🚀 go-pugleaf

In article <1262@island.uu.net> rich@island.uu.net (Rich Fanning) writes:
>In article <25675@cup.portal.com> Dick@cup.portal.com (dick a wotiz) writes:
>>You probably won't want to build a meter that will be accurate enough
>>with an inductive load like a refrigerator.  But I've seen surplus
>>real electric meters (like the one on the side of the house)
>
>Do the utility meters on the average house measure true watts or only current?
>I have always assumed they measured watts, but when I think of it now, why
>shouldn't the utility charge by the amp? It seems as if the utility
>charging for "amp-hours", rather than for watt-hours, would be in THEIR best
>interest.

Watt-hour meters measure true watts and are typically certified to meet 
tolerance specifications down to about 60% PF.  Physically, the torque 
that turns the meter disk is the product of the instantaneous voltage
and current components.  If both components are in phase, a simple
product is representative.  At the other extreme, if the power factor
is 0, ie, the current and voltage are 90 degrees out of phase as with
a purely reactive load, there is no torque generated because the voltage
and current components never coincide.

The utility is permitted to charge the basic rate for "work units" or watts.
They are also allowed to charge a power factor penalty for low power factor.
This is normally not done for residentials users, as the assumption is
that residential loads are close to resistive. 

>Utilities provide financial incentives for raising the power factor of the load
>of industrial companies.  I have always assumed that the industries get
>charged for "amp-hours".

Not in those terms.  For moderate size industrial loads, the utility will
take a representative sample of the power factor and calculate a PF penality
if the power factor is below a threshold.  For large loads, VARs (volts-amps,
reactive) are also metered (typically by using a watt meter with a reactive
element in the voltage winding to shift the phase 90 degrees.) and 
charges are calculated directly on the reactive current.

Generating stations also meter VARs but for the purpose of adjusting the
field excitation on the generator to reduce the VARS as much as is 
feasible.  Varying the field excitation on either a generator or a 
synchronous motor can make lagging VARs, leading VARs or unity PF.

John

>
>Do household and industrial customers get metered differently in this regard?

Yes


-- 
John De Armond, WD4OQC                     | The Fano Factor - 
Radiation Systems, Inc.     Atlanta, GA    | Where Theory meets Reality.
emory!rsiatl!jgd          **I am the NRA** |