Power Measurement Three Wattmeter Method
One of the method of balanced and unbalanced load is Power Measurement Three Wattmeter Method.
A wattmeter consists of (i) a low resistance current coil which is inserted in series with the line carrying the current and (ii) a high resistance pressure coil which is connected across the two points whose potential difference is to be measured.
A wattmeter shows a reading which is proportional to the product of the current through its current coil, the p.d. across its potential or pressure coil and cosine of the angle between this voltage and current.
As shown in Figure (A) in this method three wattmeters are inserted in each of the three phases of the load whether Δ-connected or Y-connected. The current coil of each wattmeter carries the current of one phase only and the pressure coil measures the phase-voltage of this phase.
Hence, each wattmeter measures the power in a single phase. The algebraic sum of the readings of three wattmeters must give the total power in the load.
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The difficulty with this method is that under ordinary conditions it is not generally feasible to break into the phases of a delta-connected load nor is it always possible.
However, it is not necessary to use three wattmeters to measure power, two wattmeters may also be used. Two Wattmeter method has also been explained in the article Power measurement Two Wattmeter Method.
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