Overmodulation Method for Torque Balance of In-Wheel Motor with Dual-Motor Dual-Inverter Considering Angle Offset

Abstract

An overmodulation strategy is proposed for an in-wheel motor where two motors and two inverters are installed to a central sum gear. In dual-motor dual-inverter single shaft systems, i.e., dual-motor propulsion systems, the output voltage magnitudes of two inverters are not the same in the overmodulation range due to the angle offset. It causes torque imbalance between two motors. In this thesis, to maintain torque balance in the overmodulation region, holding vectors are defined by taking into account the angle offset between adjacent vertices of two voltage hexagons. Around these vectors, the holding region is also defined by the holding angle. If a reference voltage vector is in the holding region, it is modulated to the holding vector. Otherwise, it is modulated to the vector on the arc of inscribed circle. For a given modulation index (MI), the holding angle is obtained based on the Fourier series expansion. Therefore, the proposed overmodulation method keeps the magnitude of output voltages of two inverters the same even if two motors have the angle offset. Further, it has the characteristic of maintaining the linearity between the reference and output MIs to the maximum MI. The effectiveness of proposed overmodulation method is demonstrated by simulation and experimental results.