RE: Input power measurement

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

Original poster: "John H. Couture by way of Terry Fritz <teslalist-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>


Paul -

I think that it is fortuitous that you have been working for a electricity
metering company and you are also interested in Tesla coils. With your
background and the electronic coilers on the List we should have the proper
TC wattmeter in the near future.

Tesla coiling definitely needs a method of measuring the input energy that
is accurate and can stand the the tough operating electrical stresses. This
will help make TC comparisons more realistic and valid.

John Couture

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-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, March 16, 2003 5:42 PM
To: tesla-at-pupman-dot-com
Subject: Re: Input power measurement


Original poster: "Paul Nicholson by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

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
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