Research On Key Technologies Of Five-phase Fault-tolerant Permanent-magnet Synchronous Machine For Pure Electric Vehicles

With the increasing energy crisis and air pollution, electric vehicles(EVs) have drawn wide interests in recent years. The safety and reliability are vitally important for EVs which are closely related to the human routine. Different from conventional hybrid EVs, electric drive system is the only power source for pure EVs. In case of electric drive system failure, the pure EVs will break down. Hence, the electric drive system with high fault-tolerant capability is one of the most important research directions for EVs. In order to improve the fault-tolerant capability, this paper investigates a kind of five-phase fault-tolerant permanent-magnet synchronous machine(PMSM). The five-phase fault-tolerant PMSM has characteristics like high reliability and high fault-tolerant capability, which can be the solution for the next generation EVs. Additionally, this technique can be extended to the fields like ship propulsion and aerospace, where high reliability is highly demanded. This paper focuses on several key problems of the five-phase fault-tolerant PMSM, and the major work can be summarized as follows:Firstly, the electromagnetic schemes and parameter calculation of the five-phase fault-tolerant PMSM are investigated. Fundamental design rules of the multi-phase PMSMs are analyzed, and the electromagnetic schemes which feature single- and double- layer fractional-slot concentrated winding(FSCW) are proposed with the application background of EVs considered. By thorough analysis of both the single- and double- layer FSCW, a novel hybrid single/double- layer FSCW is developed. Since the air-gap magnetic motive force(MMF) of FSCW has sub harmonic with pole-pair number of 1, a kind of unequal-turn FSCW is developed to cancel the sub harmonic. By comprehensive comparison of the single-layer, double-layer and hybrid single/double- layer FSCWs, the selection rules of electromagnetic schemes for the multi-phase fault-tolerant PMSM are proposed. The equations for calculating the slot-leakage inductance of FSCW are derived with the left/right-layered characteristic of FSCW considered. The conventional method for calculating the harmonic leakage inductance has characteristics like complex calculation, limited superposition dependence and low accuracy, hence, the equations for accurately calculating the harmonic leakage inductance are derived for FSCW, which provides guidance for the design of multi-phase fault-tolerant PMSM.Secondly, open-circuit fault-tolerant control strategies of the five-phase fault-tolerant PMSM are investigated. The topologies of the drive circuit of five-phase fault-tolerant PMSM are analyzed. With the application background of EVs considered, a kind of fault-tolerant control strategy, which has characteristics like reconfiguring round rotating MMF and setting no confine on neutral current, is developed to deal with different open-circuit fault conditions. A kind of open-circuit fault-tolerant control strategy, which is based on the principle of obtaining maximum ratio of torque to current loss, is developed for full-bridge controller. The abovementioned fault-tolerant control strategies are analyzed by finite-element analysis, and they are compared from aspects of output torque and efficiency. The fault-tolerant operation characteristics of five-phase fault-tolerant PMSM are investigated.Thirdly, short-circuit fault and its detection method of the five-phase fault-tolerant PMSM are investigated. The typical short-circuit faults are analyzed. To consider the influence of both the winding shape and the actual winding distribution on the short-circuit current, the analytical model of the short-circuit fault with parallel strands considered is developed. As the short-circuit fault occurs in a single strand or two parallel strands, the variation laws of the currents, copper losses and per-turn copper losses in each branch are investigated. A kind of short-circuit fault detection method which is based on a novel search coil is developed. The short-circuit faults that occur in a single strand or two parallel strands are detected by the proposed method.Fourthly, thermal and cooling problems of the five-phase fault-tolerant PMSM are investigated. The 2D finite-element thermal model of the five-phase fault-tolerant PMSM is established. The heat transfer coefficients of the outer housing and the air gap, and the equivalent process and equivalent thermal coefficients of stator windings are discussed. The effectiveness and influence region of water and forced air cooling are studied. The cooling system is designed, and the thermal field distributions of the five-phase PMSM are investigated under both rated and overload conditions. The thermal field distributions of the machine are also analyzed under postfault and short-circuit conditions.Finally, the prototype machine and controller of the five-phase fault-tolerant PMSM are built, and the test bench is established. The back electromotive force(EMF), torque and efficiency map of the prototype are measured. The magnetic isolation ability and short-circuit proof ability are evaluated. The fault-tolerant control strategies, which are based on reconfiguring round rotating MMF and obtaining maximum ratio of torque to current loss, are validated. The experimental results show that the investigated five-phase fault-tolerant PMSM enables fault-tolerant operation by adopting the developed fault-tolerant control strategies.