negative power in inductors

[Disha Karnataki]

inductors are responsible for lagging power factor & it's said that electric companies penalize for us due to this lagging power...
so when we use inductors the things that occur are:::
1)most of the power is being stored in the inductors as magnetic field & then returned to the grid --i really didn't get this point if a component is using some power & later returning it back it's the most awesome thing right???
2) there is negative power developed due to inductor ?????????? i was really baffled when this statement was told to me... though i have finished studying about inductors but i haven't still come up to any conclusion...please if someone can give physical meaning of this, i mean please don't include any mathematical equations........

 

[barry]

Not exactly. It's just that inductors mess up the way power meters work so that the 'apparent' measured amount of electricity used is not the actual amount used.

 

[FvM]

Don't stumple upon your sloppy usage of technical terms. Lagging power factor hasn't to do with "negative power". Reactive power is just oscillating energy, no net power is transferred from or to the load.

 

[permute]

The classic analogy of an inductive load:
You want cement, so you have a cement truck come out to your house. From there, you use 5 liters of cement and send the truck back. The next day, you do the same, etc... The cement company charges by volume of cement. Clearly the cost to the company is much higher when it must transport a huge amount of concrete (power) to the end user only to have most of it sent back.

In the fully reactive load analogy, all concrete (power) is returned. In the wire-loss case, some concrete spills or spoils during the round trip.


This has major infrastructure issues, as low power factor loads require larger wiring to handle the higher current. (or in the analogy, low power factor requires larger trucks.)

 

[Disha Karnataki]

so is that the power that is returned back to grid is called as negative power??

- - - Updated - - -

fvm if u want to satisfy yourself then u can go & read the link given below:
http://en.wikipedia.org/wiki/AC_power
here they have mentioned that if we consider a ""pure inductor"" then for half cycle the energy consumed is positive while in the other half cycle power consumed is negative (this negative power means that the power is returned to the grid, as u stated :""it's oscillating"")so depending on the lag or i mean depending on the inductor(whether ideal or non-ideal) power-factor is decided i.e if inductor is perfectly ideal then half cycle is positive power & other half cycle is negative power.
So IT'S TRUE THAT NEGATIVE POWER HAS TOTAL RELATION WITH THE POWER FACTOR WHICH U HAD INITIALLY DISAGREED. IF U AREN'T STILL CONVINCED PLEASE GO & READ THE LINK GIVEN ABOVE...

 

[PlanarMetamaterials]

Hi Disha Karnataki,

As FvM pointed out, you need to be very careful with the terminology you use. Negative real power is used in power systems to describe a generator. However, there is also the imaginary (or reactive) component of power (since power is inherently complex). I think what you are referring to is negative reactive power. Additional reactive power (loading) on the Grid can lead to instability and voltage drop, which is never a good thing - ideally the Grid is just some wires with only real impedance.

I also think you misunderstood what FvM was saying. Of course, the relative phase of the voltage/current (waveforms) across the load is described by the power factor.

 

[Disha Karnataki]

hi planar,
so u mean negative real power describes a generator i.e at grid only...
as generators are also inductive in nature which implies that negative real power means negative reactive power right???
if not please explain clearly what is negative real power.

 

[PlanarMetamaterials]

Sorry Disha, I was using sloppy terminology I should have said an "ideal" generator - i.e., one that only provides real and no imaginary power.

The real and reactive (imaginary) components of power are independent. Negative real power is simply "created" real power - using the convention that positive real power is "used" power - e.g. a giant resistor (with no imaginary impedance).

The difference between real and reactive power is described on your favorite website, https://en.wikipedia.org/wiki/AC_power

 

[BradtheRad]

Here is a simulation portraying the effect of an inductive load on the AC supply.

Notice how the left-hand scope traces are in sync. The resistive load does not displace the Amperage waveform.

However the right-hand traces are out of sync.

The coil opposes current flow while supply amplitude grows. Then the coil generates current as supply amplitude drops.

 

[Disha Karnataki]

thank u so much brath

The coil opposes current flow while supply amplitude grows. Then the coil generates current as supply amplitude drops.

ok now as coil generates current isn't that current useful??

 

[BradtheRad]

ok now as coil generates current isn't that current useful??

Yes, in the familiar ways:

* to generate electricity

* as a choke filter

* in transformers

* switched-coil power supply

I have Youtube videos which are animated simulations of inductor behavior. These portray flux fields building and collapsing, changing emf, current bundles flowing through wires. Different waveforms are applied.

Inductor Behavior with DC:

https://www.youtube.com/watch?v=LVNxrN4jgvs

Inductor Behavior with AC:

https://www.youtube.com/watch?v=Os3jF9UeMoE