[Pubtech] ME piece on Lower Voltages [RE-wrenches]

Wed Jul 25 17:26:25 PDT 2001

Pals:

Here's a tid-bit from another list I am on that you might find interesting
and informative (and pertinent to our list, as well). 

William

>Date: Wed, 25 Jul 2001 15:27:13 -0700
>From: Gray Frierson Haertig <gfh at haertig.com>
>Subject: Re: [Pubtech]  ME piece on Lower Voltages
>To: PUBtech Public Radio Tech Talk <pubtech at lists.wduq.org>
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>
>
>Instantaneous is always a relative term in the real world.  When it
>comes to cleaning my desk, instantaneous can be measured in months.  (c:
>
>
>A watt-hour meter is essentially an induction motor with the aluminum
>disk you see spinning in the meter as the rotor.  There are a pair of
>low impedance coils (with a core of some sort of magnetic material)
>hooked in series symmetrically off center on one side of the disk. 
>Imagine a pair of bar electromagnets pointing up and down beneath the
>aluminum disk.  They are forward of center and symmetrically spaced left
>and right of center.  These are the current poles which are hooked in
>series with one side of the line.
>
>There is another bar electromagnet above the disk in line with the other
>two poles and directly in front of the pivot.  This is a very high
>impedance coil which appears as a nearly perfect inductance.  It is the
>potential coil and is hooked across the line.  Because it's an
>inductance, the current through this coil is nearly 90° out of phase
>from the voltage across it.  Because it's not exactly 90°, there is a
>compensating coil, which is another set of turns on the same pole piece
>as the potential coil.  The ends of this coil are connected to a
>resistor which can be adjusted to bring everything into quadrature.
>
>These three poles induce eddy currents in the aluminum disk.  Assuming
>the load is resistive, the eddy currents from the potential pole are in
>quadrature with the eddy currents from the current poles.  Because all
>of this is off center from the pivot point of the disk, a torque is
>produced which causes the disk to spin.  This torque is proportional to
>the power in the circuit.
>
>In order to provide a rotor speed that is proportional to the torque, a
>braking torque must be applied to the disk that is also proportional to
>the rotor speed.  This is accomplished by a permanent magnet between
>whose pole the disk turns.  This produces eddy currents proportional to
>the rotor speed which retard the disk.
>
>If the PF isn't unity, the flux from the current and potential coils
>isn't in exact quadrature any more and the torque is proportionally reduced.
>
>With zero current in the load, if there is any asymmetry in the flux
>produced by the potential coil, the meter will tend to creep.  To
>prevent this, there are usually a number of slots cut in the perimeter
>of the disk which under the very small torque produced by the assymetry
>causes the rotor to stop with one of the slots under the potential pole
piece.
>
>Because the aluminum disk is so light, its response to changes in load
>is near enough to instantaneous that the power company isn't worrying
>about it.
>
>Gray 
>
>
>MICHAEL LeCLAIR wrote:
>> 
>> OK, I'll bite-
>> 
>> How does the mechanical device do the proper integration
>> over time of the voltage and current instantaneous values?
>> As a mechanical device with a rotational velocity it would
>> seem the word "instantaneous" would be a relative term
>> here (I'm talking about residential meters).
>> 
>> I've always wondered how those damn things worked...
>> 
>> Gray?  Harold?  Ira?
>> 
>> ML
>> 
>> Subject:                Re: [Pubtech]  ME piece on Lower Voltages
>> To:                     pubtech at lists.wduq.org
>> Send reply to:          "PUBtech Public Radio Tech Talk"
<pubtech at lists.wduq.org>
>> 
>> >
>> >
>> > On Tue, 24 Jul 2001 10:54:22 -0500 "Dupuis, Terrence E."
>> > <tdupuis at umsl.edu> writes:
>> > > Perhaps I missed this in this thread, but doesn't the power company
>> > > charge
>> > > by current draw?
>> > > The power meter principally looks at the current draw not the
>> > > voltage?
>> > >
>> >
>> >       kWH meters indeed measure kiloWATThours. They multiply the
instantaneous
>> > voltage by the instantaneous current and integrate it over time. Pretty
>> > amazing mechanical device! If you have a reactive load, it properly uses
>> > WATTS in the calculation, not VARs. Also, if you have, say, a three phase
>> > induction motor on the line, then drive the shaft to above synchronous
>> > speed, the motor is putting power back into the line (the power coming
>> > from the mechanism driving the shaft), and the kwh meter will go
>> > backwards. In California, we have "net energy billing", so if you do
>> > small scale cogeneration and dump power back into the grid, you are
>> > billed for the net energy consumed as long as you remain a net consumer.
>> > If you are a net producer instead of consumer, you are paid the wholesale
>> > rate for the electricity you dump back into the grid.
>> >
>> > Harold
>> >
>> >
>> >
>> > FCC Rules Online at http://hallikainen.com/FccRules
>> > Lighting control for theatre and television at http://www.dovesystems.com
>> >
>> > ________________________________________________________________
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>-- 
>Telecommunications Engineering
>Gray Frierson Haertig & Assoc.
>820 North River Street, Suite 100
>Portland, Oregon 97227
>503-282-2989
>503-282-3181  FAX
>gfh at haertig.com
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__________________________________________________________________
William Miller
SLO Communications: Communications and Power Systems Consulting
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email: wrmiller at slonet.org
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