RE: Watt meters

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

Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


Gerry -

The interesting thing about watt meters and watthour meters is that they
both use currents to operate them. There are three types of currents,
active, reactive, and total currents. They form a right angle triangle when
drawn on paper. The reactive current is sometimes called an imaginary
current. These are the ones to watch out for. You can take shocks from these
imaginary currents and never get hurt. When you see these currents on your
oscilloscope keep in mind that you are looking at imaginary currents and
that they do not exist. It is obvious why people have trouble understanding
how these meters work.

The engineers who design these meters are amazing people. They have to have
fantastic imaginations to design with imaginary currents. For example the
watthour meter as Jim pointed out is an electric motor that can distinguish
between active and imaginary currents. Is it a big deal to be able to select
an active current from a non existing current? And what about the wattmeter?
This meter uses at least two coils, one for voltage and one for current. The
magnetic fields of the coils are arranged to produce a wattmeter instead of
a VA meter. However, you got to admit it is not easy to design a KWH meter
that ignores imaginary currents. That is why utilities use KWH meters but
completely avoid using KVAH meters that don't ignore imaginary currents.

I am sorry that I could not better explain about KWH meters vs KVAH meters
to answer your very reasonable question. Jim and Terry did a much better
job.

John Couture

---------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, March 17, 2004 6:44 PM
To: tesla-at-pupman-dot-com
Subject: 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