Power Electronics Circuits
An inverter leg on a 600 V DC supply is operated with sinusoidal PWM at a modulation index of 0.8. Which of the following is the amplitude (peak) of the fundamental of the leg output voltage with respect to the midpoint of the DC supply?
- A240 V
- B300 V
- C480 V
Show answer and explanation
Answer: A. 240 V
The modulation index m is the reference amplitude divided by the carrier amplitude. In the linear range with m of 1 or less, the amplitude of the fundamental of the leg output voltage is m × E/2, which here is 0.8 × 600/2 = 240 V. At m = 1 it reaches its maximum of 300 V (E/2), and a higher voltage cannot be produced while staying with sinusoidal PWM. Changing the modulation index adjusts the magnitude of the AC output voltage without changing the DC voltage.
- A. Correct. 0.8 × 600 ÷ 2 = 240 V.
- B. This is E/2, the maximum at a modulation index of 1.
- C. This multiplies E rather than E/2 by the modulation index. The leg output swings between ±E/2 about the midpoint.