Electronics > Projects, Designs, and Technical Stuff
KWh meter - is there such a thing as a non-contact one?
The Soulman:
--- Quote from: Zero999 on April 03, 2019, 10:28:37 am ---
--- Quote from: EEVblog on April 03, 2019, 10:18:19 am ---
--- Quote from: soldar on April 03, 2019, 09:59:29 am ---You really need to measure (connect) voltage. Voltage variation, wave shape, reactive loads will all affect the measurement. You cannot just multiply current by an assumed voltage.
--- End quote ---
Well, you can, it's just a matter of how much error you are willing to accept.
Our mains voltage for example doesn't vary by a huge amount, so multiple a fixed value by the RMS current and you are in the ballpark.
--- End quote ---
But that will only give you apparent power, not true power. If you could obtain the phase of the sine wave by capacitive coupling, then you stand more of a chance.
--- End quote ---
With capacitive coupling you may stand a chance but the phase shift introduced by the coupling may only be calibrated out if it is the coupling is 100% repeatable.
Which it's not.
Zero999:
--- Quote from: The Soulman on April 03, 2019, 10:59:42 am ---
--- Quote from: Zero999 on April 03, 2019, 10:28:37 am ---
--- Quote from: EEVblog on April 03, 2019, 10:18:19 am ---
--- Quote from: soldar on April 03, 2019, 09:59:29 am ---You really need to measure (connect) voltage. Voltage variation, wave shape, reactive loads will all affect the measurement. You cannot just multiply current by an assumed voltage.
--- End quote ---
Well, you can, it's just a matter of how much error you are willing to accept.
Our mains voltage for example doesn't vary by a huge amount, so multiple a fixed value by the RMS current and you are in the ballpark.
--- End quote ---
But that will only give you apparent power, not true power. If you could obtain the phase of the sine wave by capacitive coupling, then you stand more of a chance.
--- End quote ---
With capacitive coupling you may stand a chance but the phase shift introduced by the coupling may only be calibrated out if it is the coupling is 100% repeatable.
Which it's not.
--- End quote ---
Providing the resistive element dominates, then the error will be minimal. For example, if the meter's input impedance is 100M and the capacitance to the mains conductor is 100pF (31M8, at 50Hz) then the phase shit will be minimal.
Shielding and filtering to remove high frequency interference will be more of a challenge, than phase shift.
The Soulman:
--- Quote from: Zero999 on April 03, 2019, 11:04:56 am ---
--- Quote from: The Soulman on April 03, 2019, 10:59:42 am ---
--- Quote from: Zero999 on April 03, 2019, 10:28:37 am ---
--- Quote from: EEVblog on April 03, 2019, 10:18:19 am ---
--- Quote from: soldar on April 03, 2019, 09:59:29 am ---You really need to measure (connect) voltage. Voltage variation, wave shape, reactive loads will all affect the measurement. You cannot just multiply current by an assumed voltage.
--- End quote ---
Well, you can, it's just a matter of how much error you are willing to accept.
Our mains voltage for example doesn't vary by a huge amount, so multiple a fixed value by the RMS current and you are in the ballpark.
--- End quote ---
But that will only give you apparent power, not true power. If you could obtain the phase of the sine wave by capacitive coupling, then you stand more of a chance.
--- End quote ---
With capacitive coupling you may stand a chance but the phase shift introduced by the coupling may only be calibrated out if it is the coupling is 100% repeatable.
Which it's not.
--- End quote ---
Providing the resistive element dominates, then the error will be minimal. For example, if the meter's input impedance is 100M and the capacitance to the mains conductor is 100pF (31M8, at 50Hz) then the phase shit will be minimal.
Shielding and filtering to remove high frequency interference will be more of a challenge, than phase shift.
--- End quote ---
--- Quote from: Zero999 on April 03, 2019, 11:04:56 am ---
--- Quote from: The Soulman on April 03, 2019, 10:59:42 am ---
--- Quote from: Zero999 on April 03, 2019, 10:28:37 am ---
--- Quote from: EEVblog on April 03, 2019, 10:18:19 am ---
--- Quote from: soldar on April 03, 2019, 09:59:29 am ---You really need to measure (connect) voltage. Voltage variation, wave shape, reactive loads will all affect the measurement. You cannot just multiply current by an assumed voltage.
--- End quote ---
Well, you can, it's just a matter of how much error you are willing to accept.
Our mains voltage for example doesn't vary by a huge amount, so multiple a fixed value by the RMS current and you are in the ballpark.
--- End quote ---
But that will only give you apparent power, not true power. If you could obtain the phase of the sine wave by capacitive coupling, then you stand more of a chance.
--- End quote ---
With capacitive coupling you may stand a chance but the phase shift introduced by the coupling may only be calibrated out if it is the coupling is 100% repeatable.
Which it's not.
--- End quote ---
Providing the resistive element dominates, then the error will be minimal. For example, if the meter's input impedance is 100M and the capacitance to the mains conductor is 100pF (31M8, at 50Hz) then the phase shit will be minimal.
Shielding and filtering to remove high frequency interference will be more of a challenge, than phase shift.
--- End quote ---
ot: congrats on your 11999 post! :-+
I think 100pf is to ambitious, according to my "back on the envelope" calculations with a probe length limit of 50mm (for practical reasons) and dual insulation (required by law) and 2,5mm2 cable the coupled capacitance is max. 10pf.
And then there are repeat-ability issues because of physical probe placement, different wire types and insulation material and thicknesses.
In a controlled (lab) environment something like this maybe doable with usable accuracy (say below 5% error) but
the probe must be calibrated in place and not be touched after.
Sounds like a nice topic for a video to me, including some experiments etc. ;)
Zero999:
--- Quote from: The Soulman on April 03, 2019, 02:36:42 pm ---I think 100pf is to ambitious, according to my "back on the envelope" calculations with a probe length limit of 50mm (for practical reasons) and dual insulation (required by law) and 2,5mm2 cable the coupled capacitance is max. 10pf.
And then there are repeat-ability issues because of physical probe placement, different wire types and insulation material and thicknesses.
In a controlled (lab) environment something like this maybe doable with usable accuracy (say below 5% error) but
the probe must be calibrated in place and not be touched after.
Sounds like a nice topic for a video to me, including some experiments etc. ;)
--- End quote ---
How did you come up with 10pF? Perhaps 100pF is a bit high, but I was thinking about a foil or length of wire wrapped around the conductor.
The input impedance of the meter could also be much higher than 100M: 1G is doable, but the higher the impedance, the more noise pick-up will become an issue.
I agree, it would be an interesting experiment to determine if this is workable.
soldar:
Many years ago I built probes, similar to multimeter probes, but with short tips made with sewing pins or needles. This will penetrate plastic insulation and leave it intact when pulled out. I find this very useful for finding where a cable is broken, identifying one cable out of several, checking for voltage, etc. This would work in this case. If I wanted to make it a bit more sophisticated I could include a fuse.
Navigation
[0] Message Index
[*] Previous page
Go to full version