On 29 Apr 2005 05:19:14 -0700, "Tim Shoppa" wrote:

The other issue here, w/ electric meters:
Are *they* being fooled by power factors, wave
forms, pulses, etc. etc.???

Residential kWH meters use two coils, one producing a field
proportional to voltage and another producing a field proportional
current. The fields are perpendicular to each other; one induces eddy
currents in the circular disk that spins around, and the other actually
moves the disk. It's the product of the two fields - true power - that
causes the disk to rotate. They're very fine devices, and the legal
specification requires them to be accurate to better than 0.1%.

The commercial ones that rack up charges based on power factor etc. are
more complicated.

Tim.


Having done metering work on power station performance tests I can say that
0.1% is at the limit of achievability for rotating kWh meters and not
achievable over a wide rage of loads or power factors without correcting for
errors. Domestic meters also do not have sufficient frequency stability or
power factor stability to be within the 0.1% meter class. These are both a
side effect of the thick disks required to generate sufficient torque to drive
the meter dials.

Domestic meters in the UK are required by law to be within +2.5% -3.5% error.
Electronic meters can be much more accurate than rotating meters for a similar
cost and will eventually completely replace electro dynamic ones.

I was very amused in 1988 when I was required by the customer to have my site
meters verified by the Indian NPL prior to a performance test at an Indian
power station. I brought my (Norma 5155 power analyzer) electronic Whr meters
to be checked at the Indian NPL with their fresh calibration certificates from
the British NPL. The chap at the Indian NPL in Delhi thanked me for the
opportunity to check his standards with my (better) ones.


Having said all of that. what your meter says is what you pay for whether its
right or wrong, so it pays to look at what it says :-)


regards
Mark Rand
RTFM