Power Electronics Circuits
In a sinusoidal PWM inverter, which of the following correctly describes what happens when the modulation index is made larger than 1 (overmodulation)?
- AThe fundamental of the output voltage keeps increasing in proportion to the modulation index, and because there are more pulses, low-order harmonics actually decrease
- BThe output voltage of the inverter becomes zero and a protection function stops the inverter
- CPulses are dropped where the reference exceeds the carrier range, so the fundamental no longer increases proportionally and low-order harmonics increase
Show answer and explanation
Answer: C. Pulses are dropped where the reference exceeds the carrier range, so the fundamental no longer increases proportionally and low-order harmonics increase
When the modulation index exceeds 1, near the peaks of the reference the reference stays above the carrier all the time, so the switches stop changing state there and pulses are dropped. The fundamental therefore no longer increases in proportion to the modulation index, and low-order harmonics such as the 5th and 7th increase. The limit as the modulation index grows is a square wave, whose fundamental amplitude is at most (4/π) × E/2 ≈ 1.27 × E/2. In the high-speed range, where as much output voltage as possible is wanted, overmodulation is sometimes used despite the added distortion.
- A. Beyond a modulation index of 1, the fundamental no longer increases proportionally and low-order harmonics increase.
- B. The output does not become zero; it approaches a square wave instead.
- C. Correct. The linear relationship breaks down and the output approaches a square wave.