Re: Watt meters

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

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!)