USCG Exam PracticeElectrical measuring devices and instrumentation
A wattmeter is installed on a switchboard to monitor a ship's generator output. Unlike simply multiplying the voltmeter and ammeter readings together, the wattmeter gives a more accurate power reading because it:
- A. Uses a current transformer to step up the line current for greater sensitivity
- ✓ Automatically accounts for the power factor through the interaction of its current and voltage coilsCorrect
- C. Reads peak values of voltage and current rather than RMS values
- D. Is connected in series with the load, giving it direct access to load current
Why B is correct
A wattmeter's deflection is proportional to the product of voltage, current, and the cosine of the phase angle between them, so it reads true power in watts and automatically accounts for power factor. A simple volt-times-amp product gives apparent power (VA), not true power (W), and will overstate power whenever the power factor is less than unity.
Cited:NEETS Mod. 3 §1-4