Overload Capability of Multiphase Machines Under Normal and Open-Phase Fault Conditions: A Thermal Analysis Approach

Multiphase drives are convenient for high-power/high-current applications as they allow the reduction of the phase current for given rated power and phase voltage. Due to their redundant structure, the multiphase drives have intrinsic open-phase fault-tolerant operation capability. This situation may happen when one or more power electronic units are turned off after a fault event, and the drive configuration allows phase disconnection. In this case, the healthy machine phases can be overloaded to keep the torque constant and without any pulsations. The goal of the work is the evaluation of the thermal parameters of the stator windings of multiphase machines to be used in the analysis of both short thermal transients and steady-state operation, during normal and open-phase faults. The approach has general validity and can be applied to any ac multiphase machine having a distributed winding configuration. The prototype used for the experimental tests is an asymmetrical 12-phase induction machine, having four 3-phase stator sets with isolated neutral points. The stator windings thermal model is obtained experimentally, by considering the mutual heat exchange phenomena among the windings when one or more winding sets are disconnected. This thermal model allows at evaluating the proper machine overloading for short thermal transients and steady-state operation in case of open-phase faults.