Meter with two consumer-side connections

[Echo the husky]

Lowered into the flood zone, you mean?

Well I nearly put 'heath and safety'. I phoned to report no power due to a problem with a key meter, the person taking the call was only concerned with the fact that the meter required a 2 tread stepladder to access it, not that there was a an elderly couple struggling with no heating, in the middle of winter. We need to get that moved right away, she kept saying!

 

[ban-all-sheds]

Mechanical meters measure true power. They integrate the instantaneous product of current and voltage.

Quoting https://en.m.wikipedia.org/wiki/Electricity_meter :

The electromechanical induction meter operates by counting the revolutions of a non-magnetic, but electrically conductive, metal disc which is made to rotate at a speed proportional to the power passing through the meter. The number of revolutions is thus proportional to the energy usage.

The disc is acted upon by two sets of coils, which form, in effect, a two phase induction motor. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees, due to the coil's inductive nature, and calibrated using a lag coil.[16] This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current, voltage and phase angle (power factor) between them. A permanent magnet acts as an eddy current brake, exerting an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power or rate of energy usage. The disc drives a register mechanism which counts revolutions, much like the odometer in a car, in order to render a measurement of the total energy used.

Well - I always thought that domestic meters were very simple devices which just measured current and voltage, and worked out watts on the assumption that they equalled VA.

 

[DetlefSchmitz]

I suspect that the meters calculate a number of samples per mains cycle of instantaneous volt * amp measurements and add them together.This would be simpler and result in a wattage value.

No - it will result in a sum of VA values.

I had assumed the reader was intelligent enough to realise that I was not giving a mathematical formula, but describing the approach. Clearly to get watts or watt hours you need to make the appropriate obvious calculations.

endecote said it best with:

Mechanical meters measure true power. They integrate the instantaneous product of current and voltage.

What I was describing is the same measurement translated to a possible microprocessor algorithm.

 

[endecotp]

the same measurement translated to a possible microprocessor algorithm.

See e.g.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD71056.pdf

Typically the "instantaneous multiplication" is done digitally, but not by a microprocessor.

 

[DetlefSchmitz]

See e.g.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD71056.pdf

Yes thanks. Now you point it out I remember examining that very data sheet a few years ago.

 

[JohnW2]

Are you deliberately lying or just don't know? Power is the product of voltage, current, and cosine of the phase angle.

BAS was surely indicating that he definitely did know, and was certainly not lying. Look at his question marks.

As I see it, he was 'questioning' (essentially rhetorically) the statement that that induction electromechanical meter measured power, and was suggesting that it probably actually measured the product of volts and amps (which, as you say, and as he obviously knows, is only the same as power if PF=1).

It would seem that you are being deliberately provocative by suggesting that he was "lying".

Kind Regards, John

 

[DetlefSchmitz]

On the other hand, he is ignoring the facts, and to my mind displaying a lot less grasp on physics than he used to. I was beginning to suspect a hacked account, had it not been for the usual OTT diatribe in another thread. John, I think you must know that Winston is only using BAS' own style of attack which he used against Winston and others when more reasoned arguments failed to work. To be honest I think it's six of one, etc.

I suspect we are heading towards a locked thread...

 

[JohnW2]

On the other hand, he is ignoring the facts, and to my mind displaying a lot less grasp on physics than he used to.

I'm not totally sure how the electro-mechanical induction meters worked/work. Rightly or wrongly, BAS clearly suspected (his response was in the form of a question, not implying certainty) that they probably measured VA, rather than power. In response to endecotp's statement "...rotate at a speed proportional to the power ...", BAS therefore responded "Power? Or the product of volts and amps?". As above, he may be wrong, but I'm not sure that he is p[articularly "ignoring facts" or demonstrating a reduced grasp of physics.

John, I think you must know that Winston is only using BAS' own style of attack which he used against Winston and others when more reasoned arguments failed to work. To be honest I think it's six of one, etc.

As you must be aware, I am the last person who would, in general, defend or support BAS, particularly in relation to his 'style'. However, I have never known him to accuse someone of "lying" (which implies a deliberate act) when the worst they may have done is made a mistake (which we all do at times).

If someone is wrong (or he believes them to be wrong), he can certainly be sarcastic, condescending, respond with rhetoric questions that might not be understood by the recipient or even make comments about their 'stupidity'/intelligence etc., but I can't recall him ever have accused an incorrect statement as representing deliberate lying.

Kind Regards, John

 

[endecotp]

In response to endecotp's statement "...rotate at a speed proportional to the power ...", BAS therefore responded "Power? Or the product of volts and amps?".

As some of you will have guessed I'm ignoring posts by maybe half the people in this thread.....

But to answer this: if you integrate the product of instantaneous current and voltage, you get true energy - and that is what the meters (electromechanical and electronic) do.

If anyone doubts this, try integrating sin t . cos t dt, i.e. the product of two sine waves 90 degrees out of phase, as voltage and current are in a purely inductive or capacitive load.

 

[JohnW2]

But to answer this: if you integrate the product of instantaneous current and voltage, you get true energy ....

Indeed, and I don't think think that anyone has doubted or disputed that mathematical fact, but ...

- and that is what the meters (electromechanical and electronic) do.

That's where (in relation to electro-mechanical meters) I wasn't sure, and BAS clearly had his doubts - hence his question.

If anyone doubts this, try integrating sin t . cos t dt, i.e. the product of two sine waves 90 degrees out of phase, as voltage and current are in a purely inductive or capacitive load.

Again, that's certainly true mathematically, the uncertainty (in my mind, and seemingly also BAS's) being whether the electro-mechanical process in electro-mechanical meters actually did/does the equivalent of such an integration. However, you seem to be saying that it does.

Kind Regards, John

 

[DetlefSchmitz]

Typically the "instantaneous multiplication" is done digitally, but not by a microprocessor.

As it happens, I just took apart one of the most commonly used meters (I won't say which here), and it uses a ATmega48 and its internal ADC.

 

[endecotp]

As it happens, I just took apart one of the most commonly used meters (I won't say which here), and it uses a ATmega48 and its internal ADC.

That's interesting! There are lots of custom chips specifically for this, some including a microcontroller and some without (and then an i2c or similar connection to a separate microcontroller). Presumably the ATmega is cheaper....

 

[JohnW2]

Typically the "instantaneous multiplication" is done digitally, but not by a microprocessor.

As it happens, I just took apart one of the most commonly used meters (I won't say which here), and it uses a ATmega48 and its internal ADC.

I'm not surprised, and I have to say that I didn't really understand exactly what endecotp meant by "not by a microprocessor". He presumably wan't suggesting that it was done by a large handful of discrete logic gates etc., so I would personally be inclined to call the chip(s) that were doing the calculations "a microprocessor". Maybe he just meant that it was not 'software programmable"?

Kind Regards, John

 

[endecotp]

John I linked to a datasheet in an earlier post. It has two sigma-delta ADCs for voltage and current, and then fixed-function DSP that multiplies and integrates - plus some other clever stuff like a high-pass filter that eliminates any DC offset in the current sensing. The result of that is a digital signal of some sort, e.g. a pulse per Joule or something. That's not a microprocessor. Typically there would also be a microprocessor that deals mostly with the display, and probably records the total energy used in non-volatile storage.

It surprises me that Analog Devices have gone to the trouble of producing a whole range of chips to do it in this way if it's also possible (and presumably cheaper) to do it with a "bog standard" microcontroller and its regular ADCs, but what do I know?

 

[DetlefSchmitz]

A very quick price comparison at one off quantities gives:
AD71056 $3.11
ATmega48 $1.45

Of course with the AD71056 you still need a microcontroller for the ir comms and display driving plus reading storage, so I guess it's a no brainer (as they say).