Research On Fault Tolerant Control For Open Winding Permanent Magnet Synchronous Motor Drive System With Open Phase Fault

Permanent magnet synchronous motors(PMSM)have been widely applied in the transportation,military industry and national defense applications due to their attractive features such as high efficiency and high power factor.However,different types of faults may occur in motor drive system,especially in some harsh environments.One of the most common fault is open phase fault caused by winding end disconnection or unstable power supply.Once the motor winding is disconnected,the phase currents are no longer symmetrical and the torque ripple will increase if the control strategy before the fault occurrence is still adopted,which will deteriorate the control performance and even endanger the whole system.Therefore,it is vital to study suitable fault-tolerant control strategy to redistribute the remaining phase currents and suppress the torque ripple.The open winding PMSM(Open Winding,OW-PMSM)with common DC bus can be considered as a natural solution for open-phase fault because the neutral point has been opened,that is,fault-tolerant control can be realized without changing the system topology after open phase fault.Except the fault tolerance capability,due to the dual terminal power supply mode,the OW system can effectively reduce the converter capacity requirements.With the advantages of high power rating,multi-level effect,flexible control mode and relatively low cost,the OWPMSM system with common dc bus has been paid much attention in recent years.However,due to the existence of zero sequence loop in the dual inverter system with common DC bus,the influence of zero sequence current and third harmonic flux must be considered in the control strategy.Meanwhile,the mathematical model will change when PMSM is under open-phase fault so that the coordinate transformations and control strategy should be redesigned accordingly in order to suppress the torque ripple.Therefore,the target of this paper is to study the modeling and control strategy for OW-PMSM with common DC bus after open-phase fault.The main innovations are as follows:1.Firstly,the fault tolerant vector control based on traditional coordinate transformation is analyzed.It is pointed out that the dq-axis voltage equations are not decoupled and the torque equation contains time-varying parameters,hence the torque ripple can not be effectively suppressed after open phase fault.Thus,the post-fault PMSM mathematical model is reconstructed in this part with considering the third harmonic flux.Two new coordinate transformation matrices are designed for the OW-PMSM system with common DC bus to keep the post-fault torque equation under dq-axis consistent with that before open phase fault as well as to get decouple dq-axis voltage equation,so that the torque control after open phase fault is more simple and accurate.2.A novel fault-tolerant control strategy for OW-PMSM considering the third harmonic flux has been proposed.Based on the new torque equation,the mechanism of harmonic torque generation is studied and the torque ripple suppression caused by the third flux linkage and dqaxis currents is achieved by injecting harmonic component into the q-axis current reference.Considering that the basic voltage vector will change after open phase fault,the voltage vector plane after open phase fault is reconstructed and the duty cycle can be calculated according to the new basic voltage vectors.Compared with the existing literature,the proposed method can minimize the torque ripple after open phase fault and the control system is relatively simple.The experimental results show the effectiveness and applicability of the proposed method.3.Model predictive control strategy based on switching state cost function under open phase fault is proposed.In order to obtain the decoupling voltage equation under dq-axis,a new voltage rotation coordinate transformation is designed.Then the dq-axis voltage equations under the new coordinate transformation are decoupled from each other and have the same form as that before open phase fault,thus the subsequent calculation burden is reduced and the control performance is improved.In order to further improve the current control performance,deadbeat model predictive control strategy is adopted.According to the multilevel characteristics of OW system and the functional relationship between phase voltages and switching states,a new cost function equations based on switching state are designed.Base on the new cost function,the switching signals for dual inverter can be directly calculated,thus the complex modulation in multi-vector model predictive control on the αβ-axis is avoided and the calculation process is simplified.Finally,the experimental results show the effectiveness and application potential of the method.