| Permanent Magnet Synchronous Motor (PMSM) has the advantages of high efficiency, high power density and aroused a widely concern in the urban rail transit. The pulsewidth modulation strategy needs to be designed carefully for a wide speed range operation and a lower output current harmonics. The hybrid PWM algorithm is widely used in high-power traction drive system, which uses asynchronous mode at low fundamental frequency, synchronous mode and special synchronous mode, such as central60°modulation and SHEPWM, at middle fundamental frequency, and near the base frequency switching to square wave mode. The principle and realization are analysed. The two special modulation technniques are campared with the regular sampled modulation at low carrier frequency ratio and also campared with each other.The special pulsewidth modulation is valid for steady-state operation. Dynamic modulation errors and high overcurrents are then encountered when the operating conditions change. To overcome this problem, the stator flux trajectory control is used The principle and realization of stator flux trajectory tracking control is analyzed particularly. And the stator flux errors can be eliminated by changing the optimal pulse sequence.A simulation model of optimal pulsewidth modulation and an experimental platform for the PMSM traction drive system is built. The results show that stator flux trajectory control for special modulation can eliminates excessive overcurrents and the harmonic distortion is lower. |
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