Sunday, October 6, 2013

Explanation of how kilowatt-hour meters work (electromechanical)

I take apart an electromechanical power meter and describe how it uses magnetic fields to measure power consumed.



5 comments:

  1. Hello, I am very interested in power monitoring.

    I have seem some power meters that only use a CT coil that is clipped on 1 leg of the house mains. My questions is how can a meter like that get an accurate kW reading with out a direct tap to the mains line for a voltage measurement?

    For example this one here, the instructions only show a clamp on meter, no direct connection to the mains.
    http://www.theowl.com/helpcentre/owl%2Busb.html

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  2. Jeff, for approximate readings, the system will assume constant voltage and power factor, so the clamp-on sensors only measure current, and the software multiplies by a constant.

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  3. Right. that is what I keep thinking. there is no way to get a real power measurement with out a voltage measurement. The problem is my application the voltage can vary 10+ V p-p so that won't be close enough.

    I think I will end up using a step down transformer, or a 100:1 voltage divider to get the voltage in a manageable ADC range and then scale it to make real power calculations.

    I have also read but don't understand some statements about current and voltage not being 90deg out of phase. The example I read was power a large motor at 4A might look very different then powering a heater at 4A. Do you have any understanding of that concept?

    Thank you so much for your insights.

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  4. Jeff, yes the concept is known as "power factor". You can search for my videos on electrical impedance if you want to know more background info. Basically, the power factor of a device depends on how it is built (whether there are any coils or capacitors involved). Typical devices range from .6 (cheap motors) to 1 (electric heaters). Devices with low power factor will consume less power than it seems if you measure their power by only looking at the current waveform.

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  5. I've often wondered how those mechanical meters worked myself. Thanks for your thorough explanation. I particularly appreciate your questions about the asymmetrical transformer. Refreshing to know that (with utmost humility) as smart as you are, no one knows everything. Thanks again.

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