In this research the behavior and mitigation of turn-to-turn faults in PMSMs are studied. The challenge with PMSMs under a turn-to-turn fault is with the rotor magnets. Typically rare earth magnets are used in PMSMs, due to the high remnant flux density and excellent coercivity. These characteristics make rare earth magnets the strongest commercially available. Thanks to this, healthy PMSMs have high torque density and are suitable for traction and power generation systems. However, what makes them excellent machines also makes them dangerous in the event of a turn-to-turn fault. The problem arises when a fault occurs and the magnetic field produced by the magnets continuously contributes to the fault by electromagnetic induction. Magnets are capable of inducing high voltages in the windings and they cannot be turned off during fault conditions. In post-fault operation, if the rotor continues spinning, even a full inverter disconnection will not assist in the mitigation of the fault current. This research demonstrates how to detect a fault and reduce the fault current by controlling the current through the remaining healthy coils. The main idea is to generate a magnetic field, using the remaining healthy coils, to cancel some of the magnetic field created by the magnets. Although magnets cannot be turned off, their effect in the faulted area can be minimized using this technique. The technique applies the principle of the field-weakening control strategy to reduce the magnetic flux linking the fault, and therefore reduce and gain a degree of control over the fault current. This mitigation technique keeps the fault current within acceptable levels and if conditions allow, the faulted PMSM can continue operating with a reduced power capacity. This has the potential of extending the post-fault life span of the motor, which could allow for continuous operation at least for a limited time.Two fault tolerant PMSMs, with fractional winding configuration, were designed to test the proposed mitigation technique. The difference between the motors is in the number of layers in the winding and the slot/pole/phase combination. One machine has a single layer winding using a 2/5 slots/pole/phase combination, while the other has a double layer winding with a 1/2 slots/pole/phase combination. Through experimental and finite elements analysis this dissertation demonstrates the effectiveness of the proposed fault mitigation technique in regulating the fault current during a turn-to-turn fault.
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