Reading of a dynamometer wattmeter connected in an AC circuit: determine what the instrument indicates under normal operation.

Introduction / Context:An electrodynamometer wattmeter has a current coil in series with the load and a pressure coil across it. The interaction torque is proportional to the product of instantaneous voltage and current averaged over a cycle. The question asks which power quantity the pointer shows in AC.

Given Data / Assumptions:

• AC steady state; frequency is constant.
• Proper connections and instrument calibration.

Concept / Approach:

Average (real) power in an AC circuit is P = V_rms * I_rms * cosφ. The dynamometer’s average torque is proportional to this P, so its steady reading corresponds to true power. Apparent power S = V_rms * I_rms differs by the power factor and cannot be read directly from a single wattmeter.

Step-by-Step Solution:

Verification / Alternative check:

For a pure inductor (cosφ ≈ 0), the wattmeter reads nearly zero, confirming it measures real—not apparent—power.

Why Other Options Are Wrong:

• Apparent/peak/instantaneous/rms power are different quantities; the instrument is not designed to indicate these directly.

Common Pitfalls:

• Equating wattmeter reading to VA without accounting for power factor.

Final Answer:

average power (true power)