Dan Lanciani wrote:
In article .com, (kevin) writes:
| So can anyone answer the op's original question? Does a mechanical
| meter overestimate when the load is imbalanced?
Yes, in the sense that the typical 4-terminal (1.5 element) meter most
often used in residential split-phase service charges the customer for
150% of the losses in the neutral on the utility's side of the meter.
(That's the simple case of one service drop from transformer to customer.
For multiple connections the analysis gets more complicated.) This seems
to surprise many people. Google for Blondel's theorem before trying to
come up with an argument that the meter does not make this error. 
I believe you, and If I understood you, when speaking of mechanical
meters the 1.5 element meter you describe has one voltage field across
the 240 volt line and two current fields, one in series with each "hot"
conductor.
I think I see what you're referring to.
Reducing it to absurdity, if you only load one side of the line, all the
return current flows through the neutral, but the voltage field still
"sees" the full 240 volts at the meter terminals even though the power
you are using isn't the product of the current drawn time half of the
240 that voltage, it's the current times half of that voltage MINUS the
voltage drop in the neutral.
So, you get charged for the power equal to that current times the "full"
voltage, when you are actually using power equal to that current times a
slightly reduced voltage.
Izzat it?
Jeff
N.B. This has nothing to do with the meter's being mechanical. You can
build a 5-terminal (2 element) mechanical meter that does not have the
error.
I'm guessing that kind of meter must have TWO voltage fields?
You can build a 4-terminal (1.5 element) electronic meter that
does have the error.
| And if so, can anyone
| think of a plausible scenario, in any possible situation, real or
| imaginary, that would let the device in the PDF picture (I can't tell
| what it is -- just some kind of metal C-shaped thing I guess) do
| anything that could even have a remotely possible chance of having even
| a miniscule, undetectable, insignificant but still non-zero effect on
| the meter?
Maybe it's the AC equivalent of the "cow magnet" to be placed on the
fuel ine of your car. :-)
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"Truth exists; only falsehood has to be invented."