USCG Exam PracticeElectrical measuring devices and instrumentation
A wattmeter reads true power in an AC circuit rather than apparent power because the instrument:
- A. Uses only a high-resistance voltage coil connected across the load
- B. Multiplies the RMS voltage by the RMS current without regard to phase
- ✓ Combines a current coil in series and a voltage coil in parallel, producing deflection proportional to voltage, current, and the cosine of the phase angleCorrect
- D. Is fed exclusively through a current transformer that corrects for power factor
Why C is correct
The wattmeter's deflection is proportional to V × I × cos(θ), where θ is the phase angle between voltage and current. This automatically accounts for power factor, which a simple volt-times-amp calculation cannot do.
Cited:NEETS Mod. 3 §1-4