USCG Exam PracticeElectrical measuring devices and instrumentation
On a shipboard switchboard, a wattmeter reads true power rather than apparent power because of which design feature?
- A. Its current coil has high resistance, which compensates for inductive loads
- ✓ It uses two coils whose combined deflection is proportional to voltage, current, and the cosine of the phase angleCorrect
- C. It is fed exclusively through potential transformers, which correct for power factor
- D. Its movement is a D'Arsonval type, which inherently responds only to real power
Why B is correct
A wattmeter uses a low-resistance current coil in series with the load and a high-resistance voltage coil across it; deflection is proportional to V × I × cos(θ), so it reads true power in watts and automatically accounts for power factor without any separate correction.
Cited:NEETS Mod. 3 §1-4