Circuit and electromagnetism basics
In a balanced three-phase system with a Y (star) connection, the phase voltage (between each phase and the neutral) is 100 V RMS. Which is closest to the line-to-line voltage (RMS)?
- AAbout 141 V
- B200 V
- CAbout 173 V
Show answer and explanation
Answer: C. About 173 V
In a balanced three-phase system the phase voltages are 120° apart, and a line-to-line voltage is the (vector) difference of two phase voltages. Its magnitude is √3 times the phase voltage: 100 V × √3 ≈ 173 V. Conversely, a Y connection with 400 V line-to-line has a phase voltage of 400/√3 ≈ 231 V. In a delta connection the line and phase voltages are equal, and the line current is √3 times the phase current.
- A. About 141 V multiplies by √2, confusing this with the RMS-to-peak conversion.
- B. 200 V adds two phase voltages as if they were in phase.
- C. Correct. 100 V × √3 ≈ 173 V.
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