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From: "Klaus Vestergaard Kragelund"
Subject: Watt meters - how does the old ones work? Watt Meter Project OT
Date: Sun, 22 Dec 2002 16:56:40 -0500
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Organization: TDC Internet
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
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?
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