Comparison Between A Three and Two Level Inverter Variable Flux Machine Drives For Traction Applications

This paper compares two electric drives, a two and three level neutral point clamped variable flux machine drive. The variable flux machine uses Aluminium Nickel Metal Cobalt (AlNiCo) Permanent Magnets (PMs) which have a lower coercive force. The PMs can thus be demagnetized and remagnetized on the fly by the use of a current pulse. The three level inverter is used due to a requirement for high performance in traction applications. Due to the complexity of the three level inverter as compared to the two level inverter and the nature of the variable flux machine, this paper conducts a comparative simulation study between the two inverters particularly during de-magnetization effects. The simulation studies were conducted at two switching frequencies, 10kHz and 20kHz respectively. Parameters of concern were the Neutral Point (NP) voltage ripple, torque ripple and the d-axis current ripple. An already existing modulation strategy for balancing the two dc bus capacitors on the three level inverter is used. Overall the three level inverter produced a NP voltage ripple of below 0.5% of the dc bus voltage even during de-magnetization stage. The three level inverter produced the lowest torque ripple of less than 7 % at a switching frequency of 20kHz. The two level inverter drive had a d-axis current ripple of more than 200% as compared to the three level inverter drive. This means that a three level inverter promises a lower iron loss contribution in the VFM.