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.

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

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.

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.

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.

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.