Uni-directional clamp meter

JohnD · 25 Jan 2015

I know how my clamp meter works, and chortled at the idea that one could detect which "direction" an AC current was flowing in.

However I was recently shown an energy monitor where the sensor clamps round a meter tail, and has a little arrow to detect if current is flowing out of or into the network.

Does anyone know how it can tell?

bernardgreen · 25 Jan 2015

the relationship between voltage and current will enable the direction of energy flow to be determined.

The clamp would need to measure current magnetically and the voltage in the conductor by capacitive coupling to a plate in the clamp. The metal core of the current sensor would be a possible plate.

JohnD · 25 Jan 2015

bernardgreen said:
the relationship between voltage and current will enable the direction of energy flow to be determined.

If you know both voltage and current, how does that enable you to determine direction?

bernardgreen · 25 Jan 2015

The relationship determines the direction of energy flow.

At the moment of the peak positive voltage the direction of the current will indicate the direction of energy flow.

AdrianUK · 25 Jan 2015

I agree with Bernard....

Voltage & current are scalar quantities (they have no associated direction - they aren't vectors)

But power is a vector & it does have an associated direction. The direction of power flow can be determined mathematically from the voltage & current waveforms. In order to get this right, the power analyser needs to have the sensors connected to the supply in a particular orientation so that it can correctly analyse the waveforms it sees. Hence the arrow on the current sensor (it also matters which is L & N for the voltage sensor).

If both are 'reversed' (on a single phase circuit) then the power direction will still be correct, if any one of them is reversed then the power flow will appear to be negative (ie flowing in the opposite direction)

On a three phase power analyser it is even more important that the voltage & current phase relationships are correct.... or some very strange answers can result which lead to a lot of head scratching until the penny drops!

JohnW2 · 25 Jan 2015

AdrianUK said:
I agree with Bernard....

So do I, in that the answer seems to rely on the phase relationship between voltage and current. However ....

AdrianUK said:
Voltage & current are scalar quantities (they have no associated direction - they aren't vectors)

Eh? Think DC - which is what AC is at any specific point in time.

AdrianUK said:
But power is a vector & it does have an associated direction. The direction of power flow ...

Perhaps I'm wrong, but that all sounds back to front to me. Current flows, but power doesn't. Current can have a direction (or phase), but power can't - what would you mean by a 'negative power', or by the 'phase of power'?

If one thinks DC, it seems quite easy to understand. If one inserted a (directional) current meter into the conductor and had a (directional) voltmeter across the 'supply cables' it would be obvious (from the 'signs' of the measurements) which direction the current was flowing. With AC, it's the same at any particular point in time, but, over time, would be manifested as the current/voltage phase relationship, wouldn't it?

Kind Regards, John

plugwash · 25 Jan 2015

Perhaps I'm wrong, but that all sounds back to front to me. Current flows, but power doesn't.

Sure it does

Current can have a direction (or phase), but power can't

Sure it can.

what would you mean by a 'negative power'

power transferred in the opposite direction from that assumed when defining the quantities in your circuit.

, or by the 'phase of power'?

The best way to view this is as a complex number (which can be thought of as either magnitude and phase or real and imaginary).

The real part represents real power flow, if it's positive you have a net transfer of energy in the one direction. If it's negative you have a net transfer of energy in the opposite direction (which direction is which depends on how you defined your signs).

The imaginary part represents what we call reactive power. This represents no net transfer of power despite there being cuurent flow. It is caused by capacitors and inductors (IIRC capacitors are considered to generate reactive power and inductors to consume it but this is just a convention)

The great thing about this is that once you represent all your voltages, currents and impedances as complex numbers (and use a consistent reference for what you consider to be zero phase) all of the circuit analysis equations that work for DC circuits also woek for your AC circuits.

JohnW2 · 25 Jan 2015

plugwash said:
Current flows, but power doesn't.

Click to expand...

Sure it does

Current can have a direction (or phase), but power can't

Click to expand...

Sure it can.

what would you mean by a 'negative power'

Click to expand...

power transferred in the opposite direction from that assumed when defining the quantities in your circuit.

I don't really get this. Power is the rate of doing work or, equivalently, the rate of consuming energy and I can't really see how that can have direction (or phase). I must be missing something.

plugwash said:
The best way to view this is as a complex number ...

For some purposes, it's more convenient to use complex numbers but, as you say, it's mathematically equivalent to considering magnitude and phase.

However, in bottom-line terms, I think we may be saying much the same - that the phase relationship between voltage and current will indicate the direction of energy transfer.

Kind Regards, John

plugwash · 25 Jan 2015

plugwash said:
The great thing about this is that once you represent all your voltages, currents and impedances as complex numbers (and use a consistent reference for what you consider to be zero phase) all of the circuit analysis equations that work for DC circuits also woek for your AC circuits.

I made a mistake here (sorry i'm rusty on this stuff). The votlage/current relationships are the same as for DC analysis but to calculate power you have to take the complex conjugate of the current before multiplying it by the voltage.

JohnW2 said:
Power is the rate of doing work or, equivalently, the rate of consuming energy and I can't really see how that can have direction

Perhaps it would make more sense if you considered it as "the rate of transfer of energy". Lets say we have a device attatched to the grid. Energy is usually transffered from the grid to the device but it can also under some circumstances be transferred from the device to the grid.

If we define a transfer of energy from the grid to the device as "positive power" than a transfer of energy from the device to the grid would be "negative power".

For some purposes, it's more convenient to use complex numbers but, as you say, it's mathematically equivalent to considering magnitude and phase.

It's possible to view a complex number as magnitude and phase but it's also possible to view it as real and imaginary parts.

In the case of power the "real and imaginary" view represents what is happening far better than the "magnitude and phase" view. The real part represents the net transfer of energy over the cycle. The imaginary part represents energy that flows in and out of the course of a cycle causing no net transfer of energy.

Though there was a mistake in my previous post. While the voltage and current equations for DC apply euqally to AC systems represented in complex numbers. In the case of the power equation we have to take the complex conjugate of the current before multiplying.

JohnW2 · 25 Jan 2015

plugwash said:
JohnW2 said:

Power is the rate of doing work or, equivalently, the rate of consuming energy and I can't really see how that can have direction

Click to expand...

Perhaps it would make more sense if you considered it as "the rate of transfer of energy".

In terms of what you're saying, that would certainly make a lot more sense, but "rate of transfer of energy" is surely not the same as "rate of consumption of energy" (energy can be transferred without being consumed), and it is the latter which AFAIAA is the physics definition of 'power'. Maybe I'm just being pedantic in trying to work to that strict defintion.

plugwash said:
If we define a transfer of energy from the grid to the device as "positive power" than a transfer of energy from the device to the grid would be "negative power".

Yes, if you created those definitions it would make sense of what you're saying - but, as above, that does not correspond with my understanding of the correct/strict definition of 'power' (whether 'positive' or 'negative').

plugwash said:
For some purposes, it's more convenient to use complex numbers but, as you say, it's mathematically equivalent to considering magnitude and phase.

Click to expand...

It's possible to view a complex number as magnitude and phase but it's also possible to view it as real and imaginary parts.

I can hardly disagree with that, since you have essentially just paraphrased what I wrote!

plugwash said:
In the case of power the "real and imaginary" view represents what is happening far better than the "magnitude and phase" view. The real part represents the net transfer of energy over the cycle. The imaginary part represents energy that flows in and out of the course of a cycle causing no net transfer of energy.

That's mathematically true but, as above, the imaginary component does not really represent true 'power' at all (per the physics definition) - since, as you say, it does not relate to the consumption of any energy.

Kind Regards, John

plugwash · 25 Jan 2015

JohnW2 said:
plugwash said:

JohnW2 said:

Power is the rate of doing work or, equivalently, the rate of consuming energy and I can't really see how that can have direction

Click to expand...

Perhaps it would make more sense if you considered it as "the rate of transfer of energy".

Click to expand...

In terms of what you're saying, that would certainly make a lot more sense, but "rate of transfer of energy" is surely not the same as "rate of consumption of energy" (energy can be transferred without being consumed), and it is the latter which AFAIAA is the physics definition

What does "consumption" mean anyway? After all energy can never be destroyed, just converted into other forms.

All a power monitoring device can tell is the rate and direction of energy transfer past the monitoring point, it can't tell where it is going or whether the receiving side of the network is going to "store" it or "consume" it.

ericmark · 26 Jan 2015

Thank you plugwash I had a few times wondered how the so called smart meter worked out the direction of energy transfer. I do wonder how many will have a rude awaking when it moves from estimated to actual power produced by solar panels.

JohnW2 · 26 Jan 2015

plugwash said:
JohnW2 said:

plugwash said:

JohnW2 said:

Power is the rate of doing work or, equivalently, the rate of consuming energy and I can't really see how that can have direction

Click to expand...

Perhaps it would make more sense if you considered it as "the rate of transfer of energy".

Click to expand...

In terms of what you're saying, that would certainly make a lot more sense, but "rate of transfer of energy" is surely not the same as "rate of consumption of energy" (energy can be transferred without being consumed), and it is the latter which AFAIAA is the physics definition

Click to expand...

What does "consumption" mean anyway? After all energy can never be destroyed, just converted into other forms.

Indeed so, and in the context of electrical energy (and electrical power), the established convention is surely that 'consumption' refers to conversion into some form other than electrical? Do you not talk about electrical heaters/appliances/whatever 'consuming energy'?

plugwash said:
All a power monitoring device can tell is the rate and direction of energy transfer past the monitoring point, ...

Exactly. That's really the point I've been making all along - all one can 'monitor' is the transfer of energy. As I said before, maybe it's pedantic of me to want to stick to physics definitions and hence be happy with that concept, but not with the concept of transfer (or movement, or direction etc.) of power (which is a rate).

plugwash said:
"... it can't tell where it is going or whether the receiving side of the network is going to "store" it or "consume" it.

FWIW, I don't think there is really any difference between 'store' and 'consume' - in context, both relate to the conversion of electrical energy into some other sort of energy - one can only 'store' electrical energy by converting it into chemical, potential, kinetic or whatever energy.

Kind Regards, John

JohnD · 26 Jan 2015

IMO the fairest way to account for solar and other domestic microgeneration is "net metering"

i.e.

you import 150 kWh from the grid, and you export 100kWh. You pay for 50kWh.

You import 150kWh, and you export 200kWh. You get paid for 50kWh.

I see the subsidy has been steadily coming down, and has now almost reached the retail price that customers pay. That should make it easier to change to a "net" payment scheme.

It may be awkward coming to an accommodation with people who are on a higher payment because they bought early panels which generated much less and cost much more.

JohnW2 · 26 Jan 2015

ericmark said:
Thank you plugwash I had a few times wondered how the so called smart meter worked out the direction of energy transfer.

I think it is particularly easy to understand if one considers the simple situation when PF=1 (i.e. no reactive components). AFAICS, in that situation, and in direct analogy with the even simpler situation with DC, current and voltage will be in-phase for energy flow in one direction or 180° out-of-phase if it is moving in the other direction. ... or is that not right?!

Kind Regards, John

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Uni-directional clamp meter

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