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From: "John Jardine"
Subject: Re: Watt meters - how does the old ones work? Watt Meter Project OT
Date: Sun, 22 Dec 2002 16:35:48 -0000
NNTP-Posting-Date: 22 Dec 2002 16:22:46 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Klaus Vestergaard Kragelund wrote in message
> 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
> inflicting magnetic fields in two positions and the third coil is
> 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
> 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
> 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?
Klaus, how could you ;-)
The meter *will* register it, as the bit of real power you thought you had
As you have not returned or balanced that initial capacitor charge you
initially took, the meter quite rightly, will charge you money for it.
In the pure interests of science and for similar reasoning I once tried
200uF of flourescent lighting caps plugged into a wall socket. Result was
zilch. In fact a small extra power was registered due to the cable heating
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