This paper presents optimal fault-tolerant control of multi-phase permanent magnet (PM) machines for open-circuit and short-circuit faults. Control techniques are developed for general n-phase PM machines to operate under any of these faulty conditions with minimum output torque ripple and stator ohmic loss. These control technique are based on instantaneous power balance theory. For open-circuit fault, the currents of the healthy phases are controlled in such a manner that the machine continues its operation optimally. A constraint which makes the summation of the phase currents to zero is considered in the derivation of the technique. A control technique is proposed for short-circuit fault-tolerant operation to mitigate the short circuit fault and continue operation under it. In this scheme, the healthy phase currents are controlled in such a way that they maintain the flux linkage in the shorted phase at zero or constant, as well as, produce the desired output torque with minimum ripple and minimum stator ohmic loss. The short circuit fault tolerant technique is also extended for the condition where sum of the phase currents are zero. Finally, a five phase PM machine is considered to demonstrate the control strategies. Optimal currents are calculated for fault-tolerant operation of the machine under various fault conditions. Simulation results are also presented for verification of the control methods.
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