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Re: Watt meters



Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Jim,

At 02:26 PM 3/17/2004, you wrote:
>At 08:15 AM 3/17/2004 -0700, you wrote:
>>Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
>>
>>Hi John,
>>
>>Is this true?  I thought that the electric meter just logged volts * amps *
>>hours (really a KVAH meter) and did not figure into it the power factor
>>(maybe assuming that residential users's power factor was one).
>>
>>Gerry R
>
>
>Nope.. it's a real clever electric motor.  Consider that the torque of a 
>motor is proportional to the armature (rotor) field multiplied by the 
>field (stator) field.  In a Permanent Magnet (PM) motor the stator field 
>is fixed, so the torque is proportional to the armature current.  In a 
>series wound motor, where the stator and armature are in series, the 
>torque is proportional to the square of the current.
>
>In a watt-hour meter, the stator field comes from the current in the wires 
>and the rotor field comes from the voltage (potential) in the circuit (I 
>may have the two backwards, but you get the idea).  So, the torque on that 
>little disk is proportional to the instantaneous product of I and 
>V.  There's a viscous drag on the disk, proportional to rotational speed 
>(created by a permanent magnet acting on the disk, oddly), so the 
>rotational speed is proportional to the torque, which is proportional to 
>the instantaneous product of V and I (or active power).
>
>Very clever, isn't it...
>
>A typical home meter actually has 4 windings  (2 for current and 2 for 
>potential), because of the neutral and the possibility of imbalance 
>between the two sides.
>
>The windings can either be energized directly, or by a small fraction of 
>the actual signals feeding that which needs to be metered.  For instance, 
>if you had your factory supplied with 14.4 kV at 100 Amps, they would 
>typically put in a 200:5 current transformer and a 14.4kV:120V potential 
>transformer and drive an off the shelf watt hour meter designed for 5A 
>current and 120V potential. (now you know why they use those current 
>transformers and potential transformers!)


Since I have worked with them raw, I have looked at them carefully inside 
and they are a marvel of engineering too!!  Little coils, gears, 
magnets...  Very accurate and can operate in any outdoor temperature, wind, 
snow, rain...  Their failure rate is just about zero, they don't drift out 
of calibration, and their lifetime is practically forever...  They are also 
pretty inexpensive.  It is still very rare to see electronic ones being 
used unless they do the radio communication thing to record the 
numbers....  I have seen more than a few electronic ones in the trash, but 
the mechanical ones never are thrown out unless someone backs a truck into 
them.

Even though everything else is electronic these days (even TV tubes, and 
street lights now...), the good old mechanical watt hour meter continues to 
dominate the market with no end in sight.  The new electronic meters just 
can't compete with the "perfect" old mechanical ones.  They are also 
practically impossible to kill with a Tesla coil or other nasty transient 
filled electrical loads ;-))

Cheers,

         Terry