Mathematical Modeling Of Duplex Three-phase Permanent Magnet Synchronous Motor For Design Considering Control Performance

Abstract

This thesis investigates the duplex three-phase permanent magnet synchronous machines (PMSM). Duplex three-phase machine is a motor with two sets of three-phase windings spatially shifted by 30 electrical degrees with independent neutral points, and supplied by two independent three phase voltage source inverters. Compared to typical three-phase machines, duplex three-phase machines can provide following advantages: reduced torque pulsation, reduced acoustic noise, reduced current per phase, and most importantly, improved reliability since the machine can continue to operate even when one winding set is damaged. The improved reliability based on the fault tolerant make the duplex 3-phase more attractive for safety-critical field, such as ship propulsion, aircraft, electric vehicle. In addition to above, another advantage that a duplex three-phase machine can provide is that it can supplied by two conventional three-phase inverters, which is commercially available. However, when a duplex three-phase PMSM is driven by two inverters, large current harmonics occur, and control becomes very difficult. In case of duplex 3-phase induction machine, the source of current harmonics is known as the harmonics of the applied voltage. But in case of synchronous machine, the problem is more serious. Since the induced voltage of the machine contains harmonics, machine itself acts as another cause of current harmonics. Various methods have been suggested in the literature. However, the interest is mainly focused on control method, and there is a lack of a research from a motor designer’s perspective. The purpose of this thesis is to identify the cause of the current harmonics in duplex three-phase PMSM and to show that it is possible to increase the control performance of the machine through proper design. A mathematical model is presented for this purpose. The model includes voltage, current, resistance, inductance and flux linkage with harmonics. Generally, it is assumed in the model for control schemes that machines are electromagnetically symmetrical, and all the harmonics in flux linkage and inductance except fundamental component are negligible. The ignored harmonics are treated as external disturbance. Because, from the perspective of motor control, the dynamics of the machine should be described as simple as possible. However, in the model presented in this thesis, the harmonics components are included in the voltage equation. If the cause of the current harmonic is identified through mathematical modeling, the design direction can be set to improve control performance. In this thesis, it is tried to reduce current harmonics of target model by applying skew or rearranging stator winding. It is worth noting that there are several other design methods that can be tried. In addition, this thesis used simulations for verification. The simulation consists of a current controller, SVPWM, and a motor. Results of the simulations were verified by comparison with the current obtained from the experiments. Details are given in appendix.