Energy - Power Meters

Most electronic meters are fairly wideband gadgets. But usually there's little or no real power in the harmonics.

If the line voltage is an undistorted sine wave, there will be no power created by any current component but the fundamental, and that includes the DC term.

John

Ok but let us suppose that we have a machine that deliberatly generates higher current harmonics. (I believe I am testing such a machine). Will the meter read low?

Hardy

On a sunny day (Sun, 28 Jun 2009 14:22:20 -0700 (PDT)) it happened HardySpicer wrote in :

There was a recent discussion about this. I think if you take samples of current and voltage every x microseconds, on a 50 - or 60 Hz mains, and multiply those, and then add those, then you get true power per unit of time. The samples need to be smaller then the higest frequency - say shortest pulse - present.

Wanted to do the PIC thing, but too much work, as Larkin already did all that I think, but probably with a 68000 ;-)

Then you haven't seen a powerline plot from an "installation" like this yet:

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On a sunny day (Sun, 28 Jun 2009 14:55:01 -0700) it happened Joerg wrote in :

Looks like it is tuned somewhere between MW and shortwave radio.

Like downstream from something that makes non-sine waveforms? Then the current harmonics matter. You'll have to read the specs on any specific electronic meters and see what their bandwidth is. Most electronic meters will be fairly wideband, just because that's the easiest way to do it.

I think classic rotating-disk meters will read low. Not sure.

John

Do all those wires generate harmonics?

John

picer

that

Yes that's right but with a normal Watt meter you would get a false reading. Suppose the current was a train of pulses for example.

Hardy

I'd imagine that the local power company could cut their distribution losses simply by having a few crews dedicated to 'cleaning up' poles like this. Just snip anything that isn't properly spliced or otherwise attached and move to the next pole. Make sure to take enough wire out so the thiefs will have to scrounge for more to make it reach the lines.

In fact, the best way to reduce this nonsense is to distribute power at

4 or 12kV and then meter one point feeding a distribution transformer. Then, along the lines of third world telephone rentals, you sell this power to a local businessperson that resells it to other people on the block. They can drag their extension cords or whatever up to the local vendor and he can work out the billing. So much per extension cord, install sub-metering, or whatever.

People are much less likely to steal from a local resident than the big, faceless corporation, there's a guy on site with an interest in keeping an eye on splices, and it opens up a business opportunity for poor people in the slums.

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If you get a chance, watch the PBS documentary 'Power Trip' about AES in Georgia. Very interesting. Nobody pays their bills.

Cheers

HardySpicer

Nyquist limit.

Sample fast enough to capture the highest harmonic for which you anticipate significant power.

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John Larkin a écrit :

Yup, they're definitely not OFHC. Monstrous cabling should use Monster cable.

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Thanks,
Fred.

s yet:

So how the hell can this work?

There is a youtube video of it working as well. The power company charge you for VI don't they so it is best to have unity power factor. I would have thought most PFs in homes are near unity anyway.

No, but the stuff that is connected to them. In countries like this there are no regs related to power factor. Or pretty much anything else for that matter.

The worst I have seen was a 13.56MHz high power RF source for materials processing. Kilowatt range. Size of the capacitors after the rectifier: Zero uF. There were none ...

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It can't.

Domestic consumers are only charged for real power, so power factor doesn't matter (rather, it only matters to the extent that it affects copper losses within the premises, which is negligible).

The above device wouldn't save the average consumer 0.1% of their bill, let alone 15%.

I designed a household meter that was intended for manufacture and sales in India, by a division of Niagra Mohawk that turned out to be run by lunatics. It used an 85-cent HC05 uP with an on-board 8 bit ADC, and inductive-coupled ASK meter reading. The readout wand had to transfer enough energy into the meter to read it when power was off, which happens a lot there.

It had a lot of anti-tamper hooks. Stealing power is more popular in India than cricket. Anybody who has ever been a military officer expects to get power free for some reason, too. And the meter readers are easily bribed.

Our BOM cost was something silly like $11.

Caps would probably have made the harmonics worse. SCR phase controls are pretty bad.

Are computer power supplies PFC yet these days? Our power line waveform is still flat on top.

John

Well, there were to big fat tubes in there and the current waveform was really ugly.

Most of them would probably have to be because AFAIK Europe has instituted a PFC mandate for anything above a certain level and that level ain't high. Somewhere around 70W I believe. Of course it is possible that they scrape off all the PFC stuff for markets currently not yet regulated in that respect, to save a few pennies.

Naturally, CFLs and such are allowed to fly under the radar screen.

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So when I was at NASA-LeRC, some labs had several fridges, microwaves, blenders, etc. I knew enough re: procurement that there HAD to be a backstory... and asked.

[The lab techs in my area had a cornpopper, but I think they chipped in themselves...]

Seems they'd been handed a mandate to test the last appearance of the above alleged energy saver in residence use. [Per the ad claims...]

And tested they did, with enthusiusm. They needed household appliances, of course.

Here is where they found it is possible to save: If you improve the power factor, you reduce the current through an induction motor, but not the power, except.... [After all, power out must equal power in, or some basic laws of phizzies get upset...] And you pay for true power.

What you DO save with that reduced current is the *slightly* reduced I^R losses in the motor windings. But those losses are such a small fraction of the whole picture; it matters only if you are a pitchman of the Billy Mayes ilk. You'll do better to close the fridge 5 seconds earlier so the light goes off faster...

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I'm not sure what an ASK is here...

I assume the voltage sampling needed direct connection to the line. Also lots of transient/overvoltage protection. Sounds ..interesting... to achieve at low parts cost..

Oh yes... Were you asked to build in a back door for the important folks?

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