Extended Overmodulation Operation of Space Vector PWM for Traction Motor Control at Low Switching Frequency

There are several benefits of fully utilization of DC link voltage in railway traction applications in terms of increasing inverter output voltage, satisfying power-torque demand and component sizing. To meet traction system requirements; mostly space vector pulse width modulation (SVPWM) is preferred since it has lower harmonic contents and larger voltage reservoir than the others such as sinusoidal PWM (SPWM). Despite extended linear range of SVPWM, it may not be sufficient for traction systems due to high power requirements and low ratio of switching frequency to inverter output frequency. To overcome this problem, an algorithm having functional in the overmodulation range of SVPWM up to square wave modulation and applicable at low sampling, switching frequencies is proposed. Proposed method has ability of soft transition to overmodulation area and covers continuously all the section. Also, calculation of modulation signals is independent from sampling period. The detailed design method is given to calculate modulation signals to be compared with carrier wave in two different overmodulation sections and implemented. Implementations are used in practical experiments to verify the effectivity of proposed method. Moreover, practical experiments to ensure stability of the method are conducted with high powered traction motor in low switching frequency.