Common-Mode Voltage Characteristics of Matrix Converter According to PWM Method

Abstract

Matrix converter (MxC) is a direct frequency conversion device without intermediate dc link voltage. Three-phase MxC consists of nine bidirectional switches that allow PWM control of output voltages and input currents. It is fully regenerative and has low input current harmonics with unity power factor. Figure 1 shows basic topology of matrix converter. It shows matrix converter-driven ac machine and common-mode noise current paths. The high change rate of the common-mode voltage has been reported as a main source of motor winding failures and motor bearing damages.We analyzed output voltage distortions due to the commutations and proposed a new three-phase modulated PWM method that improves voltage control linearity for the low speed ac drives by MxC. In the three-phase modulated PWM method pulses are controlled to avoid incomplete commutation. Detailed analysis and experimental results are presented in Fig. 2.Moreover, Fig. 3 shows the common-mode voltage FFT analysis result in each PWM method. A dashed line and a solid line show the FFT results with the two-phase and the three-phase modulated PWM methods respectively. At around switching frequency of 8 kHz, both spectrums have almost same level because dv/dt patterns are similar. However, in low frequency region, solid line shows lower common-mode voltage level as expected. Consequently, the three-phase modulated PWM shows lower common-mode voltage level especially in low frequency region.Common-mode voltages and resulting bearing currents of VSI and MxC are investigated. In two-level VSI case, peak value and dv/dt of the common-mode voltage are high. In two-phase modulated MxC, overall peak-to-peak common-mode voltage is same as two-phase modulated VSI. However, dv/dt becomes smaller than VSI. When three-phase modulated PWM method is used that is based on Emid, the peak value of common-mode voltage and dv/dt can be reduced.

Realization of SVM Methods With Carrier-Based Dipolar PWM for Matrix Converters

“…Thus, it can be concluded that the direct SVM method or <2u1d> PWM with mid-phase reference not only has good performance but is also easiest to implement for the CBDM algorithm with only one PWM mode and only one phase reference. This switching pattern is the same as that of commercial matrix converters, which were presented by Yaskawa Electric [45]. The experimental results demonstrate that SVM methods, which are otherwise complex and require intensive numerical solutions, can be easily realized using the CBDM.…”