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    #16
    The installations I worked on were industrial. Low and high voltage were in the same control panel. No idea what's required for residential. In industrial low voltage cabling exited the box through separate conduit. If you use a long run of wire to connect the CTs, you may want to include a differential amplifier circuit to subtract common mode noise. If the wire is run through metal conduit, it may not be necessary.

    Henry

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      #17
      Doe this give you any ideas? I am impressed by this guys web site. Good info.

      http://www.edcheung.com/automa/power.htm

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        #18
        Now that guys a hard core home automation enthusiast!

        Talk about "role your own" ideas.
        I'm impressed!



        Personally I wasn't really going to worry about voltage to calculate power. I was just going to set the voltage for the math at the average voltage level and then assume it's always real close to that level. Of course I could always pull the line level input from the UPS.

        I can't beleive how helpfull everyone has been with this project. Many thanks for the great advice. I'll let you all know how it comes out, I should get the CT next week sometime.

        Tim

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          #19
          The idea getting kicked around here is to measure the current. But don't you need to know the relative phase between the voltage and the current to get the real power? This is the case for non-resistive loads, and you have a lot of those in your house (AC, washer, dryer, refrigerator). I believe this is why Ed Cheung uses the IC chip... it's designed to take care of this for you.

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            #20
            You're entirely correct regarding phase angle. In fact, for accuracy the true RMS value of the current and voltage should be used. Assuming that the motor load should be somewhat constant, with exceptions, should get you a more-or-less repeatable measurement. Without a lot of fine tweaking, and monitoring non resistive loads and waveforms, you won't be able to calculate you power bill. You will be able to get relative measurements that will approximate your actual power usage, with an error factor thrown in. Even a precision measuring device has a fair amount of uncertainty built in. If periodic calibration isn't performed also, then the accuracy degrades considerably. Most times though repeatability is more valuble than accuracy. If more accuracy is needed, you can add current sensing to appliance that have non-resistive loads. If that isn't enough, you can measure phase angles and true RMS for those devices, and correct your measurements through software. You probably still won't be able to accurately calculate your power bill though.

            Henry

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              #21
              Yeah, getting the right number is tough. I would argue that even getting a good relative number is tough due to the changing fraction of power used by inductive versus non-inductive loads.

              The best method I have seen is to somehow tap the data from the util company's measurement. Most of the US has rotating disks, and those have a few holes in them. I made an early attempt to shoot modulated IR through the hole and detect the pulses on the other side. Then, the idea is to count the time between pulses to get the rotation rate. My prototype (me standing there holding the transmitter and reciever) seemed to actually work.

              There has been other discussion of this method, and I've seen a company that once sold such a thing. I haven't come back to this project since last summer, but it's on my list.

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                #22
                I may need correcting here but in domestic installations the Power factor is not an issue (except to the Power company!)

                I can only speak for the UK but domestic meters do not include the power factor in their calculation. The Power companies rely on manufacturers who make domestic electrical gear to incorporate PF correction i.e. providing capacitors with motors/inductors.

                I know that certain industries have to have PF meters fitted. Perhaps because the US domestic uses 2 phases that PF is included in the calculation? Perhaps someone can enlighten me!

                Jon
                Jon

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                  #23
                  As Jon00 said, power factor is not considered in domestic metering because the things which draw lots of power in a home are usually resistive, like for heaing or cooking. To measure the true power usage anyway, you would need what's known as a "True RMS" meter, which integrates the waveform and gives you the equivalent RMS voltage value. I would bet that comparing the voltage reading between and regular meter and a true RMS meter from your current sensing coil would hardly show any measurable reading unless you're running a large motor load.

                  -----------------------------
                  If you don't know what you're doing... do it neatly.

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                    #24
                    I am also confused. Are you guys implying that the meter on residentials is not accurate? I don't think the power company would let that be the case since it equals cash to them.

                    Or, are you implying that non-resistive loading is a small part of the usage? I would also take exception to that. Low voltage lighting doesn't appear resistive for example, and things like blowers for AC and refrigerator motors are mostly non-resistive.

                    I don't really know. Any enlightenment appreciated.

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                      #25
                      A few years ago, I purchased a device called "The Power Sentry" from Remote Measurement Systems. In fact, I think this is a direct link to it: http://www.measure.com/sensors/senso...Power%20Sentry

                      It was a little device about the size of your index finger that was attached with double-sided tape to the front of the power meter. You'd line it up with the edge of the spinning disk, and it would reflect an IR LED on it. When the black stripe came around, it would interrupt the reflection, and the device would send a pulse to my home automation controller signalling consumption of 7.5 Watts. I had the home automation software (ECS at the time) calculate how much electricity had been used for the day and month, and project what the bill would be. It worked like a champ. I haven't hooked it up at my new house, yet, but I need to get on that, soon. Worked really well.

                      Doesn't look as if this web page has been updated in a couple of years. You'll have to send a note to the company to find out if they are still available. Unfortunately, they're not going to be answering E-mail for a couple of weeks.

                      --David

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                        #26
                        Mike,

                        You got it!

                        The meters are accurate as they just measure current & voltage.

                        In DC terms this would equate to the Wattage(W=IxV) and also to AC if the power factor is 1 (resistive). With inductive loads, the power factor will increase the power supplied to the device. (Power factor=watts/volt-amps)

                        So the consumer pays for the amp/volts calculation whilst the power company pays for the actual watts used.

                        Many power companies publish the minimum power factor that they will allow for domestic (mine is 0.85) but how on earth they expect the average consumer to measure this is beyond belief!

                        Jon
                        Jon

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                          #27
                          jon00,

                          So does this mean I win if I can convert all my lighting to low voltage? I'm assuming of course, that my magnetic transformer is mostly inductive, but I think that's true.

                          When you say they "allow" a 0.85 power factor, how do they know? Is someone going to show up at my house if it looks like a big transformer instead of a big resistor? Also, doesn't the power factor change dramatically depending on what devices are running?

                          Thanks for the info.

                          Comment


                            #28
                            Mike,

                            It will cost you the same if you use normal OR low voltage lighting (which is inductive if you use step down transformers). The loser is the power company. The worse the power factor becomes, the more it costs them to provide the power. As I said earlier, you pay for the amps x volts – the power factor is not measured.

                            You win in that you know it is costing the power company more money. I am not sure this is a good thing, as the power company has to generate more power which wastes resources etc. If everyone had a poor PF - it would cost them a fortune and electricity prices would rise.

                            You are right - I do not think they check it at all for domestic but I do think it is in everyone's interest to 'try' and get the PF to 1.

                            Jon
                            Jon

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