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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Domestic KiloWatt Hour Meters
Message-ID: <1715@kitty.UUCP>
Date: Sun, 19-Apr-87 12:45:06 EST
Article-I.D.: kitty.1715
Posted: Sun Apr 19 12:45:06 1987
Date-Received: Sun, 19-Apr-87 20:38:38 EST
References: <923@mhuxh.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 78
Keywords: meters,power,watthour
Summary: Watthour meter operation

In article <923@mhuxh.UUCP>, oso@mhuxh.UUCP) (Edward J. Osoliniec) writes:
> I need some help in understanding the operation of the
> domestic KW-Hr. meters the electric utility companies
> use to calculate my electric bill every month.
> 
> It was my understanding that the KiloWatts in each
> phase of my 220 Volt service was measured, and integrated
> to obtain the KW-Hrs.
>
> Anybody have any insight??

	The most common type of watthour meter for domestic 124/240 volt
service is called a "two-element" watthour meter.  In this device, the
power through each hot wire is measured through an individual current winding,
and through an individual potential winding which measures the potential
of each hot leg to ground.
	There are therefore TWO current windings (one for each hot leg),
and TWO potential windings (one for each hot leg to ground).  The net
energy measured is the algebraic sum of the energy in each hot leg.
Since the potential of each leg is measured with respect to ground (which
is 120 volts), a 240 device which has no ground current flow shows 1/2 its
power in each leg - which is, of course, summed to read actual power.
	The watthour meter is effectively calculating:

	P = (I1)(E1) + (I2)(E2)

where P is total power, I1 is current through leg 1, E1 is voltage from
leg 1 to ground, I2 is current through leg 2, and E2 is voltage from leg 2
to ground.
	P above is actually represented as an AC magnetic field, so the
strength of this magnetic field is proportional to the instantaneous power
being consumed.  Contained within this magnetic field is a metal disc
which functions as the rotor of an induction motor; the magnetic field,
as created by the stator current and potential coils, induces eddy currents
in the metal rotor disc, thereby making it function as a motor.
	The greater the magnetic field, the faster the rotor disc turns.
The rotor disc drives a totalizing gear train to display kilowatt-hours.
There are also permanent magnets through which the rotor passes; these
magnets provide damping.
	This magnetic field/rotor arrangement provides a true integration
of instantaneous power, which therefore results in an energy measurement
as kilowatt-hours.
	Some older 120/240 volt watthour meters have only one potential
coil which measures voltage between the hot legs.  Since many loads on
a 120/240 volt power system are 120 volts only, this is not as accurate
as a meter with two potential coils.

> A friend of mine, who is an electrician by trade, claims
> that the >>DIFFERENCE<< between each phase is integrated.

	There could be some confusion in semantics in this statement, but
I am inclined to say it is oversimplified and false.  See above for actual
explanation.

> References will be helpful

	These publications used to be given away free of charge by
General Electric:

(1)	"Manual of Watthour Meters", Publication GET-1840C

(2)	"Application of Watthour Meters", Publication GET-1905D

(3)	"Manual of Demand Meters", Publication GET-2327C

(4)	"Metering Products Buyers' Guide", Publication GEP-550B

(5)	"Guide for Installing General Electric Watthour Meters",
	Publication GET-2669H

	The above materials were in my watthour measurement technical
data file.  This is NOT a "plug" for General Electric, although they
do give away helpful technical publications. :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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