Electrical Engineering › Section L · Power (ac) and resonance › Topic L.4
Wattmeters and effective resistance
A voltmeter and an ammeter tell you the apparent power, never the real power. A wattmeter measures what is actually used, and with it you can find a component's true ac resistance, which is often well above what an ohmmeter shows.
In this topic
- Why voltmeters and ammeters are not enough
- How an electrodynamometer wattmeter works
- Connections and ratings
- Digital power meters and power-factor meters
- Effective resistance
- What raises effective resistance
- Worked example: measuring a coil
- Worked example: adding an iron core
- Application: demand charges
- Common mistakes to avoid
- Key takeaways
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