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Date: Sun, 22 Dec 2002 17:17:13 +0100
From: Rene Tschaggelar
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Subject: Re: Watt meters - how does the old ones work? Watt Meter Project
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Klaus Vestergaard Kragelund wrote:
> In our part of the country of denmark we use watt meters that measures the
> power used in an appartment (to provide billing information to the power
> company). I was just curious as to how theese work exactly and what their
> limitations are.
> The power meter is one of those old types where three coils are inducing
> magnetic fields on a steel plate that rotates due to eddy currents in the
> material. I have been told years ago that the current runs through two coils
> inflicting magnetic fields in two positions and the third coil is connected
> to the mains line directly.
> The fields interact somehow to produce the average power on the load side.
> Something like P = U x I x Cos(phi). In which phi is the angle between the
> apparent power and the real power.
> So if the angle is 90 degrees - the power is zero. This leads me to a
> I am just curious if this kind of power-meter will be cheated if I connect a
> capacitor just a the start of a new period. Then at the top of the voltage
> peak the capacitor is disconnected and the stored energy is discharged into
> a load resistor. The cycle then repeats itself at the next period -
> effectively drawing energy into a load that is not registered by the power
> meter. Is this possible?
No. Loading a cap to extract power is true power and not blind power, and
measured as true power.
I lost the details of the inner workings, but they are well tested.
There are some methods to cheat, off course all illegal.
-drill a sub-mm hole and stick a few hairs in such that they rub on the
-use of a superconducting magnet to slow the rotation. Only feasible when
you're using MWh anyway.
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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