RE: Watt meters

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 19 Mar 2004, at 10:13, Tesla list wrote:

 > 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.

I have never heard of a reactive current being called imaginary but
it is anything but imaginary as someone who has connected themselves
to it can attest. The fact that a reactive current is 90 deg out of
phase with voltage does not make it unreal. There are plenty of
suffering switches and circuit breakers in the world which give
witness to that fact.

      Reactive currents are simply current flows which aren't
contributing to doing work.

Malcolm

 > 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
 >
 >
 >