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Thread: Power Factor

  1. #1
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    Default Power Factor

    Evening all. This should possibly be in "nothing to do with metalwork" but as its also electrical here goes.

    Installed a little power meter on my house genie. As you can see the PF is not always very good. Am I right in guessing the W's displayed would be true power. So the apparent power would be 935W?(that's what the genie cares about right?)

    TIA
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    Quote Originally Posted by Stustoys View Post
    Am I right in guessing the W's displayed would be true power.
    It should read true power, but what does the manual that came with the power meter say?

    PF of 0.6 seems low.
    Chris

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    Yes, True Power, which you pay for.
    Here is my calculations. My answer agrees with the screen shot.
    Power, VA, VAr, PF.jpg

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    A lightly loaded induction motor can have a Power Factor as low as 0.3

    Manufacturers of electrical equipment - air conditioners are an example - use a motor of a higher rating than is required to run the compressor. As an example, the cost difference between say - a 3 HP motor against a 2.5 HP motor is not much. This means that most of the time it is lightly loaded, and therefore has a poor Power Factor.

    The reason for this logic on the part of the manufacturer is that on very hot days when the airconditioner is working hardest, and the ambient temperature is highest the compressor will keep going, and not burn out. The outcome for the manufacturer is no warranty claims, and no lost reputation.

    The outcome for the electricity supply company is a very poor power factor most of the time which has to supplied by the electricity company at no direct charge to the customer. Indirect - yes, direct - no.

    I believe that in the UK energy is not paid for in Watts, but in VA, so there is an inbuilt direct chage for presenting a poor power factor to your electricity supplier.

    Hope this makes some sense to the non electrical Forum members.

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    Quote Originally Posted by jack620 View Post
    but what does the manual that came with the power meter say?
    You make a good point lol I will try finding the manual and have a read.
    I have a lot of LED lights, but I wouldn't have thought many of them were on.
    Things like fridges have good PF normally right?

    Quote Originally Posted by A J in WA View Post
    Yes, True Power, which you pay for.
    Well in this case I do have to pay for apparent power, at least in the sense that the genie needs to be bigger*, though I not sure how inverter gennies deal with bad PF.

    *can't really go much bigger with the genie anyway without messing with the panel again.




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    Quote Originally Posted by A J in WA View Post
    A lightly loaded induction motor can have a Power Factor as low as 0.3
    OH seems I maybe wrong about fridges, I will check them with another meter tomorrow and see what it has to say.

    One of them draws 12W all the time which I assume is a heating element that must be inside the compressor.

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    BobL is offline Member: Blue and white apron brigade
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    I recently made up 15A power box with Power meter with PF display and a C16 breaker

    PB2b.jpg

    Details here
    https://www.woodworkforums.com/f271/...lay-box-236791

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    Quote Originally Posted by Stustoys View Post
    One of them draws 12W all the time which I assume is a heating element that must be inside the compressor.
    I don't know if fridges have sump heaters Stu. Is it a frost-free fridge? If so, the 12W is probably the mechanical timer that controls the auto-defrost cycle.
    Chris

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    Quote Originally Posted by Stustoys View Post
    OH seems I maybe wrong about fridges, I will check them with another meter tomorrow and see what it has to say.

    One of them draws 12W all the time which I assume is a heating element that must be inside the compressor.
    Domestic fridge compressors do not have a heater for the sump like an air conditioning compressor.

    However they do, or could have a heater element around plastic trim to keep out condensation.
    or
    it could be the timer motor that initiates the defrost cycle

    older fridges may also have a heater in defrost pan to drive off water from defrost cycle.

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    After trying to find out what reactive power is, I have worked out most people do not know, including me.

    The best I can work out is lets say you have a device with a coil. The voltage starts at zero, reaches peak, then collapses back to zero. When the coil voltage collapses back to zero the extra non consumed energy the coil took in on the voltage rise, it induces a current flow back to the source, and it is this reverse current flow that is the problem which causes an out of sync current sine wave (otherwise how could current flow at zero volts?). It means wires have to be bigger to handle this extra current that does nothing.

    What you can do is install capacitors along the line, that takes in the excess current and gives it out when required.

    That is the best explanation I can work out after much looking at youtube videos that seem to all say something different.
    Gold, the colour of choice for the discerning person.

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    Quote Originally Posted by jack620 View Post
    Is it a frost-free fridge?
    Turns out it is auto defrost. So that explains it.....

    Its PF is around 55

    Quote Originally Posted by eskimo View Post
    However they do, or could have a heater element around plastic trim to keep out condensation.
    Where? around the door? not that I know of but then how would I know.

    Quote Originally Posted by eskimo View Post
    older fridges may also have a heater in defrost pan to drive off water from defrost cycle.
    yeah I have seen those, but this fridge has the condensate pan on top of the motor. *edit* after finding out the fridge is infact frost free


    Quote Originally Posted by .RC. View Post
    After trying to find out what reactive power is, I have worked out most people do not know, including me.
    I know what it is(or at least a more simplified version than yours)............. can I explain it? unlikely(hopefully someone will be along to correct my explanation.
    With a resistive load (say a toaster) max current and max V are in sync.
    With an inductive load(say a motor) max current lags max V
    With a capacitive load (say a capacitor lol) max current leads max V

    You have to supply the max current but you only use the W's (

    So you can have bad PF from either to much inductive load or to much capacitive load. I assume you tell the difference but adding cap and seeing if it makes things worse(if you cared). In the past I believe it was to much inductive load but more and more things these day are capacitive loads.

    And after that is goes above my head.

    Quote Originally Posted by jack620 View Post
    but what does the manual that came with the power meter say?
    It says it right there(as confirmed by A J in WA's maths) "1. Electrical parameter measurement function (voltage, current, active power, energy, frequency, power factor)"
    Further on it says Note:There are three types of power: active power (P represents / unit W), reactive power (Q represents / unit Var), apparent power (S represents / unit VA).
    And the power of our product is active power.
    Active power is calculated as: P = U * I * cosφ, cosφ represents the power factor, for purely resistive loads (such as incandescent, heater, etc.) the power factor is generally close to 1, for inductive load and capacitive load, the power factor is between 0-1; so when you test the purely resistive load, the power(P) is substantially equal to or close to U * I; but when you test inductive or capacitive loads (such as refrigerators, washing machines, televisions, computers, etc.), P

    Quote Originally Posted by BobL View Post
    I recently made up 15A power box with Power meter with PF display and a C16 breaker
    Nice. I saw those Bob, but for this job I didn't like the formfactor and the seller I was dealing with was no help on what CT I needed to add for external measurement. One thing I forgot was another genie I have has a power meter similar to yours that also measures run hours. I wanted that, but it got lost in the search of 100 different meters and I didnt notice until I was testing the installation of the one I bought that it didn't have it
    I bought one of these for general testing(as tested by bigclivelive).
    https://www.aliexpress.com/item/4000155024043.html

    Thank you to all

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    Quote Originally Posted by Stustoys View Post



    I know what it is(or at least a more simplified version than yours)............. can I explain it? unlikely(hopefully someone will be along to correct my explanation.
    With a resistive load (say a toaster) max current and max V are in sync.
    With an inductive load(say a motor) max current lags max V
    With a capacitive load (say a capacitor lol) max current leads max V

    You have to supply the max current but you only use the W's (
    But you cannot have current flowing at zero volts. AC hits zero volts during the cycle. No amps can flow then.
    Gold, the colour of choice for the discerning person.

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    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by .RC. View Post
    But you cannot have current flowing at zero volts. AC hits zero volts during the cycle. No amps can flow then.
    In a capacitive circuit, in terms of "any one point in time" it not the "absolute" volts but the "change in volts" that determines the current. So during a mains cycle, when the absolute voltage is zero the change of voltage (Gradient) is at its max. Through inductors the greatest back V generated is when the current is changing most quickly and for capacitors the greatest current is when the voltage changes most quickly.

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    That just confuses me Bob
    Gold, the colour of choice for the discerning person.

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    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by .RC. View Post
    That just confuses me Bob
    Think of a moving pendulum - the velocity is greatest when the displacement from its original rest position is zero. And its velocity is zero when it's displacement is greatest.

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