#### Measurement of Power

Introduction to Measurement of Power:

In a DC (direct current) circuit: The power in DC circuit is provided by P = V * I,

In which V stand for = Voltage

I stand for = current.

The power can be calculated basically with the help of just Ammeter and voltmeter. The below diagram shows the circuit diagram for measuring power in DC (direct current) circuit.

Diagram - Voltmeter - Ammeter method

In 1-phase AC circuit:

Power in 1-phase AC circuit is provided by P = VICosφ,

In which,

V stands for Voltage,

I stands for current and,

Cos φ stands for power factor.

Wattmeter is employed to calculate power in AC circuits. The circuit diagram is displayed in the diagram below.

Diagram - Power measurement in 1-phase AC circuit

Total power (P) = Wattmeter reading = VI Cos φ.

From this power factor can be measured as follows

Cos φ = P/ VI

3-phase AC circuit:

1) One wattmeter method:

Diagram - Single wattmeter method of power measurement in 3-phase circuit

For balanced 3-phase load that is ZR = ZY = ZB, single wattmeter might be connected into any one phase as displayed in the above diagram. This wattmeter will point out the power in that phase only. Because the load is balanced, the entire power in the 3-phae circuit will be calculated by

Total power = 3 *Wattmeter reading

ii) Two-wattmeter method:

This is most the general method for calculating power in a 3-phase, 3-wire system because it can be employed for both unbalanced loads (ZR ≠ ZY≠ ZB) and balanced (ZR = ZY = ZB) connected in either star/delta. The current coils are connected to any two of the lines, and the voltage coils are connected to another line, the one with no current coil connection, as displayed in the below diagram.

Total power = W1+ W2

Diagram - Two wattmeter method of power measurement in 3-phase circuit

iii) Three-wattmeter method:

If the load is unbalanced and the installation is 4-wire, after that 3 wattmeter are essential. The connection of wattmeter is displayed in the below diagram. Each wattmeter calculates the power in one phase and the total power will be calculated by

Total power = W1 + W2+ W3

Diagram: Three wattmeter method of power measurement in 3-phase circuit

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