Batteryless current clamps?

Current transformers are usually dumped into a load resistor aka burden resistor, to convert their output current into voltage. I'm sure the Fluke clamp-on has an internal burden resistor, and they may switch that to change ranges.

Without a burden resistor, the output voltage will be proportional to frequency and very dependent on core reluctance, which would be fatal for a clamp-on meter with a hinge and a non-repeatable air gap.

Coreless Rogowsky coils are used unloaded, but need a downstream integrator to accurately measure current.

The coolest current transformer is a second-harmonic DCCT, accurate to parts-per-million from DC to many kilohertz.

John

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Wiki's great thanks John. That second link didn't have much info on how the device works. Is the following the same thing?

(I googled second-harmonic DCCT)

George H.

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Your enthusiasm for inventing implausible straw men knows no bounds. I never made any such claim. My scepticism about you claim was purely based on the fact that you were ignoring what Joel Koltner had actually said.

The pictures were perfectly clear. It was less obvious what you were actually doing, but since I couldn't care less, this isn't any great loss.

The joke is that even if you do extract "many watts" from the power company's power lines, you won't be stealing from them. In order to be able to extract power you have to be drawing power for which you will be billed, and any extra watts you extract by transformer action is subtracted from the power you are already paying for - your paid for load will be seeing a lower drive voltage.

Joel Koltner made a rather good joke, which you have totally failed to get.

-- Bill Sloman, Nijmegen

Fig 3 is about right. The green feedback path is usually an N-turn winding to net an N-to-1 current transformer.

There are usually two physical toroids. Ib, If, and T3 are wound on both, as if they were a single core. T1 and T2 are each wound on one of the cores, in opposite directions so that there's no net coupling of the carrier frequency into T3 or the customer's Ib circuit.

The AC path (sense winding T3 and its amplifier, driving If) fight to keep the flux zero at higher frequencies. The second-harmonic system works at low frequencies, again to keep net core flux zero, which happens when Ib = N * If.

I designed one of these once. It was fun, and not all that easy.

John

If you make a 'tap' upstream of the revenue meter, even with just transformer action, you're stealing. Revenue meters (kilowatt-hour meters) have always had terminal voltage as one of their inputs. An illegal tap upstream may affect the voltage at the service entrance some small amount, but the metering will reduce the billed kWh accordingly. So regardless of the exact voltage supplied by the utility (it often varies slightly throughout the day), the amount of energy delivered at the service entrance is what is billed for. Power drawn off before the meter isn't measured and is 'stolen'.

Of course if you just 'wrap some turns around the power line' without orienting the coil properly in relation to the line, you're not going to get any power because transformer action won't work when your turns of wire are parallel to the power line's magnetic field (i.e. 'wrapped around' the power line). And I think that was John Field's point.

daestrom

Enquiring minds etc :-). I thought at first that sounded like a variation on the old fluxgate compass idea from ww2 and earlier, where the earth's magnetic field varies the saturation of a toroidal ring and the second harmonic amplitude recovered in 3 levels in 120 degree segments, but it's quite different. Here's a good article on the principle:

cdsweb.cern.ch/record/1183400/files/CERN-BE-2009-019.pdf

Page 3 of 4 for the block diagram.

Using a feedback loop to cancel the induced magnetic field. Very neat.

Something new to learn in electronics every day :-)...

Regards,

Chris

The above last line is rubbish.

Engage brain and analyse the schematic properly before posting :-( !. In fact, it is the second harmonic that is detected...

Regards,

Chris

The DCCT essentially uses a fluxgate - two of them, usually - to detect the cancellation of two currents, one the customer's current and the other a local feedback current. The null detection can be insanely sensitive, just as a fluxgate compass is insanely sensitive.

John

? "John Larkin" ?????? ??? ?????? news: snipped-for-privacy@4ax.com...

Anyway, current transformers must always be operated with the secondary shorted. In the generating facilities in Kozani, West Macedonia, where 400 kV current transformers were involved, the operators of the plant had a special indicator whether the secondary was shorted.

Some old switchboard CT's I worked on in the Navy had very thin insulator between two spring clips. Whenever we wanted to remove a meter for cal, we slip the insulator out so the two clips would short together, shorting the CT. Then we could open circuit the meter and remove it from the panel. I don't remember exactly what the blade was made of, but it's surface wasn't perfectly smooth like polished material, more porous like unglazed ceramic (of course it wasn't any metal).

The reason they built the insulator so thin was that if one accidentally open-circuited the CT without removing the wafer first, the high voltage developed by the CT would just 'punch thru' the wafer and safely short the CT. Then all you had to do to repair things was make sure you closed the circuit and replace the wafer-thin insulator blade.

Was kind of surprised when I moved to commercial power systems that they didn't use something similar. Just has to have a breakdown voltage that is low enough to avoid damaging the CT.

daestrom

Lower-power CT, like residential-metering size, 100 amps or so, will generally tolerate being unloaded. They will saturate and make two not-too-huge voltage spikes per cycle and not get very warm. The nasty part is that, once the burden is reconnected, they are very likely to wind up magnetized, which will mess up low-current accuracy.

John

Actually about equally ferrite and plastic with copper coming at about

Only active probes do that.

I can make any output ratio i want, i know how they work.

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Wrapping turns around a high voltage power line probably wouldn't be a good idea.

-- Bill Sloman, Nijmegen

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There is no CT in residential metering, the energy meter is connected directly. Even commercial / light industrial you do not see CTs in the meter circuit until 600 A, and before that you are typically at

Of course there is. I delivered over 1800 electronic submetering units for the Battery Park City apartment complex, and, trust me, we used CTs. Lots of electronic meters use CTs. You should get out more.

John