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Measurement of ThreePhase Power by TwoWattmeter MethodAuthor: E.E. Kimberly If in Fig. 95 (a) the three meter potential coil terminals at 0 be kept joined, but be removed from the neutral of the system, the readings of all wattmeters will be unchanged, because the wattmeter potential coils themselves form a balanced Yconnected circuit and so the voltage across every potential coil remains unchanged. This method of measurement is called the "floating neutral" method and is accurate on a threephase threewire or fourwire system regardless of power factor or load unbalance.
If, then, the junction of the potential leads be moved and connected to one of the line wires, as at x on line 1, the sum P_{1} + P_{2}+P_{3} will be unchanged, although the power read from wattmeter W_{1} will be zero. Thus, it is possible and feasible to measure threewire, threephase power in the circuit in Fig. 94 (a) by using only the two wattmeters W_{1} and W_{3}. This is called the twowattmeter method and may be used with convenience on any threewire system, whether Yconnected or Δconnected and whether balanced or unbalanced, as in Fig. 96.
A proof of the correctness of the twowattmeter method in measuring balanced threephase loads is as follows. For convenience Fig. 97 is drawn for a Yconnected circuit. In the vector diagram, V01 V_{0}2, and V_{0}3 are phase voltages; V_{1}2, V_{2}3, and V_{3}1 are line voltages; θ = displacement angle between a current and its respective phasetoneutral voltage.
By the threewattmeter method,
P = P_{1} + P_{2} + P_{3}
By the twowattmeter method,
P = P_{a} + P_{b}
Since the righthand members of equations (a) and (b) must be equal, if both methods are to give the same results,
But Since this equation is an identity, it follows that


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