Electronics > Projects, Designs, and Technical Stuff

KWh meter - is there such a thing as a non-contact one?

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floobydust:
The utility industry uses capacitive voltage dividers. I have seen them on 240kV power lines to monitor voltage, instead of potential transformers (PT). It's much lower cost.
But at 120VAC, it's easiest to use a resistor divider.

There are single phase DIN rail mount energy meters for under $30 on eBay. Probably burn down your house unless it uses pass-through CT's, fused voltage sampling.

EEVblog:
Not commercially that I am aware of, apart from the clamp based ones, but I know someone working on a multi vector magnetometer based design for remote non-contact switch board current measurement.
Resolution isn't great and requires calibration, but it works. Can't measure voltage of course.

soldar:
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.

EEVblog:

--- 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.

Zero999:

--- 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.

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