Re: Watt meters

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

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi Jim,

Sonofagun, learn something everyday :-))

thanks,
Gerry R


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