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Measurement of Three Phase Power

Encyclopedia
Encyclopedia
Field: Encyclopedia
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China

Three Phase Power Measurement Definition


Three phase power measurement involves using different methods to determine the total power in a three-phase circuit, depending on the number of wattmeters used.


Three Wattmeters Method


Uses three wattmeters connected to each phase and neutral line in a four-wire system to measure total power by summing individual readings.


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The circuit diagram is shown below-


This method is used for three-phase four-wire systems. The coils of the three wattmeters are connected to the respective phases, marked 1, 2, and 3. The pressure coils are connected to a common neutral point. Each wattmeter measures the product of phase current and line voltage (phase power). The total power is the sum of all wattmeter readings.

 

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Two Wattmeters Method


Employs two wattmeters and can be used for both star and delta load connections, summing the readings to determine total power.


Star connection of loads


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When the load is star connected load, the diagram is shown in below-


For star connected load clearly the reading of wattmeter one is product of phase current and voltage difference (V2-V3). Similarly the reading of wattmeter two is the product of phase current and the voltage difference (V2-V3). Thus the total power of the circuit is sum of the reading of both the wattmeters. Mathematically we can write


but we have , hence putting the value of .


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When delta connected load, the diagram is shown in below


The reading of wattmeter one can be written as


and reading of wattmeter two is


but , hence expression for total power will reduce to .


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One Wattmeter Method


Suitable only for balanced loads, using one wattmeter and switching between phases to measure power.


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Limitation of this method is that it cannot be applied on unbalanced load. So under this condition we have .


Diagram is shown below:


Two switches are used, labeled 1-3 and 1-2. Closing switch 1-3 gives the wattmeter reading as


Similarly the reading of wattmeter when switch 1-2 is closed is


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