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In this paper, a generalized pulse width (PW) modulator is proposed which can control three-leg center-split voltage source inverters (VSIs) and four-leg VSIs from two-level to multilevel topologies. VSIs with a neutral wire connection are important for power electronics applications in three-phase four-wire systems, such as active power filters and uninterruptible power supplies. For medium and large capacity applications, a multilevel three-phase four-wire VSI is a good alternative to a two-level one. However, implementing the pulse width modulation for three-phase four-wire VSIs involves complex calculations, and control algorithms differ for VSIs of different levels and topologies. The proposed generalized PW modulator controls VSIs based on voltage-second approximation, but its implementation is much easier than space vector modulation. Field programmable gate array has been chosen to implement the PW modulator due to its fast prototyping, simple hardware, and software design. Simulation and experimental results are provided. The proposed generalized PW modulator was also applied to the control system of three-phase four-wire active power filters to show its validity.