Introduction:
Voltage source inverters (VSIs) are widely used in various applications, including renewable energy systems, motor drives, and uninterruptible power supplies. PWM (Pulse Width Modulation) switching is preferred in voltage source inverters for the purpose of controlling the output voltage.

Explanation:
1. Advantages of PWM switching:
- Improved output voltage control: PWM switching allows precise control of the output voltage of the VSI. By varying the width of the pulses, the average voltage can be adjusted, enabling the VSI to generate a desired output voltage level. This is particularly important in applications where a stable and accurate output voltage is required.

2. Other benefits of PWM switching:
- Reduced harmonic content: PWM switching helps in reducing harmonic content in the output voltage of the VSI. By using high-frequency switching, the VSI can generate a nearly sinusoidal output waveform with minimal harmonic distortion. This is crucial in applications where harmonic distortion can cause interference with other electrical equipment or affect the performance of connected loads.

- Improved efficiency: PWM switching helps in improving the overall efficiency of the VSI. By controlling the switching frequency and duty cycle, the VSI can operate at higher efficiency levels, minimizing power losses and maximizing energy conversion.

- Reduced filter size: While reducing filter size is not a direct advantage of PWM switching, it is indirectly related. PWM switching allows the generation of a nearly sinusoidal output waveform, which reduces the need for extensive filtering. A smoother output waveform means that the filter requirements can be reduced, resulting in smaller and more cost-effective filters.

Conclusion:
In conclusion, PWM switching is preferred in voltage source inverters for the purpose of controlling the output voltage. PWM switching provides improved output voltage control, reduced harmonic content, improved efficiency, and indirectly contributes to reducing the filter size. These advantages make PWM switching a suitable choice for various applications where precise voltage control and high-quality output waveforms are required.