Power Electronics Circuits
A three-phase inverter on a 400 V DC supply is operated with sinusoidal PWM at a modulation index of 1. Which of the following is closest to the RMS value of the fundamental of the output line-to-line voltage?
- AAbout 141 V
- BAbout 245 V
- CAbout 283 V
Show answer and explanation
Answer: B. About 245 V
The fundamental of each phase leg output has an amplitude of m × E/2 = 200 V, whose RMS value is 200/√2 ≈ 141 V (the phase voltage). The line voltage is the difference between two phases, with a magnitude √3 times the phase voltage, so its RMS value is √3 × 141 ≈ 245 V, that is, (√3/(2√2)) × m × E ≈ 0.612 × 400 ≈ 245 V. To extend this limit slightly, adding a third harmonic to the three-phase references and space vector modulation are often used.
- A. This is the RMS phase voltage (voltage with respect to the midpoint). The line voltage is √3 times larger.
- B. Correct. 0.612 × 400 ≈ 245 V.
- C. This is the DC voltage divided by √2, which is not the formula for the line voltage with sinusoidal PWM.