Q7Power Electronics
Question
4 marks
What is pulse modulation? List the various PWM techniques. How do these differ from each other?
Answer
Pulse Width Modulation (PWM) controls the output voltage of an inverter by varying the width of voltage pulses; common techniques are single-pulse, multi-pulse, sinusoidal (SPWM), space vector (SVPWM), and hysteresis PWM.
- Single-Pulse PWM: One pulse of controllable width per half-cycle. Simple but has high harmonic content at low-order harmonics. The output voltage is Vo = (4Vdc/π)sin(d) for pulse half-width d.
- Multiple-Pulse PWM: Several equal-width pulses per half-cycle. Increases the effective frequency of harmonics (shifted to higher frequencies). Better harmonic profile than single-pulse.
- Sinusoidal PWM (SPWM): A sinusoidal reference is compared with a high-frequency triangular carrier. Natural switching points give pulses whose widths follow a sinusoidal distribution. Fundamental output = ma × Vdc. Harmonics appear at mf and sidebands — easily filtered.
- Space Vector PWM (SVPWM): Used for three-phase inverters. The reference voltage vector is synthesised from adjacent active and zero vectors in the space vector diagram. Better DC bus utilisation (15% more than SPWM), lower THD.
- Hysteresis (Bang-Bang) PWM: The actual output current is compared with a reference; the switch fires when the current error exceeds a hysteresis band. Variable switching frequency; fast transient response.