Re: Wattmeter

-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Monday, January 25, 1999 10:51 AM
Subject: Re: Wattmeter

>Original Poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>Tesla List wrote:
>An electromechanical watt-hour meter probably measures correctly the
>power for arbitrary waveforms. The mechanical inertia is just the filter
>that averages the current x voltage product, and is not a problem for
>long averages. And as the voltage is essentially a 60 Hz sinusoid for
>any reasonable current, active power flows only at 60 Hz, so other
>frequency limitations, as inductances of coils in the meter, are of
>little importance.

I've played around with a watt hour meter I picked up at the local utility
scrap yard. It has 5A full scale current (obviously designed to work with a
current transformer) and has 240 V potential winding. I was originally using
it to measure power factor on lightly loaded ac motors (which it did
admirably). However, when looking at highly irregular current waveforms
(like that into a lightly loaded capacitor input filter) where the current
is very peaky, I noticed that there were some errors, typically, reading on
the low side. I suspect it was the higher harmonic power in the current
winding getting sucked up into eddy currents, or the like.

In any case, it is easy to use. What you do need to do though, is to put a
known load on it and time how long it takes for the disc to turn. It takes a
long time to measure power if you actually use the dial, since it has 1 kWh
resolution (that means running that 1 kW coil for an hour straight....)