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In article <1676@yenta.alb.nm.us>, dt@yenta.alb.nm.us (David B. Thomas) writes:
> I was wondering what my instantaneous electricity usage was, so I went
> outside and watched my meter.  The little disc inside spun around once
> in 60 seconds, pretty much exactly.  What does this mean?  None of the
> other dials moved that I could tell.  How much energy is indicated by
> one 'spin'?

	The rotor of a conventional watthour meter is rotated by torque
caused by electromagnetic forces.  This torque is proportional to the
instantaneous power in watts (volts * amperes * power factor).  The speed
of the rotor in RPM is therefore proportional to instantaneous power in
watts.  Two coils in the watthour meter (a potential coil and a current
coil) produce an interacting magnetic field which in effect causes the
torque to be the product of volts, amperes and power factor.  A retarding
force is also placed on the rotor through the effect of two permanent
magnets.

	Watthour meters have a "watthour constant" (Kh), which is the number
of watthours represented by one revolution of the rotor disk.  There are
also other constants which describe the number of revolutions of the rotor
disk for one revolution of the least significant register digit.  There
is also a specification for rotor speed in RPM at a test rating. 

	In practice, there are various meter vendors and designs which
result in different watthour constants; this information may or may not
be stamped on the face of your meter.

	As a general rule the following formula should apply:

	INSTANTANEOUS WATTS = (DISK RPM) * (WATTHOUR CONSTANT) * 60

	Common watthour constants for 100 and 200 ampere single-phase
3-wire watthour meters are 3.6 and 7.2, respectively, but THIS DOES NOT
APPLY TO ALL WATTHOUR METERS.

	Using the above formula, if you had a 100 ampere service with
a watthour constant of 3.6, and the rotor disk moved at the speed of 
1.0 RPM, your instantaneous load in watts would be 1 * 3.6 * 60 = 216
watts.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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