The effect of sampling rates on the performance of a three phase PWM inverter and choice of appropriate sampling rates

Pulse Width Modulated (PWM) inverters are most commonly used for grid interfacing in renewable energy systems and for speed control of induction motors in Variable Frequency Drives (VFD), wherein the control computations are implemented in digital signal controllers (sample-based in nature) and are highly complicated. Sampling interval is a critical parameter in PWM control algorithms, because the controller is required to complete one set of control computations within one sampling interval. This is owing to the fact that PWM control algorithms typically involve several axis transformations during a single sampling interval, and consequently take up thousands of instruction cycles, as the trigonometric transformation equations are implemented in modern processors using iterative procedures. This paper presents the effect of sampling frequency on the performance of a PWM inverter in a VFD for speed control of an induction motor. A step by step procedure for sampling rate selection is also proposed. A simulation study is done using MATLAB/Simulink to analyze the performance of the PWM inverter under variable frequency operation. At a fundamental frequency of 50 Hz, a substantial reduction in the THD, by about 9.12% in the voltage and about 39.01% in the current is observed with the proposed increase in sampling frequency by about ten times. Also, a reduction in the % error in voltage by about 5.3% is observed.