How does an electric meter know.....?

[peter d]

Thanks to all for once again making a difficult concept easier to understand.

 

[George Stolz]

Amen to that. Not to say I fully get it yet, but I do appreciate the effort in drilling it into this thick head of mine.

 

[winnie]

Also to clarify, is that reactive power is consumed by non generating loads, it is however not considered usable, with the amount of usefulness being based on the power factor. The closer the pf is to 1.0 per unit or 100%, the less apparent power is wasted. I don't think it is accurate to say that reactive power flows back onto the system since it is consumed in things like transformers, induction motors, and HID lighting.

I think that you are correct in this.

It is fair to say that _real_ energy flows _into_ these loads for part of an AC cycle, and _out_ of these loads for part of the AC cycle. This time rate of this 'back and forth' flow of energy is what we call reactive power. But while this energy is flowing back and forth, the reactive power is moving in one direction.

By convention, reactive power is 'consumed' by inductive loads.

However a load (something that consumes real power) can be a source of reactive power, and it doesn't take a generator to produce 'reactive power'. A synchronous motor with high enough excitation can have a leading power factor, consuming real KW but supplying KVAR to the grid. A capacitor bank does not generate any power but supplies KVAR. On the other side of the coin, an induction generator will produce real KW but consume KVAR.

-Jon

 

[weressl]

How does an electric meter read true power vs. apparent power. We know that if we put a regular clamp on meter on a reactive load without PF correction it will tell us the apparent power. So how does a kilowatt-hour meter "know" the difference?

Even though small motors have lousy power factor, it has been determined that the POCO only can charge for "real" working power delivered to the resi customer. It would be not only undue burden on individual users to attempt power factor correction but wreck havoc on the harmonics, eg. power quality. This will of course change in the future wehn electronic controllers will reach donw to the common user level.

The extra cost of of needing larger wires or capacitors course is borne by the users at any rate. The POCO's like anybody else sell at their cost plus profit.