Dave Liquorice coughed up some electrons that declared:

On Sun, 07 Jun 2009 08:18:03 +0100, Tim S wrote:

Domestic meters are designed to read true power (kVAh meters do/did
exists for industrial environments where the billing was sometimes done
by the VA-h rather than a W-h).

Care to expand that for this bear of little brain when it comes to AC
power measurement and power factors.

True power is taken by integrating (or taking the sum of) a series of
instantaneous current and voltage measurements effectively multiplied. The
old magnetic disc meters did this multiplication in an analogue fashion
with a bit of clever physics to do with the way a voltage coil and a
current coil interacted on an aluminium disc, turning it into a motor,
which was balanced by a permanent magnet that tended to slow it down.

The speed of the disc gave the Watts and the integration was achieved by
tying the disc to a counter.

A VA(h) meter basically just measures RMS voltage and RMS current and
multiplies them (them being averages over a cycle or more) without regard
for any phase angle between the current and the voltage.

I have no idea how a disc meter achieved this - never seen one , only heard
of them.

In both cases, electronic meters are simpler conceptually - read current and
voltage at sub cycle samples, then apply whatever maths is desired and keep
a running total.

A capacitor is all that's needed to fix the PF on an inductive load.

Aye, that bit I do know. Has any one opened up a savaplug to find out what
is inside? I suspect not much more than a capacitor...


No idea. Probably chops the waveform in some way that I wouldn;t like to
have going into my products