# Theory of Electrical Energy Measurement

Electric energy flow into two types of circuit i.e in**DC circuit and an AC circuit.**These two different types of circuits consist of the same electrical parameter i.e voltage and current. But these two parameters are slightly different from each other with respect to AC and DC circuits. Hence the measurement of electric power in these two different circuits is different. But the theory behind the measurement of electric power is the same for

**DC and AC circuits**. Here we will discuss the

**measurement of electric power in both AC and DC**circuits.

## Theory of Electric Power Measurement

When a passive component or load is connected across the circuit it consumes electric power from the source. The power consumed by an electric circuit is given by the product of the load current flowing through the circuit and the terminal voltage of the circuit. This is the basic principle that is used for the measurement of electric power either in the DC or AC circuit.

### Electric Power Measurement In DC Circuit

Electric Power Consumed by the load in DC Circuit is calculated by the product of the dc load current drawn by the load and the voltage drop across the load. Hence the most effective method of electric power measurement voltmeter and ammeter method. The circuit diagram given below shows the two possible connection of measurement of electric power in a DC circuit.

In Figure (a) the voltmeter is connected across the supply side, the measurement of electrical power from this method is the sum of the electric power consumed by the load and electric power loss by the ammeter.

In Figure (b) the voltmeter is connected only across the load side, so the measured power by this method is the only power that is consumed by the load and a very small power loss by the voltmeter. From figure (b) the loss of power by the voltmeter is very low, this method is suitable for the measurement of electrical power for the DC circuit.

In Figure (b) the voltmeter is connected only across the load side, so the measured power by this method is the only power that is consumed by the load and a very small power loss by the voltmeter. From figure (b) the loss of power by the voltmeter is very low, this method is suitable for the measurement of electrical power for the DC circuit.

### Measurement of Power In AC Circuit

Current flowing in the AC circuit has an alternating nature i.e it goes positive for half a period of time and again negative for the next half period of time so due to this alternation of current the power flow in an ac circuit is positive for a half period of time and negative for another half period of time and for one complete period of time the average power is zero. But practically some amount of electric power flows through the circuit.

Due to this reason calculation of electric power in an electric circuit is different from the calculation of electric power in a DC circuit and is difficult also. The power consumed by the AC circuit is the product of the supply voltage, the load current flowing through the circuit, and the power factor of the circuit.

Power Consumed By AC Circuit = VICos𝜱

Where

V=Supply voltage

I = Supply Load Current

Cos𝜱= Power Factor of the circuit

𝜱=Angle between the supply voltage V and Supply load Current I

A basic circuit diagram of the electric measurement is given below. This circuit diagram consists of two different types of the coil

**(a) Current Coil and (b) Pressure Coil. The current**coil measures the load current and the pressure coil measures the voltage across the supply. The product of these two measured parameters is the instantaneous power of the AC Circuit. The Pressure Coil of the circuit is connected in series with a high-value resistor to limit current flow through it.