One wattmeter method is used to measure power in a three-circuit when

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The load is balanced in all three phases

Explanation: The one-wattmeter method is a technique used to measure the total power in a three-phase system by connecting the current coil in one line and the potential coil between that line and the neutral point (or star point). This method is valid only for balanced three-phase loads because the power consumption in each phase is assumed to be identical. Working Principle: In a balanced 3-phase star-connected system, the total power is the sum of power in each phase: $P_{total} = 3 imes P_{phase}$. By measuring the power of one phase using a wattmeter (connected between the phase wire and neutral), the total power is simply calculated as $3 imes W_1$. If the load is unbalanced, the phase powers differ, rendering this single-measurement approach invalid as it cannot account for the disparate power components of the other two phases. Key Formulas: $P_{total} = 3 imes W_1$ — Total power for balanced star/delta loads; $W_1 = V_{ph} I_{ph} \cos\phi$ — Power measured in a single phase Key Points: Applicable only to balanced 3-phase loads (star or delta).; Requires access to the neutral point for star connection.; If used on an unbalanced load, the reading will only represent the power of that specific phase.; For unbalanced loads, the two-wattmeter method (Aron's method) is standard. Advantages: Simple and cost-effective setup.; Reduces the number of instruments required for measurement. Disadvantages: Cannot measure power in unbalanced three-phase systems.; Requires a neutral connection which may not be available in delta systems. Applications: Industrial monitoring of balanced three-phase motors.; Educational laboratory experiments demonstrating power in balanced circuits. Additional Info: Option A: Power factor unity does not justify the method; it is a load condition, not a requirement for the measurement technique.. Option B: Supply frequency is irrelevant to the validity of the method; it works at any standard frequency.. Option D: Supply voltage is merely a rating parameter; it does not validate the measurement technique. Answer: C is correct — The one-wattmeter method requires the load to be balanced so that the total three-phase power can be calculated by multiplying the power of a single phase by three. Exam Tip: Always remember: Two-wattmeter method works for both balanced and unbalanced loads, but one-wattmeter method is exclusively for balanced loads.

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