In two wattmeter method, readings of both wattmeters depend on the power factor of the load. Variation in Wattmeter Reading (on lagging PF) The two wattmeter method of power measurement in three-phase circuits is suitable for every type of three-phase circuit weather circuit is balanced or unbalanced and star connected or delta connected. ![]() But in actual practice, wattmeters read the average power because of the inertia of their moving system. It can be proved that the sum of the power measured by two wattmeter W 1 and W 2 is equal to the total instantaneous power absorbed by the load. The current coils of two wattmeters are inserted in any two lines, and the potential coil is connected from its own current coil to the line without the current coil. The power measurement in three-phase, three-wire load circuits usually carried out using this method. Even a slight degree of unbalance in the loading produces a significant error in the measurement. ![]() Hence, the power measurement in three-phase, three-wire circuits can be carried out by using the one wattmeter only.īut this method has a disadvantage. In a balanced 3-wire, 3-phase load circuit the power in each phase is equal and, therefore, the total power of the circuit can be determined by multiplying the power measured in any one phase. In the case of delta connected circuits, power measurement by three wattmeter method is very difficult because phase coils of load are required to be broken for inserting the current coils of wattmeter.
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