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Curiosities: Inside a late-80s energy meter

2020-08-02 17:40:34, In: Electronics, Curiosities

And now something totally different, related more to electricity than electronics. These electricity meters were commonly installed since 1990s in many households and were used until early 2010s when they were replaced with digital meters. The principle of operation is quite simple: There are two coils: One energizes because there is mains voltage flowing through it all time. The second coil is put in series with energy receivers, so it energizes when current flows through it. If both are energized, it means that there is both voltage and current, and voltage times current equals power. Action of both coils induces eddy currents in a flat aluminum disc, it rotates and spins the counters, like in a peculiar low-power induction motor with 90-degree phase difference made by two coils.
The curent constantly present in voltage coil draws some energy, but it is not integrated to total power consumption.

A neutral wire is connected through a pass-through, while live is passed through coil. Additionally live input to counter is internally connected with one end of voltage coil and that's how these meters were connected in households. Inside, we can see that everything is installed on a metal chassis. This chassis works both as mechanical construction as well as some magnetic shielding against temparing with permanent magnets.

So, let's proceed to the voltage coil:

The voltage coil is made of thousands of turns and is made of a thin wire. It will take some energy itself but is installed the way that eddy currents will not happen and the disc will not spin. To spin the disc, additional magnetic field from a few turns of current coil is needed and this is made using a large-diameter wire. Usually the disc is compensated for its inertia by using permanent magnets pointing to the shaft, to the aluminum disc itself (which will brake the eddy currents) or a specific bearing. That's why those who want to cheat the electricity company use magnets on power meters, however there are consequences of it. The built-in magnet is re-magnetised according to the additional magnet's field and after removing the additional one it stops braking the disc effectively making it spin faster. However, this trick can be detected even easier by inspecting the chassis, as between the meter's casing and the disc there is a few centimeters of steel which may get magnetised too.

Here are additional calibration instruments. A few closed loops of wire installed on current's and voltage's coil cores can be cut to, to put simply, "unshade" enough field to get this 90-degree difference between coils needed to nicely and linearly spin the disc.

Additionally, one or two wire rounds may be present to calibrate the linearity and speed of the disc more precisely: A small coil gets power from current's coil and this power is used to power a small, one-winding turn pointing right into the disc. This winding may be put closer and farther from the disc to make the device more precise.

In some municipal conditions, there are two power pricing rates - the power drawn in peak hours costs more, and the power used during nighttime costs less. This is frequently used for electric heating. Although in modern times a radio- or clock-based tools are used, a still present approach is using the switching signal coming from the power company's clock usually installed per building, and the signal is spread to flats using an additional cable - the signal switches the high-power relay during cheaper power hours and the energy can be used then. The same signal activates a solenoid in a special two-counter energy meter to change the gear and the disc spins the other counter.

Sources: - http://elektron.pol.lublin.pl/djlj24/lm/cw05.pdf

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