Hey guys, I have been wandering on what is the mechanism used in an electricity meter to determine if the household is receiving power from the electrical grid or, for example, if a solar cell panel array or small hydroelectric system is used in the house to generate excess power, the house could be exporting energy to the grid.

I have been reading up on electrical meters and know that in the older type which uses a rotating aluminium disc to indicate power consumption. According to wikipedia, two coils are used to generate a magnetic flux, one representing the voltage and another representing the current waveforms. Another coil is used to phase shift one of these flux waveforms and this induces eddy currents in the disc.

My question is if the house is exporting power to the grid, then this disc will rotate in the other direction. But how is this achieved? I mean obviously AC power is being used here so the current and voltage waveforms are changing direction all the time anyway. I am guessing that the phase of each waveform plays a part here?

Thanks

 

This is somewhat similar to how a two phase motor(single phase capacitor run) rotates. . One field is induced by voltage, the other by current. The direction of rotation depends on which quantity leads or lags (voltage or current).

you might want to look up directional/ reverse power relays for a better understanding.