Current Transformer Electric

Electricity Meters

Electronic metering of electrical energy requires the independent sensing of both load current and supply voltage, continuous multiplication of these two quantities to produce instantaneous load power and the integration of varying load power with respect to time to derive accumulated energy usage.

In general the sensing of supply voltage is simply effected through a potentiometric resistive divider. The sensing of load current is more problematic and can be effected by a variety of well known means.

The load current can most simply be sensed by use of a resistive "shunt" in the load current path. This does suffer from instability with respect of temperature and is particularly inconvenient when it is required to reference a number of independent circuits to a common point as may be required in designs of polyphase metering equipment.

Nevertheless, especially for single phase metering, a shunt circuit with temperature compensation such as that disclosed, for example, in French Patent 2663013 can be an effective means of current sensing.

Current Transformer Electric

The current transformer is a particularly effective means of sensing load current. Typically the whole load current passes through the center of the transformer core, thereby forming one turn, and the detected secondary current is precisely equal to the load current divided by the number of turns in the secondary winding. The secondary current is effectively driven from a constant current generator, and generates a voltage in a load resistor which is then used as a precise measure of load current so forming one of the inputs to the multiplier circuit, the other being supply voltage as already discussed.