USCG Exam PracticeElectrical measuring devices and instrumentation
On a shipboard switchboard, a wattmeter reads true power rather than apparent power because:
- A. It is fed through a current transformer only, which corrects for power factor
- ✓ Its deflection is proportional to voltage times current times the cosine of the phase angle between themCorrect
- C. It uses a high-resistance voltage coil in series with the load to measure the phase angle directly
- D. It integrates the RMS voltage and RMS current over one full cycle to eliminate reactive components
Why B is correct
A wattmeter uses a current coil in series with the load and a voltage coil across it; deflection is proportional to V × I × cos(θ), which is true power in watts. This automatically accounts for power factor, something a simple volt-times-amp calculation cannot do.
Cited:NEETS Mod. 3 §1-4