Analysis and Control of Current Harmonics in Multiphase Machines in Fault-tolerant Operation against Open-phase Faults

Low-order current harmonics exist in multiphase drives due to machine and converter nonlinearity. In the healthy condition, the characteristics of current harmonics and controllers for harmonic compensation or injection has been well addressed in previous literatures. However, these problems have not been clearly clarified under fault-tolerant operation. This paper analyzed the characteristics of current harmonics under fault-tolerant operation against open-phase faults. It was found that both positive and negative sequence current harmonics exist in all the machine subspaces under asymmetrical open-phase faults, therefore the performance of traditional current harmonics control structure is degraded. Thus, we proposed an improved current harmonics control structure, in which the current harmonics control was performed on both positive and negative harmonic components in all the machine subspaces. Experiments were done on a nine-phase induction machine. The experimental results showed the proposed structure had much better harmonic control performance compared to the traditional one in open-phase conditions, which verified the analysis result and the effectiveness of the proposed structure. With the proposed current harmonics control scheme, torque ripples and winding losses can be reduced for distributed-winding machines, or torque density can be enhanced by harmonic injection for concentrated-winding machines even under open-phase faults.