Commutation effects on motor current and torque in five-phase PM BLDC drives

Five-phase PM BLDC drives are a viable solution for safety-critical applications in road traction and aeronautics sectors. A common supply technique for these drives consists in the injection of square-wave currents into the motor phases, synchronized with the flat-top portion of the back-emf to produce both constant torque and maximum torque per ampere. In practice, the currents deviate from the ideal shape during the phase commutations, which further affects the torque characteristics of the drives. This paper analyzes the current behavior during the phase commutations for the five-phase PM BLDC drives as they exhibit some differences with respect to the three-phase counterpart. Then it uses the outcomes of the current analysis to derive the effective torque developed and the torque ripple exhibited by the drive as a function of the motor speed. The base speed of the drive is also determined. Throughout the paper, the differences from the well-known operation of the three-phase PM BLDC drives are pointed out

Fault Tolerant Power Electronics Systems

Research work reported in this Ph.D. thesis is in the area of power electronics systems, specifically in the sector of electrical drives. A trustworthy operation of power electronics systems in critical applications like electric vehicles, aircrafts, satellites, and so on, has pushed engineers to develop fault-tolerant solutions. Indeed, in such applications it is necessary for the system to continue its operation, possibly with downgraded performance, even under faulty case. Present thesis reports the studied solutions to make fault-tolerant a class of electric drives under faulty conditions. It has initiated by addressing the need and importance of the usage of power electronic systems in the field of transportation sector, in particular in the automobile and aerospace industry. Permanent magnet (PM) brushless (BL) drives have become very popular thanks to their higher torque-per-ampere capabilities. Among the two different types of PM BL drives, namely those with sinusoidal back-emf (BLAC) and those with trapezoidal back-emf (BLDC), the latter ones are preferred for light-duty propulsion such as minicars and scooters, and in aeronautics as control-surface actuators. However, some concern have emerged on the use of electrical drives in such applications with regard to the fault tolerance and the power capability per volume unit. A way to effectively cope with these concerns is the adoption of multiphase drives. In this sense, a five-phase drive is a promising solution as it is the most simple multiphase structure of practical interest. The thesis starts with the study of the phase current and torque behavior in three-phase PM BLDC drive in healthy conditions. To validate the mathematical findings, a study case is used, represented by an electrical drive with in-wheel motor utilized for the propulsion of a city car. Afterwards, various types of faults in voltage source inverter (VSI) of a three-phase PM BLC drive are considered, such as one leg open, one switch open and one switch shorted. Remedial control strategies for the faults of the VSI are envisaged, that enable the three-phase PM BLDC drive to continue to operate even if in a degraded way. The resulting performance is calculated in terms of developed torque and torque ripple. The mathematical findings are substantiated with graphs obtained by simulation. A five-phase PM BLDC drive is successively considered. First, its operation and its torque capabilities are investigated in healthy conditions under ideal square-wave current supply. The torque capabilities are compared to the three-phase counterpart; torque comparison is carried out by keeping motor size constant and by considering two hypotheses: equal phase back-emf and equal phase rms current. Then, the torque available from a five-phase drive is determined under various supply modes, characterized by the conduction of a reduced number of phases; the torque available is determined by imposing an rms phase current equal to the nominal one. Moreover, the current behavior during the phase commutations for the five-phase PM BLDC drives is analyzed as they exhibit some differences with respect to the three-phase counterpart. The outcomes of the current analysis are used to derive the effective torque developed by the drive and the torque ripple exhibited as a function of the motor speed. The base speed of the drive is also determined. Also for the torque results, the differences from the well-known characteristics of the three-phase PM BLDC drives are pointed out. Lastly, an algebraic approach is developed to describe the operation of a five-phase PM BLDC drive in healthy conditions. The approach has led to the formulation of a model of the phase current supply of the motor in healthy conditions. Further, the model has been suitably adjusted to derive the mode (scheduling and magnitude) of current supplying the survival phases in the case of one or more motor open phase faults. The cases of one /two/three open phase faults have been examined and, in the case of two and three faulty phases, the cases of adjacent and non-adjacent faulty phases. For each case, the current magnitude has been found by imposing that the rms value of the current in the most solicited phase is equal to the nominal value, and the torque that the drive is able to develop as well as the maximum value of the torque ripple have been calculated. The obtained results indicate that the reduction in the motor torque as well as the extent of the torque ripple is depending, besides on the number of the faulty phases, on the relative location of the faults. The thesis work also address the evolution of electrical power generation and conversion methodologies in more electric aircraft, fault-tolerant solutions under faulty Hall sensors, and the concepts of dependability and safety aspects. The thesis work has been carried out at the Laboratory of “Electric systems for automation and automotive” headed by Prof. Giuseppe Buja. The laboratory belongs to the Department of Industrial Engineering, University of Padova, Italy