Precise Volt-Second Measuring Instrument for PWM Voltage-Source Inverters

The magnetic flux linkage is an essential parameter for many control approaches of electrical machines. In most common control algorithms, this quantity is determined either by current-based look-up tables (LUTs) or by integrating the measured pulsewidth-modulated (PWM) voltage. But accurately measuring pulse-modulated voltage is challenging due to its steep slopes, especially for SiC and GaN voltage-source inverters (VSIs) and time delays during switching. Notwithstanding, for modern position sensorless drives, knowledge of instantaneous phase voltages, and thus the magnetic flux or the volt-seconds, respectively, is required, for example for flux linkage estimation realizing high-precision field-oriented control. Common measurement techniques to determine PWM voltages are based on the sample-and-hold method executed with successive-approximation analog-to-digital converters (ADCs). However, this measurement method has the disadvantage that low-cost ADCs cannot capture high-frequency voltage components. High sampling rates of these integrated circuits (ICs) can prevent this issue but lead to rising costs. This article presents a novel measuring technique for precise volt-second sensing that considers high-frequency components in the average value of a PWM period by measuring the volt-second continuously. Thereby, it combines the advantages of analog circuit technology with the performance of a field programmable gate array (FPGA). The presented sensor prototype for volt-second sensing is based on the concept of an analog synchronous voltage-to-frequency converter (SVFC). The concept of this measuring instrument is to save additional expensive components by utilizing existing control hardware. This work shows how these savings are implemented for various applications. For this purpose, the working principle and the characteristic curve of the sensor are demonstrated and illustrated with simulation results. The implementation in hardware and software is then presented in detail before the assumptions are validated and evaluated with measurements from the test bench. Furthermore, the dynamic behavior of the sensor prototype during measurements of PWM voltages is investigated, and an analysis algorithm for the instantaneous volt-second value is developed.