Research On Overmodulation Strategies For SVPWM Inverter To Dynamic Performance Of AC Motors

With the development of power electronics, micro-processor and control technology, rotor flux oriented vector control system of AC motor has been widely used for its excellent performance. In asynchronous motor vector control system based on SVPWM inverter, when the reference value of the speed or the load changes, the reference voltage vector may be in the outside of the hexagon which is composed by the basic space vectors. Then dynamic overmodulation happens and the reference voltage vector should be limited in the hexagon by the overmodulation strategy. Different overmodulation strategy will bring different dynamic performance for the whole system. So in this paper three overmodulation strategies which have been perfected are studied by MATLAB simulation in how to influence the motor dynamic performance. Furthermore, the principle is analysed in theory.Firstly, based on the dynamic equation of three-phase asynchronous motor in the two-phase stationary frame, this paper designs the rotor flux observer with the rotor flux orientation, and so the excitation current component and the torque current component are decoupled. Also, the simulation model of asynchronous motor vector control system based on SVPWM is established. In vector control, the current control has an important impact on the system's performance. So in order to improve the system's performance, the synchronous current control and EMF forward compensation are used in the vector control system.After analysing the three existing overmodulation strategies, this paper perfects the overmodulation strategies and builds the overmodulation model of asynchronous motor vector control system. During the overmodulation, the new voltage vector of overmodulation strategy 2 or 3 is still in the outside of the hexagon when the original reference voltage vector is closed to the boundary of any two sectors. So the two overmodulation algorithms are not applicable in this region. For the above shortcomings, this paper perfects the two overmodulation strategies so that they can be used in the entire region. After that, simulation is carried out under the change of the speed and the load reference values. In the condition of acceleration and load increasing, the dynamic performance of overmodulation strategy 2 is better than 1's, and overmodulation strategy 3 is the best which has the minimal response time. In the condition of load reduction, overmodulation strategy 1 and 2 can quickly enter the stable state. However, overmodulation strategy 3 has a problem. The dynamic response time is very long. So, this strategy has its limitation.Through analysing the magnitude and the phase of the voltage vector, this paper explore the principle of how the three overmodulation strategies influence the dynamic performance which explains the reason for the different response time. In the overmodulation region, the new voltage vector's magnitude of strategy 2 is always larger than strategy 1's which leads to better dynamic performance. And in the process of acceleration and load increasing, the new voltage vector's phase of strategy 3 is always ahead of the strategy 1 and 2's. That is the reason why strategy 3 has faster response and better transient performance. However, in the process of load reduction, the phase relationship between the new voltage vector and the original voltage vector is irregular, so the overmodulation strategy 3 has the problem of long response time which needs to be improving.