Triangular Carrier Based Synchronous PWM for Operating High Power IM Drives at Odd Non-Triplen Pulse Numbers

Abstract

Two-level voltage source inverter (VSI) fed high power induction motor (IM) drives ( $\geq 750$ kW) are deployed in numerous industrial applications including traction and energy sectors. Usually, such motor drives operate at switching frequency ( $ f_{sw}$ ) restricted below 500 Hz due to high switching energy losses in VSI. For a fundamental frequency ( $ f_{1}$ ) rated at 50 Hz, pulse number ( $P$ , ratio of $f_{sw}$ and $f_{1}$ ) drops below 9 near rated $f_{1}$ . Traditionally, triangular carrier based synchronous sine triangle (SST) pulse width modulation (PWM) technique is used for these drives for maintaining reasonable line current quality and ease of implementation. Performance of SST PWM is found to be good at odd triplen (e.g. $3, 9, 15, 21$ ) $P$ due to preservation of waveform symmetries in VSI output voltages. However, large gap in $f_{sw}$ between the pulse numbers 9 and 3, forces the motor drive to operate at non-triplen pulse numbers e.g. 7 and 5, as well, over mid and high ranges of $f_{1}$ . Consequently, motor drives suffer from high current and torque ripples. This paper proposes a triangular carrier based synchronous (TCS) PWM which maintains all waveform symmetries while operating at odd non-triplen pulse numbers i.e 7 and 5. A double Fourier series based analysis is performed for the proposed PWM to compute the fundamental component. Proposed TCS PWM is implemented on a three-phase, 65 Hz, 2.2 kV, 850 kW IM drive, operated in vector control mode, over simulation. Performance of proposed TCS PWM is validated against standard SST PWM in terms of total harmonic distortion in line current ( $I_{THD}$ ), capacitor current ripple, power loss in the VSI and torque ripple. Theoretical findings are validated further experimentally on a three - phase 50 Hz, 415 V, 3.7 kW IM drive in open-loop V/f and vector control modes. Simulation and experimental results indicate that proposed TCS PWM performs significantly better than SST PWM over mid and high ranges of $f_{1}$ .