energy which it previously transferred to the load. If the retransmitted energy of the load cannot be utilized by the source, then that energy has become useless.
steveB said:So, and ideal AC source is assumed to provide real power when current and voltage are in phase. It absorbs real power when they are 180 degrees out of phase. And, the source delivers or absorbs reactive power when they are plus or minus90 degrees out of phase. Any other angles involve both real and reactive power.
... I don't get how real power is absorbed by the source when current and voltage are 180° out of phase.
And you say that the source delivers or absorbs reactive power when current and voltage are 90° out of phase. I would have said something like, "the source alternatively delivers and absorbs reactive power", instead. I know it's a minor point but I thought I should confirm it anyhow.
steveB said:OK, my wording here is ambiguous because I'm not clearly defining which current and which voltage... Yes, I like your wording better. Good point.
steveB said:If somehow the load current was 180 degrees out of phase with the load voltage, then the source would be absorbing the power.
But one thing really confuses me. In an ideal LC tank circuit if there are no resistive losses then the energy will keep on moving back and forth between the cap and inductor once the energy has been injected into the system. Suppose, there is an ac generator whose shaft can be rotated manually and to this generator a purely inductive load is connected. Cutting a long story short, from what you are saying I conclude that the generator can keep on rotating on its own because in one half cycle the generator is supplying power to the load and in the next half cycle the load is supplying power to the generator which makes the generator let rotating without any external input. I understand that what I'm saying is very close to the concept of perpetual motion machine but obviously here we are not discussing perpetual motion.
Thank you.
So, let's say we have a load which consumes 15J of real or average power, shuttles 5J of reactive power back and forth, and resistive losses in the wires etc. and friction losses of the system (source plus load) amount to 5J. So, total energy involved is 15+5+5=25J and 20J of energy is spent each second and 5J remains in the system and keep moving back and forth between the load and source. This means the source which could be an ac generator only needs to be fed with 20J of energy each second. If I have it wrong then please let me know. Thanks a lot.
Regards
PG
Thank you, MrAl, Steve.
This is what I like a lot about persons like you. There are not too many persons who could say this in reply to a person like me. I don't know why some people with scientific knowledge become a god unto themselves. In my view humility and kindness is one of the important traits of a learned and civilized person. When I go though my old threads I discover that in the past there have been many instances when I bugged you and MrAl and some other members a lot but there has not been an instance where especially you or MrAl were rude to me. No flattery intended!
I kind of get your point and it seems this text is relevant here. This is what I make out of what you said.
But one thing really confuses me. In an ideal LC tank circuit if there are no resistive losses then the energy will keep on moving back and forth between the cap and inductor once the energy has been injected into the system. Suppose, there is an ac generator whose shaft can be rotated manually and to this generator a purely inductive load is connected. Cutting a long story short, from what you are saying I conclude that the generator can keep on rotating on its own because in one half cycle the generator is supplying power to the load and in the next half cycle the load is supplying power to the generator which makes the generator let rotating without any external input. I understand that what I'm saying is very close to the concept of perpetual motion machine but obviously here we are not discussing perpetual motion. Please help me with it. Thanks.
Regards
PG
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