Re: Input power measurement

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Paul,

Good catch there in catching John in an "error".  First error John has ever 
made in my memory :o)))  Hahahha!  He has a nice "real" power meter I just 
sent him now too!!!  John's TCCG has remained almost perfectly error free 
for almost a decade!!  John usually catches us in errors, rather than us 
catching his vast knowledge!!  ;-))  John is one of the very top gurus in 
our sport...  I take his posts simply as "fact"!!

Really cool about your power meter stuff!!  I have "stuffed" lots of power 
meters ;-)), but they were the old "coil, iron, and bunch of gears" 
things...  If these new ones can flash an LED to high precision, then any 
old scope or counter can probably pick out that timing to very high 
accuracy.  Real power meters are well known for "lack" of failing...

But, I am going for these Prodigit power meters from China...  They put 
together all the good "tech" and also have the "cheap" factor too.  We just 
have to make them not blow up.  The UL, TUV, CE, guys have made these a 
little "too" fail proof...  Better to fail the meter 50% of the time rather 
than cause a house fire 0.00001% of the time thing...  Many thanks to Jim 
in mentioning these wonders to us and we hope China can fill our demand for 
them ;-))  They are pretty hard to find "in stock"...

My power meter is still coil iron and gears...  But the new water meter 
(that I ground everything too) is now all fancy electronic gizmos that the 
lady passes the wand in front of and the thing records the water usages at 
a distance...  My water bills are reasonable as opposed to 857498578457 
quadra zillion cubic light years so I guess coiling has not affected 
it...  The water meter seems to just click off every 100 gallons down here, 
but he meter on the house seems to have all the brains safely distant...

I think real accurate input power measurement is now an "easy thing"...  We 
may pop a meter know and then, but it is still "easy" nowa days...

Cheers,

         Terry



At 10:34 PM 3/16/2003 +0000, you wrote:
>Hi John,
>
>Yes, to extract the active component of the current means determining
>the portion of the current which is in-phase with the load voltage.
>
>Thus there's got to be some component which can take in both signals,
>and that component has to somehow effect an instant-by-instant
>multiplication of the two waveforms.
>
>Following your example, I've been putting some thought to how this
>might be done with readily available kit.  By chance, most of my work
>for the last several years has been for a company which designs and
>builds electricity meters, and I look after the production test and
>calibration software.  Modern domestic meters use a resistive shunt
>to sample the current and typically perform analogue multiplication
>in an ASIC followed by numeric integration in an associated uP.
>The result is very stable and remains accurate over a wide range of
>phase angles (both forward and reverse) and is fairly insensitive to
>waveform.  Electronic meters have a flashing LED on the front
>whose pulse rate indicates the active forward power.  The rate is
>stamped on the front of the meter as so many impulses per kWh.  The
>coiler would have to rig up something to time these pulses in order
>to measure the power in real time.  The alternative is to run the
>coil long enough for the kWh register to clock up a few units, which
>may be inconveniently long.  Maybe I should ask the R&D guys to knock
>together a 'special'.   They can be reprogrammed to flash at a
>much faster rate than normal, for example setting it to 1,000 flashes
>per kW of power would do nicely. Then your frequency counter watching
>the LED would read the active kW as kHz.
>
>I don't know how well they stand up the the punishment of a TC, but
>surely with all the coilers on the list here, there must be a few
>who are already runnings coils off a domestic supply which is fitted
>with solid state metering.  Has anyone ever blown their electric
>meter?
>
>Another approach would be to use an analogue multiplier IC fed from
>a current transformer and a voltage transformer.  Maybe one of the
>circuit design whiz kids could invent a standard model in kit form.
>That would give you a cheap, self contained unit which would be easy
>to calibrate, and might be better than fiddling about with modified
>electric meters and frequency counters.
>
>This issue of input power measurement is the first stumbling block
>on the road to comparative measurements on different coils, and
>really should be made to succumb to a determined effort.
>--
>Paul Nicholson
>--