Efficiency Analysis of Drive-train for an Electrified Vehicle

Doctoral thesis, 2015

In this PhD thesis, various solutions to improve the energy efficiency in the electrical drive-train of an electrified vehicle such as modifications of the propulsion inverter or control of the electric machine, are proposed and their benefits are quantified from an energy efficiency point of view. The efficiency analysis is based on modeling of a power electronics inverter, an electric machine and a battery in various drive cycles for an electrified vehicle. Several solutions are studied for the propulsion inverter. It is shown that by replacing a Silicon pn freewheeling diode in the propulsion inverter with a Silicon Carbide (SiC) diode, an average of up to 1.5 % improvement in the drive cycle efficiency can be expected. Furthermore, by replacing the Silicon IGBTs in the inverter with SiC MOSFETs, the drive cycle efficiency in NEDC can be increased between 2 to 5 percent. Several solutions to improve the efficiency in a PMSM (Permanent Magnet Synchronous Machine) are investigated. An improved MTPA (Maximum Torque per Ampere) is implemented accounting for the variable nature of the machine parameters due to saturation and temperature. Moreover, the iron losses are accounted for in the derivations of an appropriate maximum torque per ampere angle. It is shown that the low-speed and standstill torque characteristics of a selected PMSM with high current density can be improved by 7 % when using a proposed MTPA algorithm instead of an ordinary MTPA algorithm where the equivalent circuit parameters of the PMSM are updated online outside the optimization algorithm. Furthermore, the efficiency can also be improved up to 5 % at the low-speed and high-torque operating region. However, the overall energy efficiency improvement for a certain drive cycle is not significant and can be up to 0.2 %. Finally a controllable dc-link voltage for the propulsion inverter is investigated in order to quantify the energy efficiency gain for the inverter and the motor. Their drive cycle energy efficiency improvement is up to one percent when using a SiC based inverter and up to 4 percent for an IGBT based inverter.