Three Wattmeter Method of Three Phase Power Measurement - its Disadvantages

A wattmeter is an instrument used to measure the electrical power consumed by the load. It consists of a voltage coil connected across the load and a current coil connected in series with the load. In order to measure the three-phase power consumed by the load, only one wattmeter is required. But this method is limited for the 3-phase networks with balanced load only. Thus for measuring power in a three-phase network with unbalanced load, two wattmeter method or three wattmeter method is used.

Three-Wattmeter Method of Three-Phase Power Measurement :

Three wattmeter method is used when the load is unbalanced. For unbalanced load, the power in each phase will be different, thus three different wattmeters for each phase are used. Here, the sum of three wattmeters will give the total power consumed by the load.

The three wattmeter method is mostly used in the 3-phase 4-wire system (star connected load), and can also be used in delta connected load by closed delta connection of wattmeters current coils. In this method, each wattmeter connected in phase measures the power consumed by the load corresponding to that particular phase. The below shows the connection for three wattmeter method with star connected load.

Three Wattmeter Method

In the above figure, we can see that, the current coils (CC) of the three wattmeters are connected in series with the three phases R, Y, and B. The voltage coils or pressure coils (PC) of each wattmeter are connected such that, one end to their corresponding phase and the other end to the neutral line. The vector diagram of all the three-phase voltages and the currents is shown below.

Three Wattmeter Method

The wattmeters connected in the phases R, Y, and B measures the power consumption as WR, WY, and WB respectively.
Three Wattmeter Method
Therefore, the total power P consumption of the circuit is,
Three Wattmeter Method
Similarly, the connection diagram for three-phase power measurement by three wattmeter method with delta connected load is shown below.
Three Wattmeter Method
Here also the sum of all the three wattmeter readings gives the total power consumed by the load.

Disadvantages of Three-Wattmeter Method :

  • It is difficult to measure power in star connected loads where there is no availability of a neutral point for the connection of voltage coils. In such conditions, the other ends of the voltage coils are connected together forming an artificial neutral point.
  • For delta connected loads, it is difficult to use three wattmeter method. Since current coils need to be inserted in load phases in a closed delta manner as shown above.
  • The need for three wattmeters is one of the disadvantages compared to the two wattmeter method.

Also, three wattmeter method has the advantage of measuring three-phase power in both balanced and unbalanced load. But due to its limitations, it is therefore used only in a 3-phase 4-wire circuit. This drawback can be overcome by using two wattmeter method for measuring power in both balanced and unbalanced load.

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