What is Three Wattmeter Method and Its Working

An electrical device or equipment works on supplying AC current. The power consumed by the device or equipment is measured using a wattmeter ( in watts/ kilowatts/ megawatts). A wattmeter consists of 2 coils like current coil ‘CC’ of low resistance connected along with the load and potential coil “PC” which is connected across the load. The 3-phase power of a 3-phase circuit can be measured either using a 3 Wattmeter Method or a 2 Wattmeter Method or a 1 Wattmeter Method. This article describes measuring 3 phase power using a 3 wattmeter method.

What is Three Wattmeter Method?

Definition: A 3 wattmeter method is used to measure power in 3 phases using a 4 wire system which is based on Blondel’s theorem. Blondel’s theorem states that the power supplied by the “k” wire AC system is equal to the total wattmeter required less than one that is “K-1”.


The 3 phase 3 wattmeter can be constructed in 2 topologies that are either in a star topology or in delta topology. The following is the construction of 3 wattmeters using 4 wires connected in star or “Y” fashion. It consists of 3 wattmeter’s W1, W2, W3 whose pressure coils are PC1, PC2, PC3 connected to neutral point “N” and current coil CC1, CC2, CC3 is connected to the load Z1, Z2, Z3. The other end of wattmeter is connected to R, Y, B terminal.

3 Wattmeter in Star Topology Construction
3 Wattmeter in Star Topology Construction

Power Measurement

The power measurement of three wattmeters is, when supply is passed through the circuit, the total power in the circuit at balanced load condition can be calculated using the following formula

P = P1 + P2 + P3 = V1 I1 + V2 I2 + V3 I3 …….1

Where P = Total power in the circuit

P1, P2, P3 = Power in each phase

V1,V2,V3 = voltage in each phase

I1, I2, I3 = current in each phase.

Phasor Diagram of Three Wattmeter Method

The phasor diagram for 3 wattmeters’ can be represented as follows

Phasor Diagram
Phasor Diagram


The 3-wattmeter observation can be practically noted using the following table

S .NO Voltage VL (volts) Current IL (amp) Power W1 (watts) Power W2 (watts) Power W3


Total Power P = W1 + W2 + W3 Power Factor = cos φ


The following are the precautions to be taken while practically performing the experiment

  • Make sure all the connections are made properly
  • The value of current in ammeter should not exceed the value of current in wattmeter.

Advantages of Three Wattmeter Method

The following are the advantages

  • Can measure power in the individual phase
  • Can measure phase to neutral voltages
  • In the absence of neutral wire, a duplicate neutral wire can be created.

Disadvantages of Three Wattmeter Method

The following are the disadvantage

  • 3 wattmeter is used
  • Highly sensitive to heavy loads.

Applications of Three Wattmeter Method

The following are the applications

  • They are usually used in power system
  • Used in refrigerators, electrical heaters
  • Used in industries for measuring power rating and power consumption.

All the electrical appliances or equipment work on supplying AC current to them and dissipates energy. The amount of power utilized in the circuit can be measured using an electric device named wattmeter ( in terms of watts, or kilowatts, or megawatts ). This article gives an overview of the measurement of power in 3 phases using 3-watt meters’ in a star topology. The total power in the circuit can be calculated by summing all the individual wattmeter readings. The main advantage of this method is that the power in individual phases can be measured simultaneously.