### How does an electric metre work?

## How does an electric metre work?

It is designed to measure kWh, and to do so it measures

**Voltage** (Across Incoming Line)

**Current** (probably only in one leg, as the other would be symmetric with it)

**Phase** (offset between voltage sine wave and current sine wave)

Cable suitable for measuring current has to physically pass through a part of the equipment - usually a current transformer is placed around the current-carrying conductor. The most convenient designs have four large connection terminals - two in and two out (some more in 3-phase meters, of course) to make wiring operations easier. One can think of the meter as a black box in series with the supply, if that helps.

The instrument starts by continuously measuring the instantaneous power, using the calculations:

**Power = Voltage × Current × Cosine (θ)**

where is the phase offset measured between the two waves.

To convert this to kWh (which we pay for) the instantaneous power is integrated over time - for a static load that simplifies to kW x time, of course.

The resulting kWh is then stored in a non-resettable totalizer for display to the meter reader (or, in the case of smart meters, transmission to the supplier).

Mechanical meters used a pair of wound coils to drive a rotating wheel, one for voltage, one to drive a rotating wheel whose speed was the product of the two measurements, and one to measure actual power. Automatically ignored the reactive component. That wheel moved to a gear train that completed the revolutions.

## Comments

## Post a Comment