Analysis of VSI-DTC Fed 6-phase Synchronous Machines

High power drives with multiphase machine utilizes paralleled legs converters to realize required power. In addition to the reduced rating of the used semiconductor devices, higher redundancy level could be achieved. Being a relatively new technology for the combination of VSI-fed operation and salient pole synchronous machines, a through analysis is needed. This paper investigates 6-phase synchronous machines with emphasis on redundancy, fault conditions, the machine's behavior under non-sinusoidal voltage profiles and sensitivity of the design parameters. It is shown that new design thinking is required when considering converter-induced machine losses, especially those on the rotor surface. Redundancy study shows the possibility of operating the machine at half load when one supply system is totally or partially lost without exceeding machine's total losses. When studying fault scenarios, the worst case is found to be 4-phase short circuit (equivalent to 2-phase in a 3-phase machine) for the studied cases. Output from sinus supply voltage study was utilized to understand losses distribution inside the machine and establish an effective comparison with different voltage profiles. A study with square wave and DTC supplies shows higher losses in damper bar, when compared to sinus supply. Voltage profiles of two different DTC schemes are studied which show that with particular well-thought control strategies one can improve the machine losses specially those in the damper bars.