JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ADVANCED CONTROL OF ELECTRIC DRIVES Previous Question Paper (September - 2022 - R18)

Sinusoidal Pulse Width Modulation (SPWM) is a technique used to control the output voltage and frequency of an inverter by comparing a high-frequency triangular carrier wave with a low-frequency sinusoidal reference signal. The intersection points of these two waveforms determine the switching instants of the power semiconductor devices.

The principle involves:

1. A reference sine wave (modulating signal) at the desired output frequency is generated.
2. A high-frequency triangular wave (carrier signal) is generated.
3. The two signals are compared using a comparator. When the reference sine wave is greater than the carrier wave, the output is high; otherwise, it is low.
4. The resulting pulse train drives the inverter switches, ensuring the fundamental component of the output voltage is sinusoidal and proportional to the reference signal amplitude.

The modulation index, defined as the ratio of the peak amplitude of the reference signal to the peak amplitude of the carrier signal, controls the magnitude of the fundamental output voltage.

Harmonic elimination techniques in converters aim to reduce or eliminate specific lower-order harmonics from the output waveform to improve power quality and reduce motor torque pulsations. Key techniques include:

1. Selective Harmonic Elimination (SHE): This method involves calculating specific switching angles for the inverter switches such that the amplitudes of targeted low-order harmonics (e.g., 5th, 7th, 11th) are forced to zero. This requires solving a set of non-linear transcendental equations based on Fourier series analysis.
2. Multi-pulse Converters: By using phase-shifting transformers to create multiple sets of three-phase supplies with specific phase displacements, lower-order harmonics are cancelled out at the primary side of the transformer, resulting in a cleaner output.
3. Multilevel Inverters: Increasing the number of voltage levels in the output waveform naturally reduces the Total Harmonic Distortion (THD). By approximating the sine wave with more steps, the harmonic content is shifted to higher frequencies, which are easier to filter.
4. PWM Techniques: High-frequency PWM strategies shift the harmonic spectrum to much higher frequencies, far away from the fundamental frequency, allowing for the use of smaller, passive low-pass filters to effectively suppress the remaining harmonics.