# Re: Watt meters

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

At 08:15 AM 3/17/2004 -0700, you wrote:
>
>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!)

```