# Induction kWh meter theory

Hi all,
I'm wondering if I can draw on the knowledge of some of the electrical engineering experts here. This coming week I'm going to be talking to a
group of high school students about engineering. They will mostly be 17 year olds who study physics and mathematics and have a pretty good understanding of science. People who might become future scientists or engineers, or who might be drawn into IT, management consultancy or whatever. My job is to offer them a brief insight into engineering and to take them through a few questions of the kind that might be asked by university interviewers.
Usually I take along a few small machines for us examine and discuss. Last time I took a box of small electric motors in various states (some complete and working, some disassembled) and this worked well. This time I was thinking of taking a few domestic induction meters. I know that energy measurement is on their syllabus, but that the exact theory behind the meters is a little above it.
I have hunted for the oldest meter I can find, which offers a good view of the magnets and coils. As I understand it the conductive disc, which is connected to the counter via a gear train, is acted upon by three magnetic fields, all of which act perpendicular to the disc. One field is produced by a coil wound with a few turns of thick wire. This coil carries the line current and shows little inductance, so the magnetic field it produces is in phase with the line current. Another field is produced by a coil wound with many turns of thin wire. This is connected between the live and neutral of the supply, and is highly inductive, so that the magnetic field it produces is almost in quadrature with the supply voltage. I think I'm right in saying that for the meter to work, this field must be at least as strong as the field produced by the maximum allowable line current through the first coil - perhaps someone can confirm this for me? A third field is produced by strong permanent magnets. This produces a retarding torque which is proportional to the rotational speed of the disc. When the load is purely resistive, the two alternating magnetic fields are in quadrature, and the disc experiences a moving magnetic field which drags it around. When the load is purely inductive, the two fields are in phase, and the disc experiences a pulsating magnetic field which does not drag it around. Of course the usual situation is for the load to be partially resistive and partially inductive, and in this case the meter only registers the consumption of real power.
So that's my understanding of how an induction meter works. Do correct me if I've got anything wrong. Now (at last) on to my questions. The oldest meter I have has an arrangement of coils like this (the more modern meters have encapsulated coils which are small and hard to see):
|----------------| -----| |----- | | High L | | | -| |- | | | |----------------| | | | | | | | -------------------- | | Core | | ------- ------- | | | | | | | | | ---------- | | | | | | | | | | | Low | | | | | | L | | | | | | | | | | | ---------- | | | | | | | | --- ---- --- ---------------------------------- <- Disc ---------------------------- | Core | ----------------------------
Here are some pictures of the meter:
http://www.mythic-beasts.com/~cdt22/elec_meter1.jpg
http://www.mythic-beasts.com/~cdt22/elec_meter2.jpg
http://www.mythic-beasts.com/~cdt22/elec_meter3.jpg
So here are a few questions:
1. Why does this meter use the arrangement of coils and core shown above? Someone is bound to ask me.
2. Does anyone know of any eloquent, succinct explanations of induction meter theory online which I can read over?
3. Does anyone know who invented the induction meter? I was under the impression that it was Elihu Thomson, but I'm not sure of this.
I'm not firmly decided on using the induction meter as an example for discussion, but these are bright students and the meters can be found in almost every home in England, so it seems like a good topic.
I'd be interested to hear your thoughts.
Best wishes,
Chris
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Christopher Tidy wrote:

Dunno, but I suspect that the section of core between the 'high L' and the rest is to shunt the magnetic field for even _higher_ L, and to help match the high L field strength at the disk with the low L field strength. The pole pieces would be separated to help give a traveling magnetic field.

Dunno, but if you find one before someone posts it, could you post what you find?

You're missing out on an important point: The torque developed by the high-L, low-L windings will be proportional to real power used -- so it'll be slow when real power consumption is low, high when real power consumption is high, etc.
--

Tim Wescott
Wescott Design Services
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Tim Wescott wrote:

Of course. Thanks Tim. It slipped my mind while I was typing that bit about the magnetic field produced by the high-L coil being at least as strong as the maximum magnetic field produced by the low-L coil.
Best wishes,
Chris
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Hey Chris,
Can't help with the on-line explanation, but in my former occupation it was necessary to have some means of determining/guaranteeing that three phase power was being supplied in the correct phase relationship. If it was "backwards", the lift would go up when it should go down, and vice versa. Bit more to it than that of course, but prior to the contemporary solid state devices used now, the device Otis Elevator used world-wide for many years from sometime in the 1920's until well into the 1970's (and still functioning on many installations today) is referred to as a "Reverse Phase Relay", and commonly called "The J switch". It actually is just a very similar device to what you have described the meter, being a quadrature of four coils perpendicular to a 1/16 thick vertical copper disc about 5" in diameter (from memory) mounted on a horizontal needle axle . Application of power would cause the disc to rotate in one direction or the other, and the axle had a switch (the J switch) mounted to it which would shunt a contact pair allowing motor start if the phases were correct, and the disc would stall and remain stalled by the contacts until the power was removed. Phase reversal would cause it to rotate away form the contacts ands stall against a stop provided. Never made more than maybe a 20 degree rotation motion in use.
Wouldn't surprise me if you could contact any Otis branch office and ask if they have an old one kicking around that you might have or borrow fore the week.
We also used "stalling torque motors" to lift cams, but they were very ordinary in construction.
Take care.
Brian Lawson, Bothwell, Ontario. XXXXXXXXXXXXXXXXXXXXXXXXX
On Sun, 09 Jul 2006 22:43:08 +0000, Christopher Tidy

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On Sun, 09 Jul 2006 22:43:08 +0000, Christopher Tidy

--
Posted via a free Usenet account from http://www.teranews.com

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clare wrote:

<snip>
Thanks for the link. That's a very useful document.
Best wishes,
Chris
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On Mon, 10 Jul 2006 02:05:47 +0000, Christopher Tidy

The magic part of the rotating watthour meter is the fact that force is related to current multiplied by magnetic field. The current coil induces a current in the disk circulating around the central pole piece and this current interacts with the magnetic field produced by the voltage coil. All of a sudden you have a force/torque that is proportional to the instantaneous product of the voltage and the current... This is of course, the instantaneous power. The brake magnet produces a constant field and a current in the disk proportional to the speed of the disk, therefore a braking force/torque proportional to the speed of the disk. Add it all together with a bit of inertia (ignoring friction, saturation, resistivity of the disk etc) and you get a disk rotating at a speed that is proportional to the average of the power represented by the voltage and current circuits.
Mark Rand (who spent 7 years tending. calibrating and carrying kWh meters around the world for power station performance tests in a previous life) RTFM
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Mark Rand wrote:

Thanks very much. That's a nice explanation. Do you know why the designer might have chosen the interesting shape of core shown in my earlier post? I know there are several variations of this, but they all see to have one three-legged core and one two-legged core.
Best wishes,
Chris
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On Mon, 10 Jul 2006 21:55:45 +0000, Christopher Tidy

In all cases it will be so that the currents induced in the disk by one winding are circulating in the field produced by the other winding. in the one shown in your photos the currents induced by the centrally placed current winding circulate under the poles of the voltage winding.
The forces produced follow the " Fleming's left hand rule". hold our left hand so that the thumb, first finger and second finger are at right angles to each other. then you have the directions given by:_
thuMb=motion First finger=field seCond finger=current.
So you can see that with the voltage coil generating its field through the two sets of poles (down through one and up through the other, and the current induced by the field from the current coil (circulating the pole piece) going into the screen/paper on one side and out of the screen/paper on the other side, that the force generated in the disk will be in the same direction at each of the outer poles.
Also you may have noticed that the currents induced by the field from the voltage coil can interact with the field from the current coil (why not, the same rules apply to all the fields and currents involved). If I haven't completely lost the plot (quite possible), the forces produced by this combination of fields and induced currents, add up in the same direction as the other lot.
Mark Rand RTFM
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Christopher Tidy wrote:

Actually, I think I might be figuring it out now. Still got some figuring to do, but the shape is starting to make sense...
Thanks.
Best wishes,
Chris
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On Mon, 10 Jul 2006 21:55:45 +0000, Christopher Tidy
snip

For the watthour meter to respond to real power and accurately reject reactive power the current and voltage magnetic fields must be pretty precisely 90 deg apart when feeding a resistive load. (0 deg and hence no torque when feeding a purely reactive load)
The field produced by the current coil is is inherently 0 deg in phase but the field produced by a similarly wound voltage coil would lag by a lot less than 90 deg dependent on the L/R ratio of the coil.
The iron cross leg is a magnetic shunt which produces a large increase in the voltage coil inductance and this permits the current lag in the voltage coil to closely approach the ideal 90 deg lag. It is equivalent to a large external inductance in series with a conventionally configured voltage coil
Jim.
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The mechanical SAGMO units were rated at 120 years minimum. The useful life would be that of the house or more. It was like pulling teeth and kicking but when they changed over to electronic readouts... That dropped the life due to semiconductors. But the Electric companies wanted data logging, time of day..... Data logging is over the power lines - no meter readers - a computer. Time of day - multiple (2 or more) rates during a 24 hour time. Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
snipped-for-privacy@yahoo.com wrote:

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Martin sez: "But the Electric companies wanted data logging, time of day.....

Reminiscent of our Motorola days, what?
Bob Swinney
wrote:

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Yep Bob I was cleaning up the shelves for more books and ran across : 'my copy' "Load Management Study of Irrigation Consumers - Farmers Electric Cooperative Corp. Newport, Arkansas Dated Jan 1979." An Engineering Report by Allen & Hoshall of Memphis.
Remember this one - I think we were running 173MHz and had to 'swap or beg' frequency tones from another company in the area as the pine trees - the needles absorbed... So 403 ? we came. Seem to want to use those numbers and reason. I think you determined the issue and worked the transmitter problems.
Jean Seweat was the Manager at Farmers Electric at the time - The blue print copies of graphs are still good but the edges are turning...
We did a lot of good for people and enjoyed a lot of visits across the south and out to Sackoftomatoes as well!
Actually the meters I was talking about - the new time of day by Sagmo has some of my code inside - binary to bcd conversion. It was a Schlumberger / Motorola site (Moto after a while) - and I was SLB Sr. Scientist in Test Equipment (1 \$M and up type). I had machine language background (not assembly) real machine - and was contacted off my background list of abilities. So I worked some nights to develop code and guidelines so they could re-write and own the code.
Best regards, Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
Robert Swinney wrote:

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Question for Jim. . .
Is the iron cross leg an adjustable (calibrateable) feature of watthour meters? It would seem that the perm magnetic shunt would be subject to some degradation over time and thus would be "proximity" adjustable.
Bob Swinney
wrote:

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On Wed, 12 Jul 2006 11:34:02 -0500, "Robert Swinney"

I don't know but I don't think so - I would expect the primary calibration adjustment would be the strength of the permanent magnet "drag" field. Most soft magnetic materials have "u" which varies with flux density but is pretty stable against minor temperature change and time.
Jim
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Thanks Jim.
Bob Swinney
wrote:

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Hmm. Brake magnet.
What happens if the brake magnet happened to be, say, too strong for some reason?
Jim
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If they catch you it can get expensive :-). A simpler solution is a 1/64" or smaller hole in the case with a bristle sticking through it.
Mark Rand RTFM
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On Wed, 12 Jul 2006 22:54:52 +0100, Mark Rand

Hell, if they catch you, expensive won't be the end of it. Lawyers fees, bail bondsmen, etc. That's called Theft Of Energy, and they almost always prosecute.
You might end up spending some quality time - 6 months to a few years - at The Graybar Hotel. You check out when THEY say you can.
--<< Bruce >>--
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Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700