Minimizing the Power Losses of Variable-Frequency Drive by Using SiC MOSFETs

Abstract

Power semiconductors play an important role in the fast development of power electronic de-vices. During last several decades power semiconductor switches have gone through evolution from thyristors and bipolar transistors to metal-oxide-semiconductor field-effect transistors (MOSFET) and insulated-gate bipolar transistors (IGBT). Traditionally these devices have been implemented by using silicon (Si). On the other hand, Si-based technology is quite mature and major innovations with them are hard to achieve. Wide bandgap materials (WBG) such as silicon carbide (SiC) and gallium nitride (GaN) have become dominant field of study during the 2010s. They are showing outstanding electrical properties compared to older Si-based technology and are believed to be the next revolutionary generation of power semiconductors. One major field of power electronic devices are variable-frequency drives (VFD) that control the rotation speed of an electrical motor. In this thesis the usage of SiC MOSFET in the line and motor converters of VFD is studied. Studied VFD is used to control the rotation speed of an ele-vator motor and thus the speed of an elevator car. In the original device the line and motor con-verters are implemented with Si IGBT modules and during the thesis these modules are replaced with SiC MOSFET modules. Much lower power losses in the semiconductor switches are believed to be achieved because of much lower switching losses of the SiC MOSFETs. Power losses of the device are first modelled and measured with Si IGBT modules. Based on these measurements about 1 kW power losses takes place in the device at the nominal motor current, 24 A. The distribution of these power losses is analytically modelled. Based on this mod-elling, about 46 % of the total losses are taking place in the Si IGBT modules in the line and motor converter and 19 % of the losses are taking place in single-phase inductors used in the LCL-filter of the device. Power losses taking place in these inductors can be reduced by increasing the switching frequency of the line converter which enables the usage of lighter filtering. Lighter filter-ing means that lower inductance is required, and the number of turns can be reduced in these inductors. Because of lower switching losses of SiC MOSFETs, switching frequency of line con-verter can be increased without dramatically increasing the switching losses of the SiC MOSFETs. Power losses of the device are modelled with SiC MOSFETs and based on models about 28 % power savings can be achieved with SiC MOSFETs. Power losses of the device are measured with SiC MOSFETs and was seen that power losses have decreased even more than was as-sumed, about 31 %. Next, the gate voltage of the SiC MOSFETs is increased to 20 V from the original 16 V that was used with IGBTs. With this change, even higher power savings are as-sumed to be achieved because on-resistance of MOSFETs should decrease with higher gate voltage leading to lower conduction losses and switching time of the MOSFETs should decrease with higher gate voltage leading to lower switching losses. Losses are again measured and is seen that the losses are higher with increased gate voltage. Current and voltage waveforms of MOSFETs are measured with oscilloscope and is seen that higher gate voltage causes much higher current oscillation which causes additional losses in the inductors cancelling out the lower conduction losses of SiC MOSFETs. Five new variants of single-phase inductors are designed for switching frequencies between 10 and 30 kHz. With these new variants about 1,4 – 6,6 % additional power savings are assumed to be achieved with higher switching frequencies. Losses are measured with all new variants and was seen that maximally about 1,4 % power savings is achieved at 14 kHz switching frequency.