Newbie Question: Measuring electrical power: PICs and Clamp probes

Are there any designs out there for an electricity meter capable of very accurately measuring electrical power consumption, non intrusively?

Basically what I'd like to do is either build or buy some sort of clamp probe the could be attached (or wound??) around the insulated live wire of a domestic power cable, then by (presumably?) measuring the strength of the electrical field generated by the current flowing through the wire, determine and log both voltage and current, and hence electrical power.

This seems like an ideal application for a PIC and data logger. The only part I know nothing about is the clamp probe to the mains cable, and how you would convert the measured electrical field into voltage and current. How do you stop the presumably equal and opposite electrical field in the neutral wire from interfering with the measurement, if the wires are close together and can't be separated?

Is this idea feasible? Has anyone got a circuit diagram? I don't want to buy a complex and expensive power analyser, or have to touch the wires in the circuit under measurement.

Reply to
Timothy
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In message , dated Sun, 10 Sep 2006, snipped-for-privacy@tholtom.freeserve.co.uk writes

Only with considerable difficulty.

That's the problem. It is difficult to measure the current without separating the two conductors. You need a particular type of differential magnetic sensor. And it's difficult to measure the voltage non-intrusively. The purpose of the 2 layers of insulation in the cable is to prevent intrusion! The most you can do is to attach electrodes to each side of the (UK-type flat mains cable) and feed the signal picked up into a balanced high-input impedance amplifier.

Then you need to multiply the current and voltage signals together, taking into account the phase-shift (displacement power factor). The current waveform is likely to be significantly distorted by non-linear loads, and the voltage waveform will be somewhat distorted. (around 3% THD usually).

You might settle for assuming the voltage is always what you occasionally measure with a meter; it won't be exactly 230 V. Then you only need to measure the current, but what you need is the r.m.s. value of the fundamental, not of the distorted waveform. Without the voltage, you can't determine power factor, but maybe it won't be so far of 1 that it matters. In fact, this way isn't all that much easier than the first method, if at all.

It can be done, but it's not a beginners project.

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Reply to
John Woodgate

You can use a magnetic clamp-on probe to measure current, but it must go on only one of the two wires to prevent the fields from cancelling.

Measuring voltage needs some kind of direct connection.

of danger in doing this kind of project, you should have your life insurance policy made out to..

Luhan Monat

Reply to
Luhan

Why?

Reply to
Homer J Simpson

In message , dated Sun, 10 Sep 2006, Homer J Simpson writes

Sorry, that's a question for alt.rec.philosophy.

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Reply to
John Woodgate

How about this:

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Reply to
Buster

How much power? How accurately? What kind of power? What characteristics do you want to measure? Elaborate on "intrusively"

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Dave M
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Reply to
DaveM

This is what is needed usign traditional current transformer and other clamp meter constructions.

But there are also now (just became available wuite recently) clamp current meters that work with multi-core cables. Meaning you can clamp such meter over a normal 2 or 2 wire mains cable, and still get reading. The magnetic fields two wires on mains cable pretty well cancel each other, but not entirely to all directions. Those multi core cable clamp meters use some sort of special magnetic sensors to sense those small fiels.

Some technical description how this is done:

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Here are some links to products that use this technology:

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Usually you need direct connection to get reliabl readings. It is possible to get some less accurater results using capacitive sensing methods though the electrical wire insulation (maube suitable for some applications).

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Tomi Engdahl (http://www.iki.fi/then/)
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Reply to
Tomi Holger Engdahl

Current can be measured with a clamp probe, voltage is a different matter. Unless you are measuring very high voltages such as cross country transmissions lines 100,000 volts or higher, indirect reading of voltages can be very inaccurate. And since you want to have good readings for proper power consumption a hard wire connection to a good meter is the best. A note of caution unless you are qualified I would strongly advise you to have a good electrician make the connections for you. Touching the wrong things could get you very dead very quickly.

There is another problem, if you manage to build a clamp probe how would you calibrate it? Every piece of electronic equipment used in commercial and industrial applications is calibrated to set industry standards, so everyone gets the same results when making measurements with different equipment. And without that calibration how would you know if your measurements are even remotely accurate?

Reply to
Alan theTech

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