Research On Multiphase Induction Motors Under Open-circuited Fault Conditons

The high power and high reliability electric drive system is the most significant component of the integrated power system(IPS).Multiphase induction motors have broad prospect for these electric driving applications due to their simple construction,robust mechanical structure,good reliability,smaller low-frequency torque ripples,and high power with lower voltage power devices.Especially,how to analyse postfault operation performance and realize fault-tolentant control with redundancy phases after an open-circuited fault happens have been widely studied in literature.In this paper,a 15-phase induction motor driven by a full H-bridge 15-phase inverter under open-circuited fault condition is investigated.The air-gap MMFs under different open-circuited fault conditions are analysed,then steady-state operation performances with symmetrical fault and asymmetrical fault are studied respectively.Further,the dynamic mathematics model for the 15-phase induction motor under open-circuited fault condition is established.Postfault current calculation and control strategy are studied,which have been validated by experimental results of a 45-kW prototype 15-phase induction motor.First,composite current method has been employed to analyse the MMFs of 15-phase induction motor under different open-circuited fault conditions.Arbitrary spatial harmonic forward-and backward-rotating MMFs can be obtained through calculating the matching composite positive and negative sequence currents.Thus,the complicated calculation process of traditional method based on trigonometric function can be avoided.The calculated result indicates that all the harmonic MMFs are still circular rotating MMFs,though the number of harmonic MMFs increases under symmetrical open-circuited fault condition.However,all the harmonic circular rotating MMFs were distorted to ellipse rotating MMFs under asymmetrical open-circuited fault condition.Therefore,the open-circuited fault can be divided into two categories,namely symmetrical open-circuited fault and asymmetrical open-circuited fault.To analyse steady-state operation performance of the multiphase induction motor under symmetrical open-circuited fault condition,the stator and rotor parameters are recalculated.Especially,through introducing different incidence matrices,the stator slot leakage reactance,hannonic leakage reactance and end-winding leakage reactance are recalculated respectively.The equivalent circuit of the multiphase induction motor with symmetrical open-circuited fault is obtained.Then the torque regulation method for multi-phase induction motors with symmetrical fault is investigated under the precondition of constant current amplitude.The calculated result shows electromagnetic torque can be improved by regulating the supply voltage amplitude and frequency synchronously.The maximum torque matching load of the prototype 15-phase induction motor with 10-phase and 5-phase operation are evaluated.To analyse steady-state operation performance of the multiphase induction motor under asymmetrical open-circuited fault condition,composite current method is employed.The positive and negative sequence induced EMFs resulted from fundamental and hannonic forward-and backward-rotating MMFs produced by non-sinusoidal current are obtained.Then a multiphase winding loop voltage equation by matrix considering different stator topologies is established to calculate unbalanced phase currents.When a phase open-circuited fault occurs,only the fault phase resistance in stator leakage impedance matrix changes.Calculated unbalanced currents are compared with those obtained from different open-circuited fault tests of a 15-phase prototype induction machine.The good agreement between calculated and experimental results verifies the accuracy of the calculation approach.Finally,dynamic mathematics model for the multiphase induction motor under open-circuited fault condition is investigated by comparing two different models.The dynamic mathematics model for 15-phase induction motor is established based on 5-phase induction motor mathematics model.Then the postfault current with maximum torque of 5-phase induction motor is calculated under asymmetrical open-circuited fault condition.It is specially point that the torque produced by fundamental and third harmonic postfault currents is less than the one produced only by fundamental postfault currents under the precondition of same RMS current.Postfault control is based on the normal decoupling transformation,so that reconfiguration of the controller is minimized.The postfault current calculation and controller reconfiguration of 15-phase induction motor can be obtained using the same method,which realizes smooth operation of 15-phase induction motor with asymmetrical open-circuited fault.