| Multiphase fault-tolerant permanent-magnet synchronous machines and drives are key technologies for improving the reliability of drive train in battery electric vehicles.Taking advantages of nonoverlapped end winding,large winding inductance and low mutual coupling between phases,permanent-magnet synchronous machines with fractional-slot concentrated windings(FSCWs)have low fault probabilities,distinguishing isolation features and less performance degradations,making it one of the best choices for safety-critical applications.Multiphase machine system highly depends on close integrations of machine design,drive,fault analysis and post-fault control.Current research in this area is mostly focused on machine design intended for better isolation features,tradeoff between system cost and fault-tolerant capability,multiple feasibilities of post-fault control strategies and analyses of severe winding failures.This thesis examples a six-phase permanent-magnet synchronous machines with FSCWs and focuses on machine design,fault-tolerant inverter topologies and remediation methods for winding failures.The materials are summarized as follows:On machine design level,a harmonic leakage coefficient calculation method for multiphase FSCW permanent-magnet machines adopting close slot and pole combinations has been investigated.The normalized harmonic leakage coefficient calculation method for the investigated machines takes phase number,phase belt,and winding layer into acount,highlighting their impact on the harmonic leakage component of FSCW permanent-magnet machines having different slot and pole combinations.On this basis,machine candidates with high fault-tolerant capabilities have been investigated.Three typical six-phase FSCW permanent-magnet machines have been selected and compared in terms of winding-generated flux independency and magnetic coupling influenced by saturation effect.Research results show that the machine scheme with two adjacent coils per phase enjoys better magnetic isolation feature,lower magneto-motive force(MMF)harmonics and feasibility of modular stator design.On motor drive level,fault-tolerant PM machine drive converter topologies that seek to minimize the number of switches have been investigated while retaining as much of their original pre-fault performance capabilities as possible.Based on winding segregation and auxiliary switches,several six-phase drive topologies have been selected and compared with conventional half-bridge,full-bridge and redundant-leg converters,including the six-phase seven-leg,eight-leg and night-leg topologies.All investigated topologies use fewer half-bridge phase-legs than the baseline cascaded full-bridge topology.The selected six-phase converter topologies are evaluated to compare their switch parts count and post-fault performance.On this basis,the space vector pulse width modulation(SVPWM)under faulty modes for a promising six-phase eight-leg topology has been investigated.And,a reconfigurable full-bridge inverter has been designed.On post-fault control level,three key factors influencing post-fault remediation methods,i.e.copper loss minimization,neutral point constraint and exciting current waveform,have been investigated.Determined by different key factors,four post-fault remediation methods for single-phase and two-phase open-circuit faults are derived and compared.In addition,six-phase permanent-magnet machines with different phase belts(30 degree and 60 degree)are compared in terms of their fault-tolerant capabilities under winding open-circuit faults.As for winding terminal short-circuit failure,analytical modeling of short-circuit current(SCC)is developed,which shows that the SCC contains third-harmonic component.The post-fault remediation methods for a single-phase short-circuit fault are investigated based on analysis of torque pulsation due to interactions of harmonic components of SCC and back-EMF.Analytical and numerical results show the importance of counting the third harmonic component in the analytical prediction of SCC and remediation methods for winding terminal short-circuit faults.A general analytical model for inter-turn short-circuit faults is developed,which employs a novel T-type equivalent circuit.The proposed model is used in conjunction with magnetic circuit analysis to invesitigate the fault characteristics of inter-turn short-circuit faults in permanent-magnet machines with FSCWs.The impact of several key variables on the amplitude of the inter-turn SCC are discussed,including the impact of the position of short-circuited turns,the number of short-circuited turns,the motor speed,the number of turns per slot and the winding resistance,highlighting how to deal with those faults in design procedures.Besides,an inter-turn fault detection method using high-frequency signal injection has been proposed based on the equivalent circuit.Two methods that can suppress the inter-turn SCC are invesitigated,i.e.shorting the remaining turns of the same phase winding and applying controlled current in the remaining turns.Research results show the effectiveness of the method of combining the proposed equivalent circuit with magnetic circuit analysis upon fault analysis and diagnosis of winding inter-turn short-circuit faults.Finally,a six-phase permanent-magnet machine with FSCWs as well as a reconfigurable six-phase full-bridge inverters have been manufactured to build an experimental setup.Experimental varifications have been carried out to validate the fault characteristics and post-fault remediation methods of winding open-cirucit,winding terminal short-circuit and inter-turn short-circuit faults.Generally good agreements between theoretical and testing results have been achieved.It is found that the steady-state terminal SCC of the investigated prototype is 0.9 per unit.The single-turn SCC at maximum motor speed can be suppressed to 2.3 per unit by shorting the remaining turns of the same winding. |
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