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Re: Average, RMS and Power Factor made easy!



Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Tue, 9 Jan 2001, Tesla list wrote:

> Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <acmq-at-compuland-dot-com.br>
> 
> Tesla list wrote:
> > 
> > Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <free0076-at-flinders.edu.au>
> 
> > Note in the above example you might still get charged for the power you
> > aren't using... The metering systems that I have actually seen in my own
> > area won't compensate for the power factor, and I doubt that yours does
> > either, they just measure the current and charge you for it. So if you
> > draw 10A and your capacitor isn't getting warm you probably are still
> > paying for it.
> 
> This is not true. These meters are true Wattmeters. They don't look
> at power factors or RMS values, but measure the amount of energy 
> effectively consumed.

I'm glad your power company forked out for better units than mine did =)

> Old electromechanical units essentially integrate the force between 
> two coils, one with a sample of the current being consumed and another 
> with a current proportional to the voltage at the power line. This
> force is proportional to the average power being consumed.

Sounds like a variation of the electrodynamometer. I agree that it's not
hard to measure real power but the statement made was based on my own area
where they just use eddy currents to push a little aluminium wheel around.
The eddy currents are induced by an inductor in series with the supply
(maybe shunted?) and I have been told by an A-class electrician whom I
have respect for that they are usually like that in Australia, they don't
even care what RMS voltage you get fed when they measure the integrated
current. I'll ask him again though when I see him.

> (This can be done in a variety of clever mechanical arrangements.)
> More modern units do the same operation electronically, taking
> samples of the voltage and of the current, multiplying them, and 
> integrating the result.

I've used a few, and in three phase circuits too. They're cool but not
cheap =)

> 
> Antonio Carlos M. de Queiroz
> 

Happy coiling!
Darren Freeman