Power Mystery.....

Discussion in 'General Electronics Chat' started by cjdelphi, Jan 20, 2010.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    This is really quite odd, I have a power meter, once my monitor,pc, phone charger is all switched on it uses around 82 watts of power on average unless i blast out some music or recharge the phone etc etc..

    now, i also have a 90amp/h battery 12 volt deep cycle, next to that i have a basic 3.5 amp charger which will charge then float charge to keep it 14.5v, it's a sealed battery btw.

    I plug in the 3.5amp 12v charger into the row where the the rest sits, phone, computer, monitor, etc etc, so anyway...

    Before 12v charger, 82watts.

    I plugin the charger, now the power meter reads 74 watts...


    (I thought it was going mad so i repeated the test, 2 different powerboards, both give the same results, i'll even take a video for the non believers lol)
     
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    What kind of power meter is it? Is it a Kill-a-Watt?

    You say the battery is "next" to the charger; does that mean that the battery is connected to the charger so that the charger is charging the battery when it's plugged in?

    Suppose you only plug in the charger and unplug all the rest of the loads. What does the power meter say then?

    Do you have an oscilloscope and a current probe so that you can capture the waveform of the current which the power meter sees, with and without the charger load added to the rest of the loads?
     
  3. Audioguru

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    Dec 20, 2007
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    The billing meter for homes measures only the actual power used without regard to power factor. But the billing meter for industry also measures reactive power that increases its readings.

    It looks like you have a meter for industry, not for homes so some of the the capacitive reactance of the monitor is cancelled by the inductive reactance of the charger.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    It looks like you have a meter for industry, not for homes so some of the the capacitive reactance of the monitor is cancelled by the inductive reactance of the charger.

    Can you elaborate?

    That sounds like the theory behind a "lower your power bill" contraption I saw. It had the user put a coil on each side of the meter. Not on the power lines but PHYSICALLY next to the globe. I wondered if if tried to use magnetic flux to slow the rotation of the meter or something else all-together.

    cjdelphi:
    If your power bill goes down because of this, let us know. I would be one battery charging machine. "Hey friends, neighbors, need your batteries charged?" :D
     
  5. Audioguru

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    Companies in China advertise a gadget to reduce your electricity bills. It is simply a cheap capacitor but it is sold at a very high price. It might reduce your power factor if its value is correct (each inductive appliance needs its own capacitor designed for it) then it will reduce the electricity cost to the electricity company but will not reduce your home's bill.
     
  6. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    Interesting right then, I went back to the power meter.., before i plugin the charger, it shows 81watts but, power factor is 100%, been like this for the past 7 hours.

    if i add the battery charger it drops to 67 watts, but interestingly enough as pointed above, "power factor" drops to 97...

    hmm, power factor?... ugh another new thing to look up the meaning of.

    So it's the capacitor doing this is it... if my main power meter does not act on power factor, this could be reducing my power bill?

    [as for power meter, i mean a thing i plug into the wall, the outside main power meter where incoming from the transformer, i'm not looking at that, just the meter between wall-socket and power block for computer.]
     
  7. The Electrician

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    I will ask again. What is the brand name of the power meter? Is it a Kill-a-watt? Where did you buy it? Is there a web page that describes it?
     
  8. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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  9. The Electrician

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    I don't see detailed specs for the unit, but the description does say that it can monitor current as well as power and voltage.

    Could you display the current before you connect the charger (when the power is 81 watts)? Then check the current after you plug in the charger (when the power drops to 67 watts.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Here is a scan of the instructions/datasheet for what I think is the UK version of this device. It differs only in the socket arrangement from the Jaycar device.

    I also think the power factor explanation is correct and this device is supplied to commercial establishments for the reasons already outlined.
     
  11. The Electrician

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    If when you say "I also think the power factor explanation is correct", you are referring to the explanation given by Audioguru in post #3:

    "...some of the the capacitive reactance of the monitor is cancelled by the inductive reactance of the charger."

    it would seem to be contradicted by what the OP said in post #6:

    "before i plugin the charger, it shows 81watts but, power factor is 100%, been like this for the past 7 hours.

    if i add the battery charger it drops to 67 watts, but interestingly enough as pointed above, "power factor" drops to 97..."

    This would seem to indicate that there is essentially no capacitive reactance in the load (which is actually more than just a monitor; the OP said it is "...phone, computer, monitor, etc") before the charger is plugged in (100% power factor). It is only after the charger is plugged in that the power factor degrades, and if degraded power factor is responsible for an increased reading, the indication should increase when the charger is plugged in rather than decreasing.

    Furthermore, the specs you posted say that the meter measures watts. It doesn't say anything about measuring vars or apparent power. The accumulated measurement is in KWH.

    If it were intended to take into account the power factor of the load in determining the cost of electricity, there would have to be a way to insert the various penalty amounts for various power factors into the programming, and I see no indication of such a capability.

    So, I don't think the power factor explanation is correct. Something else is going on.

    I hope the OP will give us the current measurements for the two cases.
     
  12. studiot

    AAC Fanatic!

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    I said that simply because I couldn't think of better of the top of my head, but I am interested in the resolution.

    If there are any experiments I can do with this meter, before I pass it on to a client let me know? Perhaps I can compare readings with a different type of power meter.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    It would be great to see secondary measurements. Just to insure the meter isnt experiencing a display or math error. If you measure with another meter, does the AxV still equal the results on the wall meter?
     
  14. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    Before the 12v charger is hooked up to the battery right now shows...

    81watts
    0.34amps
    254volts
    pf 100%

    after i plugin charger

    72watts
    0.30amps
    254volts
    pf 95%

    one more test and that's to switch everything off (as pc is part of the block)

    and measure how much current is used by the charger alone inc pf, but off the top of my head im sure i measured it around 10watts on float.
     
  15. The Electrician

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    The best thing you could do to resolve the mystery is what I asked about in post #2:

    "Do you have an oscilloscope and a current probe so that you can capture the waveform of the current which the power meter sees, with and without the charger load added to the rest of the loads?"

    Failing that, do you have a DVM so that you could measure the current drawn by the power board with and without the charger plugged in? And also measure the line voltage.

    If the PF (power factor) is 100% without the charger plugged in, then the apparent power (product of RMS amps and volts) will be equal to the true power (as measured by the power meter).

    And, with the charger plugged in, if the PF is 97%, then the apparent power will be very close to the true power, so measuring the load current and voltage with a separate DVM will provide a good check on the power meter accuracy.

    If you don't have a DVM, then see what the power meter says the current and voltage are, with and without the charger plugged in. The product of the current and voltage is the apparent power, and should match what the power reading is when the PF is as high as 100%, and even with a PF of 97% it should be close.

    EDIT

    I see that you made the measurements I asked for and posted them while I was working on this post!

    If you multiply the volts and amps without the charger, you get 86.36 watts, compared to the watt reading of 81 watts. With a PF of 100%, those values should be identical. That's about a 6% error; the meter spec claims a 2% accuracy for the power reading.

    An important clue toward solving the mystery is that the current did go down when you plugged in the charger, so the power reading should go down too; at least the power meter is consistent in that regard.

    The only thing I can think of at this point is that the charger is somehow supplying power back to the grid. Is the charger connected to the battery when you make the measurement? What happens if the battery is not connected to the charger, and you then plug it in?
     
    Last edited: Jan 21, 2010
  16. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    I don't own an oscilloscope, but I can measure the current using my multimeter, i don't fancy doing that right now as this room has no light (long story, i broke the light fitting last week causing the bulb to drop out, got to replace both ...)

    I'll measure the current tomorrow, but I did do the test i wanted to do.

    Just the charger plugged in and nothing else reads..

    0.09amp
    PF 56
    12 watts float
    251v

    with just the monitor (CRT, oldstyle) on standby, 0.07amps, just the pc plugged in 0.08amps (never tried speakers or phone charger)

    I'll measure the current consumed with an auto ranging meter tomorrow.
     
  17. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    "The only thing I can think of at this point is that the charger is somehow supplying power back to the grid. Is the charger connected to the battery when you make the measurement? What happens if the battery is not connected to the charger, and you then plug it in?"

    yes the charger is connected to the battery... (i'll unhook the 12v battery now from the charger)

    but got to power down the computer again lol, bbinafew (i'll do just charger connected, then just charger with/without battery and connect the monitor, back with a million test results... brb)
     
  18. Audioguru

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    Dec 20, 2007
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    I think the no-name-brand meter is made by the same Chinese guy who sells over-priced capacitors to "reduce your electricity bill". Its power factor reading is backwards so it shows an industrial reduction in power when the charger (which might be capacitive) is plugged in.
     
  19. 3ldon

    Active Member

    Jan 9, 2010
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    3
    Really need to know what the current waveform looks like in both cases. it may be possible that the meter is not sampling the current often enough causing accuracy issues.
    also, if the charger is drawing a high 3rd, 5th, 7th, harmonic, then this will create a voltage depression on the incoming AC voltage, causing the meter to under report voltage, although it reports the same, we don't know how it is calculating it, or how often.

    here is a test for you, take a full wave bridge and a high value cap, such as 470uf, connect a 25 watt light bulb across it (you can guarantee that the current will be spikes, and the bulb will be consuming around 30-35 watts due to the higher voltage) and report what the meter reads.
     
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