Control of current harmonics in dual three-phase permanent magnet synchronous machine systems

Compared to conventional three-phase permanent magnet synchronous machines (PMSMs), dual three-phase (DTP) PMSMs exhibit higher power rating, power/torque sharing capability, and fault-tolerant capability, as well as lower torque ripples. They also maintain the merits of PMSMs, i.e. high power density, high torque density, and high efficiency. However, the major current harmonics are only limited by low impedance composed of resistance and leakage inductance, which makes DTP PMSMs suffer more from current harmonics. This thesis investigates the new and novel control techniques of DTP PMSMs, with particular reference to the control of current harmonics. A novel virtual impedance technique is proposed to increase the machine equivalent impedance of the DTP PMSM system. It enhances the voltage harmonic disturbance rejection capability and contributes to current harmonic suppression in a wide range of frequencies. In addition, the virtual impedance can also reduce the system sensitivity to parameter variation, which helps to enhance the dynamic performance. By using the proposed virtual impedance technique, the current harmonics can be reduced but cannot be completely eliminated or cannot track the required current harmonic references. To solve this problem, a new concept of virtual multi three-phase systems is proposed to enhance the current harmonic control in DTP PMSMs. The multiple synchronous reference frames are combined with the virtual multi three-phase systems for establishing current harmonic regulators. Compared to the existing control methods, the proposed method has higher dynamic current harmonic control capability and stability. The proposed concept is further extended to the current harmonic control of the conventional three-phase PMSMs, and also shows improved dynamic performance and stability. The scaling errors in current measurement will generate non-general current harmonics, which cannot be controlled by current harmonic regulators. An online scaling error correction method is proposed to suppress the current harmonics and torque ripples. The method is based on the injection of a high-frequency carrier voltage, and the control of corresponding high-frequency current harmonics is independent from the torque and speed regulation. The proposed method will not produce any high-frequency torque ripples since it only requires high-frequency injection in the harmonic subspace of DTP PMSMs. In contrast, three-phase PMSMs with high-frequency signal injection usually suffer from significant high-frequency torque ripples. All the investigations have been carried out by theoretical analyses, simulations, and experimental verification.