Newbie Question: Measuring electrical power: PICs and Clamp probes

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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.

 

[Phil Allison]

** Groper Alert !!!!

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

** No.


(snip drivel)

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?

** Get Maxwell to repeal his laws for you.

Is this idea feasible?

** Nope.

Has anyone got a circuit diagram?

** Ask the tooth fairy - wanker.

I don't want
to buy a complex and expensive power analyser,

** You don't want to buy a cheap and simple one either, it seems.

or have to touch the wires in the circuit under measurement.

** Why ever not ???????????

Simple power monitors exist that plug into the wall outlet and the
appliance into them.

Totally safe.

WTF is your problem ???




....... Phil

 

[John Woodgate]

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

Only with considerable difficulty.

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.

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.

 

[Luhan]

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.

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.

From the level of inexperience implied in your question, and the level

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

Luhan Monat

 

[Homer J Simpson]

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

Why?

 

[John Woodgate]

Why?

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

 

[Buster]

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

How about this:

http://www.theenergydetective.com/

 

[Don Foreman]

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.

See
http://www.analog.com/Analog_Root/s...an564.pdf#search="analog devices power meter"

As others have said, you must separate the wires to measure current
with a clamp-on probe. Such probes can be purchased. You'll also need
a direct connection for voltage measurement.

It would presumably be possible to devise a current probe having a
suitable magnetic structure that could work with non-separated
conductors, inferring both current and voltage from fields -- but the
geometry of the conductors would have to be fixed, well-known and
repeatable. It's certainly easier just to separate the conductors.

 

[John Woodgate]

It would presumably be possible to devise a current probe having a
suitable magnetic structure that could work with non-separated
conductors,

The spooks have current probes that can do this on data cables.

 

[DaveM]

Why?

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


--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

 

[Tomi Holger Engdahl]

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.

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:
http://www.suparule.com/technology_accurrent.htm

Here are some links to products that use this technology:
http://news.thomasnet.com/fullstory/465571
http://www.conpro.be/megger_multicore_digital_clamp_meter_MMC850_en.pdf
http://www.davis.com/showpage.asp?L3ID=2525
http://home.nestor.minsk.by/build/news/2005/07/2203.html

Measuring voltage needs some kind of direct connection.

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).

 

[Alan theTech]

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.

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.

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?

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?