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Energy saving device

  1. Mar 12, 2009 #1
    Hi Guys, :smile:

    I have heard of many energy saving devices available commercially, which when connected in parallel with the household mains, cause reduction in electricity bills. In smaller sizes they can be connected in parallel with any household power recepticle and cause reduction in energy consumption in any electrical appliance connected to that power recepticle.

    Can someone please provide me with any information on the working principle of such a device, since I fail to understand how can any device connected in parallel with an electrical system, load or appliance cause a reduction in energy consumption in that load or system ? I would be very grateful.

    Thanks & Regards,
    Shahvir
     
  2. jcsd
  3. Mar 12, 2009 #2

    mgb_phys

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    The only ones that work do so by stealing from your power company
    And can't be discussed here
     
  4. Mar 12, 2009 #3

    uart

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    He might just be talking about simple power factor correction. Actually this does save power, though it usually wont be reflected on your bill, it saves the power company by reducing their distribution losses.

    Domestic installations here in Australia don't pay for reactive power, only real power is metered (though I think the larger domestic appliances probably have to meet some power factor specifications to be sold).

    Industrial users however often pay a tariff that depends upon their power factor, in essence they may be charged a penalty rate if their PF is too low.
     
  5. Mar 12, 2009 #4

    mgb_phys

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    Yes I know - I received a warning for mentioning them to someone. Apparently like mod-chips or DVD region hacks we aren't allowed to discuss them here. I imagine the forum is hosted in the land of too many lawyers!

    Isn't that the point of them - they change the power factor so you receive more power than you are paying for?
     
    Last edited: Mar 12, 2009
  6. Mar 12, 2009 #5

    MATLABdude

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    I thought they just reduced the amount of imaginary power you used, and didn't touch your real power? It's just that they're completely useless for the vast majority of people, and thus all too often marketed to those that don't know any better.
     
  7. Mar 12, 2009 #6

    mgb_phys

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    Yes, just been reading up on power factor. It looks like the reason against power factor converters is that they waste electricity that you aren't paying for!
    Even then only if you have significant reactive loads.
     
  8. Mar 12, 2009 #7

    uart

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    No, quite the contrary. You are normally metered for the power you consume, not the amount of current it takes to consume that power. So when power factor correction is applied it reduces the amount of current that the power company needs to distribute to you in order to supply a given power level (wattage). The bottom line is that it saves the power company money by reducing the losses in their distribution system but saves you nothing (as a domestic consumer). Consequently there's not much incentive for a domestic consumer to do anything about power-factor correction (though not so for industrial users as I mentioned above)

    The above is how it is for domestic installations here in Australia anyway. If (elsewhere in the world) you have a system where the power companies can meter only VA (instead of Watts) and charge based only on that figure then yes adding power factor correction would cut your power bill. I'd hardly call that stealing from the power company though.
     
    Last edited: Mar 12, 2009
  9. Mar 12, 2009 #8

    uart

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    No, I wouldn't really put it like that. Power factor correctors (the simplest form of which is just a parallel capacitor) can rightly be thought of as being a local generator of reactive power (an inductive load being a consumer of reactive power).

    Basically when you have an inductive load it causes the net current to be out of phase with the supply voltage. This net current can be thought of as two component, an in phase component corresponding to (when mult by V) the Wattage being consumed and a purely inductive component (90 degree lagging) corresponding to the reactive power being consumed. A power factor correction circuit merely "generates" this reactive power component locally so that the power company doesn't need to distribute this "wasted" current. You're doing them a favour!

    As I say, here in Australia we are only billed for the Wattage consumed in domestic installations, so the poor old energy company has to supply that reactive component of the current free of charge to us (up to a limit though, by law the PF of a domestic load cant be lower than 0.8). Obviously this situation could be different in different countries, perhaps someone familiar with how energy is typically metered and billed in the US or Britain could fill us in on the details of their system.
     
    Last edited: Mar 12, 2009
  10. Mar 12, 2009 #9

    mgb_phys

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    All domestic customers get billed on assumed real power - the electric meter is just a current meter.

    Sorry I wrote my answer the wrong way round - WITHOUT a power factor converter you are wasting the power company's electricity - that you aren't paying for.
     
  11. Mar 12, 2009 #10
    Thanx guys for such an overwhelming response! one thing is for sure, these energy saving devices might be a marketing farce..as far as the individual consumer is concerned.

    Also, i just wanted to verify one thing though, does the Energy meter only record real power (Watts), or does it record apparent power (VA)?

    Kind Regards,
    Shahvir
     
  12. Mar 12, 2009 #11
    I'm 99% sure that power meters measure actual power which is reported as actual energy consumed. As in;

    Imaginary Power x Power Factor = Real Power

    Just about all residential power saving devices are going to be BS as the real ones are usually only manufactured for large amounts of power consumption (industrial) since they are only cost effective at large scales. The only exception to that rule is if you have a home with solar plant or a wind farm near by which operates in parallel with your local grid. Some of the more advanced models have power factor correction (~0.95) along with ride through capability.
     
    Last edited: Mar 12, 2009
  13. Mar 12, 2009 #12

    mgb_phys

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    Real power - it only measures current.
     
  14. Mar 12, 2009 #13
    Thanx, but why is the reactive power (or current) not measured/recorded by the Energy meter even though it passes thru the meter too ?? :confused:

    Kind Regards,
    Shahvir
     
  15. Mar 12, 2009 #14
    Maybe I'm just missing something or having another dumb moment, but how do you measure real power with only current? Don't you need to measure the peak voltage, current, and phase difference to determine real power?
     
  16. Mar 12, 2009 #15

    mgb_phys

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    Remember real power is really the real power supplied it's only the real part of the power supplied !
    Real power is just measured current * assumed peak voltage (110V/230v) and assumed purely resistive load.
     
  17. Mar 12, 2009 #16
    I built an active power factor correction circuit many years ago. They only work on induction motors like in the refrigerator. What the circuit does is monitor the power factor of the load, and reduce the motor voltage until the power factor rises to a preset value. In my case, the refrigerator motor voltage dropped to about 90 volts after a few seconds.
    Separately, I measured the watts and volt-amps of an unloaded 1/4 HP induction motor running at 120 V. Result; 110 watts, 425 volt-amps.
     
  18. Mar 12, 2009 #17
    um, no. for RLC loads, you must consider the phase between the voltage and current. to get the real power supplied, you multiply the real part of the current by the voltage. but to just look at measured current is to look at the sum of both real and imaginary current, which is always greater than the real current.

    for situations with a bunch of non-linear loads, it's not even that easy. you'd need a device that looks across the entire cycle to compute the power.
     
  19. Mar 12, 2009 #18

    russ_watters

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    Residential customers get billed for real power, commercial customers get a surcharge added if their power factor is bad (generally below 85%, I think). So such a device would benefit a commercial customer (and capacitor banks are very common in commercial/industrial buildings) and would do nothing for a residential building.
     
  20. Mar 12, 2009 #19

    russ_watters

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    Power meters are basically a totalizer function that multiplies the amperage by the voltage without measuring the phase shift. Since the amperage and voltage are not in phase if the power factor is bad, multiplying them together yields only the real power. This isn't as complicated as it sounds, though:

    Put another way, a typical residential power meter is just an electric motor wired in series with your house.
    http://en.wikipedia.org/wiki/Electricity_meter
     
  21. Mar 14, 2009 #20
    Dear Bob, :smile:

    But did your circuit save electricity or reduced your electricity bills upto some extent? plz reply, i need to know.

    Thanks & Regards,
    Shahvir
     
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