A fault tolerant motor drive for electric power steering systems

Abstract

Electric machines are becoming increasingly prevalent in safety critical transport applications, whether as both the main drive components or in auxiliary systems. An automotive electric power steering system is an auxiliary drive system that replaces conventional hydraulic systems due to its high reliability, low size and cost, high security, good road feeling, control stability and operates when required. Permanent magnet AC motors are one of the most favourable choices for this application due to their high torque and power density, low torque ripple and low acoustic noise. The main challenge with PM machines in a fault situation is the drag torque resulting from short-circuit currents. These currents are induced by fluxes from the permanent magnets. This research investigates a 12 slot 8 pole interior permanent magnet motor. It investigates different winding arrangements and winding connections for a dual-lane system and compares them to a single-lane system. The baseline motor has 4 coils in parallel per phase for a single-lane system, and 2 coils per lane per phase for a dual lane system. In a dual-lane system, the stator coils can be connected in three different arrangements which are interleaved, half-half and quarter. The half-half arrangement is the best compromise for the baseline motor, as it produces the highest average torque and medium torque ripple under a symmetrical 3-phase short-circuit fault. A modular winding was implemented on the baseline motor’s stator to reduce drag torque and torque ripple under faulted conditions. However, the stator core saturates leading to higher torque ripple and a torque drop under normal conditions. Therefore, a new modular stator was developed to overcome saturation. This gave higher torque capability due to the wider wound teeth tooth arc used, and hence a higher winding factor. The fault-tolerance of the modular stator is significantly improved due to the higher coil inductance and lower drag torque. In the constant power region, the power is significantly compromised. The knee point speed is affected as the high q-axis inductance limits the availability of the supply voltage at a lower speed. Various approaches are presented that aim to reduce the overall motor inductance or only the q-axis inductance to recover the power drop. Firstly, the baseline motor’s rotor is shaped to reduce the q-axis flux. This is not feasible as the power cannot be fully recovered and the torque ripple becomes considerably high. Secondly, the number of turns is reduced, and the input current is increased to keep the MMF input unchanged. Using this approach, the power drop is fully recovered, but a thicker wire diameter should be used for winding, and higher input current means higher ECU losses. Finally, a novel SPM motor is presented. This overcomes the constant power region torque drop through reducing the q-axis inductance. Compared to the baseline motor, the power and torque density of the motor are considerably higher. The overall stator and rotor stack lengths are shorter. As the end-windings are bigger that affects the motors overall length which might also affect the motor size and packaging. The coils and motor lanes are segregated which helps in reducing the torque ripple under faulted condition. At lower speeds the average torque available within a short circuit or single MOSFET fault within the drive stage is similar to that of the baseline motor. At higher speeds the SPM offers greater average torque capability than the baseline motor.